Revaz Ramazashvili

Magnetic quantum oscillations in the charge-density-wave state of the organic metals α-(BEDT-TTF)2MHg(SCN)4 with M = K and Tl

The low-temperature charge-density-wave (CDW) state in the layered organic metals α-(BEDT-TTF)2MHg(SCN)4 has been studied by means of the Shubnikov–de Haas and de Haas–van Alphen effects. In addition to the dominant α-frequency, which is also observed in the normal state, both the magnetoresistance and magnetic torque possess a slowly oscillating component. These slow oscillations provide a firm evidence for the CDW-induced reconstruction of the original cylindrical Fermi surface. The α-oscillations of the interlayer magnetoresistance exhibit an anomalous phase inversion in the CDW state, whereas the de Haas–van Alphen signal maintains the normal phase. We argue that the anomaly may be attributed to the magnetic-breakdown origin of the α-oscillations in the CDW state. A theoretical model illustrating the possibility of a phase inversion in the oscillating interlayer conductivity in the presence of a spatially fluctuating magnetic breakdown gap is proposed.

Z2 antiferromagnetic topological insulators with broken C4 symmetry

Abstract A two-dimensional topological insulator may arise in a centrosymmetric commensurate Neel antiferromagnet (AF), where staggered magnetization breaks both the elementary translation and time reversal, but retains their product as a symmetry. Fang et al. [6] proposed an expression for a Z 2 topological invariant to characterize such systems. Here, we show that this expression does not allow to detect all the existing phases if a certain lattice symmetry is lacking. We implement numerical techniques to diagnose topological phases of a toy Hamiltonian, and verify our results by computing the Chern numbers of degenerate bands, and also by explicitly constructing the edge states, thus illustrating the efficiency of the method.

Zeeman spin-orbit coupling in antiferromagnetic conductors

Abstract This article is a brief review of Zeeman spin-orbit coupling, arising in a low-carrier commensurate Neel antiferromagnet subject to magnetic field. The field tends to lift the degeneracy of the electron spectrum. However, a hidden symmetry protects double degeneracy of Bloch eigenstates at special momenta in the Brillouin zone. The effective transverse g-factor vanishes at such points, thus acquiring a substantial momentum dependence, which turns a textbook Zeeman term into a spin-orbit coupling. After describing the symmetry underpinnings of the Zeeman spin-orbit coupling, I compare it with its intrinsic counterparts such as Rashba coupling, and then show how Zeeman spin-orbit coupling may survive in the presence of intrinsic spin-orbit coupling. Finally, I outline some of the likely experimental manifestations of Zeeman spin-orbit coupling, and compare it with similar phenomena in other settings such as semiconducting quantum wells.

Identifying Two-Dimensional Z 2 Antiferromagnetic Topological Insulators

We revisit the question of whether a two-dimensional topological insulator may arise in a commensurate Néel antiferromagnet, where staggered magnetization breaks the symmetry with respect to both elementary translation and time reversal, but retains their product as a symmetry. In contrast to the so-called Z2 topological insulators, an exhaustive characterization of antiferromagnetic topological phases with the help of topological invariants has been missing. We analyze a simple model of an antiferromagnetic topological insulator and chart its phase diagram, using a recently proposed criterion for centrosymmetric systems [13]. We then adapt two methods, originally designed for paramagnetic systems, and make antiferromagnetic topological phases manifest. The proposed methods apply far beyond the particular examples treated in this work, and admit straightforward generalization. We illustrate this by two examples of non-centrosymmetric systems, where no simple criteria have been known to identify topological phases. We also present, for some cases, an explicit construction of edge states in an antiferromagnetic topological insulator.

Vanishing Cycles and Cartan Eigenvectors

Using the ideas coming from the singularity theory, we study the eigenvectors of the Cartan matrices of finite root systems, and of q-deformations of these matrices

Local injection of pure spin current generates electric current vortices

We show that local injection of pure spin current into an electrically disconnected ferromagnetic–normal-metal sandwich induces electric currents that run along the closed loops inside the device and are powered by the source of the spin injection. Such electric currents may significantly modify voltage distribution in spin-injection devices and induce long-range tails of spin accumulation.

Diagnosing a strong topological insulator by quantum oscillations

We show how quantum oscillation measurements of surface states in an insulator may allow to diagnose a strong topological insulator and distinguish it from its weak or topologically trivial counterpart. The criterion is defined by the parity of the number of fundamental frequencies in the surface-state quantum oscillation spectrum: an even number of frequencies implies a weak or a topologically trivial insulator, whereas an odd number points to a strong topological insulator. We also discuss various aspects and issues related to applying this criterion in practice.