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## COVER ARTICLE

Dielectric capacitors have been widely used in crucial energy storage systems of electronic power systems because of their advantages such as fast charge discharge rates, long cycle lifetimes, low losses, and flexible and convenient processingc. However, the dielectric capacitors have lower energy storage densities than electrochemical energy storage devices, which makes them difficult to meet higher application requirements for electrical engineering at the present stage. Polyvinylidene fluoride (PVDF) based polymers show great potential in achieving improved energy storage properties, which is attributed to their high dielectric constants and high breakdown strengths. This work systematically reviews PVDF-based nanocomposites for energy storage applications. Dielectric constant, breakdown strength and charge discharge efficiency are three main parameters related to energy storage properties, which are proposed to discuss their mechanisms of action and optimization strategies. Finally, the key scientific problems of PVDF-based high energy storage composites are summarized and considered, and the future development trend of dielectric capacitors is also prospected. Zheng Ming-Sheng Acta Physica Sinica.2023, 72(1): 018401.

## COVER ARTICLE

2023, 72 (1): 018401. doi: 10.7498/aps.72.20222012
Abstract +
Dielectric capacitors have been widely used in crucial energy storage systems of electronic power systems because of their advantages such as fast charge discharge rates, long cycle lifetimes, low losses, and flexible and convenient processingc. However, the dielectric capacitors have lower energy storage densities than electrochemical energy storage devices, which makes them difficult to meet higher application requirements for electrical engineering at the present stage. Polyvinylidene fluoride (PVDF) based polymers show great potential in achieving improved energy storage properties, which is attributed to their high dielectric constants and high breakdown strengths. This work systematically reviews PVDF-based nanocomposites for energy storage applications. Dielectric constant, breakdown strength and charge discharge efficiency are three main parameters related to energy storage properties, which are proposed to discuss their mechanisms of action and optimization strategies. Finally, the key scientific problems of PVDF-based high energy storage composites are summarized and considered, and the future development trend of dielectric capacitors is also prospected.
###### GENERAL
2023, 72 (1): 010201. doi: 10.7498/aps.72.20221745
Abstract +
Polarization is an important property of electromagnetic waves, and measuring their polarization properties fast and precisely is a very important issue in many applications, such as skylight polarization navigation, optical activity measurement, imaging polarimetry, spectroscopic ellipsometry, and fluorescence polarization immunoassay . The polarization measurement method based on vector optical field modulation and image processing is a new type of spatial modulation polarization detection technology. The key step of this technique moving to practical application is determined by effective polarization measuring algorithms with high speed and accuracy. In order to find out the method of fast and precisely calculating polarization direction, the principle of polarization direction measurement based on vector optical field and spatial modulation is introduced briefly, and the basic characteristics of the spatially modulated intensity distribution images are analyzed. According to the properties of spatially modulated image, we propose and implement four different polarization direction calculation methods, which are the Radon transform, intensity modulation curve detection, radial integration, and image correlation detection, and also introduce their working principles and physical thoughts elaborately. To compare the detailed performances of these four algorithms, an experimental setup is constructed to collect the images and perform the algorithm verification, and the stabilities, speeds and accuracies of the four algorithms are compared. The research results indicate that all the four methods can achieve their stable and reliable polarization direction detections. The three methods, intensity modulation curve detection, radial integration and image correlation detection, can achieve the polarization direction measuring accuracy better than 0.01° . The intensity modulation curve detection and radial integration have relatively fast calculation speed and the best comprehensive performances , and are the most promising methods to realize real-time and high-precision polarization measurement.
2023, 72 (1): 010301. doi: 10.7498/aps.72.20221524
Abstract +
In previous paper (2019 Int. J. Mod. Phys. B 33 1950197; 2020 Int. J. Mod. Phys. B 34 2050022), we presented a method to judge the entanglement of 2-qubit system. The necessary and sufficient conditions for the 2qubit system being separable are that if the relevant coefficients is positive and the principal density matrix is separable, then the system is separable, otherwise it is entangled. Now in this paper, we try to generalize this criterion to a 3-qubit system, and then, we further generalize the criterion of 3-qubit system to an N-qubit system. This is a complicated and interesting issue.
2023, 72 (1): 010302. doi: 10.7498/aps.72.20221293
Abstract +
As a peculiar resource of quantum mechanics, quantum correlation has been applied to many aspects. In quantum information processing and quantum computing, the quantum correlation plays an extremely important role, and it has been a subject of further studies, principally due to the general belief that it is a fundamental resource for different quantum information processing tasks. In addition, correlation measure is a very important physical quantity in studying the quantum correlation. A well-defined correlation measure needs to have some necessary properties. By proving these necessary properties, we can deepen our understanding of correlation measure. As one of the key concepts of quantum information theory, relative entropy is always used to measure the uncertainty contained in the state of physical system. In order to better understand the properties and applications of correlation measure based on relative entropy, in this paper, according to the properties of the min relative entropy, we give the quantum correlation measure based on min relative entropy for two-partition and k-partition. Furthermore, we prove that it satisfies some necessary properties of quantum correlation measures, including the nonnegativity, the invariance under local unitary operators, and the monotonicity under completely positive trace-preserving. By proving these properties, we show that the given correlation measure is well-defined. Security of communication has received much attention since ancient times. In today's society, the internet, instant messaging and e-commerce applications are all related to the information security, and the information security is related to the vital interests of everyone. The information encryption is one of the important methods to ensure information security. As an important way to ensure information security, quantum channel has received more and more attention. At the end of the paper, we introduce the concept of quantum channel, and discuss the influence of quantum channel on the correlation measure based on min relative entropy under k-partition. By proposing a new correlation measure and proving the effect quantum channel on the measure, we can better describe the uncertainty contained in the state of physical system.
2023, 72 (1): 010401. doi: 10.7498/aps.72.20221415
Abstract +
The SL(n,R) Toda black hole is an ideal field for us to study black hole physics because of its excellent mathematical structure and high symmetry. This work is mainly to study the Hawking radiation of SL(n,R) Toda black hole and and the problem about its related black hole information loss . For simplicity, we only consider the Hawking radiation by calculating the tunneling effect of particles with zero rest mass near the event horizon under the four-dimensional static spherical symmetric SL(n,R) Toda black hole. In the process of particle tunneling through the event horizon of the black hole, due to the conservation of energy, the mass of black hole will be changed, which will cause the event horizon to shrink. Therefore, the reaction of tunneling particles to the background space-time leads to the dynamic change of spacetime metric, that is, the self-gravitational action of the particles generates the tunneling barrier. The tunneling probability of the particle passing through the event horizon depends on the change of the black hole entropy before and after the particle exits. Under certain conditions, our results are consistent with those of RN black holes and Schwartz black holes, and the calculation results once again support the tunneling model proposed by Parikh and Wilczek. This semi-classical image shows that the new black hole radiation spectrum is not a pure heat spectrum, but there is a small deviation from the pure thermal spectrum. From the knowledge of probability theory, it can be proved that there is a correlation process between non-thermal spectra. According to the Shannon entropy definition, the black hole entropy is analogous to Shannon information entropy. We calculate the SL(n,R) Toda black hole information paradox, and find that the correlation between the particles emitted from black hole can carry information and keep the information of black hole unchanged. The specific source of this correlation, as well as the generation mechanism, remains to be further studied. The research on the problem about black hole information loss reveals that information conservation remains true when gravitational correlations among Hawking radiations are properly taken into account. Information conservation principle thus states that the Hawking radiation is unitary, which shows that the dynamics of a black hole obeys the laws of quantum mechanics. Since a black hole is a result of general relativity, the unitarity of a black hole definitely indicates the possibility of a unified gravity and quantum mechanics.

