Frontiers of Physics

ISSN 2095-0462

ISSN 2095-0470(Online)

CN 11-5994/O4

邮发代号 80-965

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Semiclassical dynamics and nonlinear charge current
Yang Gao
Frontiers of Physics    2019, 14 (3): 33404-null.   https://doi.org/10.1007/s11467-019-0887-2
摘要   PDF (1755KB)

Electron conductivity is an important material property that can provide a wealth of information about the underlying system. Especially, the response of the conductivity with respect to electromagnetic fields corresponds to various nonlinear charge currents, which have distinct symmetry requirements and hence can be used as efficient probes of different systems. To help the band-structure engineering of such nonlinear currents, a universal treatment of electron dynamics up to second order expressed in the basis of the unperturbed states are highly useful. In this work, we review the general semiclassical framework of the nonlinear charge currents.

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The art of designing carbon allotropes
Run-Sen Zhang, Jin-Wu Jiang
Frontiers of Physics    2019, 14 (1): 13401-.   https://doi.org/10.1007/s11467-018-0836-5
摘要   PDF (7542KB)

Stimulated by the success of graphene and diamond, a variety of carbon allotropes have been discovered in recent years in either two-dimensional or three-dimensional configurations. Although these emerging carbon allotropes share some common features, they have certain different and novel mechanical or physical properties. In this review, we present a comparative survey of some of the major properties of fifteen newly discovered carbon allotropes. By comparing their structural topology, we propose a general route for designing most carbon allotropes from two mother structures, namely, graphene and diamond. Furthermore, we discuss several future prospects as well as current challenges in designing new carbon allotropes.

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Graphene based functional devices: A short review
Rong Wang, Xin-Gang Ren, Ze Yan, Li-Jun Jiang, Wei E. I. Sha, Guang-Cun Shan
Frontiers of Physics    2019, 14 (1): 13603-.   https://doi.org/10.1007/s11467-018-0859-y
摘要   PDF (27911KB)

Graphene is an ideal 2D material system bridging electronic and photonic devices. It also breaks the fundamental speed and size limits by electronics and photonics, respectively. Graphene offers multiple functions of signal transmission, emission, modulation, and detection in a broad band, high speed, compact size, and low loss. Here, we have a brief view of graphene based functional devices at microwave, terahertz, and optical frequencies. Their fundamental physics and computational models were discussed as well.

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Review of borophene and its potential applications
Zhi-Qiang Wang, Tie-Yu Lü, Hui-Qiong Wang, Yuan Ping Feng, Jin-Cheng Zheng
Frontiers of Physics    2019, 14 (3): 33403-null.   https://doi.org/10.1007/s11467-019-0884-5
摘要   PDF (16562KB)

Since two-dimensional boron sheet (borophene) synthesized on Ag substrates in 2015, research on borophene has grown fast in the fields of condensed matter physics, chemistry, material science, and nanotechnology. Due to the unique physical and chemical properties, borophene has various potential applications. In this review, we summarize the progress on borophene with a particular emphasis on the recent advances. First, we introduce the phases of borophene by experimental synthesis and theoretical predictions. Then, the physical and chemical properties, such as mechanical, thermal, electronic, optical and superconducting properties are summarized. We also discuss in detail the utilization of the borophene for wide ranges of potential application among the alkali metal ion batteries, Li-S batteries, hydrogen storage, supercapacitor, sensor and catalytic in hydrogen evolution, oxygen reduction, oxygen evolution, and CO2 electroreduction reaction. Finally, the challenges and outlooks in this promising field are featured on the basis of its current development.

