arXiv subjects

Revaz Ramazashvili

Publications and source records attributed to Revaz Ramazashvili.

17 recordsLinked to original sources

arXiv2024

Electron states, bound to a texture in a Néel antiferromagnet

We study electron states, bound to topological textures such as skyrmions and domain walls in a Néel antiferromagnet. In certain limits, we find the dependence of bound states on the geometry of the texture, and estimate the bound-state contribution to its energy. This contribution proves significant compared with the purely magnetic energy, and substantially affects the equilibrium geometry of the texture. The bound-state contribution also induces a large shift of the transition line between the modulated and the uniform phase, extending the latter.

cond-mat.str-el↗

arXiv2023

Texture-induced spin-orbit coupling and Skyrmion-electron bound states in a Néel antiferromagnet

We derive effective-mass electron Hamiltonian for a Néel antiferromagnet in the presence of a smooth texture of the staggered magnetization. For certain locations of electron band extrema, the texture produces a peculiar and anomalously strong spin-orbit coupling of the scale $\hbar v / L$, with $v$ the Fermi velocity and $L$ the characteristic length scale of the texture. For a Skyrmion texture, this coupling generates electron bound states, whose energy scale is given by the gap $Δ$ in the electron spectrum. With dopant carriers, such bound states turn the Skyrmion into a charged particle, that can be manipulated by electric field.

cond-mat.str-el↗

arXiv2018

Zeeman spin-orbit coupling in antiferromagnetic conductors

This article is a brief review of Zeeman spin-orbit coupling, arising in a low-carrier commensurate Néel 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.

cond-mat.str-el↗

arXiv2016

Vanishing cycles and Cartan eigenvectors

Using the vanishing cycles of simple singularities, we study the eigenvectors of Cartan matrices of finite root systems, and of q-deformations of these matrices.

math.GR↗

arXiv2016

$Z_2$ antiferromagnetic topological insulators with broken $C_4$ symmetry

A two-dimensional topological insulator may arise in a centrosymmetric commensurate Néel antiferromagnet (AF), where staggered magnetization breaks both the elementary translation and time reversal, but retains their product as a symmetry. Fang et al.[Phys. Rev. B 88, 085406 (2013)] 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.

cond-mat.str-el↗

arXiv2016

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 both the elementary translation and time reversal, but retains their product as a symmetry. In contrast to the so-called $Z_2$ topological insulators, an exhaustive characterization of antiferromagnetic topological phases with the help of a topological invariant has been missing. We analyze a simple model of an antiferromagnetic topological insulator and chart its phase diagram based on a recently proposed criterion for centrosymmetric systems [Fang et al., Phys. Rev. B 88, 085406 (2013)]. We then adapt two methods, originally designed for paramagnetic systems, and make antiferromagnetic topological phases manifest. The proposed methods apply far beyond the particular example treated in this work, and admit straightforward generalization. We illustrate this by considering a non-centrosymmetric system, where there are no simple criteria to identify topological phases. We also present an explicit construction of edge states in an antiferromagnetic topological insulator.

cond-mat.str-el↗

arXiv2015

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.

cond-mat.str-el↗

arXiv2010

Quantum oscillations in antiferromagnetic conductors with small carrier pockets

I study magnetic quantum oscillations in antiferromagnetic conductors with small carrier pockets and show that combining the oscillation data with symmetry arguments and with the knowledge of the possible positions of the band extrema may allow us to greatly constrain or even uniquely determine the location of a detected carrier pocket in the Brillouin zone.

cond-mat.str-el↗

arXiv2009

Electric excitation of spin resonance in antiferromagnetic conductors

Antiferromagnetism couples electron spin to its orbital motion, thus allowing excitation of electron-spin transitions by an ac electric rather than magnetic field - with absorption, exceeding that of common electron spin resonance at least by four orders of magnitude. In addition to potential applications in spin electronics, this phenomenon may be used as a spectroscopy to study antiferromagnetic materials of interest - from chromium to borocarbides, cuprates, iron pnictides, and organic and heavy fermion conductors.

cond-mat.str-el↗

arXiv2009

Kramers degeneracy in a magnetic field and Zeeman spin-orbit coupling in antiferromagnets