## EDITOR'S SUGGESTION

2023, 72 (1): 010501. doi: 10.7498/aps.72.20221498
Abstract +
The negative differential thermal resistance (NDTR) effect refers to a phenomenon that may take place in a heat transport system where the heat current counterintuitively decreases as the temperature difference between heat baths increases. Understanding and controlling the NDTR properties of out-of-equilibrium systems and using them to design new functional thermal devices are the major challenges of modern science and technology, which has important theoretical significance and application prospects. Up to now, the various lattice models representing solid materials have been taken to study the NDTR properties, but the fluid models have not received enough attention. It has recently been shown that in one-dimensional hard-point gas models representing fluids, there is a mechanism for NDTR induced by heat baths. The mechanism for NDTR in such a system depends on the simple fact that decreasing the temperature of the cold bath can weaken the motion of particles and decrease the collision rate between particles and the hot bath, thus impeding thermal exchange between the cold and hot baths. In this paper, we study how this mechanism works in more general two-dimensional gas models described by multi-particle collision dynamics. The gas models we consider are in a finite rectangular region of two-dimensional space with each end in contact with a heat bath. Based on the analytical results and numerical simulations, we show that the mechanism underlying NDTR induced by heat baths is also in effect for two-dimensional gas models and is applicable for describing systems with small sizes and weak interactions. Our result, together with that previously obtained in one-dimensional gas models, provides strong evidence that gas systems can exhibit NDTR by decreasing the temperature of the heat bath, which sheds new light on the exploring direction for developing various fluidic thermal control devices.