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Graphene and other two-dimensional materials
Kostya S. Novoselov, Daria V. Andreeva, Wencai Ren, Guangcun Shan
Frontiers of Physics    2019, 14 (1): 13301-.   https://doi.org/10.1007/s11467-018-0835-6
摘要   PDF (558KB)
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Probing interlayer interactions in WSe2-graphene heterostructures by ultralow-frequency Raman spectroscopy
Yue Liu (刘月), Yu Zhou (周煜), Hao Zhang (张昊), Feirong Ran (冉飞荣), Weihao Zhao (赵炜昊), Lin Wang (王琳), Chengjie Pei (裴成杰), Jindong Zhang (张锦东), Xiao Huang (黄晓), Hai Li (李海)
Frontiers of Physics    2019, 14 (1): 13607-.   https://doi.org/10.1007/s11467-018-0854-3
摘要   PDF (9649KB)

Interlayer interactions at the heterointerfaces of van der Waals heterostructures (vdWHs), which consist of vertically stacked two-dimensional materials, play important roles in determining their properties. The interlayer interactions are tunable from noncoupling to strong coupling by controlling the twist angle between adjacent layers. However, the influence of stacking sequence and individual component thickness on the properties of vdWHs has rarely been explored. In this work, the influence of the stacking sequence of WSe2 and graphene in vdWHs of graphene-on-WSe2 (graphene/WSe2) or WSe2-on-graphene (WSe2/graphene), as well as their thickness, on their interlayer interaction was systematically investigated by ultralow-frequency (ULF) Raman spectroscopy. A series of ULF breathing modes of WSe2 nanosheets in these vdWHs were observed with frequencies highly dependent on graphene thickness. Interestingly, the ULF breathing modes of WSe2 red-shifted in graphene/WSe2 and WSe2/graphene configurations, and the amount of shift in the former was much larger than that in the latter. In contrast, no obvious ULF shift was observed by varying the twist angle between WSe2 and graphene. This indicates that the interlayer interaction is more sensitive to the stacking sequence compared with the twist angle. The results provide alternative approaches to modulate the interlayer interaction of vdWHs and, thus, tune their optical and optoelectronic properties.

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Stacking transition in rhombohedral graphite
Tataiana Latychevskaia, Seok-Kyun Son, Yaping Yang, Dale Chancellor, Michael Brown, Servet Ozdemir, Ivan Madan, Gabriele Berruto, Fabrizio Carbone, Artem Mishchenko, Kostya S. Novoselov
Frontiers of Physics    2019, 14 (1): 13608-.   https://doi.org/10.1007/s11467-018-0867-y
摘要   PDF (11234KB)

Few-layer graphene (FLG) has recently been intensively investigated for its variable electronic properties, which are defined by a local atomic arrangement. While the most natural arrangement of layers in FLG is ABA (Bernal) stacking, a metastable ABC (rhombohedral) stacking, characterized by a relatively high-energy barrier, can also occur. When both types of stacking occur in one FLG device, the arrangement results in an in-plane heterostructure with a domain wall (DW). In this paper, we present two approaches to demonstrate that the ABC stacking in FLG can be controllably and locally turned into the ABA stacking. In the first approach, we introduced Joule heating, and the transition was characterized by 2D peak Raman spectra at a submicron spatial resolution. The transition was initiated in a small region, and then the DW was controllably shifted until the entire device became ABA stacked. In the second approach, the transition was achieved by illuminating the ABC region with a train of 790-nm-wavelength laser pulses, and the transition was visualized by transmission electron microscopy in both diffraction and dark-field imaging modes. Further, using this approach, the DW was visualized at a nanoscale spatial resolution in the dark-field imaging mode.

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Transformation devices with optical nihility media and reduced realizations
Lin Xu, Qian-Nan Wu, Yang-Yang Zhou, Huan-Yang Chen
Frontiers of Physics    2019, 14 (4): 42501-null.   https://doi.org/10.1007/s11467-019-0891-6
摘要   PDF (6989KB)

Starting from optical nihility media (ONM), we design several intriguing devices with transformation optics method in two dimensions, such as a wave splitter, a concave lens, a field rotator, a concentrator, and an invisibility cloak. Though the extreme anisotropic property of ONM hinders the fabrication of these devices. We demonstrate that those devices could be effectively realized by simplified materials with Fabry–Pérot resonances (FPs) at discrete frequencies. Moreover, we propose a reduced version of simplified materials with FPs to construct a concentrator and a rotator, which is feasible in experimental fabrications. The simulations of total scattering cross-sections confirm their functionalities.

