Vol. 67, No. 12 (2018)
2018, 67 (12): 126801. doi: 10.7498/aps.67.20180846
In this article, we review the representative work that has been done by Hong-Jun Gao's group in the past two decades in Institute of Physics, Chinese Academy of Sciences. The work focuses on the construction, properties and applications of low-dimensional atomic/molecular crystals, covering the following 3 aspects. 1) Construction and growth mechanisms of low-dimensional quantum structures. Firstly, we demonstrate the fabrication and growth mechanism of a seahorse shaped fractal pattern in C60-TCNQ multilayer thin films by using the ionized-cluster-beam method. Secondly, by modifying the tip of the scanning tunneling microscope (STM), we clearly resolve the six rest atoms and twelve adatoms on a Si(111)-77 unit cell, showing the highest-resolution STM images of the Si(111)-77 surface. According to this work, we investigate the adsorption and bonding of Ge atoms on Si(111)-77 at low coverages. The configurations and growth behaviors of iron phthalocyanine molecules on Au(111) surface from sub-monolayer to bilayer are also reviewed. Furthermore, we demonstrate that organic molecules bond preferentially to different facets of the Ag(775) substrate under different deposition sequences, molecular lengths, terrace widths, and step heights. This can contribute to the design of non-templated selective functionalization of nanocrystals. 2) Reversible conductance transition and spin on-off in low-dimensional quantum structures and applications in ultrahigh-density information storage. Firstly, we implement reversible, erasable, and rewritable nano-recordings on molecular thin films as a result of conductance transition. Then we demonstrate that the Kondo resonance of iron phthalocyanine molecules on an Au(111) substrate depends strongly on adsorption configuration, and the Kondo resonance of manganese phthalocyanine molecules can reversibly switch ON and OFF via attachment and detachment of single hydrogen atom to the molecule. Moreover, we achieve the site-dependent g factor of a single magnetic molecule with sub-molecular resolution, which shows an inhomogeneous distribution of the g factor within a single molecule. These results open up new routes to realizing ultrahigh-density information storage and controlling local spin properties within a single molecule. 3) Construction, physical properties and applications of graphene and other two-dimensional atomic crystals. We start with the fabrication of a wafer-size, high-quality (almost defect free), single-crystalline graphene on Ru(0001). Then we demonstrate the structure of novel two-dimensional (2D) atomic crystals of mono-element, such as silicene,germanene, hafnene, and antimonene. Last but not least, we present the formation of intrinsically patterned bi-elements 2D materials, PtSe2 and CuSe, which can serve as templates for selective self-assembly of molecules and nanoclusters, as well as dual functionalization for catalysis or other applications. The series of work done in Hong-Jun Gao's group has laid a solid foundation in the research field of low-dimensional quantum structures and their applications.
2018, 67 (12): 128102. doi: 10.7498/aps.67.20180796
Topological semimetals have attracted much attention and become a hot subject in condensed matter physics, and single crystal growth is the basis of the physical investigation on these materials. At present, the research of topological materials has formed a cooperation circle:presenting materials by theoretical calculation; single crystal growth; verification by experiments on single crystals. Single crystal growth has become a bridge between theory and experiment. Here in this paper, we introduce the single crystal growth of the topological semimetals presented in recent years, including topological Dirac semimetals, Weyl semimetals, Node-Line semimetals and other new classes of topological materials. The detailed growth methods are summarized in this paper for each material.