## EDITOR'S SUGGESTION

2023, 72 (1): 010701. doi: 10.7498/aps.72.20221725
Abstract +
In this paper, the wavelength modulation spectroscopy (WMS) technique is modified and used for measuring methane with large absorbance. The WMS has been frequently used for gas measurement and relies on the linear relationship between the second harmonic amplitude and the gas volume concentration. However, the conventional WMS technique is only applicable for the gas whose absorbance is much smaller than 1, which is because the first-order approximation of Lambert-Beer's law is required in the derivation of the traditional WMS theory, and the first-order approximation holds only at low absorbance, hence the linear relationship between the second harmonic and the gas concentration does not hold at large absorbance. In the modified WMS in this work, it is not necessary to make any approximation to Lambert-Beer's law. The measured light is absorbed by the gas to be measured and then collected by the photodetector. The reference light is directly detected by another photodetector without being absorbed. The output signals of the two photodetectors are transmitted to the computer after implementing analog-to-digital conversion. In this way, the demodulated second harmonic signal remains linear with the gas concentration even at large absorbance. In this work, the traditional WMS theory and the modified WMS theory are both introduced, and a series of methane gas with concentration gradients are measured separately. The experimental results of the traditional WMS and the modified WMS are compared with each other. It is confirmed that the linearity in the traditional WMS theory no longer holds under large absorbance, but the improved WMS can still guarantee the linear relationship between the second harmonic and the methane concentration, which verifies the advantages of the modified scheme. Finally, through Allan's standard deviation analysis, the stability of this methane measurement system reaches the optimal value at the average time of 103.6 s, and the corresponding Allan's standard deviation is 1/26.62×10–9 volume.
###### NUCLEAR PHYSICS
2023, 72 (1): 012401. doi: 10.7498/aps.72.20221617
Abstract +
To ascertain the damage mechanism caused by high-energy proton irradiation to AlGaN/GaN power devices of enhanced Cascode structures, we study the radiation effect of enhanced Cascode structure and depletion AlGaN/GaN power devices by using 60 MeV energy protons in this work. In the case of proton injection reaching 1×1012 p/cm2, the experimental results show that the threshold voltage of the Cascode type device is negatively drifted, the transconductance decreases, and the peak leakage current increases. The threshold voltage decreases from 4.2 V to 3.0 V, with a decrement of 1.2 V, and the peak transconductance value decreases from 0.324 S/mm to 0.260 S/mm, with a decrement of about 19.75%. There is no significant change after the conventional depleted AlGaN/GaN device has been irradiated. The Cascode-type AlGaN/GaN power device is more sensitive to proton irradiation than the depletion-type AlGaN/GaN device. The Cascode-type device is sensitive to proton irradiation because of its structure connected to a silicon-based MOS tube. Proton irradiation causes the silicon-based MOS gate oxide layer to generate a large amount of net positive charge, induces an ionization damage effect, and causes threshold voltage to negatively drift and the gate leakage current to increase. The equivalent 60 MeV energy protons and cumulative injection of 1×1012 p/cm2 dose of the ${}_{}{}^{60}\rm{C}\rm{o}~\rm{\gamma }$ radiation device is used to obtain the ionization damage effect. It is found that after being irradiated by the equivalent dose ${}_{}{}^{60}\rm{C}\rm{o}~\rm{\gamma }$ ray , the device has the threshold voltage decreasing from 4.15 V to 2.15 V, with a negative drift of 2 V; transconductance peak decreases from 0.335 S/mm to 0.300 S/mm, with an approximate decrement of 10.45%. The degradation of the electrical properties of the device after being irradiated by ${}_{}{}^{60}\rm{C}\rm{o}~\rm{\gamma }$ ray is consistent with the degradation law after being irradiated by high-energy protons. In order to further verify the experimental accuracy and conclusions, the ionization energy loss and non-ionization energy loss induced by radiation in the device are obtained by Monte Carlo simulation. The simulation results show that the ionization energy loss induces silicon-based MOS to generate oxide trap charge and interfacial state trap charge, which is mainly responsible for the performance degradation of AlGaN/GaN HEMT power devices with enhanced Cascode structure.
###### ATOMIC AND MOLECULAR PHYSICS
2023, 72 (1): 013101. doi: 10.7498/aps.72.20221544
Abstract +
As a wide band gap semiconductor with perovskite structure, SnSnO3 is regarded as a promising candidate of transparent conductive oxides due to its superior properties like high transparency, non-toxicity and low price. In this work, the electronic structure of SrSnO3 is obtained through first-principles calculations based on HSE06 hybrid functional. Especially, we investigate the defect formation energy and transition levels of the intrinsic and external defects in SrSnO3. The intrinsic defects including the anti-site defects (SrSn and SnSr), the vacancy defects (VSr, VSn, and VO), and the interstitial defects (Sri, Sni and Oi) are considered while the external doping defects are taken into account, including the substitution of Li, Na, K, Al, Ga, In for Sr site, Al, Ga, In, P, As, Sb for Sn site, and N, P at O site. Subsequently, the suitable doping elements and the corresponding experimental preparation environments are pointed out. Furthermore, we discuss the mechanism of its conductance according to the energy positions of the band edges. Our calculation results demonstrate that SrSnO3 is an indirect-type semiconductor with a fundamental band gap of 3.55 eV and an optical band gap of 4.10 eV and then has a good visible light transmittance. Its valence band maximum (VBM) comes from O-2p state while its conduction band minimum (CBM) mainly originates from Sn-5s state. In consistent with the delocalized Sn-5s state at CBM, the electron effective mass is light and isotropic, which is beneficial to n-type conductance. The n-type intrinsic defects SnSr and Vo have lower defect formation energy than the p-type intrinsic defects under O-poor condition while the n-type and p-type defects with low defect formation energy are almost equal under O-rich condition. Moreover, the transition levels of SnSr and VO are both deep. Therefore, SrSnO3 cannot have a good conductance without external doping. Our calculations also demonstrate that it is hard to produce an efficient p-type external doping due to the compensation effect by VO. On the other hand, substitution of As or Sb for Sn site can result in an effective n-type external doping due to their low defect formation energy and shallow transition levels. According to the low energy positions of VBM (–7.5 eV) and CBM (–4.0 eV) of SrSnO3, we explain the reason why it is easy to realize an n-type conductance but hard to produce a high-performance p-type conductance, which follows the doping rules for wide band gap semiconductors. Finally, Sb-doped SrSnO3 is proposed as a promising candidate for n-type transparent conductive materials.
2023, 72 (1): 013102. doi: 10.7498/aps.72.20221441
Abstract +
As a new graphene-based two-dimensional semiconductor material, C3N has received extensive attention from researchers due to its excellent mechanical and electronic properties. Whether there is any difference in the phonon transport mechanism among different C3N structures remains to be further investigated. Therefore, four kinds of C3N structures with different patterns are constructed in this paper, and their thermal conduction mechanisms are studied by the non-equilibrium molecular dynamics (NEMD) method. The research results are shown as follows. 1) Among these four patterns, the C3N (M3) with the perfect structure has the highest thermal conductivity, followed by M1, and M4 has the lowest thermal conductivity. 2) Moreover, the thermal conductivities of C3N with different patterns have obviously different size and temperature effects. When the sample length is short, the phonon transport is mainly ballistic transport, while diffusion transport dominates the heat transport when the sample length further increases. As the temperature increases, Umklapp scattering dominates the heat transport, making the thermal conductivity and temperature show a 1/T trend. 3) Comparing with M3 , the patterns of M1 and M4 have large phonon band gaps, and their dispersion curves are further softened. At the same time, regardless of low-frequency or high-frequency phonons, localized features appear in the M1 and M4 (especially the M4), which has a significant inhibitory effect on thermal conductivity. This paper provides an idea for the better design of thermal management materials.
2023, 72 (1): 013402. doi: 10.7498/aps.72.20221628
Abstract +
X-ray emissionproduced by highly charged ions with the energy range near the Bohr velocity involves complicated atomic process. However, duo to the limitation of experimental conditions, the relevant researches are nearly absent. It is unclear whether the existing theory is applicable in such an energy range. This needs further exploring. In the present work, K X-ray spectra of Al excited by H+, He2+ and highly charged heavy ions I22+ and Xe20+ are investigated by using an Si drift X-ray detector in the energy range near the Bohr velocity. The X-ray production cross sections are extracted from the X-ray counts and compared with the theoretical simulations from PWBA, ECPSSR and modified BEA model. It is indicated that the cross section increases with the augment of projectile energy. With the same incident energy per nucleon, the cross section induced by highly charged heavy ions is a factor of about 104 larger than that by light ions . With the impact of H+ and He2+ ions, the K-shell electrons are mainly knocked off through the direct Coulomb ionization, and the X-ray emission cross section can be well predicted by ECPSSR theory. For the bombardment of highly charged heavy ions I22+ and Xe20+, except for the Coulomb ionization, the orbital electrons can also be excited by electron capture. The BEA simulation after being modified by both Coulomb repulsion and effective charge can well predict the X-ray production cross section.
2023, 72 (1): 013701. doi: 10.7498/aps.72.20221674
Abstract +
