arXiv subjects

Oleg A. Starykh

Publications and source records attributed to Oleg A. Starykh.

At least 37 records · Page 2Linked to original sources

arXiv2015

A Panoply of Orders from a Quantum Lifshitz Field Theory

We propose a universal non-linear sigma model field theory for one dimensional frustrated ferromagnets, which applies in the vicinity of a "quantum Lifshitz point", at which the ferromagnetic state develops a spin wave instability. We investigate the phase diagram resulting from perturbations of the exchange and of magnetic field away from the Lifshitz point, and uncover a rich structure with two distinct regimes of different properties, depending upon the value of a marginal, dimensionless, parameter of the theory. In the regime relevant for one dimensional systems with low spin, we find a metamagnetic transition line to a vector chiral phase. This line terminates in a critical endpoint from which emerges a cascade of multipolar phases. We show that the field theory has the property of "asymptotic solubility", so that a particular saddle point approximation becomes asymptotically exact near the Lifshitz point. Our results provide an analytic framework for prior numerical results on frustrated ferromagnets, and can be applied much more broadly.

cond-mat.str-el↗

arXiv2014

Unusual ordered phases of highly frustrated magnets: a review

We review ground states and excitations of a quantum antiferromagnet on triangular and other frustrated lattices. We pay special attention to the combined effects of magnetic field h, spatial anisotropy R, and spin magnitude S. The focus of the review is on the novel collinear spin density wave and spin nematic states, which are characterized by fully gapped transverse spin excitations with $S^z = \pm 1$. We discuss extensively R-h phase diagram of the antiferromagnet, both in the large-S semiclassical limit and the quantum S=1/2 limit. When possible, we point out connections with experimental findings.

cond-mat.str-el↗

arXiv2014

Phases of triangular lattice antiferromagnet near saturation

We consider 2D Heisenberg antiferromagnets on a triangular lattice with spatially anisotropic interactions in a high magnetic field close to the saturation. We show that this system possess rich phase diagram in field/anisotropy plane due to competition between classical and quantum orders: an incommensurate non-coplanar spiral state, which is favored classically, and a commensurate co-planar state, which is stabilized by quantum fluctuations. We show that the transformation between these two states is highly non-trivial and involves two intermediate phases -- the phase with co-planar incommensurate spin order and the one with non-coplanar double-${\bf Q}$ spiral order. The transition between the two co-planar states is of commensurate- incommensurate type, not accompanied by softening of spin-wave excitations. We show that a different sequence of transitions holds in triangular antiferromagnets with exchange anisotropy, such as Ba3CoSb2O9.

cond-mat.str-el↗

arXiv2013

Excitations and quasi-one-dimensionality in field-induced nematic and spin density wave states

We study the excitation spectrum and dynamical response functions for several quasi-one-dimensional spin systems in magnetic fields without dipolar spin order transverse to the field. This includes both nematic phases, which harbor "hidden" breaking of spin-rotation symmetry about the field and have been argued to occur in high fields in certain frustrated chain systems with competing ferromagnetic and antiferromagnetic interactions, and spin density wave states, in which spin-rotation symmetry is truly unbroken. Using bosonization, field theory, and exact results on the integrable sine-Gordon model, we establish the collective mode structure of these states, and show how they can be distinguished experimentally.

cond-mat.str-el↗

arXiv2013

Spin-current order in anisotropic triangular antiferromagnets

We analyze instabilities of the collinear up-up-down state of a two-dimensional quantum spin-S spatially anisotropic triangular lattice antiferromagnet in a magnetic field. We find, within large-S approximation, that near the end point of the plateau, the collinear state becomes unstable due to condensation of two-magnon bound pairs rather than single magnons. The two-magnon instability leads to a novel 2D vector chiral phase with alternating spin currents but no magnetic order in the direction transverse to the field. This phase breaks a discrete Z_2 symmetry but preserves a continuous U(1) one of rotations about the field axis. It possesses orbital antiferromagnetism and displays a magnetoelectric effect.