2018, 67 (12): 128103. doi: 10.7498/aps.67.20180732
As an emerging two-dimensional (2D) material, monolayer molybdenum disulfide films show excellent electrical and optical properties and have aroused great interest due to their potential applications in electronics and optoelectronics. In this paper, we review our works about molybdenum disulfide films in the past few years. Chemical vapor deposition (CVD) is a convenient and low-cost method to synthesize 2D materials. By oxygen-assisted CVD, the wafer-scale highly-oriented monolayer molybdenum disulfide films and large single-crystal monolayer molybdenum disulfide on various substrates have been prepared epitaxially. Preparation of high-quality monolayer molybdenum disulfide films is the key to measure its intrinsic properties and realize its large-scale applications. Besides the preparation of high-quality materials, the optimizing of transfer technique and fabrication technique are of equal importance for improving the properties of electronic and optoelectronic devices. Water-assisted lossless transfer, patterned peeling, structural change and local phase transition of monolayer molybdenum disulfide films pave the way for preparing and optimizing the functionalized devices. For example, water-assisted transfer and patterned peeling provide methods of preparing molybdenum disulfide samples with clean surfaces and interfaces. Phase transition in the contact area of field-effect transistor reduces the contact resistance effectively, which improves the electrical performance. In addition, the heterojunctions of molybdenum disulfide and other 2D materials show novel electrical and optical properties. As for the functional devices, ultrashort-channel field-effect transistors, integrated flexible thin film transistors, and humidity sensor array have been realized with monolayer molybdenum disulfide films. A grain boundary widening technique is developed to fabricate graphene electrodes for ultrashort-channel monolayer molybdenum disulfide transistors. Field-effect transistors with channel lengths scaling down to 4 nm can be realized reliably and exhibit superior performances, such as the nearly Ohmic contacts and excellent immunity to short channel effects. Furthermore, monolayer molybdenum disulfide films show excellent electrical properties in the measurement of integrated flexible thin film transistors. Under a uniaxial stain of 1%, the performance of the device shows no obvious change, revealing not only the high quality of CVD-grown molybdenum disulfide films, but also the stabilities of these flexible thin film transistor devices. Molybdenum disulfide humidity sensor array for noncontact sensation also shows high sensitivity and stability. Mobility and on/off ratio of the devices in the array decrease linearly with the relative humidity increasing, leading to a high sensitivity of more than 104. The study of monolayer molybdenum disulfide films is universal and instructive for other 2D transition metal dichalcogenides.
Real-time time dependent density functional theory with numerical atomic orbital basis set: methodology and applications
2018, 67 (12): 120201. doi: 10.7498/aps.67.20180487
Real-time time dependent density functional theory (rt-TDDFT) approach directly provides the time domain evolution of electronic wave functions together with ionic movements, presenting a versatile way of real time tracking ultrafast dynamics and phenomena either in perturbative regime or in non-perturbative regime. Thus, rt-TDDFT is a unique ab initio quantum method applicable for the exploration of strong field physics that is beyond the linear response theory. Numerical implementations of the rt-TDDFT based on planewaves and real-space grids have been demonstrated in recent years. However, the above two methods are suitable for the efficient treatment of low energy excitation on the scale of a few electron volts in a small size system. In this paper, we present a state-of-the-art real-time TDDFT approach as implemented in the time dependent ab initio package (TDAP). By employing atomic orbital basis sets, which are small in size and fast in performance, we are able to simulate a large-size system for long electronic propagation time with less computational cost while maintaining relatively high accuracy. The length and velocity-gauge of electromagnetic field are both implemented, showing the flexibility and credibility in applying our methods to various laser induced phenomena in diverse systems including solids, interfaces and two-dimensional materials. Furthermore, recently developed k-resolved algorithm ensures the possibility of handling the problems with a unit cell approach, which significantly reduces the formidable computational costs of traditional rt-TDDFT simulations. Detailed flow and implementation of this method are discussed in this paper, and several quintessential examples for applications are introduced. First, we use the present method to calculate the photoabsorption properties of armchair graphene nanoribbons and monitor the excitation details with momentum resolution. Then, we simulate laser melting of silicon, which captures the most important features of nonthermal melting observed in experiment, and further reveals that it can be attributed to drastic laser-induced change in bonding electron density and subsequent decrease in the melting barrier. After that, a model MoS2/WS2 bilayer system is used as an example to show how our method can be used to monitor the electronic dynamics in such a van der Waals heterostructure. Finally, we show the possibility of controlling the electron dynamic process to enhance high harmonic generation intensity and generate isolated attosecond pulse in monolayer MoS2 via two-color field. Most of the above examples present new ideas in their respective areas and demonstrate that our method has a great potential application in studying interesting ultrafast dynamics phenomena in a wide range of quantum systems.
