arXiv subjects

A. M. Tsvelik

Publications and source records attributed to A. M. Tsvelik.

At least 55 records · Page 3Linked to original sources

arXiv2016

Universality classes of order parameters composed of many body bound states

This theoretical paper discusses microscopic models giving rise to special types of order in which conduction electrons are bound together with localized spins to create composite order parameters. It is shown that composite order is related to the formation of a spin liquid with gapped excitations carrying quantum numbers which are a fraction of those of electron. These spin liquids are special in the sense that their formation necessarily involves spin degrees of freedom of both the conduction and the localized electrons. A detailed description of such spin liquid states is presented with a special emphasize on a demonstration of their robustness against local perturbations preserving the Lie group symmetry and the translational invariance.

cond-mat.str-el↗

arXiv2016

Fractionalized Fermi Liquid in a Kondo-Heisenberg Model

The Kondo-Heisenberg model is used for a microscopic demonstration of existence of a peculiar metallic state with unbroken translational symmetry where the Fermi surface volume is not controlled by the total electron density. I use a non-perturbative approach where the strongest interactions are taken into account by means of exact solution, and corrections are controllable. In agreement with the general requirements formulated in (T. Senthil {\it et.al.} Phys. Rev. Lett. {\bf 90}, 216403 (2003)), the resulting metallic state represents a fractionalized Fermi liquid where well defined quasiparticles coexist with gapped fractionalized collective excitations. The system undergoes a phase transition to an ordered phase (charge density wave or superconducting), at the transition temperature which is parametrically small in comparison to the quasiparticle Fermi energy.

cond-mat.str-el↗

arXiv2016

Particle Formation and Ordering in Strongly Correlated Fermionic Systems: Solving a Model of Quantum Chromodynamics

In this paper we study a (1+1)-dimensional version of the famous Nambu-Jona-Lasinio model of Quantum Chromodynamics (QCD2) both at zero and finite hadron density. We use non-perturbative techniques (non-Abelian bosonization and Truncated Conformal Space Approach). At zero density we describe a formation of fermion three-quark (nucleons and $Δ$-baryons) and boson (two-quark mesons, six-quark deuterons) bound states and also a formation of a topologically nontrivial phase. At finite hadron density, the model has a rich phase diagram which includes phases with density wave and superfluid quasi-long-range (QLR) order and also a phase of a baryon Tomonaga-Luttinger liquid (strange metal). The QLR order results as a condensation of scalar mesons (the density wave) or six-quark bound states (deuterons).

hep-th↗

arXiv2016

Orbital-Exchange and Fractional Quantum Number Excitations in an f-electron Metal Yb$_2$Pt$_2$Pb

Exotic quantum states and fractionalized magnetic excitations, such as spinons in one-dimensional chains, are generally viewed as belonging to the domain of 3d transition metal systems with spins 1/2. Our neutron scattering experiments on the 4f-electron metal Yb$_2$Pt$_2$Pb overturn this common wisdom. We observe broad magnetic continuum dispersing in only one direction, which indicates that the underlying elementary excitations are spinons carrying fractional spin-1/2. These spinons are the quantum dynamics of the anisotropic, orbital-dominated Yb moments, and thus these effective quantum spins are emergent variables that encode the electronic orbitals. The unique birthmark of their unusual origin is that only longitudinal spin fluctuations are measurable, while the transverse excitations such as spin waves are virtually invisible to magnetic neutron scattering. The proliferation of these orbital-spinons strips the electrons of their orbital identity, and we thus report here a new electron fractionalization phenomenon, charge-orbital separation.

cond-mat.str-el↗

arXiv2016

Heisenberg Necklace Model in Magnetic Field

Motivated by the experimental realizations of nearly one-dimensional spin-1/2 Heisenberg model found in chain cuprates SrCuO$_2$ and Sr$_2$CuO$_3$, which remain in a quantum-critical Luttinger liquid state down to temperatures that are much lower than in-chain exchange coupling, we consider the perturbation to this state caused by interactions with nuclear spins on the same sites. We study the low-energy sector of the Heisenberg Necklace model and estimate the effect of the coupling between the nuclear and the electronic spins on the overall spins dynamics and its dependence on the magnetic field. We find that the Necklace model has a characteristic energy scale, $Λ\sim J^{1/3}(γI)^{2/3}$, at which the coupling between spins of the necklace and the spins of the Heisenberg chain becomes strong. In the strong magnetic field $μ_B B > Λ$ the low energy spectrum is gapless, but two gapless bosonic modes have different velocities whose ratio at strong fields approaches a universal number, $\sqrt 2 +1$. In the case of Sr$_2$CuO$_3$ the energy scale $Λ$ is sizable and comparable to the Neel ordering temperature induced by the inter-chain coupling, and thus could noticeably modify the low temperature magnon dynamics. We further find that the above energy scale is insensitive to strong magnetic field, $μ_B B \gg Λ\sim J^{1/3}(γI)^{2/3}$, and therefore the interaction with nuclear spins cannot lead to unusually strong magnetic field dependence of the magnon spectrum observed by ESR in Sr$_2$CuO$_3$, which has been attributed to the magnon interaction with the Higgs mode.