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Single-step multipartite entangled states generation from coupled circuit cavities
Xiao-Tao Mo, Zheng-Yuan Xue
Frontiers of Physics    2019, 14 (3): 31602-null.   https://doi.org/10.1007/s11467-019-0888-1
摘要   PDF (939KB)

Green–Horne–Zeilinger states are a typical type of multipartite entangled states, which plays a central role in quantum information processing. For the generation of multipartite entangled states, the singlestep method is more preferable as the needed time will not increase with the increasing of the qubit number. However, this scenario has a strict requirement that all two-qubit interaction strengths should be the same, or the generated state will be of low quality. Here, we propose a scheme for generating multipartite entangled states of superconducting qubits, from a coupled circuit cavities scenario, where we rigorously achieve the requirement via adding an extra z-direction ac classical field for each qubit, leading the individual qubit-cavity coupling strength to be tunable in a wide range, and thus can be tuned to the same value. Meanwhile, in order to obtain our wanted multi-qubits interaction, xdirection ac classical field for each qubit is also introduced. By selecting the appropriate parameters, we numerically shown that high-fidelity multi-qubit GHZ states can be generated. In addition, we also show that the coupled cavities scenario is better than a single cavity case. Therefore, our proposal represents a promising alternative for multipartite entangled states generation.

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Effective models for nearly ideal Dirac semimetals
Feng Tang, Xiangang Wan
Frontiers of Physics    2019, 14 (4): 43603-null.   https://doi.org/10.1007/s11467-019-0902-7
摘要   PDF (3214KB)

Topological materials (TMs) have gained intensive attention due to their novel behaviors compared with topologically trivial materials. Among various TMs, Dirac semimetal (DSM) has been studied extensively. Although several DSMs have been proposed and verified experimentally, the suitable DSM for realistic applications is still lacking. Thus finding ideal DSMs and providing detailed analyses to them are of both fundamental and technological importance. Here, we sort out 8 (nearly) ideal DSMs from thousands of topological semimetals in Nature 566(7745), 486 (2019). We show the concrete positions of the Dirac points in the Brillouin zone for these materials and clarify the symmetryprotection mechanism for these Dirac points as well as their low-energy effective models. Our results provide a useful starting point for future study such as topological phase transition under strain and transport study based on these effective models. These DSMs with high mobilities are expected to be applied in fabrication of functional electronic devices.

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Anomalous spatial shifts in interface electronic scattering
Zhi-Ming Yu, Ying Liu, Shengyuan A. Yang
Frontiers of Physics    2019, 14 (3): 33402-null.   https://doi.org/10.1007/s11467-019-0882-7
摘要   PDF (3491KB)

The anomalous spatial shifts at interface scattering, first studied in geometric optics, recently found their counterparts in the electronic context. It was shown that both longitudinal and transverse shifts, analogous to the Goos–Hänchen and Imbert–Fedorov effects in optics, can exist when electrons are scattered at a junction interface. More interestingly, the shifts are also discovered in the process of Andreev reflection at a normal/superconductor interface. Particularly, for the case with unconventional superconductors, it was discovered that the transverse shift can arise solely from the superconducting pair potential and exhibit characteristic features depending on the pairing. Here, we briefly review the recent works in this field, with an emphasis on the physical picture and theoretical understanding.

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Majorana fermions in semiconducting nanowire and Fulde–Ferrell superconductor hybrid structures
Jia Liu, Chun Fai Chan, Ming Gong
Frontiers of Physics    2019, 14 (1): 13609-.   https://doi.org/10.1007/s11467-018-0863-2
摘要   PDF (12294KB)

The novel idea that spin-orbit coupling (SOC) and an s-wave pairing system can lead to induced pwave pairing with a strong magnetic limit, has stimulated widespread interest in searching for Majorana fermions (MFs) in semiconductor-superconductor hybrid structures. However, despite major advances in the semiconductor nanotechnology, this system has several inherent limitations that prohibit the realization and identification of MFs. We overcome these limitations by replacing the s-wave superconductor with the type-II Fulde–Ferrell (FF) superconductor, in which the center-of-mass momentum of the Cooper pair renormalizes the in-plane Zeeman field and chemical potential. As a result, MFs can be realized in semiconductor nanowires with small values of the Landé g-factor and high carrier densities. The SOC strength directly influences the topological boundary; thus, the topological phase transition and associated MFs can be engineered by an external electric field. Theoretically, almost all semiconductor nanowires can be used to realize MFs by using the FF superconductor. However, we find that InP nanowire is more suitable for the realization of MFs compared to InAs and InSb nanowires. Thus, this new scheme can take full advantage of the semiconductor nanotechnology for the realization of MFs in semiconductor-superconductor hybrid structures.