In past few years, quantum computation and quantum simulation have been developed rapidly. The research on quantum computation and quantum simulation involving medium scale number of qubits will have a development priority. In this paper, we review recent developments in those directions. The review will include quantum simulation of many-body system, quantum computation, digital quantum simulators and cloud quantum computation platforms, and quantum software. The quantum simulation of many-body system will include the simulation of quantum dynamics, time crystal and many-body localization, quantum statistical physics and quantum chemistry. The review of those results is based on our consideration to the current characteristics of quantum computation and quantum simulation. Specifically, the number of available qubits is on a medium scale from dozens to several hundreds, the fidelity of the quantum logic gate is not high enough for several thousand of operations. In this sense, the present research is at the stage from fundamental explorations to practical applications. With these in mind, we hope that this review can be helpful for the future study in quantum computation and quantum simulation.
2018, 67 (12): 120302. doi: 10.7498/aps.67.20180755
Quantum computing and quantum sensing have received much attention in recent years. As an atomic quantum system with super-long coherence time and spin-dependent optical transitions at room temperature, nitrogen-vacancy (NV) center in diamond is one of the well-studied physical systems in quantum information science. In this review, we brief the working principles and quantum control techniques of this single spin system, and also several experimental demonstrations. We focus on the following points:1) coherent manipulation of single spins with optically detected magnetic resonance; 2) main mechanism of NV election spin decoherence and schemes of coherence protection; 3) quantum sensing and quantum computing applications of spin quantum control techniques. Some open questions are discussed at the end of this review.
2018, 67 (12): 120601. doi: 10.7498/aps.67.20180847
Among various electrochemical energy storage technologies, room-temperature Na-ion batteries (NIBs) are regarded as ideal candidates in large-scale energy storage field due to advantages of abundant resources and low material cost in addition to their characteristics of high energy density and long cycle life. Since 2011, the Institute of Physics, Chinese Academy of Sciences (IOP-Chinese Academy of Sciences) has devoted to developing the cost-effective and environmental-safe NIBs, and attained many original achievements in the research of cathode, anode and electrolyte materials, and also developed Na-ion pouch cells with capacities of 1 Ah. For instance, the highly reversible Cu2+/Cu3+ redox was discovered for the first time and the low cost Na-Cu-Fe-Mn-O layered oxide cathodes have been designed accordingly; the anthracite-derived carbon anodes have been exploited via a simple one-step carbonization process with a high performance-to-price ratio; a new type of NaFSI sodium salt was first used in the non-aqueous carbonate electrolyte to significantly improve the performance of electrode materials, etc. This review summarizes the important progress and breakthroughs achieved in IOP-Chinese Academy of Sciences for materials and devices of NIBs. We hope that these contributions conduce to realizing the industrialization of NIBs.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2018, 67 (12): 124201. doi: 10.7498/aps.67.20180724
The inherent material imperfections of solid core optical fiber, for example, Kerr nonlinearity, chromatic dispersion, Rayleigh scattering and photodarkening, set fundamental limitations for further improving the performances of fiber-based systems. Hollow-core fiber (HCF) allows the light to be guided in an air core with many unprecedented characteristics, overcoming almost all the shortcomings arising from bulk material. The exploitation of HCF could revolutionize the research fields ranging from ultra-intense pulse delivery, single-cycle pulse generation, nonlinear optics, low latency optical communication, UV light sources, mid-IR gas lasers to biochemical sensing, quantum optics and mid-IR to Terahertz waveguides. Therefore, the investigations into the guidance mechanism and the ultimate limit of HCF have become a hot research topic. In the past two decades, scientists and engineers have fabricated two types of high-performance HCFs with loss figures of 1.7 dB/km and 7.7 dB/km for hollow-core photonic bandgap fiber (HC-PBGF) and hollow-core anti-resonant fiber (HC-ARF) respectively. In comparison with the twenty-years-old HC-PBGF technology, the HC-ARF that recently appeared outperforms the former in terms of broadband transmission and high laser damage threshold together with a quickly-improved loss figure, providing an ideal platform for many more challenging applications. While the guidance mechanism and fabrication technique in HC-PBGF have been well recognized, the HC-ARF still has a lot of room for improvement. At the birth of the first generation of broadband HC-ARF, the guidance mechanism was unclear, the fiber design was far from perfect, the fabrication was immature, and the optical properties were not optimized. In the past five years, we have developed an intuitive and semi-analytical model for the confinement loss of HC-ARF and managed to fabricate high-performance nodeless HC-ARF. We further employ our theoretical model and fabrication technique to well control and design other interesting properties, such as polarization maintenance and bending loss in HC-ARF. For a long time, the anti-resonant theory of light guidance has been regarded as being qualitative, and the leaky-mode-based HC-ARF have been considered to have worse performances than the guided-mode-based HC-PBGF. Our investigations in theory and experiment negative these prejudices, thus paving the way for the booming development of HC-ARF technologies in the near future.
