Disordered Weyl Semimetals and Their Topological Family

Simulation and Manipulation of Tunable Weyl-Semimetal Bands Using Superconducting Quantum Circuits

We simulated highly tunable Weyl-semimetal bands using superconducting quantum circuits. Driving the superconducting quantum circuits with microwave fields, we mapped the momentum space of a lattice to the parameter space, realizing the Hamiltonian of a Weyl semimetal. By measuring the energy spectrum, we directly imaged the Weyl points, whose topological winding numbers were further determined from the Berry curvature measurement. In addition, we manipulated the band structure with an additional pump microwave field, producing a momentum-dependent Weyl-point energy together with an artificial magnetic field, which are indispensable for generating chiral magnetic topological currents in some special Weyl semimetals and may have significant impact on topological physics.

Generalized Haldane models on laser-coupling optical lattices

We propose two generalized Haldane models on laser-coupling optical lattices. Laser-assisted nearest neighbour tunnelings generate artificial staggered magnetic flux, facilitating the realization of topological nontrivial band structures. As generalizations of Haldane model, these models support topological insulator and semimetal phases featuring high Chern numbers. We show simple rules for computing Chern numbers of our models and display the phase diagrams. Moreover, numerical calculations of energy spectra are in perfect agreement with our theoretical expectations. Our models may serve as two new family members for generalizing Haldane model on optical lattices.

A generic phase between disordered Weyl semimetal and diffusive metal

Quantum phase transitions of three-dimensional (3D) Weyl semimetals (WSMs) subject to uncorrelated on-site disorder are investigated through quantum conductance calculations and finite-size scaling of localization length. Contrary to previous claims that a direct transition from a WSM to a diffusive metal (DM) occurs, an intermediate phase of Chern insulator (CI) between the two distinct metallic phases should exist due to internode scattering that is comparable to intranode scattering. The critical exponent of localization length is ν [Formula: see text] 1.3 for both the WSM-CI and CI-DM transitions, in the same universality class of 3D Gaussian unitary ensemble of the Anderson localization transition. The CI phase is confirmed by quantized nonzero Hall conductances in the bulk insulating phase established by localization length calculations. The disorder-induced various plateau-plateau transitions in both the WSM and CI phases are observed and explained by the self-consistent Born approximation. Furthermore, we clarify that the occurrence of zero density of states at Weyl nodes is not a good criterion for the disordered WSM, and there is no fundamental principle to support the hypothesis of divergence of localization length at the WSM-DM transition.

Unified Theory of PT and CP Invariant Topological Metals and Nodal Superconductors

As PT and CP symmetries are fundamental in physics, we establish a unified topological theory of PT and CP invariant metals and nodal superconductors, based on the mathematically rigorous KO theory. Representative models are constructed for all nontrivial topological cases in dimensions d=1, 2, and 3, with their exotic physical meanings being elucidated in detail. Intriguingly, it is found that the topological charges of Fermi surfaces in the bulk determine an exotic direction-dependent distribution of topological subgap modes on the boundaries. Furthermore, by constructing an exact bulk-boundary correspondence, we show that the topological Fermi points of the PT and CP invariant classes can appear as gapless modes on the boundary of topological insulators with a certain type of anisotropic crystalline symmetry.

Novel Z(2) Topological Metals and Semimetals

We report two theoretical discoveries for Z(2) topological metals and semimetals. It is shown first that any dimensional Z(2) Fermi surface is topologically equivalent to a Fermi point. Then the famous conventional no-go theorem, which was merely proven before for Z Fermi points in a periodic system without any discrete symmetry, is generalized so that the total topological charge is zero for all cases. Most remarkably, we find and prove an unconventional strong no-go theorem: all Z(2) Fermi points have the same topological charge ν(Z(2))=1 or 0 for periodic systems. Moreover, we also establish all six topological types of Z(2) models for realistic physical dimensions.

Room Temperature Electrical Detection of Spin Polarized Currents in Topological Insulators

Topological insulators (TIs) are a new class of quantum materials that exhibit a current-induced spin polarization due to spin-momentum locking of massless Dirac Fermions in their surface states. This helical spin polarization in three-dimensional (3D) TIs has been observed using photoemission spectroscopy up to room temperatures. Recently, spin polarized surface currents in 3D TIs were detected electrically by potentiometric measurements using ferromagnetic detector contacts. However, these electric measurements are so far limited to cryogenic temperatures. Here we report the room temperature electrical detection of the spin polarization on the surface of Bi2Se3 by employing spin sensitive ferromagnetic tunnel contacts. The current-induced spin polarization on the Bi2Se3 surface is probed by measuring the magnetoresistance while switching the magnetization direction of the ferromagnetic detector. A spin resistance of up to 70 mΩ is measured at room temperature, which increases linearly with current bias, reverses sign with current direction, and decreases with higher TI thickness. The magnitude of the spin signal, its sign, and control experiments, using different measurement geometries and interface conditions, rule out other known physical effects. These findings provide further information about the electrical detection of current-induced spin polarizations in 3D TIs at ambient temperatures and could lead to innovative spin-based technologies.