Vol. 67, No. 20 (2018)
SPECIAL TOPIC—Tenth anniversary of the discovery of iron-based high temperature superconductors
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
Mode separation for multimode Lamb waves based on dispersion compensation and fractional differential
2018, 67 (20): 204301. doi: 10.7498/aps.67.20180561
With the rapid development of material science and industrial technology, the application of ultrasonic Lamb wave to the industrial nondestructive testing has received considerable attention due to its advantages of rapidness, high efficiency, high accuracy, and low cost. However, the multimode and dispersion problem of Lamb waves are still challenging. Multimode mixed Lamb wave signals are often present at the same excitation frequency in the actual detection. To separate dispersive multimode Lamb waves overlapped in time and frequency domains, a separation method based on dispersion compensation and fractional differential is presented. The multimode Lamb waves overlapped in time and frequency domains are first compensated by using the dispersion characteristic. Based on the dispersion compensation, the time-delay function is modeled. The function is used as a transfer function. Its inverse is considered as a dispersion compensation function. Then, the amplitude spectra of Lamb waves are divided into fractional order differentials. The parameters of each mode are extracted by using the fitting polynomial between the maximum amplitude and the differential order and that between the peak frequency and the differential order. Its amplitude spectrum is extracted based on its parameters. By combining with its phase spectrum, the individual mode is constructed after the dispersion has been recovered. Simulation and experiments are performed on a 1 mm-thick stainless steel plate. The oblique transducers with the angle of 26 and the central frequency of 3 MHz are used to excite the S1 and A1 mode overlapped Lamb wave signal in the plate. The transducers are coupled with the stainless steel plate by using the ultrasonic couplant. Simulation and experimental analysis show that the present method can not only achieve the separation of time-frequency overlapped multimode Lamb waves, but also guarantee the separation precision. The main advantage of the presented method is the combination of the dispersion compensation and the fractional differential, which solves the problem of mixing with other mode signals after the single mode dispersion has been compensated, and improves the extraction precision of each mode. Therefore, this method can be used for separating the time-frequency overlapped multimode Lamb waves. It is conducible to the signal processing of multi-mode Lamb wave dispersion.
2018, 67 (20): 204101. doi: 10.7498/aps.67.20180696
In this paper, a method of designing the frequency selective structure based on spoof surface plasmon polariton (SSPP) is proposed and demonstrated. According to the applications in different working bands, the designed frequency selective surface (FSS) and metallic fishbone structure array can be combined together to form a new frequency selective structure and satisfy the requirements for practical applications. Meanwhile, a dual-band-pass frequency selective structure with the property of steep cut-off frequency and high-efficiency transmission and inhibition is designed by using this method. The dual-band-pass frequency selective structure is composed of a metallic fishbone structure array and two identical FSSs. The metallic fishbone structure based on SSPP coupling can form a broadband high-efficiency transmission below the cut-off frequency of SSPP on the metallic fishbone structure. When a dual-band-pass FSS is loaded to this metallic fishbone structure array, a dual-band-pass frequency selective structure can be achieved. To improve the impedance matching of the dual-band-pass frequency selective structure, two identical FSSs are respectively loaded to the top and bottom sides of the metallic fishbone structure array. The simulated transmissivities of the dual-band-pass frequency selective structure exceed-0.5 dB in two frequency ranges:3.0-4.1 GHz and 10.5-10.9 GHz. The simulated transmissivities are lower than-10 dB in other frequency ranges:4.7-9.2 GHz and 12.1-18 GHz. The simulated transmissivities are even below-20 dB from 12.4 GHz to 15.5 GHz. The electromagnetic waves can be efficiently transmitted in the passband and restrained in the stopband. Then the dual-band-pass frequency selective structure is fabricated by using the printed circuit board technique and measured in the anechoic chamber. The measured results indicate that the real property of the dual-band-pass frequency selective structure is consistent with the simulated property and this method of designing the frequency selective structure is feasible. After filling the lightweight foam into the gap of the metallic fishbone structure, the mechanical loading property can be highly improved. Therefore, we can realize the design of combined structural and functional performance.
2018, 67 (20): 204201. doi: 10.7498/aps.67.20181144
Sheared-beam imaging (SBI) is a non-traditional imaging technique in which utilized are three sheared coherent lasers for illumination, and detector array to receive the intensity of speckle pattern reflected from the target. Finally the image of target can be reconstructed by computer algorithm from the data collected before. The SBI has some advantages in high resolution imaging for long-distance space targets. However, the wavefront distortion caused by atmospheric turbulence is a key factor affecting the imaging quality of SBI. Therefore, this paper focuses on the influence of wavefront distortion caused by atmospheric turbulence on the extraction of target spectral information. Theoretical model of the influence of wavefront distortion on imaging is established. The effects of low-order and high-order atmosphere turbulence on SBI imaging quality are analysed respectively. It turns out that low-order atmosphere turbulence does not result in poor image quality nor low-resolution, and just change the position of target on the image plane. But the image quality can be degraded when the wavefront root mean square (RMS) value at the target plane, caused by high-order atmosphere turbulence, exceeds /20. Beam emitted from larger aperture becomes more susceptible to perturbing effect, thus forming lower-quality wavefront. Considering that after passing through the atmosphere, beam also travels a long distance to reach the target surface. Targets at different heights will obtain different wavefront quality due to the diffraction of light. Thus the final wavefront quality is determined by turbulence intensity, aperture size and target height. Multi-layer phase-screen model and Hufnagel-Valley model are used to simulate the influences of near-earth (25 km) atmosphere on wavefront distortion at target plane with different imaging distances. Simulation results show that the wavefront RMS value rises with the increase of transmitting aperture diameter, and decreases with the increase of imaging distance. Transmitting aperture sizes in a range from 0.2 times r0 to twice r0 have been recommended for effective imaging by Hutchin[Hutchin R A 1993 Proc. SPIE 2029 161]. However, we find in our simulations that beams on the order of 2 r0 may cause significant wavefront error at short range target, and under some circumstances the clear image of target cannot be reconstructed. The imaging results of SBI at different laser transmitting apertures and different imaging distances are obtained, and evaluated by Strehl ratio. Imaging results show that choosing appropriate transmitting aperture size can effectively improve the imaging quality. But for the short-range targets, aperture size selection range presented by Hutchin can be too broad to be practicable. This paper suggests some approaches to choosing suitable aperture size for SBI system, and also providing a reference for the difference analysis of imaging quality for targets in different heights.
2018, 67 (20): 204203. doi: 10.7498/aps.67.20180528
A kind of dual micro-holes-based in-fiber Fabry-Perot interferometer sensor is proposed in this paper. The theoretical model of the reflection spectrum of proposed sensor is established based on the interference among four light beams, where both the relationships of the spectrum intensity with the length of micro-hole, refractive index (RI) of medium in cavity, transmission loss and reflection loss, and the characteristic parameters of fiber are demonstrated, and the temperature and RI responses of reflection spectra are also simulated. Through machining two micro-holes in single-mode fiber with 193 nm excimer laser, we fabricate the proposed fiber sensor which can be used for measuring the multi-physical quantities, and the corresponding experiments are demonstrated simultaneously. The results show that the sensor has better linear responses to temperature and RI change, and the corresponding linearity is superior to 99%. Due to having two sets of different temperature and RI sensitivities (i.e.-0.172 nm/℃ and 1050.700 nm/RIU; 0.004 nm/℃ and 48.775 nm/RIU) and better linearity, this kind of sensor can be used for measuring the temperature, the ambient RI and even the simultaneous discrimination of temperature and ambient RI. The RI and temperature resolutions are 1.010-5 RIU and 0.2℃, respectively. Furthermore, the sensor can also be used for sensing the gas pressure, and its measurement accuracy can reach to. 3 kPa. Owing to its high sensitivity, stability, small volume and easy fabrication, the sensor will be widely used in sensing technology.
Numerical investigations of interactions between shock waves and triangular cylinders in magnetic field
2018, 67 (20): 204701. doi: 10.7498/aps.67.20181127
Magnetohydrodynamic (MHD) equations are solved by using the CTU+CT (corner transport upwind + constrained transport) algorithm which guarantees the divergence-free constraint on the magnetic field. The interactions between shock wave and heavy or light triangular cylinder are investigated in detail in the cases with and without magnetic field. In the cases of hydrodynamic (B=0 T) and MHD (B=0.01 T), the numerical results indicate that heavy and light triangular cylinders have quite different wave patterns and jet structures after being impacted by a planar incident shock wave. Specifically, a regular refraction and downstream R22 jet are formed in the heavy case, whereas an irregular refraction and upstream air jet are generated in the light case. In the hydrodynamic case, the Richtmyer-Meshkov (R-M) instability and Kelvin-Helmholtz (K-H) instability are induced by the incident shock wave. Hereafter, both heavy and light density interfaces begin to roll up with a series of interfacial vortex sequences. In addition, a main vortex ring is formed in the heavy case, while a vortex dipole passing through the downstream interface is generated in the light case. In the MHD case, both heavy and light density interfaces remain smooth and interfacial vortex sequences vanish. Furthermore, the main vortex ring formed in the heavy cases and the vortex dipole generated in the light cases disappear. Moreover, in the presence of a magnetic field, a detailed investigation demonstrates that Lorentz forces give rise to the transport of baroclinic vorticities to the Alfvn waves. As a consequence, the deposition of interfacial vorticities decreases and the rolling-up of interfaces is suppressed. In the end, the vorticities are transformed into two vortex sheets travelling away from the density interfaces, and the R-M instability and K-H instability are well controlled. The quantitative investigations reveal that for both heavy and light triangular cylinders, magnetic field can accelerate the upstream interface and decelerate the downstream interface, especially for the light triangular cylinder.
