Fermi-crossing Type-II Dirac fermions and topological surface states in NiTe2

n nanostructured layered films of topological thermoelectrics n-Bi2−xSbxTe3−y Sey , thermoelectric properties in the temperature range of 4.2−300K and magnetoresistance oscillations in strong magnetic fields at low temperatures were studied. It is shown that thermoelectric efficiency in the layered n-Bi2−xSbxTe3−y Sey films increases as compared with bulk material due to both an increase in the Seebeck coefficient at the temperatures below room one, and a decrease in thermal conductivity and weakening of its temperature dependence. From the analysis of magnetoresistance oscillations, the parameters of topological surface states of Dirac fermions were determined and their influence on thermoelectric properties was estimated.

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Photoemission Spectroscopic Evidence of Multiple Dirac Cones in Superconducting BaSn3

Chinese Physics Letters ◽

10.1088/0256-307x/38/10/107403 ◽

2021 ◽

Vol 38 (10) ◽

pp. 107403

Author(s):

Zhe Huang ◽

Xianbiao Shi ◽

Gaoning Zhang ◽

Zhengtai Liu ◽

Soohyun Cho ◽

...

Keyword(s):

Surface States ◽

Photoemission Spectroscopy ◽

Rotational Axis ◽

Type I ◽

Dirac Fermions ◽

First Principles Calculations ◽

Angle Resolved Photoemission Spectroscopy ◽

Topological Quantum ◽

Topological Surface ◽

Single Material

Signatures of topological superconductivity (TSC) in superconducting materials with topological nontrivial states prompt intensive researches recently. Utilizing high-resolution angle-resolved photoemission spectroscopy and first-principles calculations, we demonstrate multiple Dirac fermions and surface states in superconductor BaSn3 with a critical transition temperature of about 4.4 K. We predict and then unveil the existence of two pairs of type-I topological Dirac fermions residing on the rotational axis. Type-II Dirac fermions protected by screw axis are confirmed in the same compound. Further calculation for the spin helical texture of the observed surface states originating from the Dirac fermions gives an opportunity for realization of TSC in one single material. Hosting multiple Dirac fermions and topological surface states, the intrinsic superconductor BaSn3 is expected to be a new platform for further investigation of topological quantum materials as well as TSC.

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Ideal type-II Weyl points in topological circuits

National Science Review ◽

10.1093/nsr/nwaa192 ◽

2020 ◽

Author(s):

Rujiang Li ◽

Bo Lv ◽

Huibin Tao ◽

Jinhui Shi ◽

Yidong Chong ◽

...

Keyword(s):

Spectral Characteristics ◽

Three Dimensional ◽

Surface States ◽

Ideal Type ◽

Type I ◽

Type Ii ◽

Ideal System ◽

Lc Resonator ◽

Topological Surface ◽

Group Velocities

Abstract Weyl points (WPs), nodal degenerate points in three-dimensional (3D) momentum space, are said to be ‘ideal’ if they are symmetry-related and well-separated, and reside at the same energy and far from nontopological bands. Although type-II WPs have unique spectral characteristics compared with type-I counterparts, ideal type-II WPs have not yet been reported due to the lack of an experimental platform with enough flexibility to produce strongly tilted dispersion bands. Here we experimentally realize a topological circuit that hosts only topological bands with a minimal number of four ideal type-II WPs. By stacking two-dimensional (2D) layers of inductor-capacitor (LC) resonator dimers with the broken parity inversion symmetry (P), we achieve a strongly tilted band structure with two group velocities in the same direction, and topological surface states in an incomplete bandgap. Our results establish an ideal system for the further study of Weyl physics and other exotic topological phenomena.

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Selective trapping of hexagonally warped topological surface states in a triangular quantum corral

Science Advances ◽

10.1126/sciadv.aaw3988 ◽

2019 ◽

Vol 5 (5) ◽

pp. eaaw3988 ◽

Cited By ~ 1

Author(s):

Mu Chen ◽

Ye-Ping Jiang ◽

Junping Peng ◽

Huimin Zhang ◽

Cui-Zu Chang ◽

...

Keyword(s):

Surface Defects ◽

Three Dimensional ◽

Surface States ◽

Real Space ◽

Dirac Fermions ◽

Selection Rules ◽

Potential Barriers ◽

Topological Surface ◽

Spin Texture ◽

Complicated Surface

The surface of a three-dimensional topological insulator (TI) hosts two-dimensional massless Dirac fermions (DFs), the gapless and spin-helical nature of which leads to their high transmission through surface defects or potential barriers. Here, we report the behaviors of topological surface states (TSS) in a triangular quantum corral (TQC) which, unlike a circular corral, is supposed to be totally transparent for DFs. By real-space mapping of the electronic structure of TQCs, both the trapping and detrapping behaviors of the TSS are observed. The selection rules are found to be governed by the geometry and spin texture of the constant energy contour of TSS upon the strong hexagonal warping in Bi2Te3. Our work indicates the extended nature of TSS and elucidates the selection rules of the trapping of TSS in the presence of a complicated surface state structure, giving insights into the effective engineering of DFs in TIs.