Coulomb crystals have applications in many areas such as quantum computing and simulation, quantum logic spectroscopy, nonlinear dynamics and chaos, phase transitions, and chemical reaction process. The structure of the Coulomb crystal and the trajectory of each trapped ion are typically determined by the parameters of the trap and the ion species. However, dark ions are often inevitable in experiment, which introduces uncertainty into the desired crystal structures and ion trajectories. Few researches have been conducted to investigate the configuration change of the crystal in the presence of dark ions and the influence of a dark ion on its surrounding ion trajectories in a multi-ion system. In this work, we utilize the molecular dynamics simulation software LAMMPS and the (py)LIon package (modified to adapt the semi-classical theory of laser cooling) for simulating the three-dimensional ion trajectories of Coulomb crystals. The formation process of 40Ca+ Coulomb crystal in a linear trap is simulated. With the micromotion and secular motion trajectories of each ion, we calculate the temperature of Coulomb crystal and the average velocity of specific ions. It is observed that the crystal structure exhibits obvious layering phenomenon when the trapped ions yield a large difference in their charge-to-mass ratio (CMR), however, layering is not obvious with a small difference in the CMR. In addition, we simulate and compare the Coulomb crystal structure formed by pure 40Ca+ ions with that formed by 40Ca+ ions mixed with a small number of dark ions including isotopic ions (44Ca+) and impurity ions (CaH+). Three different cases are investigated, namely the one-dimensional ion string, two-dimensional planar structure and three-dimensional helical structure. The results show that the ions in the neighborhood of a dark ion exhibit around micron-order position change compared with their positions before the dark ion is formed. Such a change can be measured in experiment through microscopic imaging, thereby providing a way to identify the formation of dark ions in Column crystals with a large ion number.
2023, 72 (1): 013702. doi: 10.7498/aps.72.20221363
Abstract +
Hydrogen maser uses the transition frequency of hydrogen atom at hyperfine energy level of ground state to realize precise timing. It has excellent frequency stability, especially in medium- and short-term, and low frequency drift. It has been used as high-precision frequency standard in engineering fields such as time keeping, navigation, and very long baseline interferometry. Clock transition of hydrogen maser is the transition between states of $|F = 1, m_{\rm F} = 0\rangle$ and $|F = 0, m_{\rm F} = 0\rangle$. State selection is realized by state selection magnet, through which high energy atoms are converged and low energy atoms are dispersed. In conventional magnet state-selecting system, both atoms of $|F = 1, m_{\rm F} = 0\rangle$ states, which are required for the maser transition, and useless atoms of $|F = 1, m_{\rm F} = 1\rangle$ states are focused into storage bulb, which places restrictions on the medium- and long-term frequency stability performance of hydrogen maser. In order to further improve the quality of atomic transition spectral lines and the performance of hydrogen maser, double state-selection beam optical system which is based on the Majorana transition mode is constructed through calculations and simulations. In this work, we use Majorana method to invert atomic states. The magnetic field required for Majorana transition is established by using two coils with reverse current. The two coils are separated by 71 mm, and the coil axes are aligned with the direction of atomic beam. The other two pairs of transverse Helmholtz coils are separated by 22 mm in the center of the state reversal to adjust the zero point of magnetic field, which should coincide with the atomic beam to ensure a complete reversal of atomic polarity. The state reversal region is surrounded by four magnetic shields to reduce the influence of stray magnetic fields. Relationship between selected-state magnetic field gradient and distance of magnetic poles is analyzed by simulation, and trajectories of the atoms with high and low energy under different selected-state magnetic fields are calculated. The utilization and purity of high energy state atoms entering into bulb atoms are obtained. The purity of the selected $|F = 1, m_{\rm F} = 0\rangle$ state atoms reaches 99% and the utilization rate is 58%. This is ideal for engineering applications. It effectively enhances the proportion of $|F = 1, m_{\rm F} = 0\rangle$ state atoms entering into the atomic storage bulb and ensures the utilization of atoms. We verify the state-selection beam optical system experimentally. By turning on double state-selection system the maser signal can be enhanced. By adjusting the coil current of the double state-selection system, the maser signal varies with coil current, which verifies the effectiveness of double state-selection system.
###### ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2023, 72 (1): 014201. doi: 10.7498/aps.72.20221643
Abstract +
The magneto-optical Kerr effect (MOKE) refers to the rotation of the polarization plane when a linearly polarized light is reflected at the surface of magnetic material. The MOKE reveals the magnetization of the optical properties of magnetic material and can be characterized by the dielectric tensor containing the magneto-optical constant. Thus, exploring the MOKE requires very precise determination of the magneto-optical constant. The photonic spin Hall effect (PSHE), which corresponds to the lateral and in-plane spin-dependent splitting of the beam, can be used as an effective method to characterize the magneto-optical constant due to its advantage of being extremely sensitive to changes in the physical parameters of the material. Most of the previous studies only considered the case of a single thickness of magnetic material and a single MOKE and need to introduce complex weak measurement techniques to observe the photonic spin Hall effect. In this work, we theoretically investigate the in-plane spin angular shifts in three MOKE cases in bulk and ultrathin magnetic materials. We can effectively tune the in-plane angular displacements of different magnetic material thickness by changing the magnetic field direction corresponding to different MOKEs and changing the magneto-optical constants (including amplitude and phase). The research results show that in the case of bulk and ultrathin magnetic materials, the internal spin angular displacements under different MOKEs will show different trends when the magneto-optical constants change the amplitude and phase, especially in ultra-thin magnetic material. In the lateral Kerr effect in thin material, the photon in-plane angular displacement does not affect the change of the magneto-optical constant, but in other cases, the amplitude relative to the phase has a much larger effect on the photon in-plane angular displacement. In this regard, we propose a new method which can directly determine the amplitude and phase of the magneto-optical constant by using the huge in-plane spin angular displacement without considering the weak measurements and can also judge different MOKEs according to the variation of the in-plane angular displacement in the bulk and ultrathin magnetic materials. This method not only provides a new probe for measuring magneto-optical constants but also expands the study of spin photonics.
2023, 72 (1): 014202. doi: 10.7498/aps.72.20221533
Abstract +
Lithography plays a vital important role in modern information technologies. Patterning on a nanoscale in a handy way is highly desired for both scientific and industrial purposes. In this work, we propose a convenient nanolithography method based on Fresnel diffraction patterns. We start with the explanation of the “dense-inside-sparse-outside” Fresnel diffraction fringes resulting from the apertures of finite extent, by using the fast Fourier transform algorithm through appropriately choosing the number of uniformly spaced samples. Moderately focusing the diffraction patterns via high-numerical-aperture objectives ( the method is termed the “Fresnel diffraction lithography”), the rotationally symmetric patterns with a minimum feature size of ~190 nm, and the scanning lines with a width of ~350 nm are realized, respectively, The calculation using vectorial diffraction theory suggests a better resolution when perfectly focused. This method shows good tolerance to defocus and does not require complex lens combinations or micro/nano-diffraction optical elements, Therefore, this method can find some applications in widespread areas, e.g. functional metasurfaces, as a novel and low-cost nano-patterning technology with sub-wavelength resolution and high flexibility.
2023, 72 (1): 014203. doi: 10.7498/aps.72.20221383
Abstract +
The InAs/GaSb superlattices (SPLs) is an important component of quantum cascade laser (QCL) and interband cascade laser (ICL). In particular, the upper and lower SPL waveguide layers of the ICL are alternately grown from a large number of ultra-film epitaxial layers (nm) by molecular beam epitaxy(MBE). Subtle lattice mismatch may directly lead to the deterioration of material crystal quality, and the change of thicknessand the composition of each layer will strongly affect the structural performance of device material. The optimal growth temperature of InAs/GaSb SPLs is about 420 ℃. By growing GaSb/AlSb and InAs/GaSb SPL both with 40 short periods under the substrate rotating, the thickness of GaSb layer and AlSb layer are 5.448 nm and 3.921 nm, and the thickness of InAs layer and GaSb layer are 8.998 nm and 13.77 nm, respectively. The error is within about 10%, and the optimal growth conditions of InAs/AlSb SPLs are obtained. A lattice matched 40-period InAs/AlSb superlattice waveguide layer is grown on GaSb substrate. The influence of drifting As injection on the average lattice constant of InAs/AlSb superlattice is fully considered. Under the condition of fixed SOAK time of 3 s, the As pressure is changed to 1.7 × 10–6 mbar to adjust the average lattice constants of the superlattices and achieve their matching with the GaSb substrate lattice. The experimental results show that the 0 order satellite peak of the SPL coincides with the peak of the GaSb substrate, and has a perfect lattice matching, and that the sharp second order satellite peak and the periodic structure good repeatability also indicate that the superlattice material has the excellent structural quality of the SPLs structure.
2023, 72 (1): 014204. doi: 10.7498/aps.72.20221422
Abstract +