2018, 67 (20): 204202. doi: 10.7498/aps.67.20181038
Random source is important for the security of key distribution. In this paper, a novel secure key distribution scheme based on unidirectional injection of vertical cavity surface emitting laser (VCSEL) system is proposed. In the proposed scheme, a chaotic signal without time delay signature is generated by a VCSEL subject to unidirectional optical injection, which is regarded as a master laser. The chaotic signal generated by the master VCSEL is further injected into two slave VCSELs to obtain synchronized bandwidth-enhanced chaotic signals. After that, by sampling, quantizing and XOR operation on the two synchronized chaotic signals, two key streams can be obtained. Based on the well-known spin-flip model, the time delay signature of chaotic signals generated by master VCSEL and the synchronization performance between the master VCSELs and two slave VCSELs are numerically investigated in detail. It is shown that by the unidirectional injection, the chaotic outputs can be achieved in the master VCSEL in a wide range of frequency detuning and coupling strength. More importantly, no time delay signature can be observed in the auto correlation function of the chaotic intensity time series generated by the master VCSEL. Besides, we find that high quality synchronization is achieved between the bandwidth-enhanced chaotic signals generated by two slave VCSELs under the common driving of master VCSEL. The synchronization coefficient between two slave VCSELs increases up to 0.99, and the synchronization coefficient between master VCSEL and salve VCSEL is only 0.74. Note that such a high quality synchronization between two slave VCSELs while relatively low quality synchronization between the master and slave VCSEL is conducible to ensuring the security of key distribution. In addition, the effects of tunable parameters on key bit error rate are considered, and two quantization methods are employed for comparison. Numerical simulation results show that the key bit error rate between two legitimate users is as low as 1%, and the key bit error rate between legitimate user and eavesdropper is higher than 10% in the single-threshold case; the bit error rate can even be as low as 10-6 in the double-threshold case. The influence of parameter mismatch on key bit error rate is also discussed, and it is suggested that two salve VCSELs should be finely matched to ensure low bit error rate. Finally, NIST randomness test is performed for the generated key streams. Hence, the proposed scheme enhances the security of key distribution, which is valuable for further developing the chaos communication systems.
Propagation characteristics of pseudo-Scholte waves at the interface between finite-thickness fluid layer and quasi-saturated porous half-space
2018, 67 (20): 204302. doi: 10.7498/aps.67.20180853
The propagation of interface waves at the interface between a fluid-saturated porous medium and a fluid has been extensively investigated in the last three decades due to its various and wide applications in several fields including earthquake engineering and materials testing. Although the sea floor is usually covered with porous marine sediment, the previous interface wave theories are rarely used for submarine acoustic problems for the following reasons. 1) In addition to hard porous media, unconsolidated soft porous media exist widely in the seabed, which are seldom considered in previous studies. 2) The depth of seawater is limited, and in many cases it cannot be regarded as a half-space. 3) The fluid-saturated porous medium model cannot describe the effect of a small number of bubbles caused by decomposition of organic matter in the sediment. Hence, the present paper focuses on the low-frequency pseudo-Scholte waves at the interface between an overlying fluid layer of finite thickness and a quasi-saturated porous half-space. The overlying fluid is assumed to be ideal compressible water and the quasi-saturated porous media are assumed to be sandstone and unconsolidated sediment and modeled by Biot theory. A fluid equivalent model is used to analyze the effects of the bubbles in the pores. Based on the boundary conditions, the closed-form dispersion equations of far-field interface waves are derived by using classical potential function method. The velocity and attenuation of pseudo-Scholte wave are determined by Newton iteration in a reasonable rooting interval. The analytical expressions of the displacement field and fluid pressure distribution caused by pseudo-Scholte waves are also derived. Then, based on the derived theoretical formulation, the numerical examples of calculations are presented. Our calculation results show that the stiffness of porous medium significantly affects the mode, phase velocity, displacement and fluid pressure distribution of interface waves; the phase velocity of the pseudo-Scholte wave in the finite-thickness fluid/fluid-saturated porous half-space is related to the ratio of the wavelength to the thickness of the fluid layer; the phase velocity of the shear wave is insensitive to a small number of bubbles dissolved in the pores, but the existence of bubbles has a significant influence on the phase velocity of the compressional wave and the pseudo-Scholte wave. Furthermore, the existence of bubbles can significantly affect the distribution of the pore pressure.
2018, 67 (20): 200701. doi: 10.7498/aps.67.20181330
The wolter-1 X-ray focusing mirror can reflect grazing incidence X-ray to the focal plane, which plays an important role in the astronomical detection and other fields due to its good image detecting capability. A geometric model of the optical system is established for theoretically deriving the optical path equations which is useful in this glass based focusing mirror designing, all the design parameters of the focusing mirror can be obtained by solving these equations. In the manufacturing process, the D263T glass is chosen to be the structural material of the focusing mirror due to its light weight and super smooth surface, after a slumping process, the flat glass mirror will have the shape of Wolter-1 X-ray focusing mirror. This slumping process has been used successfully in the manufacturing process of an American mission named The Nuclear Spectroscopic Telescope Array, which was launched in 2012. According to X-ray reflecting theory, the reflectivity of the Wolter-1 mirror can be improved significantly by coating metal film on the surface of the mirror. In this work, an iridium film is coated on the surface of the glass mirror through a vacuum evaporating process. In order to learn the influence of the focal spot caused by the mirror shape tolerance, the morphology of the mirror is tested by using a 3-D laser scan instrument. The results show that 50% of the total test points are located in the tolerance range of-10-10 m, in which the tolerance represents the difference between the actual lens profile and the ideal lens profile. Then the focal spot test is carried out with the help of a visible light test system:a laser collimator is installed in front of focusing mirror as an incidence light source, and a charge coupled device (CCD) is placed in the focal plane to gather the image of the focal spot, by calculating the gray level distribution of the focal spot image taken by the CCD, the energy distribution characteristic of focal spot can be obtained. The experimental results show that the focal length of the focusing mirror is 1.6 m, and the half-power surrounding diameter of the focal spot is 0.33 mm, corresponding to the angular resolution of 0.7 arc min.
2018, 1957 (20): 202801. doi: 10.7498/aps.67.20180834
Nuclear design and neutronic analysis of thermal neutron reactor need high reliable thermal neutron cross sections. Uranium mononitride (UN) is a candidate fuel material for advanced power reactor with its better thermodynamics and accident tolerance. However, in thermal neutron region, reliable thermal neutron scattering cross sections are lacked for UN, which is disadvantageous to reactor physics simulations. The scattering law of the UN fuel material may impact the thermal neutron spectrum and criticality of the reactor systems. Neutron cross sections in thermal range are correlated with energy, temperature, physical and chemical properties of the scattering medium, reflecting the phonon spectra of material itself. In this paper, based on the ab initio method of quantum mechanics, phonon density of states in UN are calculated by VASP/PHONON code, and used for integral to obtain UN heat capacity at a constant volume. Adopting this new phonon density of states, NJOY/LEAPR code is used to generate S (, ) data by thermal neutron scattering theory and NJOY/THERMR utilizes these data to produce thermal scattering matrix in order to investigate thermal kernel effect of UN. Subsequently, thermal neutron scattering cross sections of UN are generated with NJOY code system. Comparison with uranium dioxide (UO2) in the traditional PWR is done. Results indicate that optimized lattice parameter are in good agreement with the database; the optical modes are well separated from the acoustic modes compared with UO2; heat capacity at a constant volume is consistent with experimental value; the inelastic and elastic cross sections of 238U in UN are lower than those of 238U in UO2. N in UN only deals with incoherent part in elastic cross sections. As the temperature increases, elastic cross sections of UN decrease while inelastic ones increase, and cross sections approach to free atom cross section at high energies. Considering the limitations of 14N, the scattering law and inelastic scattering cross sections are also under investigation using 15N in UN compound. This paper's conclusion fulfill the vacancy of thermal neutron scattering cross sections of UN, which laid a foundation for systematic study on the neutronics properties of UN fuel in the light water reactors as well as for the design of new neutron moderators and neutron filer.
