Higher Chern number states in curved periodic nanowires.

The coupling between the spin and momentum degrees of freedom due to spin-orbit interactions (SOI) suggests that the strength of the latter can be modified by controlling the motion of the charge carriers. In this paper, we investigate how the effective SOI can be modulated by constraining the motion of charge carriers to curved waveguides thereby introducing real-space geometric curvature in their motion. The change in the SOI can in turn induce topological phase transitions in the system.

Phase analysis on the error scaling of entangled qubits in a 53-qubit system.

We have studied carefully the behaviors of entangled qubits on the IBM Rochester with various connectivities and under a "noisy" environment. A phase trajectory analysis based on our measurements of the GHZ-like states is performed. Our results point to an important fact that entangled qubits are "protected" against environmental noise by a scaling property that impacts only the weighting of their amplitudes.

Electrically tunable valley polarization in Weyl semimetals with tilted energy dispersion.

Tunneling transport across electrical potential barriers in Weyl semimetals with tilted energy dispersion is investigated. We report that the electrons around different valleys experience opposite direction refractions at the barrier interface when the energy dispersion is tilted along one of the transverse directions. Chirality dependent refractions at the barrier interface polarize the Weyl fermions in angle-space according to their valley index.

Curvature induced quantum phase transitions in an electron-hole system.

In this work, we study the effect of introducing a periodic curvature on nanostructures, and demonstrate that the curvature can lead to a transition from a topologically trivial state to a non-trivial state. We first present the Hamiltonian for an arbitrarily curved nanostructure, and introduce a numerical scheme for calculating the bandstructure of a periodically curved nanostructure. Using this scheme, we calculate the bandstructure for a sinusoidally curved two-dimensional electron gas.

Newton's second law in spin-orbit torque.

Spin-orbit torque (SOT) refers to the excitation of magnetization dynamics via spin-orbit coupling (SOC) under the application of a charged current. In this work, we introduce a simple and intuitive description of the SOT in terms of spin force. In Rashba SOC system, the damping-like SOT can be expressed as [Formula: see text], in analogy to the classical torque-force relation, where R is the effective radius characterizing the Rashba splitting in the momentum space.

Inherent orbital spin textures in Rashba effect and their implications in spin-orbitronics.

The Rashba effect gives rise to the key feature of chiral spin texture. Recently it was demonstrated that the orbital angular momentum (OAM) texture forms the underlying basis for Rashba spin texture. Here we solve a model Hamiltonian of a generic p-orbital system in the presence of crystal field, internal spin-orbit coupling (SOC) and inversion symmetry breaking (ISB), and demonstrate, in addition to OAM and spin texture, the existence of orbital projection (OP) of the spin texture in a general Rashba system.

Influence of Fermi arc states and double Weyl node on tunneling in a Dirac semimetal.

Most theoretical studies of tunneling in Dirac and the closely related Weyl semimetals have modeled these materials as single Weyl nodes described by the three-dimensional Dirac equation [Formula: see text]. The influence of scattering between the different valleys centered around different Weyl nodes, and the Fermi arc states which connect these nodes are hence not evident from these studies. In this work we study the tunneling in a thin film system of the Dirac semimetal NaBi consisting of a central segment with a gate potential, sandwiched between identical semi-infinite source and drain segments.

Effect of surface state hybridization on current-induced spin-orbit torque in thin topological insulator films.

We investigate the thickness optimization for maximum current-induced spin-orbit torque (SOT) generated by topological surface states (TSS's) in a bilayer system comprising of a ferromagnetic layer coupled to a thin topological insulator (TI) film. We show that by reducing the TI thickness, two competing effects on the SOT are induced: (i) the torque strength is stronger as the bulk contribution is decreased; (ii) on the other hand, the torque strength becomes suppressed due to increasing hybridization of the surface states. The latter is attributed to the opposite helicities of the coupled TSS's.

Effective Hamiltonian for surface states of topological insulator nanotubes.

In this work we derive an effective Hamiltonian for the surface states of a hollow topological insulator (TI) nanotube with finite width walls. Unlike a solid TI cylinder, a TI nanotube possesses both an inner as well as outer surface on which the states localized at each surface are coupled together. The curvature along the circumference of the nanotube leads to a spatial variation of the spin orbit interaction field experienced by the charge carriers as well as an asymmetry between the inner and outer surfaces of the nanotube.

Klein tunneling in Weyl semimetals under the influence of magnetic field.

Klein tunneling refers to the absence of normal backscattering of electrons even under the case of high potential barriers. At the barrier interface, the perfect matching of electron and hole wavefunctions enables a unit transmission probability for normally incident electrons. It is theoretically and experimentally well understood in two-dimensional relativistic materials such as graphene.

Dirac semimetal thin films in in-plane magnetic fields.

In this work we study the effects of in-plane magnetic fields on thin films of the Dirac Semimetal (DSM) NaBi where one of the in-plane directions is perpendicular to the k-separation between the two Weyl nodes that exist for each spin orientation. We show numerically that the states localized near the surfaces of these thin films are related to the Fermi arc states in semi-infinite slabs. Due to the anisotropy between the two in-plane directions, the application of a magnetic field along these directions have differing effects.

Gauge Physics of Spin Hall Effect.

Spin Hall effect (SHE) has been discussed in the context of Kubo formulation, geometric physics, spin orbit force, and numerous semi-classical treatments. It can be confusing if the different pictures have partial or overlapping claims of contribution to the SHE. In this article, we present a gauge-theoretic, time-momentum elucidation, which provides a general SHE equation of motion, that unifies under one theoretical framework, all contributions of SHE conductivity due to the kinetic, the spin orbit force (Yang-Mills), and the geometric (Murakami-Fujita) effects.

Robustness of topological Hall effect of nontrivial spin textures.

We analyze the topological Hall conductivity (THC) of topologically nontrivial spin textures like magnetic vortices and skyrmions and investigate its possible application in the readback for magnetic memory based on those spin textures. Under adiabatic conditions, such spin textures would theoretically yield quantized THC values, which are related to topological invariants such as the winding number and polarity, and as such are insensitive to fluctuations and smooth deformations. However, in a practical setting, the finite size of spin texture elements and the influence of edges may cause them to deviate from their ideal configurations.

Magnetic random access memory (MRAM).

The high density and high speed nonvolatile MTJ MRAMs are reviewed from perspective of the reading and writing operation. The reading operation of the MRAM with different sensing schemes and cell array structures is discussed, in particular the reference resistance generating schemes which are introduced to maximize the cell efficiency and reading reliability. The high density, low cost cross-point cell layout structures are analyzed systematically.