Hermite-Gaussian (HG) beams have many important applications in the optical frontier, and the limited output power of the high-purity HG beams is partly due to the small gain volume of the mode. The commonly used off-axis end-pumped scheme offers a narrow gain volume whose diameter is about a hundred microns. In this work, a new method of generating the HG beams based on a slab resonator that has a large mode volume is proposed and experimentally demonstrated. According to the optical resonator theory, the intra-cavity modes in thickness and width direction of the slab resonator are restricted by inserting two size-adjustable apertures, respectively. The one-dimensional HG beam generation is mainly guaranteed by the size of the aperture along the thickness direction of the slab, which matches the diameter of the fundamental mode. The different order one-dimensional HG beams are obtained by refined intra-cavity mode modulation. Since the higher-order modes are less sensitive to the misalignment of the cavity mirror than the lower-order modes, and the manipulation of the modes-loss at different orders is achieved by combining the tilt control of the coupled output mirror and the size control of intra-cavity apertures. By adjusting the optical gain and loss in the resonant cavity, the single mode wins the competition of laser modes. Therefore, high-purity one-dimensional HG beams with 0 to 9 orders (HG00 to HG09) are generated. The pump module is comprised of a two-dimensional laser diode array which offers face-pumping to the large surface of the slab, therefore the width of the mode volume is extended to several millimeters. By further incorporating the 100mm-level long slab, the total gain volume is much larger than the counterpart in the off-axis pumping scheme. In this work, the output power of the highest order HG09 mode increases up to 244 mW. Owing to the large gain volume and uniform gain distribution caused by the face-pumped slab, the purity of high order HG modes is quite good. The correlation coefficient $\rho$ between the measured intensity distribution and the theoretical value is larger than 0.95. The beam quality factor ${M}^{2}$ is also in good agreement with the theoretical one. Finally, a conversion from Hermite-Gaussian beams to the donut-shaped Laguerre-Gaussian beams is realized by using an astigmatic mode converter. Hopefully, power scaling of the HG beam output is also expected by employing cascaded slab amplifiers, and the approach in this paper provides a novel solution for generation of high power HG beams.
2023, 72 (1): 014205. doi: 10.7498/aps.72.20221530
Abstract +
In cavity ring-down technique, cavity maladjustment has an essential effect on the measurement of intracavity loss. Several adjustment criterions have been adopted to achieve the optimal cavity state. However, experimental study shows that these criterions may correspond to different cavity states, which means that there is discrepancy between different criterions. In view of this problem, a model of intracavity propagation of Gaussian beam is established based on the angular spectrum propagation theory. This model is tested by numerical simulation and experimental research together. In the simulation, the true value of intracavity loss can be known beforehand. The two-dimensional angular scanning is carried out for certain cavity mirror. The two-dimensional distributions of the measure value of intracavity loss and the transmission light intensity are obtained simultaneously. These distributions are both nonlinear and multi-extremum, which will doubtlessly increase the difficulty in realizing the cavity adjustment. By comparing the distributions , we do find the discrepancy between the largest transmission light intensity and the least measured intracavity loss. Meanwhile both of these two states may be not corresponding to the true value in fact. After statistical studies, the relative error of the least measured intracavity loss is (–37.01±11.79) ppm, whereas the relative error of the largest transmission intensity is (–2.70±0.89) ppm. The criterion of the largest transmission intensity shows better stability and repeatability. This model is further tested in a folded cavity ring-down setup. The similar scanning procedure is carried out. A major problem in the experiment is that the true value of intracavity loss cannot be known. So only the repeatability precision of the measured intracavity loss can be analyzed. The statistical results of the largest light intensity and the least measured intracavity loss are ±29.32 ppm and ±70.71 ppm, respectively. The criterion of the largest transmission intensity has better repeatability, which is basically consistent with the simulation result. In this way the rationality of this model can be verified to some degree. In this paper, the criterion of the largest transmission intensity is recommended in the cavity ring-down technique. Furthermore, this model can be a reference for the research of intracavity optical field response, intracavity optical field transmission, unstable resonator alignment, etc.
2023, 72 (1): 014206. doi: 10.7498/aps.72.20221232
Abstract +
We have studied the self-focusing and filamentation of vortex laser beams propagating in underdense plasmas with different values of the topological charge and initial laser powers. The self-focusing dynamics of vortex laser beams is closely related to the topological charge, which has attracted widespread attention. Based on the paraxial approximation of the Helmholtz equation, the steady-state solution of vortex beams propagating in underdense plasmas is deduced, and the expression for critical power of vortex laser beams is obtained. Furthermore, using the split-step Fourier method to solve the wave equation, we analyze the numerical images of vortex laser beams propagating in underdense plasma under different parameters. The simulation results show that the critical power for self-focusing is positively correlated with the topological charge of vortex laser beams. When the initial laser power is high enough, the vortex laser beam will first be focused into a thin ring, and then the modulation amplitude increases continuously, which eventually leads to the ring structure breaking into filaments. The number of filaments has an integer multiple relationship with the topological charge. In the process of filamentation, the radius and the maximum light intensity of vortex laser beam both change drastically. After the filamentation process is completed, the vortex laser beam continues to propagate with a new topological type. We further increase the incident laser power and find that the number of filaments of the vortex laser beam increases. The increased number of filaments is the value of its topological charge at each time. With the development of filament instability, higher-order modulation instability can be excited in the later stage, and the intensity of filaments will exhibit angular modulation. Our results show that in compared with the standard Gaussian beam, the propagation behavior of vortex laser beams in underdense plasmas is much more stable under the same power, wavelength and plasma parameters. The propagation characteristics of vortex laser beams are helpful to the theoretical and experimental study of stimulated backward Raman amplification of ultra-strong vortex beam in underdense plasmas.
2023, 72 (1): 014207. doi: 10.7498/aps.72.20221709
Abstract +
Optical frequency combs (OFCs) each consist of a set of equally spaced discrete frequency components, and they have been widely applied to many fields such as metrology, optical arbitrary waveform generation, spectroscopy, optical communication, and THz generation. In this work, we propose a scheme for generating broadband and tunable OFCs based on a 1550 nm vertical-cavity surface-emitting laser (VCSEL) under pulsed current modulation and optical injection. Firstly, a pulsed electrical signal is utilized to drive a 1550 nm-VCSEL into the gain-switching state with a broad noisy spectrum. Next, a continuous optical wave is further introduced for generating broadband and tunable OFC. Under injection light with power of 18.82 µW and wavelength of 1551.8570 nm, and pulsed electrical signal with a frequency of 0.5 GHz and pulse width of 200 ps, an OFC with a bandwidth of 82.5 GHz and CNR of 35 dB is experimentally acquired, and the single sideband phase noise at the 0.5 GHz reaches –123.3 dBc/Hz at 10 kHz. Moreover, the influences of injection light wavelength, frequency and width of pulse electrical signal on the performance of generated OFC are investigated. The experimental results show that OFCs with different comb spacings can be obtained by varying the frequency of pulsed electrical signal. For the frequency of pulsed current signal varying in a range of 0.25 GHz–3 GHz, the bandwidth of generated OFCs can exceed 60 GHz through selecting optimized injection optical wavelength and width of pulse electrical signal.
2023, 72 (1): 014208. doi: 10.7498/aps.72.20221752
Abstract +
Multi-wavelength confocal lens is an indispensable part of optical system, the traditional optical confocal system is often added by a certain number of optical lenses, or uses a different combination of optical lenses of different materials to implement multi-wavelength co-focusing,making the system possess a larger volume and weight, which, however, is difficult to meet the requirements for high integration and miniaturization of the system. As an optical element composed of two-dimensional planar subwavelength micro-element structure, the metalens has the advantages of flatness, ultra-thinness and regulating light waves, and has great potential applications in highly integrated and miniaturized optical confocal systems. According to relevant research reports, it is known that the existing research schemes of multi-wavelength confocal metalens have some shortcomings, such as relatively complex structure and relatively low focusing efficiency. In this work, a kind of metalens composed of simple micro-element structure is proposed and designed, which can simultaneously realize the long infrared dual wavelength confocal function. Based on the generalized Snell's law and the transmission phase modulation mechanism, a scientific evaluation function is established to select the optimal array of micro-elements structure to form a metalens. With the elliptical nano silicon column in a simple micro-element structure, the wavefront phase of the long infrared dual wavelength in the orthogonal linear polarization state can be adjusted independently and efficiently , while reducing the wavelength crosstalk and improving the focusing efficiency. The design results show that the proposed metalens achieves dual wavelength co-focused with a wavelength of 10.6 and 9.3 μm, and has a high focusing efficiency, The focusing spot is close to the diffraction limit. The quantitative analysis of the redundancy of the structural parameters of the metalens micro-element structure is made, and the trend of its influence on the focusing efficiency and the allowable deviation range of the micro-element structure parameters are obtained, which provides a theoretical basis for further precisely controlling the device fabrication. The matalens designed in this work is expected to meet the requirements for integration and miniaturization of long infrared optical confocal system, and has important applications in laser surgery, industrial cutting and welding and other fields.