ATOMIC AND MOLECULAR PHYSICS
Experimental and theoritical research on the dynamical transmission of 30 keV H+ ions through polycarbonate nanocapillaries
2018, 67 (20): 203401. doi: 10.7498/aps.67.20181062
The ions with different incident energies transmitting through insulating nanocapillaries are studied in various configurations. For the low energy ions transmitting through nanocapillaries, Stolterfoht et al.[2002 Phys. Rev. Lett. 88 133201] have observed the guiding effect. Subsequent studies revealed that the self-organizing charge patches on the capillary wall inhibit charge exchange and the ions are transmitted along the capillary axis direction. The high energies of ions transmitting through nanocapillaries are measured, the main transmission mechanism is multiple random inelastic collisions below the surface, and the charge patches will not affect the transmitted ions trajectories. The transmission features of the intermediate energy ions are different from those of the low and high energy ions. The ion beams with intermediate energies have many applications, so it is necessary to understand the transmission features of the intermediate energy ions though nanocapillaries. Recent studies have focused on the transmission of the intermediate energies ions through the nanocapillaries. In the present work, we investigate thie transmission features, such as the two-dimensional transmitted angular distributions, the charge states and position distributions, and the evolution of the relative transmission rate and the charge purity of 30 keV H+ transmitting through nanocapillaries in a polycarbonate membrane at the angles of-1 and-2. The experimental data clearly show that the transmitted H+ ions consist of the transmitted scattering H+ ions, which are located around the direction of the incident beam, and the transmitted guiding H+ ions, which are located around the direction of the capillary axis. With the charges depositing in the capillary, the proportion of the transmitted scattering H+ ions increases and the proportion of the transmitted guiding H+ ion decreases, which directly demonstrates the dynamical evolution of the scattering ions and the guiding ions. To understand the competition between the transmitted scattering ions and the transmitted guiding ions and the physical picture of the intermediate energy ions transmitting through the insulating nanocapillaries, the trajectories of the H+ ions in the capillary and the potential distribution and electric field intensity distribution in the capillary are numerically simulated. The results show that the potential distributions and electric field intensitiesy are different for H+ ions transmitting through nanocapillaries at various tilt angles, and the simulation results are in good agreement with the experimental data. The experimental and simulation results give us a further insight into the mechanisms of guiding and scattering in intermediate energy ions transmitting through nanocapillaries.
2018, 67 (20): 203701. doi: 10.7498/aps.67.20180908
On the basis of the stimulated Raman adiabatic passage technology, we study the conversion of ultracold atoms into diatomic molecules by using a square-shaped pulse field. By the method of adiabatic fidelity, we analyze the dynamical evolution process of the coherent population trapping state for the atom-molecule conversion system. We introduce two adiabatic fidelities to describe the efficiency of ultracold atom-molecule conversion, i.e.:1) the final adiabatic fidelity, which gives the value of the adiabatic fidelity at the end of the evolution:the closer to 1 it is, the higher the conversion efficiency is; 2) the final maximum adiabatic fidelity, which denotes the maximum value that can be achieved at the end of evolution, indicating the highest conversion efficiency. With these two quantities, we discuss how to achieve higher adiabatic fidelity for the coherent population trapping state through optimizing the pulse-delay time and the pulse-laser intensity of the stimulated Raman adiabatic passage. In addition, we also discuss the effects of the width of pulses on the ultracold atom-molecule conversion efficiency and the feasibility of continuous light. It is shown that the final adiabatic fidelity of the coherent population trapping state demonstrates a large periodic oscillation with the pulse-laser intensity. By calculating and analyzing the final adiabatic fidelity and the final maximum adiabatic fidelity, we obtain the conditions for higher efficiency conversion, which gives the best choice of the pulse-laser intensity, the pulse-delay time, and the width of pulses. The results show that the scheme of square-shaped pulses we discussed has obvious advantages compared with that of Gaussian-shaped pulses, which can achieve high adiabatic fidelity and realize higher ultracold atom-molecule conversion efficiency via employing the pulse-laser field with low intensity. Further detailed comparison between the square-shaped pulses and the Gaussian-shaped pulses is also given. Particularly, we find that the final adiabatic fidelity shows a periodic oscillation with the pulse width, which means that the high efficiency atom-molecule conversion can be achieved by using a pulse field with small width. Moreover, we find that the high efficiency conversion can also be achieved by using special continuous light under certain conditions.
Multiphoton ionization dissociation dynamics of iodoethane studied with velocity map imaging technique
2018, 67 (20): 203301. doi: 10.7498/aps.67.20181468
Halogenated alkanes destroy the ozone layer, and iodoethane is one of the important representative halogenated alkanes. Time-of-flight mass spectrometry and velocity map imaging technique are used for investigating the photoionization dissociation dynamics of iodoethane, induced by 800 nm femtosecond laser. The dissociation mechanisms of iodoethane are obtained and discussed by analyzing the velocity distributions and angular distributions of the fragment ions generated in the dissociation. The measurements by time-of-flight mass spectrometry show that iodoethane cations generates C2H5+, I+, CH2I+, C2H2+, C2H3+ and C2H4+. The fragments related to CI bond fragmentation are C2H5+ ions and I+ ions, and the dissociation mechanisms are C2H5I+ C2H5++I and C2H5I+ C2H5+I+ respectively. Comparison between the configurations before and after ionization shows that the CI bond length is 0.2220 nm before ionization and turns longer and becomes 0.2329 nm after ionization. This indicates that the CI bond becomes more unstable after ionization and is more prone to dissociation. Moreover, the velocity map images of C2H5+ and I+ ions are acquired, from which the speed and angular distribution of C2H5+ and I+ are obtained. The analysis of speed distribution of the fragment ions shows that there are two channels, i.e. high energy channel and low energy channel in the dissociation process for producing C2H5+ and I+ ion. The difference between the ratios of the high energy channel and the low energy channel is small, indicating that the high energy channel and the low energy channel of the two dissociation processes are similar. According to the further analysis of the angular distribution of the fragment ions, it is found that the anisotropy parameter of C2H5+ is close to 0 (isotropic), the production channel of which may correspond to the slow vibration predissociation process. The anisotropy parameters of I+ ions are higher, which may be due to the rapid dissociation process on the repulsive potential energy surface. In addition, the density functional theory is used to calculate the configuration change of the iodoethane molecule before and after ionization, the energy level and oscillator strength for the ionic state in order to obtain more insights into the photodissociation dynamics.