cond-mat.str-el↗

arXiv2013

Ground States of Spin-1/2 Triangular Antiferromagnets in a Magnetic Field

We use a combination of density matrix renormalization group calculations and analytical approaches to study a simplified model for a spatially anisotropic spin-1/2 triangular lattice Heisenberg antiferromagnet: the three-leg triangular spin tube (TST). The model is described by three Heisenberg chains, with exchange constant J, coupled antiferromagnetically with exchange constant J' along the diagonals of the ladder system, with periodic boundary conditions in the shorter direction. We determine the full phase diagram of this model as a function of spatial anisotropy, J'/J, and magnetic field. We find a rich phase diagram, which is dominated by quantum states - phases corresponding to the classical ground state appears only in a small region. Among the dominant phases generated by quantum effects are commensurate and incommensurate coplanar quasi-ordered states, which appear in the vicinity of the isotropic region for most fields, and in the high field region for most anisotropies. The coplanar states, while not classical ground states, can be understood semiclassically. Even more strikingly, the largest region of phase space is occupied by a spin density wave phase, which has incommensurate collinear correlations along the field. This phase has no semiclassical analog, and may be ascribed to enhanced one-dimensional (1d) fluctuations due to frustration. Cutting across the phase diagram is a magnetization plateau, with a gap to all excitations and up up down spin order, with a quantized magnetization equal to 1/3 of the saturation value. In the TST, this plateau extends almost but not quite to the decoupled chains limit. Most of the above features are expected to carry over to the two dimensional system, which we also discuss. At low field, a dimerized phase appears, which is particular to the 1d nature of the TST, and which can be understood from quantum Berry phase arguments.

cond-mat.str-el↗

arXiv2012

Half-metallic magnetization plateaux

We propose a novel interaction-based route to half-metal state for interacting electrons on two-dimensional lattices. Magnetic field applied parallel to the lattice is used to tune one of the spin densities to a particular commensurate with the lattice value in which the system spontaneously `locks in' via van Hove enhanced density wave state. Electrons of opposite spin polarization retain their metallic character and provide for the half-metal state which, in addition, supports magnetization plateau in a finite interval of external magnetic field. Similar half-metal state is realized in the finite-U Hubbard model on a triangular lattice at 1/3 of the maximum magnetization.

cond-mat.str-el↗

arXiv2011

Deformed triangular lattice antiferromagnets in a magnetic field: role of spatial anisotropy and Dzyaloshinskii-Moriya interactions

Recent experiments on the anisotropic spin-1/2 triangular antiferromagnet Cs_2CuBr_4 have revealed a remarkably rich phase diagram in applied magnetic fields, consisting of an unexpectedly large number of ordered phases. Motivated by this finding, we study the role of spatial anisotropy, Dzyaloshinskii-Moriya interactions, and quantum fluctuations on the magnetization process of a triangular antiferromagnet, coming from the semiclassical limit. The richness of the problem stems from two key facts: 1) the classical isotropic model exhibits a large accidental ground state degeneracy, and 2) these three ingredients compete with one another and split this degeneracy in opposing ways. Using a variety of complementary approaches, including extensive Monte Carlo numerics, spin-wave theory, and an analysis of Bose-Einstein condensation of magnons at high fields, we find that their interplay gives rise to a complex phase diagram consisting of numerous incommensurate and commensurate phases. Our results shed light on the observed phase diagram for Cs_2CuBr_4 and suggest a number of future theoretical and experimental directions that will be useful for obtaining a complete understanding of this material's interesting phenomenology.

cond-mat.str-el↗

arXiv2011

Interaction Effects in Topological Superconducting Wires Supporting Majorana Fermions

