Frontiers of Physics

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ISSN 2095-0462 (Print)
ISSN 2095-0470 (Online)
CN 11-5994/O4
Postal Subscription Code 80-965
2019 Impact Factor: 2.502
Recent Advances in Topological Materials (Eds. Yugui Yao, Xiangang Wan, Shengyuan A. Yang, Hua Chen)
 
Starting from the discovery of topological insulators in 2004, the field of topological materials has emerged as the frontier of the current research. It has greatly deepened our fundamental understanding of the band theory of materials. A variety of topological phases of matter have been predicted, and a number of such materials along with exotic physical properties have been discovered in experiment. The field has attracted wide interest because it also has an intrinsic interdisciplinary nature, not only limited to condensed matter physics, but also connected to high-energy physics, astrophysics, chemistry, materials science, and intimately related to application fields such as device and electronic engineering. It holds huge promise as building blocks for the next-generation electronics industry, based on the concept of “topological electronics”.
 
In view of these developments, we, together with the editorial office of the journal “Frontiers of Physics”, have decided that it is timely to edit a special issue dedicated to the topic of “Recent Advances in Topological Materials”. The scope of this focus issue in Frontiers of Physics would cover all of the aspects from theoretical, computational, to experimental progress in the field. This special issue will present the major recent progress in this field from the best teams all over the world. We do hope that the issue will form a broad overview of the current state of this cutting-edge field.
 
Specific topics of interest covered in this issue include
 ●  Weyl and Dirac semimetals
 ●  Novel topological semimetals beyond Weyl and Dirac
 ●  Spintronics with topological materials
 ●  Progress on 2D topological insulators
 ●  Dirac superconductor
 ●  Majorana fermions and topological superconductivity
 ●  Transport in topological materials
 ●  Magnetism and topological materials
 ●  High order topological insulators
 ●  Anomalous Hall and magneto-optic effect in topological materials
 ●  Topological states in cold atom systems
 
We are looking for high profile scientists from China and overseas to contribute Review, Mini-Review, Perspective, or Research Article in the foresaid areas. Please feel free to choose a striking topic that best fits the issue. Co-authorship is welcome. There is no strict length limit for each article, and for each review at least 15 pages length is highly expected.
 
The tentative published date of the special issue is the end of 2018, so we expect that the review articles will be submitted by October 30, 2018. The sample article (TEX template) can be downloaded via http://journal.hep.com.cn/fop/EN/column/column15258.shtml and the new manuscript can be submitted online through http://cn.manuscriptcentral.com/fop. All PDFs of the special issue will be openly accessed at http://journal.hep.com.cn/fop, and a copy of the volume will be mailed to all participants.
 
Sincerely,
 
Guest Editors
Yugui Yao, Beijing Institute of Technology, ygyao@bit.edu.cn
Xiangang Wan, Nanjing University, xgwan@nju.edu.cn
Shengyuan A. Yang, Singapore University of Technology and Design, shengyuan_yang@sutd.edu.sg
Hua Chen, Colorado State University, huachen@colostate.edu

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Quantum transport in topological semimetals under magnetic fields (II)
Hai-Peng Sun, Hai-Zhou Lu
Front. Phys. .  2019, 14 (3): 33405.   https://doi.org/10.1007/s11467-019-0890-7
Abstract   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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Anomalous spatial shifts in interface electronic scattering
Zhi-Ming Yu, Ying Liu, Shengyuan A. Yang
Front. Phys. .  2019, 14 (3): 33402.   https://doi.org/10.1007/s11467-019-0882-7
Abstract   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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Semiclassical dynamics and nonlinear charge current
Yang Gao
Front. Phys. .  2019, 14 (3): 33404.   https://doi.org/10.1007/s11467-019-0887-2
Abstract   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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New topological semimetal candidate of nonsymmorphic PdSb2 with unique six-fold degenerate point
Guangcun Shan, Hai-Bin Gao
Front. Phys. .  2019, 14 (4): 43201.   https://doi.org/10.1007/s11467-019-0909-0
Abstract   PDF (1153KB)
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Topological gapless matters in three-dimensional ultracold atomic gases
Yong Xu
Front. Phys. .  2019, 14 (4): 43402.   https://doi.org/10.1007/s11467-019-0896-1
Abstract   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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Dephasing effects in topological insulators
Junjie Qi, Haiwen Liu, Hua Jiang, X. C. Xie
Front. Phys. .  2019, 14 (4): 43403.   https://doi.org/10.1007/s11467-019-0907-2
Abstract   PDF (2297KB)