Theoritical research on optical Stark deceleration and trapping of neutral molecular beams based on modulated optical lattices
2018, 67 (20): 203702. doi: 10.7498/aps.67.20181348
According to the optical Stark deceleration theory of using a stationary quasi-cw red-detuned optical lattice to slow and trap an arbitrary pulsed molecular beam, we propose a novel idea of using a modulated optical lattice instead of a stationary one to realize a multistage optical Stark deceleration. We analyze the motion of the decelerated molecules inside the optical decelerator, and study the dependence of the velocity of the decelerated molecular packet on the synchronous phase angle and the number of the deceleration stages (i.e. half the number of the optical-lattice cells) by using the Monte-Carlo method. The simulation results show that it takes longer time for the molecules to reach the detector as the number of the deceleration stages increases. The decelerated molecular wave packets are gradually separated from the large wave packets of the original molecular velocity distribution. And the higher the number of the deceleration stages, the lower the decelerated molecular speed is. In addition, we also study the influence of the initial phase angle of synchronous molecules under the same conditions. It is demonstrated that the higher the initial phase angle of synchronous molecules, the lower the decelerated molecular speed is and the smaller the number of molecules in the deceleration wave packet, so the phase space is compressed. The result also shows that the modulated optical Stark decelerator does not have the process of molecular free flight, and thus improving the efficiency of deceleration for molecules. The ultra-cold molecules can be trapped in the optical lattice by rapidly turning off the modulation signal of the lattice. Comparing with the previous scheme, the doubled number of the deceleration stages is reached in the same optical lattice length since a modulated optical lattice is used. For a length of optical lattice of 3.71 mm, theoretical simulation results demonstrate that the speed of methane molecules is decelerated from 280 m/s to 172 m/s. Comparing with the previous results from 280 m/s to 232 m/s, the deceleration effect is improved by 26%. Our scheme can not only obtain an ultra-colder molecular packet under the same molecular-beam parameters and deceleration conditions, but also be directly used to trap the slowed cold molecules after the deceleration without needing to use other techniques for molecular trapping.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
Theroy and observation of bidirectional leader of lightning: Polarity asymmetry, instability, and intermittency
2018, 67 (20): 205201. doi: 10.7498/aps.67.20181079
A lightning discharge in a thundercloud usually starts with a locally breakdown process (preliminary breakdown) followed by a widely extending leader process. In the early 1960 s, from the view of fundamental electrostatics[Kasemir H W 1960 J. Geophys. Res. 65 1873] suggested that the lightning leader is initiated by an electrodeless discharge with a zero-net-charge conductive channel extending bidirectionally in the ambient electric field of the thundercloud, i.e.the bidirectional leader theory. However, the bidirectional leader theory has just been recognized by lightning researchers since the late 1980 s, when airplane[Mazur V 1989 J. Geophys. Res. 94 3326] and rocket-triggered lightning experiments have proven that the bidirectional leader theory provides the best common physical basis for explaining a variety of lightning processes. Nevertheless, challenges still remain in other properties of the bidirectional leader theory, such as the polarity asymmetry, the sustainability, instability and restrike of a leader channel, which are all the key concerns of lightning researches. In the present paper, we first briefly review the concept and development of the bidirectional leader theory and its appearances in various lightning processes, especially in the negative stepped leader and recoil leader process. By reviewing a variety of field observation data of inception thresholds and propagation properties of long gap spark discharges, we put forward and emphasize an alternative polarity asymmetry of the lightning leader:the polarity asymmetry of continuity, i.e., the continuity of positive leader and the intermittency of negative leader. Based on the detailed discussion, we then argue that the stepwise positive leader may be restrikes due to temporary instability of a continuous positive leader, while the intermittency of negative leader manifests not only a relatively regular stepped leader but also a more erratic negative recoil leader. We further propose that the positive leader-streamer may play a unique role in igniting all kinds of intermittent events in lightning, including the initiation of lightning with the fast positive streamer, the formation of space leader in front of a negative stepped leader with the secondary positive streamer, and the reformation of recoil-leader in an instability leader channel due to its low threshold of inception and propagation. Finally, we discuss the self-adjustable ability of a bidirectional leader, suggesting that the potential drop along the streamer channel in front of the leader tip may be an important factor for stabilizing the bidirectional leader channel.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2018, 67 (20): 206101. doi: 10.7498/aps.67.20181039
A method of using energy loss to reconstruct the density is presented with protons at intermediate and high energy for proton radiography, and the equation and condition of density reconstruction are given based on the Bethe-Bolch formula. For the intermediate and high energy proton radiography, the stopping power of material is changed slowly within a certain energy range, and the stopping power can be approximated as a constant, then the multi-material object can be reconstructed by using the energy loss information. In this work, the protons at 1.6 GeV which can be obtained by China Spallation Neutron Source are used in the radiography, and the energy loss information is used in the reconstruction, and the Geant 4 is applied to Monte Carlo simulation. From the theoretical calculation and the Geant4 simulation, it can be seen that when the protons energy ranges from 1.45 GeV to 1.6 GeV the stopping power of material can be approximately constant, and the relative change of material stopping power is less than 1%, thus the stopping power of material is only dependent on the incident proton energy, and the density of the multimaterial object can be reconstructed by the energy loss information. The proton scanning imaging system which can avoid blurring image caused by multiple coulomb scattering at the receiving plane is used in the proton radiography to obtain the energy loss information. In the imaging system, two energy detectors are employed to record the incident energy and exit energy of protons, the object is scanned by the protons with a certain step length, and the object is rotated 180 or 360. The energy loss distribution of the object can be obtained by the scanning imaging system, and the density of the object can be reconstructed by solving corresponding equations. The Geant 4 is used to simulate the proton scanning imaging system. In the simulation, the object is the scaling french test object (FTO) that the areal density is 113 g/cm2, the protons are monoenergetic at 1.6 GeV, the scanning interval is 0.5 mm, and the rotation angle is 0.9. The results of the density reconstruction of the scaling FTO are in good agreement with the true values.
2018, 67 (20): 206402. doi: 10.7498/aps.67.20180966
Dry granular materials consist of a collection of macroscopic discrete particles interacting solely via contact forces. By changing the external conditions, the granular packing displays rich phenomena ranging from fluid-like properties to jamming glassy behavior and to aging observed when these grains are trapped in a frozen state. Once the grains contact liquid, the force between the liquid and grains has an influence on the mechanical properties of the wet granular materials, and some mechanical behaviors are quite different from those of the dry granular materials. However, the underlying mechanism of the complex dynamics of granular assemblies is still not completely understood. In this paper, mechanical spectroscopy (the shear modular G and the related energy dissipation tan) of NaCl wet granular system is investigated with different liquid content (weight fraction) under the constant temperature 25℃ and air humidity by a modified low-frequency inverted torsion pendulum. The NaCl wet granular system also displays jamming behavior when subjected to an external vibration with increasing intensity, which is quiet similar to dry granular matter. With the increase of water content, all the spectra of tan and modular G show a peak at the water content about 11% (critical water content). At the same time, the applied shear force has little influence on the positions of these peaks. All of these behaviors illustrate that the main interaction forces among granular matters in the system are changed at the moment.
Behavior characteristics from self-organization to criticality caused by cumulative damage leading to instability of locked segments in seismogenic fault system
2018, 67 (20): 206401. doi: 10.7498/aps.67.20180614
Each of the seismogenic locked segments in a well-defined seismic zone can accumulate high strain energy to bring about a major earthquake. Hence, better understanding the physical implication of self-organized criticality in the locked segment (rock) failure process is crucial to achieving insights into such issues as earthquake predictability and so on. We point out that there exist two critical points in the locked segment fracturing process. The first critical point is volume expansion point, which is the starting point of self-organization, at which a rupture event with high energy occurs. It can be regarded as the only identified precursor to macroscopic rupture of locked segment. The second critical point is the peak strength point, namely, the instability point, at which a major earthquake which is able to generate obvious surface rupture zones takes place. According to our previous research on the theoretical relationship of strain ratio between the two points as well as the constrained expressions concerning earthquake magnitudes and elastic strain energy, also known as the theory about the brittle failure of multiple locked segments in a seismogenic fault system, we can predict some characteristic earthquakes occurring at the first and the second critical point of locked segment, e.g., the 2004 Sumatra-Andaman MW9.0 earthquake in Indonesia, the 2008 Sichuan MS8.1 earthquake in China, and the 2011 Tōhoku MW9.0 earthquake in Japan. This was obtained by retrospectively analyzing the earthquake cases in 62 seismic zones covering the circum-Pacific seismic belt and the Eurasia seismic belt. The present results show that the self-organized process before the locked segment (rock) instability must arise due to its heterogeneity; there exists a causal link between the self-organization and criticality; it is possible to predict some large earthquakes (e.g. characteristic earthquakes) just because of the existence of self-organized process. We emphasize here that the damage process between the two critical points is not transient behavior, usually a long-term process; the evolution of characteristic earthquakes follows a deterministic rule; there is no probability with which small earthquakes can cascade into a large event (e.g. characteristic earthquakes). In summary, this study can help to comprehend the evolutionary mechanism of characteristic earthquakes, provide a physical basis of understanding the generation process of earthquakes, and clarify such issues as the identification of earthquake types and predictability of earthquakes.
SPECIAL TOPIC—Tenth anniversary of the discovery of iron-based high temperature superconductors
2018, 67 (20): 207102. doi: 10.7498/aps.67.20181401
We perform an in-plane optical spectroscopy measurement on iron-based superconductor Li0.8Fe0.2ODFeSe single crystal. At room temperature, the low frequency optical conductivity shows an incoherent characteristic; the Drude component is absent. With temperature decreasing, the Drude component develops and narrows rapidly. A well-defined plasma edge is observed in reflectance spectrum at temperature below 100 K, indicating a dramatically reduced scattering rate. The spectral weight contributed from free carriers is even smaller than that of FeSe single crystal. A number of phonon modes are visible in the measured spectra. We also observe clear spectral change below 160 cm-1 at 10 K, associated with the formation of superconducting energy gap in the superconducting state. The energy scale of the superconducting gap is comparable to the value measured by angle-resolved photoemission spectroscopy technique. Like FeSe and other iron pnictides, a clear temperature-induced spectral weight transfer at high energy is observed for Li0.8Fe0.2ODFeSe, indicating the presence of strong correlation effect.
2018, 67 (20): 207401. doi: 10.7498/aps.67.20181818
Since the discovery of iron-based superconductors in 2008, it has been a hot topic to research the pairing mechanism of superconductivity. Scanning tunneling microscopy (STM) can be used to detect the electronic information in nano-scale, hence, it is an important tool to do research on superconductivity. In recent 10 years, many valuable works have been carried out by STM in iron-based superconductors. In this paper, we try to make a brief introduction of the STM works in iron-based superconductors. Since the iron-based superconductors have multiple bands and superconducting gaps, the Fermi surface topology can change significantly among different materials. There are some evidences to prove a nodeless s-wave pairing in the optimally-doped iron-based superconductors with both electron and hole pockets by STM experiments. Furthermore, it has been demonstrated that FeSe-based materials with only electron pockets also have a sign-change order parameter, which provides a robust evidence for the unified picture of the electron pairing in iron-based superconductors. Besides, STM experiments provide fruitful information about the novel electronic properties including the electronic nematicity, shallow band effect, and possible topological superconductivity. Finally, we also give perspectives about the STM studies in iron based superconductors.
