We combine transport, magnetization, and torque magnetometry measurements to investigate the electronic structure of ZrTe5 and its evolution with temperature. At fields beyond the quantum limit, we observe a magnetization reversal from paramagnetic to diamagnetic response, which is characteristic of a Dirac semi-metal. We also observe a strong non-linearity in the magnetization that suggests the presence of additional low-lying carriers from other low-energy bands. Finally, we observe a striking sensitivity of the magnetic reversal to temperature that is not readily explained by simple band-structure models, but may be connected to a temperature dependent Lifshitz transition proposed to exist in this material.
Two-dimensional ($2$D) semi-Dirac systems, such as $2$D black phosphorus and arsenene, can exhibit a rich topological phase transition between insulating, semi-Dirac, and band inversion phases when subjected to an external modulation. How these phase transitions manifest within the quantum transport and shot noise signatures remain an open question thus far. Here, we show that the Fano factor converges to the universal $F\approx0.179$ at the semi-Dirac phase, and transits between the sub-Poissonian ($F\approx1/3$) and the Poissonian shot noise ($F\approx1$) limit at the band inversion and the …
We consider the optical and transport properties in a model two-dimensional Hamiltonian which describes the merging of two Dirac points. At low energy, in the presence of an energy gap parameter $Δ$, there are two distinct Dirac points with linear dispersion, these are connected by a saddle point at higher energy. As $Δ$ goes to zero, the two Dirac points merge and the resulting dispersion exhibits semi-Dirac behaviour which is quadratic in the $x$-direction ("nonrelativistic") and linear the $y$-direction ("relativistic").In the clean limit for each direction ($x,y$) the contribution of the i…
Polymorph of 8-Pmmn borophene exhibits anisotropic tilted Dirac cones. In this work, we explore the consequences of the tilted Dirac cones in magnetotransport properties of a periodically modulated borophene. We evaluate modulation induced diffusive conductivity by using linear response theory in low temperature regime. The application of weak modulation (electric/magnetic or both) gives rise to the magnetic field dependent non-zero oscillatory drift velocity which causes Weiss oscillation in the longitudinal conductivity at low magnetic field. The Weiss oscillation is studied in presence of a…
In this paper, we give a systematic theoretical study on the Landau levels (LLs) and magneto-optical conductivity Re$(σ_{αα})$ in a gapped Dirac semimetal model with mixed linear and parabolic dispersions under a magnetic field, which was recently proposed by Jiang \textit{et al.} [Phys. Rev. Lett. {\bf125}, 046403 (2020)] to explain the experimental magnetoinfrared spectroscopy in the three-dimensional ZrTe$_5$ crystal. We find that the strong magnetic field can drive the LLs become noninverted and thus the strong topological insulator phase in ZrTe$_5$ turns to be a trivial insulator. In the…
We investigate the entanglement entropy (EE) and charge fluctuations in models where the low energy physics is governed by massless Dirac fermions. We focus on the response to flux insertion which, for the EE, is widely assumed to be universal, \emph{i.e.}, independent of the microscopic details. We provide an analytical derivation of the EE and charge fluctuations for the seminal example of graphene, using the dimensional reduction of its tight-binding model to the one-dimensional Su-Schrieffer-Heeger model. Our asymptotic expression for the EE matches the conformal field theory prediction. W…
Many applications for topological insulators (TIs) as well as new phenomena require devices with reduced dimensions. While much progress has been made to realize thin films of TIs with low bulk carrier density, nanostructures have not yet been reported with similar properties, despite the fact that size confinement should help reduce contributions from bulk carriers. Here we demonstrate that Bi2Se3 nanoribbons, grown by a simple catalyst-free physical-vapour deposition, have inherently low bulk carrier densities, and can be further made bulk-free by size confinement, thus revealing the high mo…
Lifshitz transition is a kind of topological phase transition in which the Fermi surface is reconstructed. It can occur in the two-dimensional (2D) tilted Dirac materials when the energy bands change between the type-I phase ($0<t<1$) and the type-II phase ($t>1$) through the type-III phase ($t=1$), where different tilts are parametrized by the values of $t$. In order to characterize the Lifshitz transition therein, we theoretically investigate the longitudinal optical conductivities (LOCs) in type-I, type-II, and type-III Dirac materials within linear response theory. In the undoped case, the…