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New models for multi-dimensional stable vortex solitons
Hidetsugu Sakaguchi
Frontiers of Physics    2019, 14 (1): 12301-.   https://doi.org/10.1007/s11467-018-0857-0
摘要   PDF (9725KB)
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Quantum transport in topological semimetals under magnetic fields (II)
Hai-Peng Sun, Hai-Zhou Lu
Frontiers of Physics    2019, 14 (3): 33405-null.   https://doi.org/10.1007/s11467-019-0890-7
摘要   PDF (4045KB)

We review our recent works on the quantum transport, mainly in topological semimetals and also in topological insulators, organized according to the strength of the magnetic field. At weak magnetic fields, we explain the negative magnetoresistance in topological semimetals and topological insulators by using the semiclassical equations of motion with the nontrivial Berry curvature. We show that the negative magnetoresistance can exist without the chiral anomaly. At strong magnetic fields, we establish theories for the quantum oscillations in topological Weyl, Dirac, and nodal-line semimetals. We propose a new mechanism of 3D quantum Hall effect, via the “wormhole” tunneling through the Weyl orbit formed by the Fermi arcs and Weyl nodes in topological semimetals. In the quantum limit at extremely strong magnetic fields, we find that an unexpected Hall resistance reversal can be understood in terms of the Weyl fermion annihilation. Additionally, in parallel magnetic fields, longitudinal resistance dips in the quantum limit can serve as signatures for topological insulators.

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Topological insulator: Spintronics and quantum computations
Mengyun He, Huimin Sun, Qing Lin He
Frontiers of Physics    2019, 14 (4): 43401-null.   https://doi.org/10.1007/s11467-019-0893-4
摘要   PDF (2060KB)

Topological insulators are emergent states of quantum matter that are gapped in the bulk with timereversal symmetry-preserved gapless edge/surface states, adiabatically distinct from conventional materials. By proximity to various magnets and superconductors, topological insulators show novel physics at the interfaces, which give rise to two new areas named topological spintronics and topological quantum computation. Effects in the former such as the spin torques, spin-charge conversion, topological antiferromagnetic spintronics, and skyrmions realized in topological systems will be addressed. In the latter, a superconducting pairing gap leads to a state that supports Majorana fermions states, which may provide a new path for realizing topological quantum computation. Various signatures of Majorana zero modes/edge mode in topological superconductors will be discussed. The review ends by outlooks and potential applications of topological insulators. Topological superconductors that are fabricated using topological insulators with superconductors have a full pairing gap in the bulk and gapless surface states consisting of Majorana fermions. The theory of topological superconductors is reviewed, in close analogy to the theory of topological insulators.

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Nonequilibrium and morphological characterizations of Kelvin–Helmholtz instability in compressible flows
Yan-Biao Gan, Ai-Guo Xu, Guang-Cai Zhang, Chuan-Dong Lin, Hui-Lin Lai, Zhi-Peng Liu
Frontiers of Physics    2019, 14 (4): 43602-null.   https://doi.org/10.1007/s11467-019-0885-4
摘要   PDF (6362KB)

We investigate the effects of viscosity and heat conduction on the onset and growth of Kelvin–Helmholtz instability (KHI) via an efficient discrete Boltzmann model. Technically, two effective approaches are presented to quantitatively analyze and understand the configurations and kinetic processes. One is to determine the thickness of mixing layers through tracking the distributions and evolutions of the thermodynamic nonequilibrium (TNE) measures; the other is to evaluate the growth rate of KHI from the slopes of morphological functionals. Physically, it is found that the time histories of width of mixing layer, TNE intensity, and boundary length show high correlation and attain their maxima simultaneously. The viscosity effects are twofold, stabilize the KHI, and enhance both the local and global TNE intensities. Contrary to the monotonically inhibiting effects of viscosity, the heat conduction effects firstly refrain then enhance the evolution afterwards. The physical reasons are analyzed and presented.