2018, 67 (20): 207407. doi: 10.7498/aps.67.20181543
Like the superconductivities in other unconventional superconductors, high-temperature superconductivity in the iron pnictide often emerges after the static antiferromagnetic order has been suppressed, and is always accompanied by strong spin fluctuations. Therefore, understanding the magnetism and its origin could be an important premise for ascertaining the microscopic mechanism of iron-based superconductivity. Neutron scattering, as a powerful tool for studying magnetic ordering and spin dynamics in condensed matters, plays an essential role in understanding the relationship between magnetism and superconductivity in iron-based superconductors. In this paper, we review the neutron scattering results for iron pnictides, including static magnetic structures, magnetic phase transitions, spin excitations and electronic nematicity, and discuss their relationship with superconductivity.
2018, 67 (20): 207402. doi: 10.7498/aps.67.20181256
With high transition temperature Tc (~38 K), high upper critical field Hc2 ( 100 T), superior transport Jc (~106 A/cm2) and extremely small anisotropy (1.5-2.0), the 122-type iron-based superconductors show great promise in high-field applications such as next-generation high energy physics accelerator and high-field magnetic resonance imaging (MRI). Power-in-tube (PIT) method is widely adopted to fabricate the iron-based superconducting wires and tapes due to low cost and easiness of large-scale fabrication. In the past few years, substantial efforts have been made to improve the transport performances of 122-type iron-based superconducting wires and tapes by ex-situ PIT technique. In this review, the recent progress of 122-type iron-based superconducting wires and tapes is presented. Firstly, we focus on the techniques for fabricating high-performance 122-type wires and tapes. We also discuss the key factors affecting the final performances of wires and tapes during the PIT process, including the preparation of high-quality precursor, the effect of chemical doping, the improvement of core density and grain connection. Recently, due to the improving of degree of c-axis texture and connectivity of grains, the transport Jc value of 122/Ag tapes reached 1.5105 A/cm2 at 4.2 K and 10 T, which exceeds the practical level of 105 A/cm2 and demonstrates their promise in high-field applications. Then, the progress of practical application of 122-type wires and tapes is summarized. In order to reduce the fabrication cost and improve the mechanical strengths of superconducting wires and tapes, an additional outer sheath such as Fe, Cu and stainless steel was used in combination with Ag. Besides, a favourable transport Jc was also obtained in the Cu-, or Fe-sheathed 122 tapes. For round wires, the highest Jc value reached 3.8104 A/cm2 in Cu/Ag composite sheathed wires at 4.2 K and 10 T, obtained by the hot-isostatic-press technology. From the viewpoint of practicality, the fabrication of multifilamentary wires and tapes is an indispensable step. The 7-, 19-and 114-filament 122 wires and tapes were successfully fabricated by the PIT method, and these multifilamentary tapes exhibited weak field dependence of Jc. Based on the experience of high-performance short samples and multifilamentary wires process, the scalable rolling process has been used to produce the first 115-m-long 7-filament Sr1-xKxFe2As2/Ag superconducting tape, confirming the great potential for large-scale manufacture. Moreover, the mechanical property, anisotropy and superconducting joint of 122 tapes are also studied. Finally, a perspective for the future development of 122-type wires and tapes in practical applications is given.
2018, 67 (20): 207403. doi: 10.7498/aps.67.20181393
The 112-type (Ca, RE)FeAs2 (RE=rare earth) superconductors are very special among the iron-based superconductors for their particular crystal structures with arsenic chain configuration and attractive electronic phase diagram with the coexistence of superconductivity and antiferromagnetism upon carrier doping, while the chemical phases are absent for the low doping level or undoped parent compound. Here we report the single crystal growth method and physical characterizations for the newly discovered Eu 112 type parent compound EuFeAs2. The single crystal of EuFeAs2 is grown by high temperature solution method through using CsCl as the flux under the constant temperature of 800℃ with the molar ratio of the starting materials Eu:Fe:As:CsCl=1:1:4:18. The as-grown crystal is shinyplatelike piece with a typical size of 1 mm1 mm0.2 mm, and quite stable in air. The chemical composition of EuFeAs2 crystal is confirmed by energy-dispersive X-ray spectroscopy. The single crystal X-ray diffraction analysis at room temperature indicates that EuFeAs2 crystallizes into an orthorhombic crystal structure with the space group Imm2 (No. 44), and the refined lattice parameters are a=21.285(9) , b=3.9082(10) , c=3.9752(9) , which are different from those of the Ca 112 compound, but similar to those of unique zigzag As-As chain configuration presented in the layered crystal structure. Electrical resistivity measurements show three anomalies near 110 K, 98 K, and 46 K. The former two anomalies with relatively high temperature imply that the structural and antiferromagnetic transitions are related to Fe2+ sublattice, which is similar to other iron-based parent compounds. The low temperature anomaly at 46 K is attributed to the antiferromagnetic transition of Eu2+ sublattice, which is also confirmed by the corresponding transition observed in the direct current magnetic susceptibility measurement. The magnetic susceptibility of EuFeAs2 exhibits obvious anisotropy blow 46 K when the magnetic field is parallel or perpendicular to the bc plane, while the exact orientation of the Eu2+ moment needs further studying. The discovery of EuFeAs2 provides a new platform for further studying the unique crystal structure and electronic state phase diagrams in the 112-type iron-based superconducting family, and may shed new light on the correlations between superconductivity and magnetism.
2018, 67 (20): 207404. doi: 10.7498/aps.67.20181319
Among the iron-based superconductors, the structural simplest FeSe and its derived materials have received much attention in recent years due to the great tunability of the superconducting transition temperature (Tc). The relatively low Tc 8.5 K of FeSe can be raised to over 40 K via the interlayer intercalations such as AxFe2-ySe2 (A=K, Rb, Cs, Tl), Lix(NH3)yFe2Se2, and (Li1-xFex)OHFeSe. Although the monolayer FeSe/SrTiO3 is reported to have a Tc as high as 65 K, none of the Tc values of these FeSe-derived bulk materials has exceeded 50 K at ambient pressure so far. In order to explore other routes to further enhance Tc of FeSe-based materials, we recently performed the detailed high-pressure study of two intercalated FeSe high-Tc superconductors, namely (Li0.84Fe0.16)OHFe0.98Se and Li036(NH3)yFe2Se2, by using a cubic anvil cell apparatus. We find that the applied high pressure first suppresses the superconducting phase (denoted as SC-I) and then induces a second high-Tc superconducting phase (denoted as SC-Ⅱ) above a critical pressure Pc (~5 GPa for (Li0.84Fe0.16)OHFe0.98Se and 2 GPa for Li036(NH3)yFe2Se2). The highest Tc values in the SC-Ⅱ phases of these two compounds can reach~52 K and 55 K, respectively, the latter of which is the highest in the FeSe-based bulk materials, and is very close to the highest Tc of FeAs-based high-Tc superconductors. Our high-precision resistivity data of (Li0.84Fe0.16)OHFe0.98Se also uncover a sharp transition of the normal state from Fermi liquid for SC-I to non-Fermi liquid for SC-Ⅱ phase. In addition, the reemergence of high-Tc SC-Ⅱ phase under pressure is found to be accompanied with a concurrent enhancement of electron carrier density. Interestingly, we find a nearly parallel scaling behavior between Tc and the inverse Hall coefficient for the SC-Ⅱ phases of both (Li0.84Fe0.16)OHFe0.98Se and Li0.36(NH3)yFe2Se2. In the case without structural transition below 10 GPa, the observed enhancement of carrier density in SC-Ⅱ should be ascribed to an electronic origin presumably associated with pressure-induced Fermi surface reconstruction. Our work demonstrates that high pressure offers a distinctive means to further raise the maximum Tc values of intercalated FeSe-based materials.
2018, 67 (20): 207406. doi: 10.7498/aps.67.20181355
The key structural unit of iron-based superconductors (FeSCs) is the Fe2X2 (X refers to a pnictogen or a chalcogen element) layer which stacks alternately along the crystallographic c axis with other spacer layers. This structural feature makes it possible to find FeSCs via rational material design. In this paper, we first review the crystal structure of FeSCs along with the relevant progress. Then we summarize several rules for designing the intergrowth structures. The rules include the following points. 1) Lattice match between the intergrowth layers should be good enough. Quantitatively, the lattice mismatch, defined as =2(aA-aB)/(aA + aB), where aA and aB are respectively the lattice parameters of the two constituent compounds, should be no larger than~2%. 2) The charge transfer between the intergrowth layers is mostly essential, which acts as the glue that combines the constituent layers together. Such a charge transfer also induces the extra charge carriers in the superconducting key layer to give rise to superconductivity without extrinsic doping (so-called self doping). 3) For the structure with similar yet crystallographically distinct sites, one needs to avoid forming solid solutions. 4) Each intergrowth layer is preferably thermodynamically stable. 5) The designed structure can be preliminary evaluated with the hard and soft acids and bases conception and ab initio calculations. Following these empirical rules, we introduce and analyze five examples, namely, (Li0.8Fe0.2OH)FeSe, Ba2Ti2Fe4As4O, 42214-type Ln4Fe2As2Te1-xO4 (Ln=Pr, Sm, Gd), 1144-type AkAeFe4As4 (Ak=K, Rb, Cs; Ae=Ca, Sr, Eu), and 12442-type AkCa2Fe4As4F2 and AkLn2Fe4As4O2 (Ak=K, Rb, Cs; Ln=Nd-Ho). For the last 12442-type compounds, we also discuss the unusual relation between superconducting transition temperature and crystallographic parameters. We conclude that the structural-design approach may serve as an effective route, not only for discovering new FeSCs but also for exploring other relevant functional materials with similar crystal structures.
