scholarly journals Terahertz signatures of ultrafast Dirac fermion relaxation at the surface of topological insulators

2021 ◽  
Vol 6 (1) ◽  
Author(s):  
S. Kovalev ◽  
K.-J. Tielrooij ◽  
J.-C. Deinert ◽  
I. Ilyakov ◽  
N. Awari ◽  
...  
Keyword(s):  
Fermi Level ◽  
Topological Insulators ◽  
Surface States ◽  
Dirac Fermion ◽  
Dirac Fermions ◽  
Bulk Carriers ◽  
Information And Communication ◽  
Protected Surface ◽  
Saturation Effects ◽  
Conversion Efficiencies

AbstractTopologically protected surface states present rich physics and promising spintronic, optoelectronic, and photonic applications that require a proper understanding of their ultrafast carrier dynamics. Here, we investigate these dynamics in topological insulators (TIs) of the bismuth and antimony chalcogenide family, where we isolate the response of Dirac fermions at the surface from the response of bulk carriers by combining photoexcitation with below-bandgap terahertz (THz) photons and TI samples with varying Fermi level, including one sample with the Fermi level located within the bandgap. We identify distinctly faster relaxation of charge carriers in the topologically protected Dirac surface states (few hundred femtoseconds), compared to bulk carriers (few picoseconds). In agreement with such fast cooling dynamics, we observe THz harmonic generation without any saturation effects for increasing incident fields, unlike graphene which exhibits strong saturation. This opens up promising avenues for increased THz nonlinear conversion efficiencies, and high-bandwidth optoelectronic and spintronic information and communication applications.

scholarly journals Fermi-level-dependent charge-to-spin current conversion by Dirac surface states of topological insulators

2016 ◽  
Vol 12 (11) ◽  
pp. 1027-1031 ◽  
Author(s):  
K. Kondou ◽  
R. Yoshimi ◽  
A. Tsukazaki ◽  
Y. Fukuma ◽  
J. Matsuno ◽  
...  
Keyword(s):  
Fermi Level ◽  
Topological Insulators ◽  
Spin Current ◽  
Surface States

scholarly journals A Chemical Theory of Topological Insulators

2019 ◽  
Author(s):  
Angel Martín Pendás ◽  
Julia Contreras-García ◽  
Fernanda Pinilla ◽  
José Daniel Mella ◽  
Carlos Cárdenas ◽  
...  
Keyword(s):  
Topological Insulators ◽  
Surface States ◽  
Chemical Theory ◽  
Protected Surface ◽  
Symmetry Protected ◽  
Chemical Description

This article presents a chemical description of a simple topological insulators model in order to translate concepts such as "symmetry protected", "surface states" to the chemistry vocabulary

scholarly journals Generation of spin-polarized hot electrons at topological insulators surfaces by scattering from collective charge excitations

2021 ◽  
Vol 4 (1) ◽  
Author(s):  
Khalil Zakeri ◽  
Janek Wettstein ◽  
Christoph Sürgers
Keyword(s):  
Topological Insulators ◽  
Spin Asymmetry ◽  
Surface States ◽  
Inelastic Channel ◽  
Dirac Fermions ◽  
Elastic Channel ◽  
Fundamental Physics ◽  
Spintronic Devices ◽  
Spin Polarized ◽  
New Ideas

AbstractTopological insulators (TIs) are materials which exhibit topologically protected electronic surface states, acting as mass-less Dirac fermions. Beside their fascinating fundamental physics, TIs are also promising candidates for future spintronic devices. In this regard, generation of spin-polarized currents in TIs is the first and most important step towards their application in spin-based devices. Here we demonstrate that when electrons are scattered from the surface of bismuth selenide, a prototype TI, not only the elastic channel but also the inelastic channel is strongly spin dependent. In particular collective charge excitations (plasmons) excited at such surfaces show a large spin-dependent electron scattering. Electrons scattered by these excitations exhibit a high spin asymmetry, as high as 40%. The observed effect opens up new possibilities to generate spin-polarized currents at the surface of TIs or utilize the collective charge excitations to analyze the electrons’ spin. The results are also important to understand the spin polarization of the photo-excited electrons excited at TIs surfaces. Moreover, our finding will inspire new ideas for using these plasmonic excitations in the field of spin-plasmonics.