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Dimensional crossover and band topology evolution in ultrathin semimetallic NiTe2 films

npj 2D Materials and Applications ◽

10.1038/s41699-021-00218-z ◽

2021 ◽

Vol 5 (1) ◽

Author(s):

Joseph A. Hlevyack ◽

Liang-Ying Feng ◽

Meng-Kai Lin ◽

Rovi Angelo B. Villaos ◽

Ro-Ya Liu ◽

...

Keyword(s):

Film Thickness ◽

Ultrathin Films ◽

Molecular Beam ◽

Surface States ◽

Single Layer ◽

Dirac Fermions ◽

Dirac Semimetal ◽

Emergent Phenomena ◽

Device Applications ◽

Topological Surface

AbstractNickel ditelluride (NiTe2), a recently discovered Type-II Dirac semimetal with topological Dirac fermions near the Fermi energy, is expected to exhibit strong thickness-mediated electronic tunability and intrinsic two-gap superconductivity in the single-layer limit. Realizing such intriguing phenomena requires the fabrication of ultrathin NiTe2 films and an understanding of the underlying physics that is still under debate. By conducting experimental band mappings of ultrathin films prepared with molecular beam epitaxy, we reveal spectroscopic evidence for the dimensionality crossover of single-crystalline ultrathin NiTe2 films as a function of film thickness. As the film thickness increases from one to five layers, the gap in the conical topological surface states closes. Comparisons of experimental to first-principles results also highlight difficulties in fabricating atomically smooth single-layer NiTe2 films. Our results not only provide further impetus for studying emergent phenomena in NiTe2 but also underscore the limitations of fabricating NiTe2 films for device applications.

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Топологические поверхностные состояния фермионов Дирака в термоэлектриках n-Bi-=SUB=-2-=/SUB=-Te-=SUB=-3-y-=/SUB=-Se-=SUB=-y-=/SUB=-

Физика и техника полупроводников ◽

10.21883/ftp.2019.05.47557.15 ◽

2019 ◽

Vol 53 (5) ◽

pp. 654

Author(s):

Л.Н. Лукьянова ◽

И.В. Макаренко ◽

О.А. Усов ◽

П.А. Дементьев

Keyword(s):

Dirac Point ◽

Energy Gap ◽

Surface States ◽

Surface Concentration ◽

Scanning Tunneling ◽

Fermi Velocity ◽

Dirac Fermions ◽

Tunneling Microscopy ◽

Topological Surface ◽

Local Distortions

Topological surface states of Dirac fermions in n-Bi2Te3−ySey thermoelectrics L.N. Lukyanova, I.V. Makarenko, O.A. Usov, P.A. Dementev Ioffe Institute, 194021 St. Petersburg, Russia Abstract In n-Bi2Te3 and n-Bi2Te3−ySey thermoelectrics, the surface states of Dirac fermions of the interlayer van der Waals plane (0001) were studied by scanning tunneling microscopy (STM) and spectroscopy. The surface morphology and modulated line profiles of the images in tunneling microscopy is determined by local distortions of the surface electron states density and depend on the composition. The Dirac point ED of the studied compositions is localized in the energy gap, and it shifts to the top of the valence band with increase of Se content in the solid solutions. The dependence between Dirac surface state parameters (the Dirac point position, the Fermi velocity, the fermion surface concentration) and thermoelectric properties (Seebeck coefficient and the power factor) is established for the thermoelectrics studied.

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Coexistence of type-II and type-IV Dirac fermions in SrAgBi

Modern Physics Letters B ◽

10.1142/s0217984921501815 ◽

2021 ◽

pp. 2150181

Author(s):

Tian-Chi Ma ◽

Jing-Nan Hu ◽

Yuan Chen ◽

Lei Shao ◽

Xian-Ru Hu ◽

...

Keyword(s):

High Energy Physics ◽

Surface States ◽

High Energy ◽

Dirac Fermion ◽

Dirac Fermions ◽

Type Ii ◽

Linear Dispersion ◽

Space Groups ◽

Type Iv ◽

Poincaré Symmetry

Relativistic massless Weyl and Dirac fermions have isotropic and linear dispersion relations to maintain Poincaré symmetry, which is the most basic symmetry in high-energy physics. The situation in condensed matter physics is less constrained; only certain subgroups of Poincaré symmetry — the 230 space groups that exist in 3D lattices — need be respected. Then, the free fermionic excitations that have no high-energy analogues could exist in solid state systems. Here, We discovered a type of nonlinear Dirac fermion without high-energy analogue in SrAgBi and named it type-IV Dirac fermion. The type-IV Dirac fermion has a nonlinear dispersion relationship and is similar to the type-II Dirac fermion, which has electron pocket and hole pocket. The effective model for the type-IV Dirac fermion is also found. It is worth pointing out that there is a type-II Dirac fermion near this new Dirac fermion. So we used two models to describe the coexistence of these two Dirac fermions. Topological surface states of these two Dirac points are also calculated. We envision that our findings will stimulate researchers to study novel physics of type-IV Dirac fermions, as well as the interplay of type-II and type-IV Dirac fermions.

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