2018, 67 (20): 207408. doi: 10.7498/aps.67.20181496
Since the high-Tc superconductivity in iron-based superconductors was found in 2008, numerous new iron-based superconductors have been discovered. Of them, FeSe-based superconductors receive the most attention due to their unique properties. Here, we briefly introduce the structure and physical properties of two newly found FeSe-based superconductors, i.e. (Li, Fe) OHFeSe and (CTA)x FeSe. The former is synthesized by the hydrothermal method, while the latter is synthesized by electrochemical intercalation method. Moreover, we also introduce the tuning of electronic properties of FeSe by electric-double-layer and solid-ion-conductor based transistors.
2018, 67 (20): 207409. doi: 10.7498/aps.67.20181651
The discovery of Fe-based superconductor in 2018 opened an illustrious chapter in the history of high temperature superconductors. Over the past ten years, many progresses on experiments, theories and applications have been achieved in the studies of Fe-based superconductors, which have greatly enriched the basic knowledge on the superconductivity of high temperature (Tc) superconductors and laid a solid foundation for uncovering superconducting mechanism of high-Tc superconductors and expanding their applications. In this review article, we present some important progresses and new phenomena/physics exhibited in the pressurized Fe-based superconductors, including pressure-induced superconductivity, pressure-induced reemergence of superconductivity, pressure-enhanced superconducting temperature, the prediction on the highest superconducting temperature for Fe-based superconductors via high pressure studies, the effect of the separated phase structure on the superconductivity and the discovery of a bi-critical point between antiferromagnetic and superconducting phases. It is expected that these high pressure experimental results on Fe-based superconductors, together with the results reported in the same issue through other experimental and theoretical methods, can aid to outline a more complete physical picture for a more comprehensive and deeper understanding on Fe-based superconductors.
2018, 67 (20): 207411. doi: 10.7498/aps.67.20181692
Since the discovery of high-temperature superconductivity in cuprates, finding more unconventional superconductors and understanding their superconducting pairing mechanism has been an important theme in condensed matter physics. Recently, ternary Cr-based superconductors A2Cr3As3 (A=K, Rb, Cs) and ACr3As3 (A=K, Rb) were reported, which own quasi-one-dimensional crystal structure, containing[(Cr3As3)-] linear chains. A2Cr3As3 belongs to P6m2 space group, and ACr3As3 crystallizes in a centrosymmetric structure with the space group P63/m. Many experiments, such as nuclear magnetic resonance, London penetration depth, show that A2Cr3As3 is an unconventional superconductor. However, these A2Cr3As3 compounds are extremely unstable in air. Here, we study the superconducting gap of the air-stable RbCr3As3 single crystal, using ultralow-temperature thermal conductivity measurement. The resistivity of RbCr3As3 single crystal shows a superconducting transition temperature Tczero at 6.6 K. The normal-state resistivity data from 20 K to 8 K are fitted to (T)=0 + AT2, which gives a residual resistivity of 0=781 cm. Then, the thermal conductivity of RbCr3As3 single crystal is measured at temperature down to 80 mK and in magnetic fields up to 9 T. In zero field, residual linear term 0/T=7.5 WK-2cm-1 is observed, which is about 24% of its normal-state value, suggesting nodes in the superconducting gap. At low field, the 0/T of RbCr3As3 shows a relatively faster field dependence than single-gap s-wave superconductors. These results reveal that RbCr3As3 is likely an unconventional superconductor with superconducting gap nodes, although the exact superconducting gap symmetry and structure for this quasi-one-dimensional superconductor needs further investigation.
2018, 67 (20): 207412. doi: 10.7498/aps.67.20181701
FeSe-based superconductors, as an important part of the family of iron-based superconducting materials, have attracted intensive research interest in the field of condensed matter physics. The exploration and preparation of such superconducting materials is the basis for studying their physical properties. At present, the exploration of FeSe-based superconducting materials mainly focuses on intercalated materials and epitaxial single-layer FeSe films. Among them, the intercalated FeSe-based superconducting materials have unique properties and are numerous in variety. This paper introduces a series of FeSe-based high-temperature superconducting materials discovered in recent years, covering KxFe2Se2, AxNH3FeSe, LiOHFeSe and organic molecular intercalation FeSe, etc., their properties and impacts are also briefly described.
2018, 67 (20): 207413. doi: 10.7498/aps.67.20181768
Copper oxide superconductors and iron-based superconductors are two important families of high temperature superconductors. Their high-temperature superconductivity mechanism is a long-standing issue and still in hot debate in the field of condensed matter physics. The extensive and in-depth exploration of iron-based superconductors and their comparative study with copper oxide high-temperature superconductors are of great significance for the development of new quantum theory, the solution of high-temperature superconducting mechanism, the exploration of new superconductors and practical applications of superconductors. The macroscopic properties of materials are determined by their microscopic electronic structure. Revealing the microscopic electronic structure of high temperature superconductors is fundamental for understanding high temperature superconductivity. Angle-resolved photoelectron spectroscopy, due to its unique simultaneous energy, momentum and even spin resolving ability, has become the most direct and powerful experimental tool for detecting the microscopic electronic structure of materials, and has played an important role in the study of iron-based high-temperature superconductors. The revealing and discovery of the Fermi surface topology, superconducting energy gap and its symmetry, three-dimensionality, orbital selectivity, and electronic coupling mode in different iron-based superconductor systems provide an important basis for identifying and proposing new theory of iron-based superconductivity to solve high temperature superconductivity mechanism.
2018, 67 (20): 207414. doi: 10.7498/aps.67.20181586
111-typed iron based superconductors have three members: LiFeAs, NaFeAs and LiFeP. The family of LiFeAs itself does not show any long range magnetic order but become superconductor without chemical doping. NaFeAs displays the separation of structural and magnetic transition, suitable to investigate the origin of the two transitions. LiFeP has been proved to be a nodal superconductor. The structure of 111 compounds consists of[FeAs/P] layers intercalated with two alkali metal layers, which makes single crystals easy to be cleaved into two equal counterparts with non-polar surface and thus is favored by the surface characterization techniques, such as the research of angleresolved photoemission experiment and scanning tunneling microscope measurement. Up to now, fruitful results have been achieved about the study of 111 family. In this paper, we summarize recent progresses on this family.
New progress of FeSe-based superconducting single crystals and films: Spin nematicity, electronic phase separation, and high critical parameters
2018, 67 (20): 207410. doi: 10.7498/aps.67.20181638
High-quality superconducting single crystals and thin films play an important role in the basic research and application of high-Tc superconductivity. In these two aspects, iron-based superconductors feature the merit of rich physical phenomena and high superconducting critical parameters (including the transition temperature Tc, the upper critical field Hc2 and the critical current density Jc). By developing ion-exchange and ion-de-intercalation method, we successfully synthesize a series of high-quality and sizable (Li,Fe)OHFeSe and FeSe single crystal samples. We observe Ising spin nematicity (below Tsn), and the universal linear relationship between Tc and Tsn in FeSe single crystals, indicating that the superconductivity is closely related to the spin nematicity driven by stripe antiferromagnetic spin fluctuations. In (Li,Fe)OHFeSe single crystals, we observe the coexistence of an AFM state (below Tafm~125 K) together with the SC state. We explain the coexistence by electronic phase separation, similar to that in high-Tc cuprates and iron arsenides, and establish a complete phase diagram for (Li,Fe)OHFeSe system. Here, we also make a brief introduction about our latest progress in growing a high-quality single-crystalline superconducting film of (Li,Fe)OHFeSe. The film is prepared by a hydrothermal epitaxial method. The high crystalline quality of the film is demonstrated by x-ray diffraction results, showing a single (001) orientation with a small crystal mosaic of 0.22 in terms of the full width at half maximum of the rocking curve, as well as an excellent in-plane orientation by the -scan of (101) plane. Its bulk superconducting transition temperature Tc of 42.4 K is characterized by both zero electrical resistance and diamagnetization measurements. Based on systematic magnetoresistance measurements, the values of upper critical field Hc2 are estimated at 79.5 T and 443 T for the magnetic field perpendicular and parallel to the ab plane, respectively. Moreover, a large critical current density Jc of a value over 0.5 MA/cm2 is achieved at~20 K. Such a (Li,Fe)OHFeSe film is not only important for the fundamental research for understanding the high-Tc mechanism, but also promises the high-Tc superconductivity applications, especially in high-performance electronic devices and large scientific facilities such as superconducting accelerator.