## EDITOR'S SUGGESTION

2023, 72 (1): 014209. doi: 10.7498/aps.72.20221290
Abstract +
Global climate change and the formation of the Antarctic ozone hole have prompted people to pay attention to the changes in atmospheric ozone content. The global continuous observation of ozone is achieved by retrieving the global total column concentration from nadir satellite data. In this work, the weighted multiplication algebraic algorithm is combined with the radiative transfer model SCIATRAN, by using the 2011 Chappuis-Wulf band SCIAMACHY limb radiation data to retrieve the stratospheric ozone profile between 15- and 40 km altitude, solving the ozone global stratified observation problems. In the ozone global stratification map, the whole process of the global transmission of ozone formed in low latitude regions to high latitude regions is observed, which is directly related to the Brewer-Dobson circulation. During the most severe period of the Antarctic ozone hole from September to October, the Antarctic polar vortex has an obvious hindering effect on ozone transmission, and the polar vortex has a “transparent wall” effect. On the one hand, it is difficult to transfer ozone from the equatorial region to the Antarctic region for replenishment. On the other hand, the retention of ozone-depleting substances over the Antarctic region leads to the acceleration of ozone depletion, and the combination of low replenishment and high depletion contributes to the Antarctic ozone hole. Compared with the global total column concentration of ozone, the observation of global ozone stratification is very valuable for scientific research and will promote the detailed study of the whole process of ozone formation, transmission, and consumption.
2023, 72 (1): 014210. doi: 10.7498/aps.72.20221600
Abstract +
Non-line-of-sight (NLOS) imaging is an emerging technology for optically imaging the objects blocked beyond the detector's line of sight. The NLOS imaging based on light-cone transform and inverted method can be regarded as a deconvolution process. The traditional Wiener filtering deconvolution method uses the empirical values or the repeated attempts to obtain the power spectral density noise-to-signal ratio (PSDNSR) of the transient image: each hidden scene has a different PSDNSR for NLOS imaging, so the prior estimation is not appropriate and repeated attempts make it difficult to quickly find the optimal value. Therefore, in this work proposed is a method of estimating the PSDNSR by using the mid-frequency information of captured transient images for Wiener filtering to achieve NLOS imaging. In this method, the turning points between the mid-frequency domain and the high-frequency domain of the transient image amplitude spectrum are determined, and then the PSDNSR value is solved by analyzing the characteristics and relationship among the noise power spectra at the low, middle and high frequency. Experiments show that the PSDNSR estimated by NLOS imaging algorithm based on Wiener filtering of mid-frequency domain has a better reconstruction effect. Compared with other methods, the algorithm in this work can directly estimate PSDNSR in one step, without iterative operations, and the computational complexity is low, therebysimplifying the parameter adjustment steps of the Wiener filtering deconvolution NLOS imaging algorithm based on light-cone transform. Therefore the reconstruction efficiency can be improved on the premise of ensuring the reconstruction effect.
2023, 72 (1): 014301. doi: 10.7498/aps.72.20221547
Abstract +
The nonlinear effect of high-intensity sound waves produces the acoustic radiation force (ARF), which are used for acoustic levitation and manipulation practical. With no special requirement for the physical and chemical properties of the controlled objects, acoustic levitation owns a promising application prospect. The common levitation scheme includes the standing-wave system and phased-array levitation system. The standing-wave system has poor performance in the aspects of the degree of spatial freedom, the ARF along the non-axial direction, and the levitation stability. The phased-array system requires a complex control system and a high production cost. Here, we propose a single-side acoustic levitation system based on the paired confocal focused transducers. By driving the transducer pairs with reverse phase mode, two anti-phase focused spherical waves interfere with each other, resulting in constant sound pressure of 0 Pa at the focus. The resulting potential well can achieve stable particle capturing and levitating. First, we verifed the theoretical feasibility of the system according to Huygens' principle. Then, using the finite element method, we analyzed the influences of structural and driving parameters on the sound field distribution, such as the angle between the transducer axis and the central axis of the structure and the excitation phase modes. Finally, we demonstrated the particle trappings under two kinds of excitation phase modes of the levitation system experimentally. The results show that, 1) the intensity of the dominating potential well reaches a strongest value when the structural angle is 45°; 2) as the excitation phases are 0, 0, π, and π, the sound field owns three potential wells which can capture three clusters of quartz sands, the primary potential well is stronger than the secondary one; 3) as the excitation phases are 0, π/2, π, and 3π/2, the sound field owns one potential well and captures one cluster of quartz sands. The isosurface of wave intensity around the potential well is more comprehensive than in the previous phase mode. The four-phase excitation improves the levitation stability better. The proposed levitation scheme can realize stable single- or multi-position capture of high-density objects in the fluid. Moreover, it has the advantages of low cost and a high degree of freedom.
2023, 72 (1): 014302. doi: 10.7498/aps.72.20221748
Abstract +
The scattering of sound waves by underwater vortex flow filed is the basic problem of sound waves propagating in complex flow fields, which has important significance in implementing underwater target detection and sound imaging of flow field. The theoretical analysis model and numerical calculation method are established for the problem of sound scattering modulation in underwater low frequency oscillating vortex flow fields, and the generation mechanism and time frequency and space characteristics of the scattering modulation sound field are explored. Firstly, based on the wave equation of the moving medium, in the first-order approximation the wave equation is decomposed into the flow-sound coupling term and the non flow-sound coupling term by introducing a potential function, and the flow-sound coupling term is analyzed in the frequency domain, revealing the underwater oscillating vortex flow field. Secondly, the discontinuous Galerkin numerical calculation method is used to solve the wave equation of the moving medium, and the sound propagation process in the underwater low frequency oscillating vortex flow field is numerically simulated. Under the condition of low Mach number, the effects of incident sound frequency, the oscillation frequency of the vortex flow field, and the scale of the vortex core on the time-frequency and space characteristics of the scattering modulating sound fields of vortex flow field are analyzed, and theoretical analysis model is used to explain the characteristics. The research results show that under the condition of low Mach number, the scattering of sound wave by oscillating vortex flow field can produce a scattering modulated sound field containing the harmonic of oscillating frequency side frequency modulation. The amplitude of the scattered sound pressure changes periodically with time, and the forward scattering is much stronger than the backward scattering. The fundamental frequency scattering modulation is much stronger than the frequency doubling scattering modulation. With the increase of the frequency of the incident sound wave and the scale of the vortex core, the intensity of the scattering modulating sound field increases, and the spatial distribution of the total scattering sound field has symmetry and an obvious main lobe, the main lobe is gradually sharpened, the azimuth angle of the main lobe is close to the propagation direction of the incident wave. When the frequency ratio is much greater than 1, the vortex flow field oscillation frequency has little effect on the spatial distribution of the sound field intensity of scattering modulating sound field.
###### PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2023, 72 (1): 015201. doi: 10.7498/aps.72.20221197
Abstract +
Mixing between shell material and gas fuel, caused by hydrodynamic instability, isolated defects, or kinetic effects, is the key to understand the degradation of implosion performance in the research of inertial confinement fusion. Understanding the mixing mechanism and reducing its impact is of extreme importance to achieve the ignition and high gain. The impact of mixing morphology on thermonuclear reaction rate in sub grid level has gradually attracted people’s attention in recent years due to its direct influence on burn rate and fusion process, the study on physical model of thermonuclear reaction rate in different mix morphology has important scientific significance and application value. In the paper, the dependence of thermonuclear reaction rate on mass distribution of different fuel concentrations at sub grid scale is derived. Based on thermodynamic equilibrium and ideal gas equation of state, the physical law of the evolution of the thermonuclear reaction rate with mix morphology under the dominance of diffusion mixing is revealed through analytical formula and numerical solution of diffusion equation in one-dimensional spherical geometry. It is convinced that the mixing amount directly affects the thermonuclear reaction rate by mainly affecting the volume fraction of the fuel, and the mixing diffusion time determined by heterogeneous mixing scale and diffusion coefficient directly affects the evolution behavior of the thermonuclear reaction rate. Furthermore, based on mutual diffusion coefficient obtained from direct simulation of diffusion process by Monte Carlo method, the difference of impact to thermonuclear reaction rate for low-Z Carbon and high-Z gold mixing is quantitatively investigated. Heterogeneous mix size with 0.1 μm, 0.01 μm respectively for the low-Z and high-Z mixing can be treated as atomic mix in burn rate aspect, and heterogeneous mix size with 10 μm, 1 μm respectively for the low-Z and high-Z mixing can be treated as ideal chunk mix in burn rate aspect, and heterogeneous mix size in the middle state needs to be evaluated by using the heterogeneous mixing model of thermonuclear reaction rate in the paper. Finally, the physical model is compared with 3D simulation results of the heterogeneous mixing effect experiment called “MARBLE Campaign” carried out on OMEGA laser facility, which is designed as a separated reactant experiments and capsules are filled with deuterated foam and HT gas pores of different size, covering typical mix morphology from atomic mix to chunk mix, which validate the reliability of the theoretical evaluation about the evolution of mixing morphology and its impact to thermonuclear reaction rate. This work is significant for the design and improvement of inertial confinement fusion mixing effect experiment in China.
###### CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2023, 72 (1): 016801. doi: 10.7498/aps.72.20221630
Abstract +
Fluorination of graphene is one of the most effective methods to improve the corrosion protection of graphene coatings. In this work, the diffusion and penetration behaviors of O atoms on fully fluorinated graphene (CF) and partially fluorinated graphene (C4F) are investigated by using the method of searching for NEB transition state . The effects of F atoms on the corrosion resistance of fluorinated graphene films are also analyzed r. The results show that the adsorption of F atoms can effectively inhibit the diffusion of O atoms on graphene. On C4F, the F atoms are distributed in a para-top position, which greatly increases the surface diffusion energy barrier of O atoms. Moreover, it is difficult for the adsorbed O atoms to diffuse to different sp2 C rings through the obstruction of F atoms. The energy barrier of the horizontal diffusion of O atoms even reaches 2.69 eV in CF. And with the increase of F atoms, the stable structure of graphene is gradually destroyed, the ability of C-atom layer to bar the penetration behaviors of O atoms decreases greatly. Furthermore, the interfacial adhesion work of pure graphene, CF and C4F films with Cu(111) surfaces are calculated, as well as the electronic structures of the composite interface are investigated by using first-principles calculations. The interfacial adhesion work of the Cu/G, Cu/C4F and Cu/CF interfaces are 2.626 J/m2, 3.529 J/m2 and 3.559 J/m2, respectively. The calculations show that the bonding of C4F and C4F with Cu substrate are stronger than pure graphene with Cu substrate, and the interfacial adhesion work increases with the augment of F atom adsorption concentration. The calculation of the density of states also conforms that the interaction between Cu and C atoms of the Cu/C4F interface is stronger than that at the Cu/CF interface. Bader charge analysis shows that the charge transfer at the Cu/C4F interface and the Cu/CF interface increase comparing with that at the Cu/G interface, and Cu/C4F interface has more charge transfer, in which Cu—C bonds are formed.
###### CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2023, 72 (1): 017101. doi: 10.7498/aps.72.20221653
Abstract +
In this paper, the structures of chalcogenide glasses GexGa8S92–x (x = 24%, 26.67%, 29.6%, 32% and 36%) at a fixed Ga atomic content of 8% are studied by high-resolution X-ray photoelectron spectroscopy and Raman scattering spectra. In order to quantify the evolutions of the different structural units in GexGa8S92–x glasses, the number of double peaks in the Ge 3d, Ga 3d and S 2p spectra are determined by iterative fitting method, the binding energy and the full width at half maximum of each peak, and the relative ratio of the integral area of each decomposed peak to that of the whole area of the X-ray photoelectron spectroscopy are thus achieved. On the other hand, the Raman scattering spectra of GexGa8S92–x glass are decomposed into multiple Gaussians based on the structural units. We use the iterative method to simulate the position of peak center, full width at half maximum, and height of each Raman peak. By analyzing the evolution of each unit structure in the glasses, it is found that the network structure of glass network is mainly formed by S atom bridging the tetrahedral structure of GeS4 and GaS4. The S chains or rings structural units are formed in Ge24Ga8S68 glass, indicating that S atoms are in excess in the chemical composition of the glass, so there are enough S atoms around Ge and Ga atoms, forming heteropolar Ge—S and Ga—S bonds. With the gradual increase of Ge content, S chains or rings structure units rapidly disappear in Ge26.67Ga8S65.33 glass. The Ge—Ge homopolar bonds in the ethane-like structure S3Ge—GeS3 and the MM (Ge—Ge, Ga—Ga or Ge—Ga) homopolar bonds in the S3Ge/Ga—Ga/GeS3 structure simultaneous appear in the Ge29.6Ga8S62.4 glass, and the number of structures increases gradually with the increase of Ge content. This is mainly due to the insufficient number of S atoms in the Ge-Ga-S glass. Once S atoms are lacking, the excess Ge and Ga atoms can only combine with themselves to form the homopolar bond MM. It can be concluded below. Firstly, Ge and Ga atoms appear mainly in the form of 4-coordination, while S atoms occur mainly in the form of 2-coordination in the chalcogenide glasses of GexGa8S92–x. Secondly, the existence of MM bond leads the nanophase to separate, and the ordering degree of glass network structure to decrease .
2023, 72 (1): 017201. doi: 10.7498/aps.72.20221183
Abstract +
Yttrium iron garnet (YIG), as a room temperature ferrimagnetic insulator with low damping and narrow ferromagnetic resonance linewidth, has been the research hotspot in spintronics because of its spin transport properties. Bi is one of the most common doping elements used in YIG, and some researches have proved that it can tune the magnetic properties of YIG. Previous studies of BixY3–xFe5O12 thin films focused on the evolutions of their structures, morphologies, and magnetic characteristics. Yet, the effects of Bi3+ substitution of Y3+ on spin transport in YIG thin films have not been systematically studied. The regulation of YIG spin transport by doping is expected to provide a new idea for the spintronics exploration of Pt/YIG system. In this work, we prepare a series of BixY3–xFe5O12 films with different doping ratios by spin coating. And we investigate the effects of Bi3+ on morphology, structure and spin transport properties of YIG films. The results show that Bi doping does not change the crystal structure of YIG. The absorption of the film increases and the bandgap decreases with the increase of doping ratio. The X-ray photoelectron spectroscopy (XPS) indicates the co-existence of Bi3+ and Bi2+. The regulation of Bi doping on spin transport is reflected in the fact that the magnon diffusion length of BixY3–xFe5O12 films is significantly smaller than that of pure YIG films. Meanwhile, we find that the obvious spin Hall magnetoresistance can still be detected in the Pt/BixY3–xFe5O12 heterostructure, and the amplitude is the largest when x = 0.3.