Among the broad spectrum of systems predicted to exhibit topological superconductivity and Majorana fermions, one-dimensional wires with strong spin-orbit coupling provide one of the most promising experimental candidates. Here we investigate the fate of the topological superconducting phase in such wires when repulsive interactions are present. Using a combination of Density Matrix Renormalization Group, bosonization, and Hartree-Fock techniques, we demonstrate that while interactions degrade the bulk gap -consistent with recent results of Gangadharaiah et al.- they also greatly expand the parameter range over which the topological phase arises. In particular, we show that with interactions this phase can be accessed over a broader chemical potential window, thereby leading to greater immunity against disorder-induced chemical potential fluctuations in the wire. We also suggest that in certain wires strong interactions may allow Majorana fermions to be generated without requiring a magnetic field.

cond-mat.supr-con↗

arXiv2010

Extreme sensitivity of a frustrated quantum magnet: Cs_2CuCl_4

We report a thorough theoretical study of the low temperature phase diagram of Cs_2CuCl_4, a spatially anisotropic spin S=1/2 triangular lattice antiferromagnet, in a magnetic field. Our results, obtained in a quasi-one-dimensional limit in which the system is regarded as a set of weakly coupled Heisenberg chains, are in excellent agreement with experiment. The analysis reveals some surprising physics. First, we find that, when the magnetic field is oriented within the triangular layer, spins are actually most strongly correlated within planes perpendicular to the triangular layers. This is despite the fact that the inter-layer exchange coupling in Cs_2CuCl_4 is about an order of magnitude smaller than the weakest (diagonal) exchange in the triangular planes themselves. Second, the phase diagram in such orientations is exquisitely sensitive to tiny interactions, heretofore neglected, of order a few percent or less of the largest exchange couplings. These interactions, which we describe in detail, induce entirely new phases, and a novel commensurate-incommensurate transition, the signatures of which are identified in NMR experiments. We discuss the differences between the behavior of Cs_2CuCl_4 and an ideal two-dimensional triangular model, and in particular the occurrence of magnetization plateaux in the latter. These and other related results are presented here along with a thorough exposition of the theoretical methods, and a discussion of broader experimental consequences to Cs_2CuCl_4 and other materials.

cond-mat.str-el↗

arXiv2010

Phase diagram of the frustrated spin ladder

We re-visit the phase diagram of the frustrated spin-1/2 ladder with two competing inter-chain antiferromagnetic exchanges, rung coupling J_\perp and diagonal coupling J_\times. We suggest, based on the accurate renormalization group analysis of the low-energy Hamiltonian of the ladder, that marginal inter-chain current-current interaction plays central role in destabilizing previously predicted intermediate columnar dimer phase in the vicinity of classical degeneracy line J_\perp = 2J_\times. Following this insight we then suggest that changing these competing inter-chain exchanges from the previously considered antiferromagnetic to the ferromagnetic ones eliminates the issue of the marginal interactions altogether and dramatically expands the region of stability of the columnar dimer phase. This analytical prediction is convincingly confirmed by the numerical density matrix renormalization group and exact diagonalization calculations as well as by the perturbative calculation in the strong rung-coupling limit. The phase diagram for ferromagnetic J_\perp and J_\times is determined.

cond-mat.stat-mech↗

arXiv2008

Spatially Anisotropic Heisenberg Kagome Antiferromagnet

We study the quasi-one-dimensional limit of the spin-1/2 quantum Heisenberg antiferromagnet on the kagome lattice. The lattice is divided into antiferromagnetic spin-chains (exchange J) that are weakly coupled via intermediate "dangling" spins (exchange J'). Using one-dimensional bosonization, renormalization group methods, and current algebra techniques the ground state is determined in the limit J'<<J. We find that the dangling spins and chain spins form a spiral with O(1) and O(J'/J) static moments, respectively, atop of which the chain spins exhibit a smaller O[(J'/J)^2] antiferromagnetically ordered component along the axis perpendicular to the spiral plane.

cond-mat.str-el↗

arXiv2008

Quantum stabilization of 1/3-magnetization plateau in Cs_2CuBr_4

We consider the phase diagram of a spatially anisotropic 2D triangular antiferromagnet in a magnetic field. Classically, the ground state is umbrella-like for all fields, but we show that the quantum phase diagram is much richer and contains a 1/3 magnetization plateau, two commensurate planar states, two incommensurate chiral umbrella phases, and, possibly, a planar state separating the two chiral phases. Our analysis sheds light on several recent experimental findings for the spin-1/2 system Cs_2CuBr_4.