cond-mat.str-el↗

arXiv2016

Probing Strong Correlations with Light Scattering: the Example of the Quantum Ising model

In this paper we calculate the nonlinear susceptibility and the resonant Raman cross section for the paramagnetic phase of the ferromagnetic Quantum Ising model in one dimension. In this region the spectrum of the Ising model has a gap $m$. The Raman cross section has a strong singularity when the energy of the outgoing photon is at the spectral gap $ω_{f} \approx m$ and a square root threshold when the frequency difference between the incident and outgoing photons $ω_{i} -ω_{f} \approx 2m$. The latter feature reflects the fermionic nature of the Ising model excitations.

cond-mat.other↗

arXiv2016

Kondo physics from quasiparticle poisoning in Majorana devices

We present a theoretical analysis of quasiparticle poisoning in Coulomb-blockaded Majorana fermion systems tunnel-coupled to normal-conducting leads. Taking into account finite-energy quasiparticles, we derive the effective low-energy theory and present a renormalization group analysis. We find qualitatively new effects when a quasiparticle state with very low energy is localized near a tunnel contact. For $M=2$ attached leads, such "dangerous" quasiparticle poisoning processes cause a spin $S=1/2$ single-channel Kondo effect, which can be detected through a characteristic zero-bias anomaly conductance peak in all Coulomb blockade valleys. For more than two attached leads, the topological Kondo effect of the unpoisoned system becomes unstable. A strong-coupling bosonization analysis indicates that at low energy the poisoned lead is effectively decoupled and hence, for $M>3$, the topological Kondo fixed point re-emerges, though now it involves only $M-1$ leads. As a consequence, for $M=3$, the low-energy fixed point becomes trivial corresponding to decoupled leads.

cond-mat.mes-hall↗

arXiv2015

Low Energy Properties of the Kondo chain in the RKKY regime

We study the Kondo chain in the regime of high spin concentration where the low energy physics is dominated by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. As has been recently shown (A. M. Tsvelik and O. M. Yevtushenko, Phys. Rev. Lett 115, 216402 (2015)), this model has two phases with drastically different transport properties depending on the anisotropy of the exchange interaction. In particular, the helical symmetry of the fermions is spontaneously broken when the anisotropy is of the easy plane type (EP). This leads to a parametrical suppression of the localization effects. In the present paper we substantially extend the previous theory, in particular, by analyzing a competition of forward- and backward- scattering, including into the theory short range electron interactions and calculating spin correlation functions. We discuss applicability of our theory and possible experiments which could support the theoretical findings.

cond-mat.mes-hall↗

arXiv2015

Quantum Phase Transition and Protected Ideal Transport in a Kondo Chain

We study the low energy physics of a Kondo chain where electrons from a one-dimensional band interact with magnetic moments via an anisotropic exchange interaction. It is demonstrated that the anisotropy gives rise to two different phases which are separated by a quantum phase transition. In the phase with easy plane anisotropy, Z$_2$ symmetry between sectors with different helicity of the electrons is broken. As a result, localization effects are suppressed and the dc transport acquires (partial) symmetry protection. This effect is similar to the protection of the edge transport in time-reversal invariant topological insulators. The phase with easy axis anisotropy corresponds to the Tomonaga-Luttinger liquid with a pronounced spin-charge separation. The slow charge density wave modes have no protection against localizatioin.

cond-mat.mes-hall↗

arXiv2015

Studying the Perturbed Wess-Zumino-Novikov-Witten SU(2)k Theory Using the Truncated Conformal Spectrum Approach

We study the $SU(2)_k$ Wess-Zumino-Novikov-Witten (WZNW) theory perturbed by the trace of the primary field in the adjoint representation, a theory governing the low-energy behaviour of a class of strongly correlated electronic systems. While the model is non-integrable, its dynamics can be investigated using the numerical technique of the truncated conformal spectrum approach combined with numerical and analytical renormalization groups (TCSA+RG). The numerical results so obtained provide support for a semiclassical analysis valid at $k\gg 1$. Namely, we find that the low energy behavior is sensitive to the sign of the coupling constant, $λ$. Moreover for $λ>0$ this behavior depends on whether $k$ is even or odd. With $k$ even, we find definitive evidence that the model at low energies is equivalent to the massive $O(3)$ sigma model. For $k$ odd, the numerical evidence is more equivocal, but we find indications that the low energy effective theory is critical.

