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Alexei Tsvelik

Brookhaven National Laboratory, Condensed Matter Physics, Faculty Member

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Papers by Alexei Tsvelik

Spectral properties of the one-dimensional two-channel Kondo lattice model

Journal of Magnetism and Magnetic Materials, 2001

We have studied the energy spectrum of a one-dimensional Kondo lattice, where the localized magne... more We have studied the energy spectrum of a one-dimensional Kondo lattice, where the localized magnetic moments have SU(N) symmetry and two channels of conduction electrons are present. At half filling, the system is shown to exist in two phases: one dominated by RKKY-exchange interaction effects, and the other by Kondo screening. A quantum phase transition point separates these two regimes at temperature T = 0. The Kondo-dominated phase is shown to possess soft modes, with spectral gaps much smaller than the Kondo temperature.

Can magnetic field transform a superconductor into a Bose metal

arXiv (Cornell University), May 5, 2020

We discuss whether a simple theory of superconducting stripes coupled by Josephson tunneling can ... more We discuss whether a simple theory of superconducting stripes coupled by Josephson tunneling can describe a metallic transport, once the coherent tunneling of pairs is suppressed by the magnetic field. For a clean system, the conclusion we reached is negative: the excitation spectrum of preformed pairs consists of Landau levels, and once the magnetic field exceeds a critical value, the transport becomes insulating. As a speculation, we suggest that a Bose metal can exist in disordered systems provided that the disorder is strong enough to localize some pairs. Then the coupling between propagating and localized pairs broadens the Landau levels, resulting in a metallic conductivity. Our model respects the particle-hole symmetry, which leads to a zero Hall response. And intriguingly, the resulting anomalous metallic state has no Drude peak and the spectral weight of the cyclotron resonance vanishes at low temperatures.

format_quoteThe study identifies a peculiar metallic state with zero Hall response that arises after superconductivity is suppressed.format_quote

Confinement and deconfinement of spinons in a frustrated spin-12Heisenberg model

Physical Review B, 2004

In this publication I discuss the phase diagram of a frustrated spin-1/2 Heisenberg model suggest... more In this publication I discuss the phase diagram of a frustrated spin-1/2 Heisenberg model suggested in A. A. Nersesyan and A. M. Tsvelik, Phys. Rev. B67, 024422 (2003). The phase diagram contains (π, 0) and (π, π) antiferromagnetic phases separated by the Valence Bond Crystal (VBC) state. I argue that the point of the phase diagram with deconfined spinons, predicted in the aforementioned work, is situated in the middle of VBC state, at the point where the dimerization order parameter changes sign.

format_quoteTwo scenarios for deconfined spinons in the CF model are ultimately unachievable, indicating model uniqueness versus literature norms.format_quote

Anisotropic spin-1/2 Heisenberg chain with open boundary conditions

Physical Review B, 1995

We study the exact solution of the anisotropic spin-1/2 Heisenberg chain with a boundary magnetic... more We study the exact solution of the anisotropic spin-1/2 Heisenberg chain with a boundary magnetic field in the region where the bulk excitations are gapless. It is shown that near the boundary a bound state is created which is underscreened when γ < π/3 and which at low temperatures behaves like a single spin weakly coupled to the bulk. The IR fixed point in this case belongs to the universality class of the underscreened anisotropic Kondo model. We argue that the same fixed point appears in the boundary sine-Gordon model when the scaling dimension of the boundary term 2/3 < ∆ < 1.

Instabilities of the Abrikosov-Suhl resonance

Physica B: Condensed Matter, 1994

We consider the possibility that instabilities of the Abrikosov-Suhl resonance lead to new fixed ... more We consider the possibility that instabilities of the Abrikosov-Suhl resonance lead to new fixed point behavior of the Kondo effect in a lattice environment. In one scenario, a pairing component to the resonant scattering develops in the Kondo lattice, leading to an odd frequency superconductor. We discuss experiments that can discriminate between this picture and d-wave pairing, and its relationship to the non Fermi liquid fixed point of the overscreened Kondo model.