scholarly journals Confinement Properties of the 3D Topological Insulators Bi₂Se₃, Bi₂Te₃, and Sb₂Te₃

2021 ◽  
Author(s):  
◽  
Markus Kotulla
Keyword(s):  
Magnetic Field ◽  
Topological Insulators ◽  
Surface States ◽  
Cold Atom ◽  
Zeeman Splitting ◽  
Dirac Fermion ◽  
Topological Phases ◽  
Edge States ◽  
Experimental Realization ◽  
Band Inversion

<p>Recent discoveries have spurred the theoretical prediction and experimental realization of novel materials that have topological properties arising from band inversion. Such topological insulators have conductive surface or edge states but are insulating in the bulk. How the signatures of topological behavior evolve when the system size is reduced is noteworthy from both a fundamental and an application-oriented point of view, as such understanding may form the basis for tailoring systems to be in specific topological phases. This thesis investigates the softly confined topological insulator family of Bi₂Se₃ and its properties when subjected to an in-plane magnetic field. The model system provides a useful platform for systematic study of the transition between the normal and the topological phases, including the development of band inversion and the formation of massless-Dirac-fermion surface states. The effects of bare size quantization, two-dimensional-subband mixing, and electron-hole asymmetry are disentangled and their corresponding physical consequences elucidated.  When a magnetic field is present, it is found that the Dirac cone which is formed in surface states, splits into two cones separated in momentum space and that these cones exhibit properties of Weyl fermions. The effective Zeeman splitting is much larger for the surface states than for the bulk states. Furthermore, the g-factor of the surface states depends on the size of the material. The mathematical model presented here may be realizable experimentally in the frame of optical lattices in ultra cold atom gases.</p>

scholarly journals Zero-Energy Modes, Fractional Fermion Numbers and The Index Theorem in a Vortex-Dirac Fermion System

Symmetry ◽  
2020 ◽  
Vol 12 (3) ◽  
pp. 373 ◽  
Author(s):  
Takashi Yanagisawa
Keyword(s):  
Topological Insulators ◽  
Majorana Fermion ◽  
Dirac Fermion ◽  
Dirac Fermions ◽  
Flux Quantum ◽  
Zero Energy ◽  
Energy Excitation ◽  
Fermion Number ◽  
Quantum Vortex ◽  
Energy Mode

Physics of topological materials has attracted much attention from both physicists and mathematicians recently. The index and the fermion number of Dirac fermions play an important role in topological insulators and topological superconductors. A zero-energy mode exists when Dirac fermions couple to objects with soliton-like structure such as kinks, vortices, monopoles, strings, and branes. We discuss a system of Dirac fermions interacting with a vortex and a kink. This kind of systems will be realized on the surface of topological insulators where Dirac fermions exist. The fermion number is fractionalized and this is related to the presence of fermion zero-energy excitation modes. A zero-energy mode can be regarded as a Majorana fermion mode when the chemical potential vanishes. Our discussion includes the case where there is a half-flux quantum vortex associated with a kink in a magnetic field in a bilayer superconductor. A normalizable wave function of fermion zero-energy mode does not exist in the core of the half-flux quantum vortex. The index of Dirac operator and the fermion number have additional contributions when a soliton scalar field has a singularity.

scholarly journals Glide symmetry protected higher-order topological insulators from semimetals with butterfly-like nodal lines

2021 ◽  
Vol 7 (1) ◽  
Author(s):  
Xiaoting Zhou ◽  
Chuang-Han Hsu ◽  
Cheng-Yi Huang ◽  
Mikel Iraola ◽  
Juan L. Mañes ◽  
...  
Keyword(s):  
Topological Insulator ◽  
Topological Insulators ◽  
Surface States ◽  
Nodal Line ◽  
Higher Order ◽  
Dirac Fermion ◽  
Space Groups ◽  
Nodal Lines ◽  
Topological Semimetals ◽  
Symmetry Protected

