arXiv subjects

A. S. Mishchenko

Publications and source records attributed to A. S. Mishchenko.

48 records · Page 3Linked to original sources

arXiv2003

Optical conductivity of the Frohlich polaron

We present accurate results for optical conductivity of the three dimensional Frohlich polaron in all coupling regimes. The systematic-error free diagrammatic quantum Monte Carlo method is employed where the Feynman graphs for the momentum-momentum correlation function in imaginary time are summed up. The real-frequency optical conductivity is obtained by the analytic continuation with stochastic optimization. We compare numerical data with available perturbative and non-perturbative approaches to the optical conductivity and show that the picture of sharp resonances due to relaxed excited states in the strong coupling regime is ``washed out''by large broadening of these states. As a result, the spectrum contains only a single-maximum broad peak with peculiar shape and a shoulder.

cond-mat.str-el↗

arXiv2002

Self-Trapping of Polarons in the Rashba-Pekar Model

We performed quantum Monte Carlo study of the exciton-polaron model which features the self-trapping phenomenon when the coupling strength and/or particle momentum is varied. For the first time accurate data for energy, effective mass, the structure of the polaronic cloud, dispersion law, and spectral function are available throughout the crossover region. We observed that self-trapping can not be reduced to hybridization of two states with different lattice deformation, and that at least three states are involved in the crossover from light- to heavy-mass regimes.

cond-mat.str-el↗

arXiv2001

Diagrammatic Quantum Monte Carlo for Two-Body Problem: Exciton

We present a novel method for precise numerical solution of the irreducible two-body problem and apply it to excitons in solids. The approach is based on the Monte Carlo simulation of the two-body Green function specified by Feynman's diagrammatic expansion. Our method does not rely on the specific form of the electron and hole dispersion laws and is valid for any attractive electron-hole potential. We establish limits of validity of the Wannier (large radius) and Frenkel (small radius) approximations, present accurate data for the intermediate radius excitons, and give evidence for the charge transfer nature of the monopolar exciton in mixed valence materials.

cond-mat.str-el↗

arXiv2001

Magnetic frustrations in the face centered cubic antiferromagnet NiS2

Neutron scattering experiment on NiS2 single crystal revealed a honeycomb pattern of the intensity distribution in reciprocal lattice space (continuous-line structure along the fcc zone boundary) providing the first direct evidence for nearly frustrated antiferromagnetism (AF) on the face centered cubic (fcc) lattice. A small but finite lattice distortion below 30.9 K lifts the degeneracy of the magnetic ground state due to the frustration and eventually result in the coexistence of the type I and the type II AF long range orderings, which are mutually incompatible in the fcc symmetry at higher temperatures.

cond-mat.str-el↗

arXiv2001

Quasi-degenerate self-trapping in one-dimensional charge transfer exciton

The self-trapping by the nondiagonal particle-phonon interaction between two quasi-degenerate energy levels of excitonic system, is studied. We propose this is realized in charge transfer exciton, where the directions of the polarization give the quasi-degeneracy. It is shown that this mechanism, unlike the conventional diagonal one, allows a coexistence and resonance of the free and self-trapped states even in one-dimensional systems and a quantitative theory for the optical properties (light absorption and time-resolved luminescence) of the resonating states is presented. This theory gives a consistent resolution for the long-standing puzzles in quasi-one-dimensional compound A-PMDA.

cond-mat.str-el↗

arXiv1999

Comprehensive study of Frohlich polaron

A detailed study of the Frohlich polaron model is performed on the basis of diagrammatic quantum Monte Carlo method. The method is further developed both quantitatively (performance) and qualitatively (new estimators), and is enhanced by spectral analysis of the polaron Green function, within a novel approach. We present the best up to date results for the binding energy, and for the first time make available precise data for the effective mass, including the region of intermediate and strong couplings. We look at the structure of the polaron cloud and answer such questions as the average number of phonons in the cloud and their number/momentum distribution. The spectral analysis reveals non-trivial structure of the spectral density at intermediate and large coupling: the spectral continuum features pronounced peaks that we attribute to unstable excited states of the polaron.

