Negative Longitudinal Magnetoresistance from the Anomalous N=0 Landau Level in Topological Materials

Room-Temperature Spin Transport in Cd3As2

As the need for ever greater transistor density increases, the commensurate decrease in device size approaches the atomic limit, leading to increased energy loss and leakage currents, reducing energy efficiencies. Alternative state variables, such as electronic spin rather than electronic charge, have the potential to enable more energy-efficient and higher performance devices. These spintronic devices require materials capable of efficiently harnessing the electron spin. Here we show robust spin transport in Cd₃As₂ films up to room temperature. We demonstrate a nonlocal spin valve switch from this material, as well as inverse spin Hall effect measurements yielding spin Hall angles as high as θ_SH = 1.5 and spin diffusion lengths of 10-40 μm. Long spin-coherence lengths with efficient charge-to-spin conversion rates and coherent spin transport up to room temperature, as we show here in Cd₃As₂, are enabling steps toward realizing actual spintronic devices.

Magnetotransport signatures of Weyl physics and discrete scale invariance in the elemental semiconductor tellurium

The study of topological materials possessing nontrivial band structures enables exploitation of relativistic physics and development of a spectrum of intriguing physical phenomena. However, previous studies of Weyl physics have been limited exclusively to semimetals. Here, via systematic magnetotransport measurements, two representative topological transport signatures of Weyl physics, the negative longitudinal magnetoresistance and the planar Hall effect, are observed in the elemental semiconductor tellurium. More strikingly, logarithmically periodic oscillations in both the magnetoresistance and Hall data are revealed beyond the quantum limit and found to share similar characteristics with those observed in ZrTe5 and HfTe5 The log-periodic oscillations originate from the formation of two-body quasi-bound states formed between Weyl fermions and opposite charge centers, the energies of which constitute a geometric series that matches the general feature of discrete scale invariance (DSI). Our discovery reveals the topological nature of tellurium and further confirms the universality of DSI in topological materials. Moreover, introduction of Weyl physics into semiconductors to develop "Weyl semiconductors" provides an ideal platform for manipulating fundamental Weyl fermionic behaviors and for designing future topological devices.

Weak Antilocalization and Anisotropic Magnetoresistance as a Probe of Surface States in Topological Bi2TexSe3-x Thin Films

Topological materials, such as the quintessential topological insulators in the Bi2X3 family (X = O, S, Se, Te), are extremely promising for beyond Moore's Law computing applications where alternative state variables and energy efficiency are prized. It is essential to understand how the topological nature of these materials changes with growth conditions and, more specifically, chalcogen content. In this study, we investigate the evolution of the magnetoresistance of Bi2TexSe3-x for varying chalcogen ratios and constant growth conditions as a function of both temperature and angle of applied field. The contribution of 2D and 3D weak antilocalization are investigated by utilizing the Tkachov-Hankiewicz model and Hakami-Larkin-Nagaoka models of magnetoconductance.

Anomalous Thermoelectric Effects of ZrTe_{5} in and beyond the Quantum Limit

Thermoelectric effects are more sensitive and promising probes to topological properties of emergent materials, but much less addressed compared to other physical properties. We study the thermoelectric effects of ZrTe_{5} in a magnetic field. The presence of the nontrivial electrons leads to the anomalous Nernst effect and quasilinear field dependence of thermopower below the quantum limit. In the strong-field quantum limit, both the thermopower and Nernst signal exhibit exotic peaks. At higher magnetic fields, the Nernst signal has a sign reversal at a critical field where the thermopower approaches zero. We propose that these anomalous behaviors can be attributed to the gap closing of the zeroth Landau bands in topological materials with the band inversion. Our understanding to the anomalous thermoelectric properties in ZrTe_{5} opens a new avenue for exploring Dirac physics in topological materials.

Growth and Strain Engineering of Trigonal Te for Topological Quantum Phases in Non-Symmorphic Chiral Crystals

Strained trigonal Te has been predicted to host Weyl nodes supported by a non-symmorphic chiral symmetry. Using low-pressure physical vapor deposition, we systematically explored the growth of trigonal Te nanowires with naturally occurring strain caused by curvature of the wires. Raman spectra and high mobility electronic transport attest to the highly crystalline nature of the wires. Comparison of Raman spectra for both straight and curved nanowires indicates a breathing mode that is significantly broader and shifted in frequency for the curved wires. Strain induced by curvature during growth therefore may provide a simple pathway to investigate topological phases in trigonal Te.