2018, 67 (12): 124202. doi: 10.7498/aps.67.20180564
Strong terahertz (THz) radiation of MV/cm can be generated from air via two-color laser scheme. In this paper, we introduce three recent theoretical and experimental researches conducted by Wang et al., in which they explored the long-standing problem of THz generation mechanism and extended the scheme with uncommon frequency ratio. In the widely-studied two-color laser scheme, the frequency ratio of the two lasers is usually fixed at 2/1=1:2. In 2013 they predicted according to the plasma current model, for the first time, that the two-color scheme can be extended to a new frequency ratio 1:2n, where n is an positive integer. In 2017 they found that the frequency ratio can be further extended to much broader values. In that year, their experiments showed, for the first time, efficient THz generation with new ratios of 2/1=1:4 and 2:3. They observed that the THz polarization can be adjusted by rotating the longer-wavelength laser polarization, but the polarization adjustment becomes inefficient by rotating the other laser polarization, which is inconsistent with the symmetric nature in the susceptibility tensor required by the multi-wave mixing theory; the THz energy shows similar scaling laws with different frequency ratios, which is inconsistent with the scaling predicted according to the multi-wave mixing theory. These experimental results are in agreement with the plasma current model and particle-in-cell simulations. Therefore, their studies not only push the development of the two-color scheme, but also show that the THz generation mechanism should be mainly attributed to the plasma current model.
2018, 67 (12): 124203. doi: 10.7498/aps.67.20180706
When a short laser pulse passes through transparent medium, the spectrum may be broadened due to nonlinear optical effects, and a coherent octave supercontinuum may be generated under certain conditions. Such a supercontinuum may be compressed into a femtosecond few-cycle pulse, which has many applications in ultrafast optics and beyond. Spectral broadening has been achieved experimentally in gases, liquids, and solids. Current mainstream technique of supercontinuum generation is to send multi-cycle femtosecond pulses through inert-gas-filled hollow-core fibers. However, due to the limitation of the core diameter, the hollow-core fiber cannot work with high-energy laser pulses. With a much higher nonlinear index of refraction, solid-state material is naturally a more promising candidate for supercontinuum generation, but it is difficult to obtain a near-octave spectrum in one piece of solid without filamentation. The optical Kerr effect in solids triggers self-phase modulation (SPM) which induces desired spectral broadening as well as self-focusing, thus causing the laser intensity to rise drastically with substaintial multiphoton excitation and ionization leading to plasma formation. This behavior results in filamentation and optical breakdown, and eventually permanent damage to the material occurs if the laser pulse energy is high enough. Using a thin plate of dielectrics may minimize the effect of self-focusing-the beam exits from the nonlinear medium before it starts to shrink and causes damage. However, one thin plate does not provide enough nonlinear effect to generate a broad spectrum. To prevent disastrous self-focusing while achieving spectral broadening, using multiple Kerr elements has been proposed theoretically and demonstrated experimentally at microjoule to millijoule level. In such a configuration, a femtosecond laser pulse is being spectrally broadened via SPM in the thin plates, while self-focusing converges the beam in each plate but the focal spot is located outside the plate. Once the converging beam passes through its focal spot in air, the beam diverges and enters the next plate to repeat this process until the spectral broadening stops after several elements. Using this method, octave supercontinuum with energies at microjoule to millijoule level has been experimentally obtained in a spectral range covering near-ultraviolet to mid-infrared. In this paper, we review the development of supercontinuum generation in multiple thin solid plates, outline the principle of supercontinuum generation in this new type of thin solid medium, brief the experiments using this new method in recent years, and look into the prospects for its development.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