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Photon-phonon squeezing and entanglement in a cavity optomechanical system with a flying atom
Jun-Hao Liu, Yu-Bao Zhang, Ya-Fei Yu, Zhi-Ming Zhang
Frontiers of Physics    2019, 14 (1): 12601-.   https://doi.org/10.1007/s11467-018-0861-4
摘要   PDF (4383KB)

We study the quadrature squeezing and entanglement in a cavity optomechanical system (COMS). In our model, a flying atom sequentially passes through and interacts with the COMS and a Ramsey pulse zone, and subsequently the atomic state is detected. In this way, the photon-phonon squeezing and entanglement can be generated. The dynamic evolution of the squeezing and entanglement in the presence of losses are examined by using the master equation method.

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Bohm’s approach to quantum mechanics: Alternative theory or practical picture?
A. S. Sanz
Frontiers of Physics    2019, 14 (1): 11301-.   https://doi.org/10.1007/s11467-018-0853-4
摘要   PDF (7170KB)

Since its inception Bohmian mechanics has been generally regarded as a hidden-variable theory aimed at providing an objective description of quantum phenomena. To date, this rather narrow conception of Bohm’s proposal has caused it more rejection than acceptance. Now, after 65 years of Bohmian mechanics, should still be such an interpretational aspect the prevailing appraisal? Why not favoring a more pragmatic view, as a legitimate picture of quantum mechanics, on equal footing in all respects with any other more conventional quantum picture? These questions are used here to introduce a discussion on an alternative way to deal with Bohmian mechanics at present, enhancing its aspect as an efficient and useful picture or formulation to tackle, explore, describe and explain quantum phenomena where phase and correlation (entanglement) are key elements. This discussion is presented through two complementary blocks. The first block is aimed at briefly revisiting the historical context that gave rise to the appearance of Bohmian mechanics, and how this approach or analogous ones have been used in different physical contexts. This discussion is used to emphasize a more pragmatic view to the detriment of the more conventional hidden-variable (ontological) approach that has been a leitmotif within the quantum foundations. The second block focuses on some particular formal aspects of Bohmian mechanics supporting the view presented here, with special emphasis on the physical meaning of the local phase field and the associated velocity field encoded within the wave function. As an illustration, a simple model of Young’s two-slit experiment is considered. The simplicity of this model allows to understand in an easy manner how the information conveyed by the Bohmian formulation relates to other more conventional concepts in quantum mechanics. This sort of pedagogical application is also aimed at showing the potential interest to introduce Bohmian mechanics in undergraduate quantum mechanics courses as a working tool rather than merely an alternative interpretation.

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Time-resolved imaging of mode-conversion process of terahertz transients in subwavelength waveguides
Yao Lu, Qiang Wu, Qi Zhang, Ri-De Wang, Bin Zhang, Wen-Juan Zhao, Deng Zhang, Hao Xiong, Cheng-Liang Yang, Ji-Wei Qi, Chong-Pei Pan, Jing-Jun Xu
Frontiers of Physics    2019, 14 (4): 42502-null.   https://doi.org/10.1007/s11467-019-0892-5
摘要   PDF (9629KB)

We studied the mode-conversion process of terahertz pulses from a planar subwavelength waveguide to a tilted rectangular subwavelength waveguide. An unusual wavefront rotation, which led to an extra conversion time, was observed using a time-resolved imaging technique. We simulated the mode conversion process by a finite-difference time-domain method, and the results agreed well with the experiments. According to the simulations, the conversion time was demonstrated to become longer as the tilt angle or width of the rectangular waveguide increased. This work provides the possibility to optimize the future high-speed communications and terahertz integrated platforms.