## EDITOR'S SUGGESTION

2023, 72 (1): 017301. doi: 10.7498/aps.72.20221512
Abstract +
Serial triple quantum dot (STQD) systems have received extensive attention in the past decade, not only because quantum dot scaling up is an indispensable ingredient for integrations, but also due to the fact that specific charge states of STQD can be employed to achieve fast full-electrical manipulation of spin qubits. For the latter, a comprehensive understanding of the relationship between neighboring charge occupancy states of STQD is essential for three-electron exchange-only spin qubit-based quantum computations. Charge stability diagram is usually employed to map out the charge occupation states about the plunger gate voltages of STQDs and to study the degeneracy among charge occupation states. Experimentally, two- rather than three-dimensional charge stability diagram was obtained in a lot of early studies by keeping one of plunger gates unchanged to reduce complexity. The obtained two-dimensional diagram can only provide limited information and is subject to blurred boundary of charge occupation states due to the low tunneling current and the energy level broading effects. It is, therefore, challenge to searching for the working points where quantum manipulation can be performed promptly and accurately.In principle, three-dimensional charge occupation stability diagram can be efficiently constructed by numerical simulations based on constant interaction (CI) model. In this study, we calculate the electrochemical potential of STQD about three plunger gate voltages by using the CI model-based capacitance network to reproduce any desired two-dimensional charge stability diagram. The simulated diagram not only well accords with the diagrams obtained from the early experimental data of STQD, but also provides high clarity of the charge state boundaries with tunable parameters. The systematical study of two-dimensional charge stability diagram reviews the energy degeneracy triple and quadruple points of STQD charge occupation states and concludes the energy degeneracy points in three types to compare with experimental data. For each of the energy degeneracy points, we discuss both the electron and hole transport by using the electrochemical potential alignment schematics. We reveal the common and unique triple points of STQD in comparison with those of double quantum dot. The quadruple points of STQD are also addressed in the manipulation of quantum cellular automata and quantum logical gate. The comprehensive understanding of these energy degeneracy points can efficiently guide experiments to build an optimal working point of the STQD system for quantum computations and simulations.
2023, 72 (1): 017302. doi: 10.7498/aps.72.20221253
Abstract +
Noise is a signal. Low-frequency noise with a 1/f-type spectral density (1/f noise) has been observed in a wide variety of systems. There are plenty of physical processes under the 1/f noise phenomenon. It is not only a useful tool for scientific research, but also a quantitative probe for the performance of electronic devices. In this paper, the 1/f noise models are summarized from the general mathematical forms to physical processes. Based on Markov process and diffusion process, two general mathematical models of 1/f noise are introduced respectively. On this basis, tracing the development history, several typical physical models are described, including Mc Whorter model, Hooge model, Voss-Clarker model, Dutta-horn model, interference model and unified Hung model. The advent of the two-dimensional material graphene offers unique opportunities for studying the mechanism of 1/f noise. In the fact of the cloudy and even contradictory conclusions from different reports, this paper combs the consensus accepted widely. An analysis model based on three-level classification for the graphene low-frequency noise study is built, which divides the noise into intrinsic background 1/f noise, 1/f-like noise and Lorentz-like noise. Typical research on the related mechanism at each level is analyzed, and the dominant mechanisms are summarized. Further, we focus on the gate-modulated characteristic spectrum shape of 1/f noise from different reported experiments, which may be a key to the material internal scattering mechanism and charge distribution. The experimental measurements show that the characteristic shape is variable, and mainly exists in three forms: V-type, Λ-type and M-type. Through the comparative analysis of graphene cleanliness, bias current (voltage) and other experimental parameters, the possible causes of the complexity and variability of the characteristic shape are analyzed, showing that the main reason may be that the experimental parameters are not strictly controlled, and the selection of measuring point is unreasonable. In order to capture the accurate noise characteristics and reveal the noise mechanism clearly, a standard 1/f noise measurement paradigm is proposed in this work to guide the effective research on graphene 1/f noise and the distinction betweenintrinsic noise and extrinsic noise. The standard paradigm includes three processes. The first process is to prepare suspended graphene samples, the second one is to remove the surface contamination by using the methods such as current annealing, and the third one is to test the curve of the 1/f noise amplitude versus the bias voltage or current. Based on this curve, suitable test points can be selected for different measurement schemes. The proposed standard intrinsic background 1/f noise measurement paradigm may be expected to clarify and reveal the characteristics of graphene 1/f noise.
2023, 72 (1): 017401. doi: 10.7498/aps.72.20221252
Abstract +
The superconducting solenoid with constant large current is exposed to an alternating magnetic field during the acceleration of the superconducting maglev train, which will cause flux jump of the superconducting solenoid. It can reduce the current-carrying capacity of the solenoid, and generate a lot of heat and make the temperature of the superconducting solenoid rise sharply, which will make the whole superconducting coils quenched. Thus the research of flux jump has very important scientific significance. Nb3Sn superconducting wire is a composite structure composed of multiple superconducting filaments、copper and epoxy resin. In this paper, the magneto-thermal instability behavior of a three-dimensional superconducting wires under alternating magnetic fields and constant current is studied by using a two-dimensional model in which the net current of each filament is constrained to zero. By analyzing the effect of amplitude and frequency of alternating magnetic field on flux jump of a Nb3Sn superconducting wire, we find that when the magnetic field amplitude keeps unchanged, the magnetic field threshold of the initial flux jump changes non-monotonically with the frequency. While the frequency keeps unchanged, the threshold of the initial flux jump changes monotonously with the amplitude of the alternating magnetic field. In addition, with the decreasing applied field, the frequency range for flux jump first increases then decreases to certain critical frequency when the superconducting wire does not have flux jumps. The results of this paper can provide a theoretical basis for regulating the magneto-thermal instability of superconducting wires.
2023, 72 (1): 017801. doi: 10.7498/aps.72.20221341
Abstract +
A series of rare earth Dy3+, Tb3+, Eu3+ singly doped Gd2Te4O11 (GTO) tellurite phosphors with intrinsic polarity are prepared by hydrothermal method. The phase structures, morphologies and thermal stabilities of these phosphors are characterized. Their luminescence properties are tested in detail. The results show that all those phosphors are crystalized into single phase of digadolinium tellurite with short rod-like shape. The maximum size in the axial direction is microns. The phosphor has good thermal stability. For the GTO:Dy3+, the fluorescence emission under UV excitation is mainly located in the yellow-green region. The optimal doping concentration corresponding to the strongest excitation and emission is 2.5%, and the CIE color coordinates are (0.39, 0.43). The fluorescence decay curve shows that the lifetime of the GTO:Dy3+ on 4F9/2 energy level decreases gradually with doping concentration of Dy3+ increasing, which may be related to the cross relaxation (CR) between Dy3+ ions. For the GTO:Eu3+, the fluorescence emission under UV excitation is mainly located in the red region and orange-red region. The emission intensity is enhanced with the doping concentration of Eu3+ increasing. When the doping concentration is 10%, the CIE color coordinates are (0.62, 0.38), which are located in the orange-red region with high color purity. The fluorescence lifetime of Eu3+ on 5D0 energy level is hardly affected by the change of Eu3+ doping concentration. For the GTO:Tb3+, with the increase of the Tb3+ concentration, the fluorescence emission under UV excitation changes from blue-violet region to yellow-green region, which can be ascribed to the influence of CR between Tb3+ ions. The fluorescence decay behavior reveals that the Tb3+ ions on 5D4 excited state may undergo energy transfer and reabsorption, which can deviate the fluorescence decay from the single exponential model. When the concentration of Tb3+ is 0.5%, the sample exhibits white light emission with the CIE color coordinates of (0.33, 0.35) and color rendering index of 86. The measurements of temperature-dependent emission spectra show that the above-mentioned phosphors have good luminescent thermal stability. The internal quantum efficiencies (IQEs) of those three types of phosphors are measured, and the IQE of GTO:Eu3+ is better than those of GTO:Dy3+ and GTO:Tb3+. There is still much room for improvement in the luminescent performance of all these phosphors. These phosphors have potential to be used in UV-excited white LEDs.