cond-mat.str-el↗

arXiv2015

Majorana boundary modes in Josephson junctions arrays with gapless bulk excitations

The search for Majorana bound states in solid-state physics has been limited to materials which display a gap in their bulk spectrum. We show that such unpaired states appear in certain quasi-one-dimensional Josephson junctions arrays with gapless bulk excitations. The bulk modes mediate a coupling between Majorana bound states via the Ruderman-Kittel-Yosida-Kasuya mechanism. As a consequence, the lowest energy doublet acquires a finite energy difference. For realistic set of parameters this energy splitting remains much smaller than the energy of the bulk eigenstates even for short chains of length $L \sim 10$.

cond-mat.mes-hall↗

arXiv2015

Two-leg SU(2n) spin ladder: A low-energy effective field theory approach

We present a field theory analysis of a model of two SU(2n)-invariant magnetic chains coupled by a generic interaction preserving time reversal and inversion symmetry. Contrary to the SU(2)-invariant case the zero-temperature phase diagram of such two-leg spin ladder does not contain topological phases. Only generalized Valence Bond Solid phases are stabilized when n>1 with different wave vectors and ground-state degeneracies. In particular, we find a phase which is made of a cluster of 2n spins put in an SU(2n) singlet state. For n=3, this cluster phase is relevant to ytterbium ultracold atoms, with an emergent SU(6) symmetry, loaded in double well optical lattice.

cond-mat.str-el↗

arXiv2014

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

I discuss a one-dimensional model of interacting fermions which collective excitations are Z$_N$-parafermions. 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.

cond-mat.str-el↗

arXiv2014

Multi-channel Kondo impurity dynamics in a Majorana device

We study the multi-channel Kondo impurity dynamics realized in a mesoscopic superconducting island connected to metallic leads. The effective 'impurity spin' is non-locally realized by Majorana bound states and strongly coupled to lead electrons by non-Fermi liquid correlations. We explore the spin dynamics and its observable ramifications near the low-temperature fixed point. The topological protection of the system raises the perspective to observe multi-channel Kondo impurity dynamics in experimentally realistic environments.

cond-mat.mes-hall↗

arXiv2014

Composite charge order in the pseudogap region of the cuprates

We study the Ginzburg-Landau free energy functional for two coupled U(1) charge order parameters describing two non-equivalent charge orders with wave vector ${\bf Q}$ detected in X-ray and STM measurements of underdoped cuprates. We do not rely on a mean-field analysis, but rather utilize a field-theoretical technique suitable to study the interplay between vortex physics and discrete symmetry breaking in two-dimensional systems with U(1) symmetry. Our calculations support the idea that in the clean systems there are two transitions: from a high temperature disordered state into a state with a composite charge order which breaks time-reversal symmetry, but leaves U(1) fields disordered, and then into a state with quasi long range order in the U(1) fields.

cond-mat.supr-con↗

arXiv2014

An integrable model with parafermion zero energy modes

Parafermion zero energy modes are a vital element of fault-tolerant topological quantum computation. Although it is believed that such modes form on the border between topological and normal phases, this has been demonstrated only for Z$_2$ (Majorana) and Z$_3$ parafermions. I consider an integrable model where such demonstration is possible for Z$_N$ parafermions with any $N$. The procedure is easely generalizable for more complicated symmetry groups.

cond-mat.str-el↗

arXiv2014

Incoherent Soliton Excitations and Spin-Charge Separation in Blue Bronze

We present new high resolution angle resolved photoemission (ARPES) data for K$_{0.3}$MoO$_3$ (blue bronze) and propose a novel theoretical description of these results. The observed Fermi surface, with two quasi-one-dimensional sheets, is consistent with a ladder material with a weak inter-ladder coupling. Hence, we base our description on spectral properties of one-dimensional ladders. The marked broadening of the ARPES lineshape, a significant fraction of an eV, is interpreted in terms of spin-charge separation. A high energy feature, which is revealed for the first time in the spectra near the Fermi momentum thanks to improved energy resolution, is seen as a signature of a higher energy bound state of soliton excitations on a ladder.

cond-mat.str-el↗

arXiv2014

Bethe ansatz solution of the topological Kondo model

Conduction electrons coupled to a mesoscopic superconducting island hosting Majorana bound states have been shown to display a topological Kondo effect with robust non-Fermi liquid correlations. With $M$ bound states coupled to $M$ leads, this is an SO($M$) Kondo problem, with the asymptotic high and low energy theories known from bosonization and conformal field theory studies. Here we complement these approaches by analyzing the Bethe ansatz equations describing the exact solution of these models at all energy scales. We apply our findings to obtain nonperturbative results on the thermodynamics of $M\rightarrow M-2$ crossovers induced by tunnel couplings between adjacent Majorana bound states.

cond-mat.str-el↗