2018, 67 (20): 207101. doi: 10.7498/aps.67.20181455
Iron-based superconductors and topological quantum states have been two important research frontiers in condensed matter physics in recent years. It is a very significant question whether the nontrivial topological phenomena can occur in iron-based superconductors. In this paper, the basic characteristics of the electronic structure of iron-based superconducting are analyzed from three aspects:crystal symmetry, effective model near the high symmetry points in Brillouin zone, and spin-orbit coupling interaction. On this basis, this paper focuses on how the nontrivial topological quantum states occur in the normal state, the states with long-range order near superconducting state and the superconducting state. Furthermore, the relevant theoretical models and results are introduced in detail, the related experimental progress is reviewed, and the development in this field is prospected.
2018, 67 (20): 207415. doi: 10.7498/aps.67.20181681
We report on the observation of a superconducting gap of about 14-15 meV, significantly enlarged compared with the value of 2.2 meV for bulk FeSe, in monolayer FeSe film interfaced with MgO epitaxial on SrTiO3(001) substrate by using the scanning tunneling microscopy. While the MgO exhibits the same work function as SrTiO3 substrate, the gap magnitude is in coincidence with that of surface K-doped two-unit-cell FeSe film on SrTiO3(001), suggesting that the interface enhanced superconductivity might be attributed to cooperation of interface charge transfer driven by band bending with interface electron-phonon coupling as discovered at FeSe/TiO2 interfaces. On the other hand, the observation of such an enlarged superconducting gap, complementary to our previous transport observation of an onset superconducting transition temperature of 18 K in monolayer FeSe film on a bulk MgO substrate, implies that FeSe/MgO interface is likely to be a new interface high-temperature superconducting system, providing a new platform for investigating the mechanism of interface hightemperature superconductivity.
2018, 67 (20): 207405. doi: 10.7498/aps.67.20181541
The similarities between the Fe-based superconductors and cuprate superconductors imply a possible unified picture of high temperature superconductivity. However, various chemical doping effects in Fe-based superconductors can lead to qualitatively similar phase diagrams that show diverse and complicated details, which pose great challenges of establishing a unified picture. Studying how chemical doping affects the electronic structure and superconductivity, and finding the real universal control parameter for superconductivity, are very important for establishing a unified picture and revealing the mechanism of high temperature superconductivity. In this article, we review a series of angle resolved photoemission studies on the chemical doping effect in Fe-based superconductors, involving both type I Fe-based superconductors with both electron and hole Fermi pockets, and type Ⅱ Fe-based superconductors with only electron Fermi pockets, and involving chemical doping of hetero-valent doping, isovalent doping, and chemical doping at different sites in unit cell. Comprehensive studies and analysis are conducted from various aspects of doping effects, including Fermi surfaces, impurity scattering, and electron correlation, and their roles in evolving the superconductivity. Electron correlation is found to be a universal electronic parameter behind the diverse phase diagrams of Fe-based superconductors, which naturally explains the qualitatively similar phase diagrams of various Fe-base superconductors despite of doping them in different ways. The electron correlation in Fe-based superconductors is closely related to both the carrier type of dopant and the lattice structure parameters, such as bond length. The different impurity scattering effects and different structures may affect the optimal Tc and thus leading to the diversity and complexity in the phase diagram. Fermi surface topology and its evolution with doping may play a secondary role in determining Tc. In order to enhance the Tc, one needs to optimize a moderate electronic correlation while minimizing the impurity scattering in the Fe-anion layer. Our results explain many puzzles and controversies and provide a new view for understanding the phase diagrams, resistivity behaviors, superconducting properties, etc. Our findings also strongly challenge the weak coupling theories based on the Fermi surface nesting, but favors the strong-coupling pairing scenario, where the competition between the electron kinetic energy and the local correlation interactions is a driving parameter of superconducting phase diagram. Like the t-J model of cuprates, in the picture of local antiferromagnetic exchange pairing, superconductivity appears in Fe-based superconductor when the electron correlation strength is at a moderate level. If the correlation is too weak, the system cannot exhibit superconductivity and remains metallic at low temperature. If the correlation is too strong, magnetic order appears in type I Fe-based superconductor, while type Ⅱ Fe-based superconductor shows a bandwidth-control correlated insulating state. The control parameter of the phase diagram is carrier doping for cuprates, but electron correlation strength for Fe-based superconductors. Our experimental results give a unified understanding of iron-based superconductors as a bandwidth-controlled system.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2018, 67 (20): 207301. doi: 10.7498/aps.67.20181106
Epsilon-near-zero mode provides a new path for tailoring light-matter interactions on a nanoscale because of its unique characteristics and ability to be used in many scientific fields. Among these applications, broadband absorption has aroused the considerable interest in photonic research. In this paper, we first show that the surface plasmon resonance is excited by the metal disk array structure without dysprosium-doped cadmium oxide nanolayer, and the structure achieves the local effect of light at a certain wavelength. In addition, in order to be able to use this new technique to achieve a broadband absorption, we take advantage of the surface plasmon resonance to excite the epsilonnear-zero mode which cannot be excited under normal incidence but has a very large density of states. Then, we show that over one order of magnitude increase in the absorption band of a periodically patterned metal-dielectric-metal structure can be obtained by integrating a dysprosium-doped cadmium oxide material into the insulating dielectric gap region. We analyze the absorption band at mid-infrared wavelength comprising plasmonic metamaterial resonators and epsilon-near-zero modes supported by dysprosium-doped cadmium oxide material. The two resonance modes lie in the weak coupling regime and achieve a 470 nm wideband light absorption. Finally, we perform numerical simulations by using the finite-difference-time-domain method to investigate the relationship between the epsilon-near-zero mode and the surface plasmon resonance mode. It is sure that the whole broadband mightily has the local effect of light. The epsilon-near-zero mode mainly is excited at the short wavelength of the broadband, and the surface plasmon resonance mode mainly focuses on long wavelength of the broadband. The simulation demonstrates that the two resonance modes are coupled to achieve a broadband absorption. Additionally, the dielectric constants are tunable by doping density, resulting in plasma frequency change, where the real part of the dielectric constant becomes zero at plasma frequency. Broadband absorption theoretically can be realized in any band of mid-infrared wavelength due to plasma frequency changing. Broadband absorption relaxes the single wavelength condition in previous absorption studies, and compared with the narrowband absorption, broadband absorption at present has many applications, such as in absorbers, sensors, filters, coherent thermal emitters, microbolometers, photodectors, solar cells, fingerprint recognition, energy harvesting devices, etc.
Linear dependence of magnetocaloric effect on magnetic field in Mn0.6Fe0.4NiSi0.5Ge0.5 and Ni50Mn34Co2Sn14 with first-order magnetostructural transformation
2018, 67 (20): 207501. doi: 10.7498/aps.67.20180927
The study on the field dependence of magnetocaloric effect (MCE) is considered to be of fundamental and practical importance, since it not only guides us in understanding and optimizing the MCE, but also helps us estimate the MCE for higher magnetic field which is not available in some laboratories. The magnetic field (0H) dependence of magnetic entropy change (△SM) has been studied extensively in many materials with second-order magnetic transition. However, the field dependence of MCE for first-order magnetic transition (FOMT) materials has not been sufficiently studied due to their complexity and diversity. In the present work, polycrystalline Mn0.6Fe0.4NiSi0.5Ge0.5, Ni50Mn34Co2Sn14, and LaFe11.7Si1.3 compounds with FOMT are prepared, and the magnetic and magnetocaloric properties are investigated systematically. In order to avoid a spurious △SM, the M-0H curves are measured in a loop process. The M-0H curves are corrected by taking into account the demagnetization effect, i.e. Hint=Hext-NdM. It is found that the -△SM follows a linear relationship -△SM=-△S0 +0H with the variation of magnetic field in Mn0.6Fe0.4NiSi0.5Ge0.5 compound when 0H 1 T. In addition, it is also noted that the △SM is approximately proportional to the square of 0H at low field. The origin of this linear relationship between △SM and 0H at high field and the deviation at low field are discussed by numerically analyzing the Maxwell relation. In addition to the △SM peak value, it is found that other △SM values at different temperatures also follow the linear relation at high field by performing the same numerical analysis. Moreover, it is found that the fitted △SM curve matches the experimental data very well. This result indicates that the linear relationship between △SM and 0H could be utilized to predict the △SM for higher magnetic field change when the field is lower than the saturation field. The applicability of this linear relationship is also verified in other systems with first-order magnetostructural transformation, such as Ni50Mn34Co2Sn14. However, it fails to describe the field dependence of △SM in LaFe11.7Si1.3, which exhibits a strong field dependence of transition temperature. Consequently, our study reveals that a linear dependence of △SM on 0H could occur in magnetostructural transition materials, which show the field independence of transition temperature.