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Topological gapless matters in three-dimensional ultracold atomic gases
Yong Xu
Frontiers of Physics    2019, 14 (4): 43402-null.   https://doi.org/10.1007/s11467-019-0896-1
摘要   PDF (10266KB)

Three-dimensional topological gapless matters with gapless degeneracies protected by a topological invariant defined over a closed manifold in momentum space have attracted considerable interest in various fields ranging from condensed matter materials to ultracold atomic gases. As a highly controllable and disorder free system, ultracold atomic gases provide a versatile platform to simulate topological gapless matters. Here, the current progress in studies of topological gapless phenomena in three-dimensional cold atom systems is summarized in the review. It is mainly focused on Weyl points, structured (type-II) Weyl points, Dirac points, nodal rings and Weyl exceptional rings in cold atoms. Since interactions in cold atoms can be controlled via Feshbach resonances, the progress in both superfluids for attractive interactions and non-interacting cold atom gases is reviewed.

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Hydrothermal synthesis, structure and magnetic properties of Ru doped La0.5Sr0.5MnO3
Ling-Ling Wang, Jia-Nan Chu, Xuan Zhang, Yong-Hui Ma, Qiu-Cheng Ji, Wei Li, Hui Zhang, Gang Mu, Xiao-Ming Xie
Frontiers of Physics    2019, 14 (1): 13604-.   https://doi.org/10.1007/s11467-018-0860-5
摘要   PDF (4094KB)

Synthesis, structure and magnetic properties of Ru doped perovskite structured manganite La0.5Sr0.5MnO3 were investigated experimentally. A hydrothermal method was used for the preparation of the samples. A high-temperature annealing process was also employed to make a comparison. A slightly enhancement of the unit cell volume was observed with the increase of Ru concentration. Scanning electron microscopy shows that the materials are made up of cube-shaped particles with dimension of several micrometers. Importantly, it is found that both the Curie temperature TC and saturation moment can be reduced by Ru doping. The value of coercive field is not affected by the introduction of Ru.

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A two-density approach to the general many-body problem and a proof of principle for small atoms and molecules
Thomas Pope, Werner Hofer
Frontiers of Physics    2019, 14 (2): 23604-.   https://doi.org/10.1007/s11467-018-0872-1
摘要   PDF (866KB)

An extended electron model fully recovers many of the experimental results of quantum mechanics while it avoids many of the pitfalls and remains generally free of paradoxes. The formulation of the manybody electronic problem here resembles the Kohn–Sham formulation of standard density functional theory. However, rather than referring electronic properties to a large set of single electron orbitals, the extended electron model uses only mass density and field components, leading to a substantial increase in computational efficiency. To date, the Hohenberg–Kohn theorems have not been proved for a model of this type, nor has a universal energy functional been presented. In this paper, we address these problems and show that the Hohenberg–Kohn theorems do also hold for a density model of this type. We then present a proof-of-concept practical implementation of this method and show that it reproduces the accuracy of more widely used methods on a test-set of small atomic systems, thus paving the way for the development of fast, efficient and accurate codes on this basis.

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Enhancing the thermoelectric performance of Bi2S3: A promising earth-abundant thermoelectric material
Ye Chen, Dongyang Wang, Yuling Zhou, Qiantao Pang, Jianwei Shao, Guangtao Wang, Jinfeng Wang, Li-Dong Zhao
Frontiers of Physics    2019, 14 (1): 13601-.   https://doi.org/10.1007/s11467-018-0845-4
摘要   PDF (12552KB)

Recently, bismuth sulfide (Bi2S3) has attracted much attention in the thermoelectric community owing to its abundance, low cost, and advanced properties. However, its poor electrical transport properties have prevented Bi2S3 devices from realizing high thermoelectric performance. In this work, our motivation is to decrease the large electrical resistivity, which is recognized as the origin of the low ZT value in undoped Bi2S3. We combined melting and spark plasma sintering (SPS) in a continuous fabrication process to produce Bi2S3–xSex (x = 0, 0.09, 0.15, 0.21) and Bi2S2.85–ySe0.15Cly (y = 0.0015, 0.0045, 0.0075, 0.015, 0.03) samples. Our results show that Se alloying at S sites can narrow the band gap and activate intrinsic electron conduction, leading to a high power factor of ~2.0 μW·cm–1·K–2 at room temperature in Bi2S2.85S0.15, about 100 times higher than that of undoped Bi2S3. Moreover, our further introduction of Cl atoms into the S sites resulted in a second-stage optimization of carrier concentration and simultaneously reduced the lattice thermal conductivity, which contributed to a high ZT value of ~0.6 at 723 K for Bi2S2.835Se0.15Cl0.015. Our results indicate that high thermoelectric performance could be realized in Bi2S3 with earth-abundant and low-cost elements.

