Frontiers of Physics

ISSN 2095-0462

ISSN 2095-0470(Online)

CN 11-5994/O4

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In situ analysis of magnesium alloy using a standoff and double-pulse laser-induced breakdown spectroscopy system
Yong Xin (辛勇),Lan-Xiang Sun (孙兰香),Zhi-Jia Yang (杨志家),Peng Zeng (曾鹏),Zhi-Bo Cong (丛智博),Li-Feng Qi (齐立峰)
Frontiers of Physics    2016, 11 (5): 115207-.   https://doi.org/10.1007/s11467-016-0619-9
摘要   PDF (4190KB)

To monitor the components of molten magnesium alloy during the smelting process in real time and online, we designed a standoff double-pulse laser-induced breakdown spectroscopy (LIBS) analysis system that can perform focusing, collecting and imaging of long-range samples. First, we tested the system on solid standard magnesium alloy samples in the laboratory to establish a basis for the online monitoring of the components of molten magnesium alloy in the future. The experimental results show that the diameters of the focus spots are approximately 1 mm at a range of 3 m, the ablation depth of the double-pulse mode is much deeper than that of the single-pulse mode, the optimum interpulse delay of the double pulse is inconsistent at different ranges, and the spectral intensity decays rapidly as the range increases. In addition, the enhancement effect of the double pulse at 1.89 m is greater than that at 2.97 m, the maximum enhancement is 7.1-fold for the Y(I)550.35-nm line at 1.89 m, and the calibration results at 1.89 m are better than those at 2.97 m. At 1.89 m, the determination coefficients (R2) of the calibration curves are approximately 99% for Y, Pr, and Zr; the relative standard deviations (RSDs) are less than 10% for Y, Pr, and Zr; the root mean square errors (RMSEs) are less than 0.037% for Pr and Zr; the limits of detection (LODs) are less than 1000 ppm for Y, Pr, and Zr; and the LODs of Y, Pr, and Zr at 2.97 m are higher than those at 1.89 m. Additionally, we tested the system on molten magnesium alloy in a magnesium alloy plant. The calibration results of the liquid magnesium alloy are not as favorable as those of the sampling solid magnesium alloys. In particular, the RSDs of the liquid magnesium alloy are approximately 20% for Pr and La. However, with future improvements in the experimental conditions, the developed system is promising for the in situ analysis of molten magnesium alloy.

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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 rise of two-dimensional MoS2 for catalysis
Jun Mao (毛军), Yong Wang (王勇), Zhilong Zheng (郑智龙), Dehui Deng (邓德会)
Frontiers of Physics    2018, 13 (4): 138118-.   https://doi.org/10.1007/s11467-018-0812-0
摘要   PDF (56346KB)

Two-dimensional (2D) MoS2 is used as a catalyst or support and has received increased research interest because of its superior structural and electronic properties compared with those of bulk structures. In this article, we illustrate the active sites of 2D MoS2 and various strategies for enhancing its intrinsic catalytic activity. The recent advances in the use of 2D MoS2-based materials for applications such as thermocatalysis, electrocatalysis, and photocatalysis are discussed. We also discuss the future opportunities and challenges for 2D MoS2-based materials, in both fundamental research and industrial applications.

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A survey of dark matter and related topics in cosmology
Bing-Lin Young
Frontiers of Physics    2017, 12 (2): 121201-.   https://doi.org/10.1007/s11467-016-0583-4
摘要   PDF (80604KB)

This article presents an extensive review of the status of the search of the dark matter. The first eight sections are devoted to topics in dark matter and its experimental searches, and the rest to selected topics in astrophysics and cosmology, which are intended to supply some of the needed background for students in particle physics. Sections 9 and 13 are introductory cosmology. The three astrophysical topics, Big Bang nucleosynthesis Section 10, Boltzmann transport equation and freeze out of massive particles Section 11, and CMB anisotropy Section 12 can all be studied in analytical approaches when reasonable approximations are made. Their original analytically forms, to which this article follows very closely, were given by particle physicists. Dark matter is an evolving subject requiring timely update to stay current. Hence a review of such a subject matter would undoubtedly have something wanting when it appears in print. It is hoped that this review can form a humble basis for those graduate students who would like to pursue the subject of dark matter. The reader can use the extensive table of contents to see in some details the materials covered in the article.