cond-mat.str-el↗

arXiv2008

Spin-orbital effects in magnetized quantum wires and spin chains

We present analysis of the interacting quantum wire problem in the presence of magnetic field and spin-orbital interaction. We show that an interesting interplay of Zeeman and spin-orbit terms, facilitated by the electron-electron interaction, results in the spin-density wave (SDW) state when the magnetic field and spin-orbit axes are orthogonal. We show that this instability is enhanced in a closely related problem of Heisenberg spin chain with asymmetric uniform Dzyaloshinskii-Moriya (DM) interaction. Magnetic field in the direction perpendicular to the DM anisotropy axis results in staggered long-range magnetic order along the orthogonal to the applied field direction. We explore consequences of the uniform DM interaction for the electron spin resonance (ESR) measurements, and point out that they provide way to probe right- and left-moving excitations of the spin chain separately.

cond-mat.mes-hall↗

arXiv2008

Spin-orbit mediated anisotropic spin interaction in interacting electron systems

We investigate interactions between spins of strongly correlated electrons subject to the spin-orbit interaction. Our main finding is that of a novel, spin-orbit mediated anisotropic spin-spin coupling of the van der Waals type. Unlike the standard exchange, this interaction does not require the wave functions to overlap. We argue that this ferromagnetic interaction is important in the Wigner crystal state where the exchange processes are severely suppressed. We also comment on the anisotropy of the exchange between spins mediated by the spin-orbital coupling.

cond-mat.mes-hall↗

arXiv2007

Spinons and triplons in spatially anisotropic frustrated antiferromagnets

The search for elementary excitations with fractional quantum numbers is a central challenge in modern condensed matter physics. We explore the possibility in a realistic model for several materials, the spin-1/2 spatially anisotropic frustrated Heisenberg antiferromagnet in two dimensions. By restricting the Hilbert space to that expressed by exact eigenstates of the Heisenberg chain, we derive an effective Schrödinger equation valid in the weak interchain-coupling regime. The dynamical spin correlations from this approach agree quantitatively with inelastic neutron measurements on the triangular antiferromagnet Cs_2CuCl_4. The spectral features in such antiferromagnets can be attributed to two types of excitations: descendents of one-dimensional spinons of individual chains, and coherently propagating "triplon" bound states of spinon pairs. We argue that triplons are generic features of spatially anisotropic frustrated antiferromagnets, and arise because the bound spinon pair lowers its kinetic energy by propagating between chains.

cond-mat.str-el↗

arXiv2007

Spin-orbit induced spin-density wave in a quantum wire

We present analysis of the interacting quantum wire problem in the presence of magnetic field and spin-orbit interaction. We show that an interesting interplay of Zeeman and spin-orbit terms, facilitated by the electron-electron interaction, results in the spin-density wave (SDW) state when the magnetic field and spin-orbit axes are orthogonal. This strongly affects charge transport through the wire: with SDW stabilized, single particle backscattering off an nonmagnetic impurity becomes irrelevant. Sensitivity of the effect to the direction of the magnetic field can be used for experimental verification of this proposal.

cond-mat.str-el↗

arXiv2007

Ordering in spatially anisotropic triangular antiferromagnets

We investigate the phase diagram of the anisotropic spin-1/2 triangular lattice antiferromagnet, with interchain diagonal exchange J' much weaker than the intrachain exchange J. We find that fluctuations lead to a competition between (commensurate) collinear antiferromagnetic and (zig-zag) dimer orders. Both states differ in symmetry from the spiral order known to occur for larger J', and are therefore separated by quantum phase transitions from it. The zero-field collinear antiferromagnet is succeeded in a magnetic field by magnetically-ordered spin-density-wave and cone phases, before reaching the fully polarized state. Implications for the anisotropic triangular magnet Cs_2CuCl_4 are discussed.

cond-mat.str-el↗