cond-mat.stat-mech↗

arXiv1999

Thermodynamics of CeNiSn at low temperatures and in weak magnetic fields

Detailed experimental and theoretical studies of the low-temperature specific heat, magnetic susceptibility, thermal expansion and magnetostriction of the orthorhombic compound CeNiSn are presented. All anomalies observed in the thermodynamic and magnetic properties of CeNiSn are explained in a framework of a model of metallic Kondo lattice with well developed spin-liquid-type excitations. The pseudogap behavior of these excitations appears due to interplay between spin-fermions and soft crystal-field (CF) states. The thermodynamic relations for the spin liquid are derived. Together with the explanation of inelastic neutron scattering spectra given earlier within a same approach these studies of the low-temperature thermodynamics and magnetic response give a consistent description of the nature of anomalies in the low-temperature thermodynamics of perfect and imperfect CeNiSn crystals.

cond-mat.str-el↗

arXiv1998

Magnetic bound states in the quarter-filled ladder system $α'-NaV_{2}O_{5}$}

Raman scattering in the quarter-filled spin ladder system alpha'-NaV_2O_5 shows in the dimerized singlet ground state ($T \leq T_{SP}=35K$) an unexpected sequence of three magnetic bound states. Our results suggest that the recently proposed mapping onto an effective spin chain for $T > T_{SP}$ has to be given up in favor of the full topology and exchange paths of a ladder in the dimerized phase for $T < T_{SP}$. As the new ground state we propose a dynamic superposition of energetically nearly degenerate dimer configurations on the ladder.

cond-mat.str-el↗

arXiv1997

Relations between asymptotic and Fredholm representations

We prove that for matrix algebras $M_n$ there exists a monomorphism $(\prod_n M_n/\oplus_n M_n)\otimes C(S^1) \to {\cal Q} $ into the Calkin algebra which induces an isomorphism of the $K_1$-groups. As a consequence we show that every vector bundle over a classifying space $Bπ$ which can be obtained from an asymptotic representation of a discrete group $π$ can be obtained also from a representation of the group $π\times Z$ into the Calkin algebra. We give also a generalization of the notion of Fredholm representation and show that asymptotic representations can be viewed as asymptotic Fredholm representations.

funct-an↗

arXiv1996

Interplay between Heavy Fermions and Crystal Field Excitation in Kondo Lattices. Low-Temperature Thermodynamics and Inelastic Neutron Scattering Spectra of CeNiSn

The microscopic theory of interaction between the heavy fermions and the crystal field excitations in Kondo lattices is presented. It is shown that the heavy-fermion spectrum scaled by the Kondo temperature $T_K$ can be modified by the crystal field excitations with the energy $Δ_{CF}$ provided the inequality $Δ_{CF}<T_K$ is realized. On the base of general description of excitation spectrum the detailed qualitative and quantitative explanation of anisotropic inelastic neutron scattering spectra and low-temperature specific heat of orthorhombic CeNiSn is given. The theory resolves the apparent contradiction between the metallic conductivity and the gap-wise behavior of thermodynamic properties and spin response of CeNiSn at low temperatures.

cond-mat.str-el↗

arXiv1996

Spin liquid in 3D Kondo lattice. High temperature regime

The mechanism explaining the key role of AFM correlations in formation of the heavy fermion state is offered. It is shown that in the case of $(T^*-T_N)/T_N << 1$ the critical spin fluctuations transform the mean-field second-order transition to RVB state into the crossover from high-temperature paramagnetic behavior of localized spins to strongly correlated spin liquid with quasi itinerant character of susceptibility. Thus the spin liquid state {\it by its origin} is close to magnetic instability, so either short-range or long-range AFM order should arise at low T.

cond-mat↗

arXiv1994

Mean Field Description of Kondo-Stabilized Spin Liquid in Kondo Lattice with Heavy Fermions

Microscopic mechanism of forming the spin liquid of neutral fermions in Kondo lattices with nearly integer valence of the f-electrons in presented. It is shown that the competition between the Kondo scattering and the indirect exchange interaction can result in stabilization of a state with separated spin and charge excitations with a Fermi-type spectrum for both of them. The origin of strong antiferromagnetic fluctuations is briefly discussed.

cond-mat↗