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Laser-induced breakdown spectroscopy in Asia
Zhen-Zhen Wang (王珍珍),Yoshihiro Deguchi (出口祥啓),Zhen-Zhen Zhang (张臻臻),Zhe Wang (王哲),Xiao-Yan Zeng (曾晓雁),Jun-Jie Yan (严俊杰)
Frontiers of Physics    2016, 11 (6): 114213-.   https://doi.org/10.1007/s11467-016-0607-0
摘要   PDF (5652KB)

Laser-induced breakdown spectroscopy (LIBS) is an analytical detection technique based on atomic emission spectroscopy to measure the elemental composition. LIBS has been extensively studied and developed due to the non-contact, fast response, high sensitivity, real-time and multi-elemental detection features. The development and applications of LIBS technique in Asia are summarized and discussed in this review paper. The researchers in Asia work on different aspects of the LIBS study in fundamentals, data processing and modeling, applications and instrumentations. According to the current research status, the challenges, opportunities and further development of LIBS technique in Asia are also evaluated to promote LIBS research and its applications.

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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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Recent advances in MXene: Preparation, properties, and applications
Lei Jin-Cheng(雷进程), Zhang Xu(张旭), Zhou Zhen(周震)
Frontiers of Physics    2015, 10 (3): 107303-null.   https://doi.org/10.1007/s11467-015-0493-x
摘要   PDF (677KB)

Owing to the exceptional properties of graphene, intensive studies have been carried out on novel two-dimensional (2D) materials. In the past several years, an elegant exfoliation approach has been used to successfully create a new family of 2D transition metal carbides, nitrides, and carbonitrides, termed MXene, from layered MAX phases. More recently, some unique properties of MXene have been discovered leading to proposals of potential applications. In this review, we summarize the latest progress in development of MXene from both a theoretical and experimental view, with emphasis on the possible applications.

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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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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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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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Two-dimensional materials: Emerging toolkit for construction of ultrathin high-efficiency microwave shield and absorber
Mingjun Hu, Naibo Zhang, Guangcun Shan, Jiefeng Gao, Jinzhang Liu, Robert K. Y. Li
Frontiers of Physics    2018, 13 (4): 138113-.   https://doi.org/10.1007/s11467-018-0809-8
摘要   PDF (39068KB)

Two-dimensional (2D) materials generally have unusual physical and chemical properties owing to the confined electro-strong interaction in a plane and can exhibit obvious anisotropy and a significant quantum-confinement effect, thus showing great promise in many fields. Some 2D materials, such as graphene and MXenes, have recently exhibited extraordinary electromagnetic-wave shielding and absorbing performance, which is attributed to their special electrical behavior, large specific surface area, and low mass density. Compared with traditional microwave attenuating materials, 2D materials have several obvious inherent advantages. First, similar to other nanomaterials, 2D materials have a very large specific surface area and can provide numerous interfaces for the enhanced interfacial polarization as well as the reflection and scattering of electromagnetic waves. Second, 2D materials have a particular 2D morphology with ultrasmall thickness, which is not only beneficial for the penetration and dissipation of electromagnetic waves through the 2D nanosheets, giving rise to multiple reflections and the dissipation of electromagnetic energy, but is also conducive to the design and fabrication of various well-defined structures, such as layer-by-layer assemblies, core–shell particles, and porous foam, for broadband attenuation of electromagnetic waves. Third, owing to their good processability, 2D materials can be integrated into various multifunctional composites for multimode attenuation of electromagnetic energy. In addition to behaving as microwave reflectors and absorbers, 2D materials can act as impedance regulators and provide structural support for good impedance matching and setup of the optimal structure. Numerous studies indicate that 2D materials are among the most promising microwave attenuation materials. In view of the rapid development and enormous advancement of 2D materials in shielding and absorbing electromagnetic wave, there is a strong need to summarize the recent research results in this field for presenting a comprehensive view and providing helpful suggestions for future development.