Nematic fluctuations in iron-based superconductors studied by resistivity change under uniaxial pressure
2018, 67 (12): 127401. doi: 10.7498/aps.67.20180627
Antiferromagnetic, nematic and superconducting phases have been widely found in iron-based superconductors. The study on their relationships is thus crucial for understanding the low-energy physics and high-temperature superconductivity. The so-called nematic phase represents a spontaneous in-plane rotational symmetry breaking of the electronic states, which results in strong in-plane anisotropic properties. We have developed a uniaxial pressure device, which enables us to obtain nematic susceptibility by studying the resistivity change under uniaxial pressure at low temperature. In this paper, we brief two of our recent researches on nematic fluctuations in iron-based superconductors. The first research shows the presence of a nematic quantum critical point in BaFe2-xNixAs2, which exhibits several characteristics, including the zero mean-field nematic transition temperature x=0.11, broad hump feature in the nematic susceptibility in overdoped samples, strongest nematic susceptibility along the (100) direction at x=0.11, and the divergence of zero-temperature nematic susceptibility at x=0.11 for uniaxial pressure along both the (110) and (100) directions. We further study the nematic susceptibility in many other iron-based superconductors and find that the ordered moment at zero temperature linearly scales with nematic Curie constant, which is obtained from the Curie-Weiss-like temperature dependence of nematic susceptibility in these materials. Accordingly, we propose a universal phase diagram for iron-based superconductors, where superconductivity is achieved by suppressing the long-range antiferromagnetic order in a hypothetical parent compound though the enhancement of nematic fluctuations by doping, including both carrier doping and isovalent doping. Our results suggest that nematic fluctuations play a very important role in both the antiferromagnetism and superconductivity in iron-based superconductors.
2018, 67 (12): 127501. doi: 10.7498/aps.67.20180712
The magnetoelectric coupling effect in materials provides an additional degree of freedom of physical states for information storage and shows great potential in developing a new generation of memory devices. We use an alternative concept of nonvolatile memory based on a type of nonlinear magnetoelectric effects showing a butterfly-shaped hysteresis loop. The state of magnetoelectric coefficient, instead of magnetization, electric polarization, or resistance, is utilized to store information. Because this memory concept depends on the relationship between the charge and magnetic flux, it is actually the fourth fundamental circuit memory element in addition to memristor, memcapacitor, and meminductor, and is defined as memtranstor. Our experiments in memtranstor comprised of the[Pb(Mg1/3Nb2/3)]0.7[PbTiO3]0.3(PMN-PT)/Terfenol-D and Ni/PMN-PT/Ni multiferroic heterostructures clearly demonstrated that the magnetoelectric coefficient can be repeatedly switched not only between positive and negative polarities but also between multilevel states by applying electric fields, confirming the feasibility of this principle. In addition to nonvolatile memory, the nonvolatile logic functions, such as NOR and NAND and synaptic plasticity functions, such as long-term potentiation/depression and spiking-time-dependent plasticity are implemented in a single memtranstor by engineering the applied electric-field pulses. The combined functionalities of memory, logic, and synaptic plasticity enable the memtranstor to serve as a promising candidate for future computing systems beyond von Neumann architecture.