New developments in the theoretical treatment of low dimensional strongly correlated systems

Reports on progress in physics. Physical Society (Great Britain), Jan 9, 2017

We review two important non-perturbative approaches for extracting the physics of low- dimensiona... more We review two important non-perturbative approaches for extracting the physics of low- dimensional strongly correlated quantum systems. Firstly, we start by providing a comprehensive review of non-Abelian bosonization. This includes an introduction to the basic elements of confor- mal field theory as applied to systems with a current algebra, and we orient the reader by presenting a number of applications of non-Abelian bosonization to models with large symmetries. We then tie this technique into recent advances in the ability of cold atomic systems to realize complex symme- tries. Secondly, we discuss truncated spectrum methods for the numerical study of systems in one and two dimensions. For one-dimensional systems we provide the reader with considerable insight into the methodology by reviewing canonical applications of the technique to the Ising model (and its variants) and the sine-Gordon model. Following this we review recent work on the development of renormalization groups, ...

Anomalous Hall effect at the Lifshitz transition in ZrTe5

Physical Review B

Zirconium pentatelluride ZrTe5 is a topological semimetal. The presence of a temperature induced ... more Zirconium pentatelluride ZrTe5 is a topological semimetal. The presence of a temperature induced Lifshitz transition, in which the Fermi level goes from the conduction band to the valence band with increasing temperature, provides unique opportunities to study the interplay between Fermi-surface topology, dynamics of Dirac fermions, and Berry curvature in one system. Here we present a combined experimental and theoretical study and show that a low energy model can be used to understand the complicated Hall response and large anomalous Hall effect observed in ZrTe5 over a wide range of temperature and magnetic field. We found that the the anomalous Hall contribution dominates the Hall response in a narrow temperature window around the Lifshitz transition, away from which the orbital contribution dominates. Moreover, our results indicate that a topological phase transition coexists with the Lifshitz transition. Our model provides a unifying framework to understand the Hall effect in semimetals with large Zeeman splitting and non-trivial topology.

Novel Magnetism from the Spin-Orbit-Coupling Strength Alternation

Magnetic and Orbital Excitations in Sr $ _3 $ CuIrO $ _6$

We report resonant inelastic x-ray scattering studies of the one-dimensional Sr 3 CuIrO 6 at the ... more We report resonant inelastic x-ray scattering studies of the one-dimensional Sr 3 CuIrO 6 at the Ir L 3 edge, with ∆E=40 meV. At high energies, we find peaks at 0.6 eV, 0.9 eV, 3 eV, 4 eV and 6 eV. These peaks are non-dispersive, well-defined excitations. On the basis of quantum chemistry calculations, we are able to identify the first two as excitations within the t 2g manifold and the next two as between the t 2g and e g manifolds, together with charge transfer excitations from the O 2p to the Ir 5d. The 6 eV feature is another t 2g-e g excitation. From these we are able to determine both the spin-orbit and non-cubic crystal field splittings. We find that they are of comparable strength. In addition, magnetic excitations are observed, corresponding to excitations of the j 1/2 isospin. We find these are highly dispersive along the chain direction with a bandwidth of 20 meV and a gap of 30 meV. These results allow a complete modeling of the spin and orbital degrees of freedom in this model compound and we conclude an atomic, spin-orbit coupled description works well.

Anomalies in the pseudogap phase of the cuprates: competing ground states and the role of umklapp scattering

Reports on Progress in Physics, 2019

Over the past two decades, advances in computational algorithms have revealed a curious property ... more Over the past two decades, advances in computational algorithms have revealed a curious property of the two-dimensional Hubbard model (and related theories) with hole doping: the presence of close-in-energy competing ground states that display very different physical properties. On the one hand, there is a complicated state exhibiting intertwined spin, charge, and pair density wave orders. We call this 'type A'. On the other hand, there is a uniform d-wave superconducting state that we denote as 'type B'. We advocate, with the support of both microscopic theoretical calculations and experimental data, dividing the high-temperature cuprate superconductors into two corresponding families, whose properties reflect either the type A or type B ground states at low temperatures. We review the anomalous properties of the pseudogap phase that led us to this picture, and present a modern perspective on the role that umklapp scattering plays in these phenomena in the type B materials. This reflects a consistent framework that has emerged over the last decade, in which Mott correlations at weak coupling drive the formation of the pseudogap. We discuss this development, recent theory and experiments, and open issues.