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Lattice quantum chromodynamics and baryon-baryon interactions
Tetsuo Hatsuda
Frontiers of Physics    2018, 13 (6): 132105-.   https://doi.org/10.1007/s11467-018-0829-4
摘要   PDF (2012KB)

After briefly reviewing the theoretical concepts and numerical methods in lattice QCD, recent simulation results of the hadron masses and hadron interactions with nearly physical quark masses are presented. Special emphasis is placed on the baryon-baryon interactions on the basis of the HAL QCD method where the integro-differential equation for the equal-time Nambu–Bethe–Salpeter amplitude plays a key role to bridge a gap between the multi-baryon correlation and the scattering observable such as the phase shift.

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Tuning up-conversion luminescence in Er3+-doped glass ceramic by phase-shaped femtosecond laser field with optimal feedback control
Lian-Zhong Deng, Yun-Hua Yao, Li Deng, Huai-Yuan Jia, Ye Zheng, Cheng Xu, Jian-Ping Li, Tian-Qing Jia, Jian-Rong Qiu, Zhen-Rong Sun, Shi-An Zhang
Frontiers of Physics    2019, 14 (1): 13602-.   https://doi.org/10.1007/s11467-018-0858-z
摘要   PDF (2284KB)

Tuning the color output of rare-earth ion doped luminescent nanomaterials has important scientific significance for further extending applications in color displays, laser sources, optoelectronic devices, and biolabeling. In previous studies, pre-designed phase modulation of the femtosecond laser field has been proven to be effective in tuning the luminescence of doped rare-earth ions. Owing to the complex light–matter interaction in the actual experiment, the dynamic range and optimal efficiency for color tuning cannot be determined with the pre-designed phase modulation. This article shares the development of an adaptive femtosecond pulse shaping method based on a genetic algorithm, and its use to manipulate the green and red luminescence tuning in an Er3+-doped glass ceramic under 800-nm femtosecond laser field excitation for the first time. Experimental results show that the intensity ratio of the green and red UC luminescence of the doped Er3+ ions can be either increased or decreased conveniently by the phase-shaped femtosecond laser field with an optimal feedback control. The physical control mechanisms for the color tuning are also explained in detail. This article demonstrates the potential applications of the adaptive femtosecond pulse shaping technique in controlling the color output of doped rare-earth ions.

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Impurity-induced bound states as a signature of pairing symmetry in multiband superconducting CeCu2Si2
Dong-Dong Wang, Bin Liu, Min Liu, Yi-Feng Yang, Shi-Ping Feng
Frontiers of Physics    2019, 14 (1): 13501-.   https://doi.org/10.1007/s11467-018-0852-5
摘要   PDF (3169KB)

The notion of multiband superconductivity with dominant two-gap features has been recently applied to the unconventional superconductor CeCu2Si2 for challenging the previously accepted concept of nodal d-wave pairing. In the proposed study, the realistic multiband Fermi surface topology of CeCu2Si2 was obtained through first-principles calculations, and analysis was conducted with an effective two-band hybridization model including detailed band structure. Within the T-matrix approximation, the obtained calculation results show that different pairing candidates, including fully gapped s-wave, loop-nodal s-wave, and d-wave pairings, could yield qualitatively distinct features characterized by impurity-induced bound states. These features can be verified through high-resolution scanning tunneling microscopy or spectroscopy and provide corroborative justification that would be beneficial for the ongoing research regarding the superconducting gap symmetry of CeCu2Si2 at ambient pressure.