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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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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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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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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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Negative magnetoresistance in Weyl semimetals NbAs and NbP: Intrinsic chiral anomaly and extrinsic effects
Yupeng Li,Zhen Wang,Pengshan Li,Xiaojun Yang,Zhixuan Shen,Feng Sheng,Xiaodong Li,Yunhao Lu,Yi Zheng,Zhu-An Xu
Frontiers of Physics    2017, 12 (3): 127205-.   https://doi.org/10.1007/s11467-016-0636-8
摘要   PDF (1187KB)

Chiral anomaly-induced negative magnetoresistance (NMR) has been widely used as critical transport evidence for the existence of Weyl fermions in topological semimetals. In this mini-review, we discuss the general observation of NMR phenomena in non-centrosymmetric NbP and NbAs. We show that NMR can arise from the intrinsic chiral anomaly of Weyl fermions and/or extrinsic effects, such as the superimposition of Hall signals; field-dependent inhomogeneous current flow in the bulk, i.e., current jetting; and weak localization (WL) of coexistent trivial carriers. The WL-controlled NMR is heavily dependent on sample quality and is characterized by a pronounced crossover from positive to negative MR growth at elevated temperatures, resulting from the competition between the phase coherence time and the spin-orbital scattering constant of the bulk trivial pockets. Thus, the correlation between the NMR and the chiral anomaly need to be scrutinized without the support of complimentary techniques. Because of the lifting of spin degeneracy, the spin orientations of Weyl fermions are either parallel or antiparallel to the momentum, which is a unique physical property known as helicity. The conservation of helicity provides strong protection for the transport of Weyl fermions, which can only be effectively scattered by magnetic impurities. Chemical doping with magnetic and non-magnetic impurities is thus more convincing than the NMR method for detecting the existence of Weyl fermions.

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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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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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The delay time of gravitational wave – gamma-ray burst associations
Bing Zhang
Frontiers of Physics    2019, 14 (6): 64402-null.   https://doi.org/10.1007/s11467-019-0913-4
摘要   PDF (697KB)

The first gravitational wave (GW) – gamma-ray burst (GRB) association, GW170817/GRB 170817A, had an offset in time, with the GRB trigger time delayed by ~1.7 s with respect to the merger time of the GW signal. We generally discuss the astrophysical origin of the delay time, Δt, of GW-GRB associations within the context of compact binary coalescence (CBC) – short GRB (sGRB) associations and GW burst – long GRB (lGRB) associations. In general, the delay time should include three terms, the time to launch a clean (relativistic) jet, Δtjet; the time for the jet to break out from the surrounding medium, Δtbo; and the time for the jet to reach the energy dissipation and GRB emission site, ΔtGRB. For CBC-sGRB associations, Δtjet and Δtbo are correlated, and the final delay can be from 10 ms to a few seconds. For GWB-lGRB associations, Δtjet and Δtbo are independent. The latter is at least ~10 s, so that Δt of these associations is at least this long. For certain jet launching mechanisms of lGRBs, Δt can be minutes or even hours long due to the extended engine waiting time to launch a jet. We discuss the cases of GW170817/GRB 170817A and GW150914/GW150914-GBM within this theoretical framework and suggest that the delay times of future GW/GRB associations will shed light into the jet launching mechanisms of GRBs.

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12 superconducting qubits for quantum walks
Heng Fan, Xiaobo Zhu
Frontiers of Physics    2019, 14 (6): 61201-null.   https://doi.org/10.1007/s11467-019-0915-2
摘要   PDF (780KB)
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Quantum transport in topological semimetals under magnetic fields
Hai-Zhou Lu,Shun-Qing Shen
Frontiers of Physics    2017, 12 (3): 127201-.   https://doi.org/10.1007/s11467-016-0609-y
摘要   PDF (1444KB)

Topological semimetals are three-dimensional topological states of matter, in which the conduction and valence bands touch at a finite number of points, i.e., the Weyl nodes. Topological semimetals host paired monopoles and antimonopoles of Berry curvature at the Weyl nodes and topologically protected Fermi arcs at certain surfaces. We review our recent works on quantum transport in topological semimetals, according to the strength of the magnetic field. At weak magnetic fields, there are competitions between the positive magnetoresistivity induced by the weak anti-localization effect and negative magnetoresistivity related to the nontrivial Berry curvature. We propose a fitting formula for the magnetoconductivity of the weak anti-localization. We expect that the weak localization may be induced by inter-valley effects and interaction effect, and occur in double-Weyl semimetals. For the negative magnetoresistance induced by the nontrivial Berry curvature in topological semimetals, we show the dependence of the negative magnetoresistance on the carrier density. At strong magnetic fields, specifically, in the quantum limit, the magnetoconductivity depends on the type and range of the scattering potential of disorder. The high-field positive magnetoconductivity may not be a compelling signature of the chiral anomaly. For long-range Gaussian scattering potential and half filling, the magnetoconductivity can be linear in the quantum limit. A minimal conductivity is found at the Weyl nodes although the density of states vanishes there.

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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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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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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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