AbstractMost topological insulators (TIs) discovered today in spinful systems can be transformed from topological semimetals (TSMs) with vanishing bulk gap via introducing the spin-orbit coupling (SOC), which manifests the intrinsic links between the gapped topological insulator phases and the gapless TSMs. Recently, we have discovered a family of TSMs in time-reversal invariant spinless systems, which host butterfly-like nodal-lines (NLs) consisting of a pair of identical concentric intersecting coplanar ellipses (CICE). In this Communication, we unveil the intrinsic link between this exotic class of nodal-line semimetals (NLSMs) and a $${{\mathbb{Z}}}_{4}$$ Z 4 = 2 topological crystalline insulator (TCI), by including substantial SOC. We demonstrate that in three space groups (i.e., Pbam (No.55), P4/mbm (No.127), and P42/mbc (No.135)), the TCI supports a fourfold Dirac fermion on the (001) surface protected by two glide symmetries, which originates from the intertwined drumhead surface states of the CICE NLs. The higher order topology is further demonstrated by the emergence of one-dimensional helical hinge states, indicating the discovery of a higher order topological insulator protected by a glide symmetry.

scholarly journals Mapping propagation of collective modes in Bi2Se3 and Bi2Te2.2Se0.8 topological insulators by near-field terahertz nanoscopy

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Eva Arianna Aurelia Pogna ◽  
Leonardo Viti ◽  
Antonio Politano ◽  
Massimo Brambilla ◽  
Gaetano Scamarcio ◽  
...  
Keyword(s):  
Topological Insulators ◽  
Near Field ◽  
Surface States ◽  
Longitudinal Optical ◽  
Collective Modes ◽  
Protected Surface ◽  
Topological Surface ◽  
The Rich ◽  
Nano Imaging ◽  
New Research

AbstractNear-field microscopy discloses a peculiar potential to explore novel quantum state of matter at the nanoscale, providing an intriguing playground to investigate, locally, carrier dynamics or propagation of photoexcited modes as plasmons, phonons, plasmon-polaritons or phonon-polaritons. Here, we exploit a combination of hyperspectral time domain spectroscopy nano-imaging and detectorless scattering near-field optical microscopy, at multiple terahertz frequencies, to explore the rich physics of layered topological insulators as Bi2Se3 and Bi2Te2.2Se0.8, hyperbolic materials with topologically protected surface states. By mapping the near-field scattering signal from a set of thin flakes of Bi2Se3 and Bi2Te2.2Se0.8 of various thicknesses, we shed light on the nature of the collective modes dominating their optical response in the 2-3 THz range. We capture snapshots of the activation of transverse and longitudinal optical phonons and reveal the propagation of sub-diffractional hyperbolic phonon-polariton modes influenced by the Dirac plasmons arising from the topological surface states and of bulk plasmons, prospecting new research directions in plasmonics, tailored nanophotonics, spintronics and quantum technologies.

scholarly journals Confinement Properties of the 3D Topological Insulators Bi₂Se₃, Bi₂Te₃, and Sb₂Te₃

2021 ◽  
Author(s):  
◽  
Markus Kotulla
Keyword(s):  
Magnetic Field ◽  
Topological Insulators ◽  
Surface States ◽  
Cold Atom ◽  
Zeeman Splitting ◽  
Dirac Fermion ◽  
Topological Phases ◽  
Edge States ◽  
Experimental Realization ◽  
Band Inversion

<p>Recent discoveries have spurred the theoretical prediction and experimental realization of novel materials that have topological properties arising from band inversion. Such topological insulators have conductive surface or edge states but are insulating in the bulk. How the signatures of topological behavior evolve when the system size is reduced is noteworthy from both a fundamental and an application-oriented point of view, as such understanding may form the basis for tailoring systems to be in specific topological phases. This thesis investigates the softly confined topological insulator family of Bi₂Se₃ and its properties when subjected to an in-plane magnetic field. The model system provides a useful platform for systematic study of the transition between the normal and the topological phases, including the development of band inversion and the formation of massless-Dirac-fermion surface states. The effects of bare size quantization, two-dimensional-subband mixing, and electron-hole asymmetry are disentangled and their corresponding physical consequences elucidated.  When a magnetic field is present, it is found that the Dirac cone which is formed in surface states, splits into two cones separated in momentum space and that these cones exhibit properties of Weyl fermions. The effective Zeeman splitting is much larger for the surface states than for the bulk states. Furthermore, the g-factor of the surface states depends on the size of the material. The mathematical model presented here may be realizable experimentally in the frame of optical lattices in ultra cold atom gases.</p>

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

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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