## EDITOR'S SUGGESTION

2023, 72 (1): 018501. doi: 10.7498/aps.72.20221584
Abstract +
The exchange bias has a crucial influence on the key performance parameters of magneroresistive sensor, which has wide applications in many fields. This paper presents a method that uses the Joule heating effect combined with a magnetic field to modulate the exchange bias in magnetic multilayers. By this method, we systematically modulate the in-plane exchange bias field (Heb) in the inverted (Co/Pt)n/Co/IrMn structure (n + 1 is the repetition of the Co layers), here the thickness of the Pt layer is smaller than that of the Co layer. In these inverted structures, the Heb can be continuously modulated by changing the amplitude of a pulse current IDC (an in-plane magnetic field Hp) after fixing an Hp (IDC). In more detail, the Heb deceases gradually by increasing the IDC and its polarity of the Heb can be reversed finally, which will not disappear even under a large IDC. Furthermore, if both the amplitude and direction of IDC (Hp) are changed, with a Hp (IDC) fixed, a reversal of Heb can be realized from positive (negative) to negative (positive) direction under a large IDC. From here, one may find that the modulation of the exchange bias in our text is totally different from the normal case one thinks, where the Heb becomes zero under a large enough IDC due to the pure heating effect. Therefore, we believe that the above results show that our method can modulate in situ the linear field range and sensitivity, which has important significance in guiding the optimization of the performance parameters of magneroresistive sensors.

## EDITOR'S SUGGESTION

2023, 72 (1): 018801. doi: 10.7498/aps.72.20221461
Abstract +
In recent years, CH(NH2)2PbI3 (FAPbI3) has received extensive attention due to the suitable band gap, becoming the most attractive photoelectric functional material in perovskite solar cells. However, the traditional perovskite layer prepared by formamidine iodide (FAI) and lead iodide (PbI2) has inaccurate stoichiometric ratio, high defect density, low stability, and low crystallinity, which makes it challenging to improve the performance of perovskite solar cells further. In this paper, the perovskite film prepared by FAPbI3 single crystal has high crystallinity, high stability, accurate stoichiometric ratio and low defect density. The single crystal derived perovskite film has a large grain size and few grain boundaries, resulting in fewer defects in the grain boundaries, which improves the short-circuit current density (JSC) and open-circuit voltage (VOC) of perovskite solar cells, and greatly improves the photoelectric conversion efficiency. This work provides an efficient strategy for fabricating perovskite solar cells with high stability, high crystallinity, and low defect density.
2023, 72 (1): 018802. doi: 10.7498/aps.72.20221120
Abstract +
Perovskite/organic integrated solar cell possesses the perovskite active layer with wide band gap that can absorb high energy photons, while the lower energy photons can pass through the perovskite layer and be absorbed by the organic active layer with narrow band gap. By introducing a Bulk heterojunction (BHJ) consisting of perovskite materials and near-infrared (NIR) organic semiconductor materials in the visible light region, the enhanced short-circuit current density of organic cells can be obtained while maintaining the high open-circuit voltage of perovskite-type devices. We prepare perovskite/organic integrated solar cells by directly deposing the narrow band gap organic active layer PC20BDTDPP:PC71BM on CH3NH3PbI3. The CH3NH3PbI3/PC20BDTDPP:PC71BM integrated solar cell can widen the perovskite absorption spectra, thereby increasing the near-infrared light absorption. The results show that the short-circuit current density of the integrated solar cell increases to 23.90 mA/cm2, the optical response is widened to 920 nm, the external quantum efficiency reaches 85% in the visible region, and is close to 55% in the near infrared region (800–900 nm), and the energy conversion efficiency of the device increases up to 20.30%. The integrated current density, quantum efficiency, and energy conversion efficiency of the best device are the highest values ever reported in perovskite/organic integrated solar cells. At room temperature of 25 ℃ and humidity of 30%, the efficiency of the device decreases to 95% of the original efficiency after 350 h, showing excellent device stability. The results show that it is an effective method to improve the near-infrared absorption of perovskite solar cells and improve the performance of perovskite/organic integrated solar cells through material combination and device structure optimization. The present research provides theoretical guidance and experimental basis for the development of perovskite/ organic integrated cells with high efficiency and stability in the future.