Development of novel high-resolution electron energy loss spectroscopy and related studies on surface excitations
2018, 67 (12): 127901. doi: 10.7498/aps.67.20180689
High-resolution electron energy loss spectroscopy (HREELS) is a powerful technique to probe vibrational and electronic excitations at solid surfaces. A monochromatic electron beam incident on the crystal surface may interact with the vibrations of adsorbed molecules, surface phonons or electronic excitations before being back-scattered. By analyzing the energy and momentum of the scattered electrons, we can obtain the information about the chemical bonds, lattice dynamics, occupation of electronic states, and surface plasmons. However the application of traditional HREELS to dispersion analyses is restricted by its point-by-point measurement of the energy loss spectrum for each momentum. Recently, a new strategy for HREELS was realized by utilizing a specially designed lens system with a double-cylindrical monochromator combined with a commercial Scienta hemispherical electron energy analyzer, which can be used to simultaneously measure the energy and momentum of the scattered electrons. The new system possesses improved momentum resolution, high detecting efficiency and high sampling density with no loss in energy resolution. The new HREELS system was employed to study the mechanism of the superconductivity enhancement at FeSe/SrTiO3 interface. By surface phonon measurements on samples with different film thickness, it is revealed that the electric field associated with phonon modes of SrTiO3 substrate can penetrate into FeSe film and interact with the electrons therein, playing the key role in the superconductivity enhancement. The surface collective modes of three-dimensional topological insulator was also studied by using this new HREELS system. A highly unusual acoustic plasmon mode is revealed on the surface of a typical three-dimensional topological insulator Bi2Se3. This mode exhibits an almost linear dispersion to the second Brouillion zone center without reflecting lattice periodicity, and it remains prominent over a large momentum range, with unusually weak damping unseen in any other system. This observation indicates that the topological protection exists not only in single-particle topological states but also in their collective excitations. The application of the new HREELS system with the ability to measure large momentum range with high-efficiency, will definitely promote the development of related researches on condensed matter physics.
2018, 67 (12): 127101. doi: 10.7498/aps.67.20180767
The interplay among spin, orbital and lattice in a strongly-correlated electron system attracts a lot of attention in the community of condensed matter physics. The competition and collaboration of these effects result in multiple ground states, such as superconductivity, quantum criticality state, topological phase transition, metallic-insulating transition, etc. As is well known, the spin-orbital coupling is an interaction between the spin angular moment and orbit angular moment. In quantum mechanics, the spin-orbital coupling can be described as an additional interaction in the Hamitonian. For a compound containing heavy elements, the spin-orbital interaction becomes nontrival and can influence the ground states. For instance, in 4d/5d based superconductors, the superconducting pairing mechanism might be significantly different from that of conventional Bardeen-Cooper-Schrieffer superconductor. In this paper, we will summarize the structures and physical properties of several typical 4d/5d transition metal-based superconductors and discuss the intrinsic relationship between them. Importantly, the strength of anionic covalent bonds can determine the phase transition and superconductivity, which will be highlighted here.
2018, 67 (12): 127402. doi: 10.7498/aps.67.20180343
Many exotic phenomena in strongly correlated electron systems, such as unconventional superconductivity, metal-insulator transition, and quantum criticality, take place in the intermediate regime between localized and itinerant electronic state. To understand the electronic behaviors near the localized-to-itinerant crossover remains a challenging problem in condensed matter physics. The Ru5+ cubic pyrochlores A2Ru2O7 (A=Cd, Cd, Hg) constitute such a system that the Ru-4d electrons acquire characters of both itinerancy and localization. In addition, the magnetic Ru5+ ions that are situated on the vertices of corner-shared tetrahedral lattice are expected to experience strong geometrical frustration given an antiferromagnetic (AF) arrangement. In this work, we investigate the cubic pyrochlore Cd2Ru2O7, which develops a peculiar metallic state below the AF transition. We synthesize a series of Pb-doped Cd2-xPbxRu2O7 (0 x 2) polycrystalline samples under high-pressure condition, and study the effects of Pb doping on their crystal structure and physical properties. Although the Pb2Ru2O7 pyrochlore is a Pauli paramagnetic metal, we find that the substitution of 10% Pb2+ for Cd2+ in Cd1.8Pb0.2Ru2O7 converts the metallic state of Cd2Ru2O7 into an insulating ground state, in a manner similar to the consequence of exerting hydrostatic pressure or substituting 10% Ca2+ for Cd2+ ions as we found recently. We propose that the electronic state of Cd2Ru2O7 be located at the itinerancy to localization crossover, and the introduction of chemical disorder via Pb2+ substitution may enhance the localized character and induce the metal-to-insulator transition. Our results further demonstrate that the cubic Ru5+ pyrochlore oxides offer an important paradigm for studying the exotic physics of correlated electrons on the border of (de)localization in the presence of strong geometrical frustration.