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Log-periodic quantum oscillations in topological or Dirac materials
Huichao Wang, Yanzhao Liu, Haiwen Liu, Jian Wang
Frontiers of Physics    2019, 14 (2): 23201-null.   https://doi.org/10.1007/s11467-018-0878-8
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Convergent and divergent beam electron holography and reconstruction of adsorbates on free-standing two-dimensional crystals
T. Latychevskaia, C. R. Woods, Yi Bo Wang, M. Holwill, E. Prestat, S. J. Haigh, K. S. Novoselov
Frontiers of Physics    2019, 14 (1): 13606-.   https://doi.org/10.1007/s11467-018-0851-6
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Van der Waals heterostructures have been lately intensively studied because they offer a large variety of properties that can be controlled by selecting 2D materials and their sequence in the stack. The exact arrangement of the layers as well as the exact arrangement of the atoms within the layers, both are important for the properties of the resulting device. However, it is very difficult to control and characterize the exact position of the atoms and the layers in such heterostructures, in particular, along the vertical (z) dimension. Recently it has been demonstrated that convergent beam electron diffraction (CBED) allows quantitative three-dimensional mapping of atomic positions in three-dimensional materials from a single CBED pattern. In this study we investigate CBED in more detail by simulating and performing various CBED regimes, with convergent and divergent wavefronts, on a somewhat simplified system: a two-dimensional (2D) monolayer crystal. In CBED, each CBED spot is in fact an in-line hologram of the sample, where in-line holography is known to exhibit high intensity contrast in detection of weak phase objects that are not detectable in conventional in-focus imaging mode. Adsorbates exhibit strong intensity contrast in the zero and higher order CBED spots, whereas lattice deformation such as strain or rippling cause noticeable intensity contrast only in the first and higher order CBED spots. The individual CBED spots can thus be reconstructed as typical in-line holograms, and a resolution of 2.13 Å can in principle be achieved in the reconstructions. We provide simulated and experimental examples of CBED of a 2D monolayer crystal. The simulations show that individual CBED spots can be treated as in-line holograms and sample distributions such as adsorbates, can be reconstructed. Individual atoms can be reconstructed from a single CBED pattern provided the later exhibits high-order CBED spots. The experimental results were obtained in a transmission electron microscope (TEM) at 80 keV on free-standing monolayer hBN containing adsorbates. Examples of reconstructions obtained from experimental CBED patterns at a resolution of 2.7 Å are shown. CBED technique can be potentially useful for imaging individual biological macromolecules, because it provides a relatively high resolution and does not require additional scanning procedure or multiple image acquisitions and therefore allows minimizing the radiation damage.

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Phase transition of the frustrated antiferromagntic J1-J2-J3 spin-1/2 Heisenberg model on a simple cubic lattice
Ai-Yuan Hu, Huai-Yu Wang
Frontiers of Physics    2019, 14 (1): 13605-.   https://doi.org/10.1007/s11467-018-0831-x
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We have comprehensively investigated the frustrated J1-J2-J3 Heisenberg model on a simple cubic lattice. This model allows three regimes of magnetic order, viz., (π; π; π), (0; π; π) and (0; 0; π), denoted as AF1, AF2, and AF3, respectively. The effects of the interplay of neighboring couplings on the model are studied in the entire temperature range. The zero temperature magnetic properties of this model are discussed utilizing the linear spin wave (LSW) theory, nonlinear spin wave (NLSW) theory, and Green’s function (GF) method. The zero temperature phase diagrams evaluated by the LSW and NLSW methods are illustrated, and are observed to exhibit different parameter boundaries. In certain regions and along the parameter boundaries, the possible phase transformations driven by the parameters are discussed. The results obtained using the LSW, NLSW, and GF methods are compared with those obtained using the series expansion (SE) method, and are observed to be in good agreement when the value of J2 is not close to the parameter boundaries. The ground state energies obtained using the LSW and NLSW methods are close to that obtained using the SE method. At finite temperatures, only the GF method is employed to evaluate the magnetic properties, and the calculated phase diagram is observed to be identical to the classical phase diagram. The results indicate that at the parameter boundaries, a temperature-driven first-order phase transition between AF1 and AF2 may occur along the boundary line. Along the AF1-AF3 and AF2-AF3 boundary lines, AF3 is less stable than AF1 and AF2. Our calculated critical temperature agrees with that obtained using Monte Carlo simulations and pseudofermion functional renormalization group scheme.

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