Spin-dependent scattering induced negative magnetoresistance in topological insulator Bi2Te3 nanowires

Studies of negative magnetoresistance in novel materials have recently been in the forefront of spintronic research. Here, we report an experimental observation of the temperature dependent negative magnetoresistance in Bi₂Te₃ topological insulator (TI) nanowires at ultralow temperatures (20 mK). We find a crossover from negative to positive magnetoresistance while increasing temperature under longitudinal magnetic field. We observe a large negative magnetoresistance which reaches −22% at 8 T. The interplay between negative and positive magnetoresistance can be understood in terms of the competition between dephasing and spin-orbit scattering time scales. Based on the first-principles calculations within a density functional theory framework, we demonstrate that disorder (substitutional) by Ga⁺ ion milling process, which is used to fabricate nanowires, induces local magnetic moments in Bi₂Te₃ crystal that can lead to spin-dependent scattering of surface and bulk electrons. These experimental findings show a significant advance in the nanoscale spintronics applications based on longitudinal magnetoresistance in TIs. Our experimental results of large negative longitudinal magnetoresistance in 3D TIs further indicate that axial anomaly is a universal phenomenon in generic 3D metals.

Theory for the negative longitudinal magnetoresistance in the quantum limit of Kramers Weyl semimetals

Negative magnetoresistance is rare in non-magnetic materials. Recently, negative magnetoresistance has been observed in the quantum limit of β-Ag₂Se, where only one band of Landau levels is occupied in a strong magnetic field parallel to the applied current. β-Ag₂Se is a material that hosts a Kramers Weyl cone with band degeneracy near the Fermi energy. Kramers Weyl cones exist at time-reversal invariant momenta in all symmorphic chiral crystals, and at a subset of these momenta, including the Γ point, in non-symmorphic chiral crystals. Here, we present a theory for the negative magnetoresistance in the quantum limit of Kramers Weyl semimetals. We show that, although there is a band touching similar to those in Weyl semimetals, negative magnetoresistance can exist without a chiral anomaly. We find that it requires screened Coulomb scattering potentials between electrons and impurities, which is naturally the case in β-Ag₂Se.

Forbidden Backscattering and Resistance Dip in the Quantum Limit as a Signature for Topological Insulators

Identifying topological insulators and semimetals often focuses on their surface states, using spectroscopic methods such as angle-resolved photoemission spectroscopy or scanning tunneling microscopy. In contrast, studying the topological properties of topological insulators from their bulk-state transport is more accessible in most labs but seldom addressed. We show that, in the quantum limit of a topological insulator, the backscattering between the only two states on the Fermi surface of the lowest Landau band can be forbidden at a critical magnetic field. The conductivity is determined solely by the backscattering between the two states, leading to a resistance dip that may serve as a signature for topological insulator phases. More importantly, this forbidden backscattering mechanism for the resistance dip is irrelevant to details of disorder scattering. Our theory can be applied to revisit the experiments on Pb_{1-x}Sn_{x}Se, ZrTe_{5}, and Ag_{2}Te families, and will be particularly useful for controversial small-gap materials at the boundary between topological and normal insulators.

Topological Phase Transition-Induced Triaxial Vector Magnetoresistance in (Bi1-xInx)2Se3 Nanodevices

We report the study of a triaxial vector magnetoresistance (MR) in nonmagnetic (Bi_1-xIn_x)₂Se₃ nanodevices at the composition of x = 0.08. We show a dumbbell-shaped in-plane negative MR up to room temperature as well as a large out-of-plane positive MR. MR at three directions is about in a -3%:-1%:225% ratio at 2 K. Through both the thickness and composition-dependent magnetotransport measurements, we show that the in-plane negative MR is due to the topological phase transition enhanced intersurface coupling near the topological critical point. Our devices suggest the great potential for room-temperature spintronic applications in, for example, vector magnetic sensors.

Negative Magnetoresistance without Chiral Anomaly in Topological Insulators

An intriguing phenomenon in topological semimetals and topological insulators is the negative magnetoresistance (MR) observed when a magnetic field is applied along the current direction. A prevailing understanding to the negative MR in topological semimetals is the chiral anomaly, which, however, is not well defined in topological insulators. We calculate the MR of a three-dimensional topological insulator, by using the semiclassical equations of motion, in which the Berry curvature explicitly induces an anomalous velocity and orbital moment. Our theoretical results are in quantitative agreement with the experiments. The negative MR is not sensitive to temperature and increases as the Fermi energy approaches the band edge. The orbital moment and g factors also play important roles in the negative MR. Our results give a reasonable explanation to the negative MR in 3D topological insulators and will be helpful in understanding the anomalous quantum transport in topological states of matter.