2018, 67 (20): 207416. doi: 10.7498/aps.67.20180940
Of all iron-based superconductors, FeSe possesses the simplest structure whereas its superconducting critical temperature can be remarkably enhanced. Compared with bulk sample fabrication, the film preparation process is very precise and controllable. Although FeSe monolayer films exhibit a high Tc, they are unstable in air, and ex-situ measurements are very difficult. Therefore, the stable films with~100 nm in thickness can serve as good candidates to explore the mechanisms of iron-based superconductors. There is no doubt that the fabrication of high-quality FeSe thin films is of significance. The pulsed laser deposition (PLD) technique has more advantages in the growth of FeSe thick films than any other film fabrication technology, because of its high efficiency and wide adaptability. In this work, we systematically optimize the growth conditions of FeSe thin film fabricated by PLD. The main results are as follows. 1) The optimal growth temperature is 350℃, where the film has the best crystallinity and the highest Tc. 2) High-quality -FeSe epitaxial thin films with the thickness ranging from 10 to 320 nm have been successfully prepared on twelve types of substrates:CaF2, LiF, SrTiO3, MgO, BaF2, TiO2, LaAlO3, MgF2, Nb-SrTiO3, LSAT, LaSr(AlO4) and MgAl2O4. The Tc for the films on CaF2 with the same thickness of 160 nm can be tuned from 2 K to 14 K. 3) The Tc of the FeSe thick films may be precisely tuned by the Fe/Se ratio which is affected by the proportion of the nominal components of the target, the laser energy density and the ablation off-stoichiometry of target. 4) The surface morphology measurement, cleavability and transferability experiments of films are performed. In addition, it is worth of mentioning that there is a significant positive correlation between Tc, lattice constant c and residual resistivity ratio (RRR), as evidenced through a detailed statistical analysis of the data from more than 1500 samples. Since c and RRR are usually associated with the vacancies or defects, we conclude that the superconductivity of -FeSe thin films is closely related to the ratio of Fe to Se. Moreover, the first principle simulation shows that 0.5% increase of Fe content does lead to a change of 0.05 of c. However, according to the angle-resolved photoelectron spectroscopy experiment, there is no obvious change near the point in the hole energy band, but the energy band changes significantly at the M point. This variation of electronic structures cannot be explained by electron filling which lifts up the Fermi energy. Therefore, the specific relationship among the superconductivity, lattice structure and electronic structure of FeSe thin films remains to be clarified. Such a series of high-quality -FeSe films offers a chance to further explore the nature of FeSe-based superconductors.
2018, 67 (20): 207701. doi: 10.7498/aps.67.20181091
Piezoelectric functional materials have been extensively studied and employed in numerous devices. With the rapid development of modern industries, such as power plants, aerospace, automotive, renewable energy and material processing industries, the high temperature piezoelectric materials that can work in extreme environments are in great demand. Piezoelectric materials including piezoelectric single crystals, ceramics and films, are at the heart of electromechanical actuation and sensing devices. A variety of applications where piezoelectric actuators and sensors operate at elevated temperatures (T 200℃) would be extremely desired. The actuators need to work efficiently with high strokes, torques, and forces while operating under relatively harsh conditions. These include high-temperature fans and turbines, motors for valves or natural gas industries, kiln automation, and actuators for automotive engines such as fuel injectors and cooling system elements. Yet, the majority of industrial actuator applications are at or below the 250℃ temperature limit. In addition to the increase in operational temperatures of piezoelectric motors and actuators, a future area of interest is high-temperature MEMS research, which can be used for high-temperature valving. On the other hand, the piezoelectric sensors have been widely used for structural health monitoring applications. This is due to their wide bandwidth, versatility, simplicity, high rigidity, high stability, high reproducibility, fast response time, wide operating temperature range, insensitivity to electric and magnetic fields, the capacity for miniaturization and minimal dependence on moving parts and low power consumption, and wide piezoelectric materials and mechanisms selections, which will greatly benefit the sensing applications. In addition to the temperature usage range, the piezoelectric sensors must withstand the harsh environments encountered in space, engine, power plants, and also need to possess high sensitivity, resistivity, reliability, stability and robustness. In order to use the piezoelectric materials for a specific high temperature application, many aspects need to be considered together with piezoelectric properties, such as phase transition, thermal aging, thermal expansion, chemical stability, electrical resistivity, and the stability of properties at elevated temperature. In this paper, ferroelectric materials with high Curie point, including perovskite-type ferroelectrics, bismuth layer structured ferroelectrics, tungsten-bronze structured ferroelectrics, together with non-ferroelectric piezoelectric single crystals, are surveyed. The crystal structure characteristics, high temperature piezoelectric properties, and recent research progress are discussed. A series of high temperature piezoelectric devices and their applications are reviewed, including high temperature piezoelectric detectors, sensors, transducers, actuators, etc. Finally, recent important research topics, the future development of high temperature piezoelectric materials and the potential new applications are summarized.
Research progress of light out-coupling in organic light-emitting diodes with non-period micro/nanostructures
2018, 67 (20): 207801. doi: 10.7498/aps.67.20181209
Organic light-emitting diodes (OLEDs) possess a number of advantages such as low power consumption, light weight, wide color gamut, high response speed, and high contrast ratio. They have received widespread attention due to their tremendous commercial applications in the fields of full-color flat panel display and solid-state lighting. Although nearly 100% internal quantum efficiency of OLED has been achieved through adopting phosphorescence or thermally activated delayed fluorescence emitters. However, the majority of light generated in an emitting layer is confined within the whole device but does not escape into air due to the induced surface plasmons at the interface between metal and dielectric layers as well as the differences in refractive index between layers of OLED structures including air, glass substrate, transparent electrode as well as organic or inorganic layers. The external quantum efficiency for an OLED with a flat glass substrate is limited to~20%. A low light out-coupling efficiency severely restricts the development and application of OLED. Therefore, enhancing the light out-coupling efficiency of OLED via light extraction technology offers the greatest potential for achieving a substantial increase in the external quantum efficiency of OLED and has been one of the most attractive projects. Up to now, lots of light out-coupling technologies such as micro-lens arrays, photonic crystal, Bragg mirrors and periodic grating have been suggested to enhance the out-coupling efficiency of OLEDs. However, the periodic light out-coupling structures have a limitation that the electroluminescence intensity and spectrum of OLED usually depend on the viewing angle. The angular dependence of the emission characteristic does not hold true for actual display applications due to its deviation from the Lambertian intensity distribution. In this review, we present recent research progress of using non-period micro/nanostructures to improve the light out-coupling efficiency of OLED. In contrast to the emission directionality for OLED using periodic light out-coupling structures, the luminance distribution and spectral stability of OLED based on non-period micro/nanostructures are insensitive to viewing angle. Various light out-coupling techniques such as random micro/nano lens structure, light scattering medium layer, polymer porous scattering films, random concave-convex corrugated structure, and random buckled structure are summarized and discussed. These techniques have the potential applications in displays and solid-state lighting. Finally, summary and prospects regarding to light-coupling techniques of OLEDs are presented.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2018, 67 (20): 209401. doi: 10.7498/aps.67.20180504
Plasma, as a special state of matter, has an effect on its inner conductors. Practically in the plasma environment, the effect may induce surface to charge and discharge, and may degrade the performance of spacecraft. Therefore, this effect needs to be further studied in the electromagnetic compatibility. The energy in a conductors' system is a key factor of the effect, which can also be used to depict the system consisting of relevant conductors and plasma environment. In order to investigate the essence of the system, the variational method is adopted. So with considering the electromagnetic compatibility and protection of this system, the energy of related conductors should be estimated by the theoretical method in the plasma environment. In the stochastic movement, electrons are faster than the irons. Therefore, the negative energy is cumulated. Considering the definition of capacitance, the system energy can be represented by the conductor capacitance and charging potential. Meanwhile, from the plasma kinetic theory, the potential can be obtained in the steady state. Thus, the relations among electromagnetic parameters of conductors, environmental features of plasma, and systematic energy are established, from which the corresponding Collin principle is also investigated. The principle indicates the system essence in the complex electromagnetic environment. In order to illustrate the utility of the variational principle, a simple cubic model is theoretically analyzed directly. From the typical instance, the relation between the geometric dimension and electric energy is illustrated, which is in consistence with the results in the early literature. The relation between secondary electrons and systematic energy is also analyzed. Starting from these theoretical investigations, in order to estimate the complicated structures, the analysis needs to be generalized further. With the assistance of discrete technology, the numerical method is established for analyzing the system energy of the complex conductive system in plasma environment. The generalized method is based on the equation with integral operator, in the calculation of which the method of moment is practically employed. As an application, the estimated energy of cube in plasma environment is compared with the theoretical estimation and the numerical estimation, which are in good agreement with each other. And then a composited structure is numerically analyzed. Obviously, the vairational analysis is beneficial to investigating the physical and principal regulation for conductors in the plasma environment, and the generalized method has wide potential applications in controlling the energy of complex charged conductors, electromagnetic protection, compatibility engineering in plasma environment, etc.