In this article, I analyze the symmetries and degeneracies of electron eigenstates in a commensurate collinear antiferromagnet. In a magnetic field transverse to the staggered magnetization, a hidden anti-unitary symmetry protects double degeneracy of the Bloch eigenstates at a special set of momenta. In addition to this `Kramers degeneracy' subset, the manifold of momenta, labeling the doubly degenerate Bloch states in the Brillouin zone, may also contain an `accidental degeneracy' subset, that is not protected by symmetry and that may change its shape under perturbation. These degeneracies give rise to a substantial momentum dependence of the transverse g-factor in the Zeeman coupling, turning the latter into a spin-orbit interaction. I discuss a number of materials, where Zeeman spin-orbit coupling is likely to be present, and outline the simplest properties and experimental consequences of this interaction, that may be relevant to systems from chromium to borocarbides, cuprates, hexaborides, iron pnictides, as well as organic and heavy fermion conductors.

cond-mat.str-el↗

arXiv2008

Kramers degeneracy in a magnetic field and Zeeman spin-orbit coupling in antiferromagnetic conductors

In this article, I study magnetic response of electron wavefunctions in a commensurate collinear antiferromagnet. I show that, at a special set of momenta, hidden anti-unitary symmetry protects Kramers degeneracy of Bloch eigenstates against a magnetic field, pointing transversely to staggered magnetization. Hence a substantial momentum dependence of the transverse g-factor in the Zeeman term, turning the latter into a spin-orbit coupling, that may be present in materials from chromium to borocarbides, cuprates, pnictides, as well as organic and heavy fermion conductors.

cond-mat.str-el↗

arXiv2004

Broken particle-hole symmetry at atomically flat a-axis YBa2Cu3O7-d interfaces

We have studied quasiparticle tunneling into atomically flat a-axis films of YBa2Cu3O7-d and DyBa2Cu3O7-d through epitaxial CaTiO3 barriers. The junction heterostructures were grown by oxide molecular beam epitaxy and were carefully optimized using in-situ monitoring techniques, resulting in unprecedented crystalline perfection of the superconductor/insulator interface. Below Tc, the tunneling conductance shows the evolution of a large unexpected asymmetrical feature near zero bias. This is evidence that superconducting YBCO crystals, atomically truncated along the lobe direction with a titanate layer, have intrinsically broken particle-hole symmetry over macroscopically large areas.

cond-mat.supr-con↗

arXiv2002

Ward identities for disordered metals and superconductors

This article revisits Ward identities for disordered interacting normal metals and superconductors. It offers a simple derivation based on gauge invariance and recasts the identities in a new form that allows easy analysis of the quasiparticle charge conservation (as e.g. in a normal metal) or non-conservation (as e.g. in a d-wave superconductor).

cond-mat.dis-nn↗

arXiv2000

Level correlations in disordered superconducting grains

I study the quasiparticle level correlations in a grain of a weakly disordered d-wave superconductor, and show that, in a wide intermediate energy range, they are characterized by a novel type of universal behavior.

cond-mat.str-el↗

arXiv1999

Quantum Lifshitz Point

I study a quantum Lifshitz point in a three-dimensional itinerant antiferromagnet, in particular the scaling of the Néel temperature, the correlation length, the staggered susceptibility, the specific heat coefficient and the resistivity. At low temperatures, the model is shown to have the inverse staggered susceptibility and the resistivity varying as T$^{5/4}$, and the specific heat coefficient varying as T$^{1/4}$.

cond-mat.str-el↗

arXiv1997

Anomalous Behavior at a Superconducting Quantum Critical Point

Motivated by pressure experiments on UBe_{13} and Tl_{2}Ba_{2}CaCu_{2}O_8, we discuss low-temperature effects of the pairing interaction at a superconducting quantum critical point in a clean system. We point out that measurements at this quantum critical point can provide a diagnostic tool to mark out non-BCS mechanisms of superconductivity.

cond-mat↗

arXiv1997

Superconducting Quantum Critical Point

We study the properties of a quantum critical point which develops in a BCS superconductor when pair-breaking suppresses the transition temperature to zero. The pair fluctuations are characterized by a dynamical critical exponent z=2. Except for very low temperatures, anomalous contribution to the conductivity is proportional to the square root of T in three dimensions, but to 1/T in two dimensions. At lowest temperatures, the conductivity correction varies as T to the power 1/4 in three dimensions, and as ln(1/T) in two.

cond-mat↗