$Anomalies in the pseudogap phase of the cuprates Figure 18. (a) The simulated dispersion for d-wave homogeneous superconductivity with order parameter V = 30 meV. The quasiparticle energy for a given momentum state k is given by E(k) = \/[e(k)]? + [A(k)]?. Cuts are along (z,—7) — (a,0) — (a,7). The red (blue) curve is for the true normal (gapped) state. The spectral weight is indicated by the curve thickness. The back-bending (or saturation) of the dispersion and kp are indicated in the panel. Note that the back-bending momentum in the gapped state remains aligned with ke. (b) Summary of the experimentally measured intensity maximum dispersions at different temperatures. (c) Simulation of the experimental dispersion with and without the pseudogap within the framework of the YRZ model with short-range spin correlations (Kidd et al. unpublished).$

Z $ _N $ parafermion zero modes without Fractional Quantum Hall effect

I discuss a one-dimensional model of interacting fermions which collective excitations are ZNpara... more I discuss a one-dimensional model of interacting fermions which collective excitations are ZNparafermions. The phase diagram of this model contains ground states with Charge Density Wave and superconducting quasi long range order. Boundaries between these phases contain zero energy parafermion modes.

format_quoteSmall deviations from integrability led to identifying relevant operators, linking to non-Abelian excitations in the model.format_quote

Superconductivity and charge-density waves in a quasi-one-dimensional spin-gap system

Physical Review B, 2002

We consider a model of spin-gapped chains weakly coupled by Josephson and Coulomb interactions. C... more We consider a model of spin-gapped chains weakly coupled by Josephson and Coulomb interactions. Combining such non-perturbative methods as bosonization and Bethe ansatz to treat the intra-chain interactions with the Random Phase Approximation for the inter-chain couplings and the first corrections to this, we investigate the phase diagram of this model. The phase diagram shows both charge density wave ordering and superconductivity. These phases are separated by line of critical points which exhibits an approximate an SU(2) symmetry. We consider the effects of a magnetic field on the system. We apply the theory to the material Sr2Ca12Cu24O41 and suggest further experiments.

format_quoteCalculated phase transition temperature interacts critically with inter-chain couplings; instability occurs at K_c = 1 with equal bare susceptibilities for CDW and superconductivity.format_quote

Coexistence of orbital degeneracy lifting and superconductivity in iron-based superconductors

Physical Review B, 2014

In contrast to conventional superconducting (SC) materials, superconductivity in high-temperature... more In contrast to conventional superconducting (SC) materials, superconductivity in high-temperature superconductors (HTCs) usually emerges in the presence of other fluctuating orders with similar or higher energy scales 1-5 , thus instigating debates over their relevance for the SC pairing mechanism. In iron-based superconductors (IBSCs), local orbital fluctuations have been proposed to be directly responsible for the structural phase transition 6,7 and closely related to the observed giant magnetic anisotropy and electronic nematicity 8-12. However, whether superconductivity can emerge from, or even coexist with orbital fluctuations, remains unclear. Here we report the angle-resolved photoemission spectroscopy (ARPES) observation of the lifting of symmetry-protected band

format_quoteAnalysis shows connection between ferro-orbital correlations and superconductivity in Li(Fe 1-x Co x )As.format_quote

Realization of Haldane's Exclusion Statistics in a Model of Electron-Phonon Interactions

Physical Review Letters, 1999

We discuss an integrable model describing one-dimensional electrons interacting with twodimension... more We discuss an integrable model describing one-dimensional electrons interacting with twodimensional anharmonic phonons. In the low temperature limit it is possible to decouple phonons and consider one-dimensional excitations separately. They have a trivial two-body scattering matrix and obey fractional statistics. As far as we know the original model presents the first example of a model with local bare interactions generating purely statistical interactions between renormalized particles. As a by-product we obtain non-trivial thermodynamic equations for the interacting system of two-dimensional phonons.

Long-range magnetic ordering in Na2IrO3

Physical Review B, 2011

We report a combined experimental and theoretical investigation of the magnetic structure of the ... more We report a combined experimental and theoretical investigation of the magnetic structure of the honeycomb lattice magnet Na2IrO3, a candidate for a realization of a gapless spin-liquid. Using resonant x-ray magnetic scattering at the Ir L3-edge, we find 3D long range antiferromagnetic order below TN =13.3 K. From the azimuthal dependence of the magnetic Bragg peak, the ordered moment is determined to be predominantly along the a-axis. Combining the experimental data with first principles calculations, we propose that the most likely spin structure is a "zig-zag" structure.