The current response to an electromagnetic field in a Weyl or Dirac semimetal becomes nonlocal due to the chiral anomaly activated by an applied static magnetic field. The nonlocality develops under the conditions of the normal skin effect and is related to the valley charge imbalance generated by the joint effect of the electric field of the impinging wave and the static magnetic field. We elucidate the signatures of this nonlocality in the transmission of electromagnetic waves. The signatures include enhancement of the transmission amplitude and its specific dependence on the wave's frequenc…
We theoretically investigate the features of Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between two magnetic impurities, mediated by the interfacial bound states inside a domain wall (DW). The latter separates the two regions with oppositely signed inversion symmetry broken terms in graphene and Weyl semimetal. The DW is modelled by a smooth quantum well which hosts a number of discrete bound states including a pair of gapless, metallic zero-energy modes with opposite chiralities. We find clear signatures of these interfacial chiral bound states in spin response (RKKY exchange i…
Optical conductivity signature of Van Hove singularity in altermagnetic topological systems
Jiangsu University of Science and Technology（江苏科技大学）；Hubei University（湖北大学）；Southern University of Science and Technology (SUSTech)（南方科技大学）；Nanyang Technological University（南洋理工大学）
Using polarized optical and magneto-optical spectroscopy, we have demonstrated universal aspects of electrodynamics associated with Dirac nodal-lines. We investigated anisotropic electrodynamics of NbAs$_2$ where the spin-orbit interaction triggers energy gaps along the nodal-lines, which manifest as sharp steps in the optical conductivity spectra. We show experimentally and theoretically that shifted 2D Dirac nodal-lines feature linear scaling $σ_1 (ω)\simω$, similar to 3D nodal-points. Massive Dirac nature of the nodal-lines are confirmed by magneto-optical data, which may also be indicative…
Voltage characteristics of hydrodynamic Dirac electron nozzles with supersonic flow
Imec；Peter Grünberg Institute (PGI-9), Forschungszentrum Jülich（于利希研究中心）；Instituut voor Theoretische Fysica, Department of Physics and Astronomy, KU Leuven（荷语鲁汶大学）；Department of Physics and Materials Science, University of Luxembourg（卢森堡大学）；Institut für Theoretische Physik und Astrophysik, Universität Würzburg（维尔茨堡大学）
The field of topological materials science has recently been focussing on three-dimensional Dirac semimetals, which exhibit robust Dirac phases in the bulk. However, the absence of characteristic surface states in accidental Dirac semimetals (DSM) makes it difficult to experimentally verify claims about the topological nature using commonly used surface-sensitive techniques. The chiral magnetic effect (CME), which originates from the Weyl nodes, causes an $\textbf{E}\cdot\textbf{B}$-dependent chiral charge polarization, which manifests itself as negative magnetoresistance. We exploit the exten…
We theoretically investigate the emergence of non-hermitian physics at the heterojunction of a type-II Dirac semi-metal (DSM) and a dirty superconductor (DSC). The non-hermiticity is introduced in the DSM through the self-energy term incorporated via the dirtiness of the superconducting material. This causes the spectra of the effective Hamiltonian to become complex, which gives rise to the appearance of the exceptional points (EPs). This complex self energy, apart from having a frequency dependence, also acquires spatial dependence as well, which is unique and can provide interesting effects …
A three-dimensional topological insulator features a two-dimensional surface state consisting of a single linearly-dispersive Dirac cone. Under broken time-reversal symmetry, the single Dirac cone is predicted to cause half-integer quantization of Hall conductance, which is a manifestation of the parity anomaly in quantum field theory. However, despite various observations of quantization phenomena, the half-integer quantization has been elusive because a pair of equivalent Dirac cones on two opposing surfaces are simultaneously measured in ordinary experiments. Here we demonstrate the half-in…
Robust Signatures of Fragile Topology in Nonmagnetic Materials
Institute for Theoretical Solid State Physics, IFW Dresden（德累斯顿弗劳恩霍夫研究所固态物理理论研究所）；Würzburg-Dresden Cluster of Excellence ctd.qmat（维尔茨堡-德累斯顿卓越集群量子物质材料）；Institute for Theoretical Physics Amsterdam, University of Amsterdam（阿姆斯特丹大学阿姆斯特丹理论物理研究所）；Kyiv Academic University（基辅学术大学）；Institute for Theoretical Physics, TU Dresden（德累斯顿工业大学理论物理研究所）
Interaction of Phonons and Dirac Fermions on the Surface of Bi2Se3: A Strong Kohn Anomaly
我们首次测量了强三维拓扑绝缘体Bi 2 Se 3的（001）面声子色散曲线。利用相干氦束表面散射技术对表面声子进行了测量。结果揭示了一个突出的签名的异国情调的金属狄拉克费米子准粒子，包括一个强大的科恩异常。签名是明显的低能量各向同性凸色散表面声子分支的频率最大值为1.8太赫兹，并具有V形最小值在约2kF，定义了科恩异常。理论分析将这种色散分布归因于表面狄拉克费米子对表面声子激发的重正化。狄拉克费米子的贡献，这个重整化的库仑型微扰模型的条款。
Link between thermodynamic correlation signatures and superconductivity in twisted trilayer graphene
IBM Quantum（IBM量子）；Department of Physics, Stanford University（斯坦福大学物理系）；Geballe Laboratory for Advanced Materials（吉巴尔先进材料实验室）；Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory（斯坦福材料与能源科学研究所，SLAC国家加速器实验室）；Research Center for Electronic and Optical Materials, National Institute for Materials Science（物质材料研究机构电子与光学材料研究中心）；Research Center for Materials Nanoarchitectonics, National Institute for Materials Science（物质材料研究机构材料纳米架构中心）