New progress on FeSe-based superconductors: high-quality and high-critical-parameter (Li, Fe)OHFeSe thin film
2018, 67 (12): 127403. doi: 10.7498/aps.67.20180770
High-quality superconducting thin films play an important role in the application and basic research of high-Tc superconductivity. In these aspects, iron-based superconductors feature the merits 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). The recently discovered high-Tc (Li,Fe)OHFeSe superconductor proves to be an important material for the studies of the mechanism and application of unconventional high-Tc superconductivity. However, due to the hydroxyl ion inherent in the compound, none of the conventional high-temperature synthesis methods is applicable for (Li,Fe)OHFeSe materials in bulk and thin film forms. Recently, by developing a hydrothermal ion-exchange technique, we have synthesized for the first time big and high-quality single crystals of (Li,Fe)OHFeSe (2015 Phys. Rev. B 92 064515). Here in this paper, we brief our most recent progress on growing a high-quality single-crystalline superconducting film of (Li,Fe)OHFeSe (2017 Chin. Phys. Lett. 34 077404). The film has been prepared on a LaAlO3 substrate by a hydrothermal epitaxial method. The high crystalline quality of the film is verified by X-ray diffraction (XRD). The XRD measurements show 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 revealed by the -scan of (101) plane. Its bulk superconducting transition temperature Tc of 42.4 K is determined by both zero electrical resistance and diamagnetism measurements. Based on systematic magnetoresistance measurements, the upper critical field Hc2 is estimated to be 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 therefore is not only important for the fundamental research for understanding the high-Tc mechanism, but also promising for the applications in high-performance electronic devices and large scientific facilities such as superconducting accelerator.
Quantum coherence measurement with femtosecond time-resolve two-dimensional electronic spectroscopy: principles, applications and outlook
2018, 67 (12): 127801. doi: 10.7498/aps.67.20180783
Two-dimensional electronic spectroscopy is a kind of nonlinear optical spectroscopy with both high time resolution and high frequency resolution. It can be used to observe the complex dynamics of a condensed molecular system. Meanwhile it is a very powerful tool to study the coherence between the electronic states or electronic and vibration states. In 2007, Flemming's group reported the long-lived quantum coherence observed in the energy transfer process in the light-harvesting antenna protein complex Fenna-Matthews-Olson at 77 K by means of two-dimensional electronic spectroscopy. Though it has been proved not to arise from the pure electronic coherence later, this discovery has greatly stimulated the exploration of the coherent energy transfer pathways possibly existing in the natural and artificial photosynthetic systems, and this is still a very active area nowadays. Here in this paper we briefly review the principle and set-up of the two-dimensional electronic spectroscopy, and also some of its applications in investigating coherent energy transfer in the photosynthetic and artificial systems, aiming to bring this novel spectroscopic tool into a wider application.
2018, 67 (12): 127201. doi: 10.7498/aps.67.20180906
Spin logic has advantages of nonvolatility, CMOS compatibility and fast speed, thus it has become a promising alternative solution to realizing non von Neumann computing architectures. Here in this paper we show two hopeful spin logic solutions based on spin Hall effect and spin orbit torques. First basic Boolean logic and storage functions are realized in a spin Hall logic device. Furthermore, utilizing symmetric requirements for magnetic fields and applied current, programmability of the spin logic device among 5 different Boolean logic functions, AND, OR, NOT, NOR and NAND, is even realized. The demonstration of programmable spin Hall logic can advance the birth and development of practical spin logic devices and circuits.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
Anomalous light-to-electricity conversion of low dimensional semiconductor in p-n junction and interband transition quantum well infrared detector
2018, 67 (12): 128101. doi: 10.7498/aps.67.20180588
Recently, high localized carrier extraction efficiency and enhanced absorption coefficient were observed in low-dimensional semiconductor within a p-n junction. In this work, we report the discovery and verification of the phenomenon, and the performance of the first photon detector based on the interband transition of strained InGaAs/GaAs quantum wells (QWs). By introducing the resonant excitation photoluminescence, the same phenomena are observed in several different material systems. More than 95% of the photoexcited carriers escape from InGaN/GaN QWs, and 87.3% in InGaAs/GaAs QWs and 88% in InAs/GaAs quantum dots are observed. The external quantum efficiency of the device is measured to be 31% by using an absorption layer with only 100 nm effective thickness in the case without an anti-reflection layer. Using such a high value of quantum efficiency, an absorption coefficient of 3.7104 cm-1 is calculated, which is obviously larger than previously reported values. The results here demonstrate the possibility of fabricating high-performance and low-cost infrared photon detectors.