Topological insulators, a class of typical topological materials in both two dimensions and three dimensions,are insulating in bulk and metallic at surface. The spin-momentum locked surface states and peculiar transport properties exhibit promising potential applications on quantum devices, which generate extensive interest in the last decade. Dephasing is the process of the loss of phase coherence, which inevitably exists in a realistic sample. In this review, we focus on recent progress in dephasing effects on the topological insulators. In general, there are two types of dephasing processes: normal dephasing and spin dephasing. In two-dimensional topological insulators, the phenomenologically numerical investigation shows that the longitudinal resistance plateaus is robust against normal dephasing but fragile with spin dephasing. Several microscopic mechanisms of spin dephasing are then discussed. In three-dimensional topological insulators, the helical surface states exhibit a helical spin texture due to the spin-momentum locking mechanism. Thus, normal dephasing has close connection to spin dephasing in this case, and gives rise to anomalous “gap-like” feature. Dephasing effects on properties of helical surface states are investigated.

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Equivariant PT-symmetric real Chern insulators
Y. X. Zhao
Front. Phys. .  2020, 15 (1): 13603.   https://doi.org/10.1007/s11467-019-0943-y
Abstract   PDF (737KB)

It was understood that Chern insulators cannot be realized in the presence of PT symmetry. In this paper, we reveal a new class of PT-symmetric Chern insulators, which has internal degrees of freedom forming real representations of a symmetry group with a complex endomorphism field. As a generalization to the conventional 2n-dimensional Chern insulators with integer n≥1, these PT-symmetric Chern insulators have the n-th complex Chern number as their topological invariant, and have a Zclassification given by the equivariant orthogonal K theory. Thus, in a fairly different sense, there exist ubiquitously Chern insulators with PT symmetry. By generalizing the Thouless charge pump argument, we find that, for a PT-symmetric Chern insulator with Chern number υ, there are equally many υ flavors of coexisting left- and right-handed chiral modes. Chiral modes with opposite chirality are complex conjugates to each other as complex representations of the internal symmetry group, but are not isomorphic. For the physical dimensionality d = 2, the PT-symmetric Chern insulators may be realized in artificial systems including photonic crystals and periodic mechanical systems.

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Type-II topological metals
Si Li, Zhi-Ming Yu, Yugui Yao, Shengyuan A. Yang
Front. Phys. .  2020, 15 (4): 43201.   https://doi.org/10.1007/s11467-020-0963-7
Abstract   PDF (2737KB)

Topological metals (TMs) are a kind of special metallic materials, which feature nontrivial band crossings near the Fermi energy, giving rise to peculiar quasiparticle excitations. TMs can be classified based on the characteristics of these band crossings. For example, according to the dimensionality of the crossing, TMs can be classified into nodal-point, nodal-line, and nodal-surface metals. Another important property is the type of dispersion. According to degree of the tilt of the local dispersion around the crossing, we have type-I and type-II dispersions. This leads to significant distinctions in the physical properties of the materials, owing to their contrasting Fermi surface topologies. In this article, we briefly review the recent advances in this research direction, focusing on the concepts, the physical properties, and the material realizations of the type-II nodal-point and nodal-line TMs.

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Advances on topological materials
Qian Niu
Front. Phys. .  2020, 15 (4): 43601.   https://doi.org/10.1007/s11467-020-0979-z
Abstract   PDF (537KB)
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Effective models for nearly ideal Dirac semimetals
Feng Tang, Xiangang Wan
Front. Phys. .  2019, 14 (4): 43603.   https://doi.org/10.1007/s11467-019-0902-7
Abstract   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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