2018, 67 (12): 128501. doi: 10.7498/aps.67.20180757
The failure problems, associated with capacity fade, poor cycle life, increased internal resistance, abnormal voltage, lithium plating, gas generation, electrolyte leakage, short circuit, battery deformation, thermal runaway, etc., are the fatal issues that restrict the performances and reliabilities of the lithium batteries. The main tasks of failure analysis of lithium batteries are to accurately diagnose, which is vital for revealing the failure modes or failure mechanisms. These information has profound significance for improving the performances and technology of lithium batteries. In order to have a comprehensive understanding of the recent progress on failure analysis research of lithium batteries, the failure analyses from the respect of definition, phenomenon, reason, analysis content, process, difficulty, etc. are briefly reviewed. We hope this review will helpful to the researchers engaged in the field of failure analysis as well as battery field.
2018, 67 (12): 128801. doi: 10.7498/aps.67.20180657
After the continuous research on the discovering new materials based on theoretical methods and material genome initiative, the high-throughput simulation platform is established. With this new research mode and platform, the screening, optimization and design of lithium battery materials are realized by using lithium migration properties as criteria. The attempt at introducing machine learning method into material design is also made. With the high-throughput bond-valence calculations, two coating materials for Li-rich cathode are found, the modified -Li3PS4 and a new layered oxysulfide as novel lithium superionic conductors are designed, and the relationship between the volume change of electrode during delithiation and the atomic structure is investigated. The application of the material genome method to the development of lithium battery materials provides the possibility to promote this new research and development model in other types of materials.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2018, 67 (12): 126101. doi: 10.7498/aps.67.20180681
Owing to combining the properties of both metal and glass, metallic glasses exhibit superior physical and mechanical properties along with exotic phenomena, so they have a wide application prospect in many areas. In addition, their continuously adjustable composition and simple disordered atomic structure provide ideal model material systems for the study of fundamental questions commonly existing in glassy materials. The discovery of metallic glasses that can form bulk materials has pushed the relevant research to the frontier of condensed matter physics and material science. The EX4 group of the Institute of Physics, Chinese Academy of Sciences, has devoted to the study of glassy materials and physics for many years, and made important contributions to this field. In this paper, we summarize our recent progress of metallic glasses, including the relaxation behavior and stability, surface dynamics, materials functionalities, and new method on materials discovery.
In situ transmission electron microscopy studies on nanomaterials and HfO2-based storage nanodevices
2018, 67 (12): 126802. doi: 10.7498/aps.67.20180731
Advanced transmission electron microscopy combined with in situ techniques provides powerful ability to characterize the dynamic behaviors of phase transitions, composition changes and potential variations in the nanomaterials and devices under external electric field. In this paper, we review some important progress, in this field, of the explanation of structural transition path caused by the Joule heating in C60 nanowhikers, the clarification of electron storage position in charge trapping memory and the direct evidences of the oxygen vacancy channel and the conductive filament formation in resistive random access memory. These studies could improve an understanding of the basic mechanism of nanomaterial and device performance, and also demonstrate the diversity of the functions of transmission electron microscopy in microelectronic field.
2018, 67 (12): 126803. doi: 10.7498/aps.67.20180844
The morphological evolutions of gold nanofilm on the suspended graphene is investigated before and after an annealing process, and two important phenomena are observed. First, the layer number of suspended graphene can be determined by the morphological change of gold nanofilm, and it is noteworthy that as-observed results without the substrate supporting effect are completely contrary to previously reported results of the graphene supported by the substrate. Second, after a rapid and careful annealing process, the gold nanofilm on the suspended graphene shows a liquid-like behavior as if the water is on the lotus leave surface. The mechanisms behind these phenomena are discussed in detail. These results provide very useful information for many applications such as metal intercalation in graphene, electronic contact between metal and graphene, fabrication of patterned suspended graphene device, etc.