arXiv subjects

A. M. Shvaika

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

At least 19 recordsLinked to original sources

arXiv2025

Out-of-time ordered correlation functions for the localized $f$ electrons in the Falicov-Kimball model

We provide an exact evaluation of the out-of-time correlation (OTOC) functions for the localized $f$-particle states in the Falicov-Kimball model within dynamical mean-field theory. Different regimes of quantum chaos and quantum scrambling are distinguished by the winding numbers of the block Toeplitz matrices used in the calculation. The similarities of these fermionic OTOCs and their logarithmic derivatives for time evolution with the OTOCs for quantum spin models with disorder are also discussed.

cond-mat.str-el↗

arXiv2020

Thermoelectric properties of Mott insulator with correlated hopping at microdoping

An influence of the localization of itinerant electrons induced by correlated hopping on the electronic charge and heat transport is discussed for the lightly doped Mott insulator phase of the Falicov-Kimball model. The case of strongly reduced hopping amplitude between the sites with occupied f-electron levels, when an additional band of localized d-electron states could appear on the DOS in the Mott gap, is considered. Due to the electron-hole asymmetry and anomalous features on the DOS and transport function induced by correlated hopping, a strong enhancement of the Seebeck coefficient is observed at low temperatures, when the flattened dependence is displayed in a wide temperature range.

cond-mat.str-el↗

arXiv2018

Interpreting pulse-shape effects in pump-probe spectroscopies

The effect of the pulse-shape on pump-probe spectroscopies is examined for the simplest model of noninteracting fermions on an infinite-dimensional hypercubic lattice. The probe-modified density of states follows the time evolution of the pump and displays narrowing and Floquet-like sidebands at the pump maximum, whereas the photoelectron spectra are also strongly affected by the nonequilibrium occupation of the single-particle states due to the excitation from the pump. The nonequilibrium Raman cross section is derived, and the nonresonant one in both the $A_{1\mathrm{g}}$ and $B_{1\mathrm{g}}$ symmetries contains a number of peaks at the pump maximum, which can be attributed to an interference effect or Brillouin scattering off the time variations of the stress tensor. Both the "measured" occupation of single-particle states and the ratio of Stokes to anti-Stokes peaks are strongly modified by the probe-pulse width, which must be included in the interpretation of experimental results.

cond-mat.str-el↗

arXiv2018

X-Ray Photoemission Spectroscopy in the Falicov-Kimball model

We calculate the finite temperature X-ray photoemission spectroscopy for the Falicov-Kimball model using a Weiner-Hopf sum equation approach. In the metallic state, the core-hole spectral function shows two side peaks corresponding to the creation of a core-hole on an empty site (or a doubly occupied site) and also has two nearly degenerate central peaks (because of our choice of the model parameters) corresponding to the creation of a core-hole on a singly occupied site. The nearly doubly degenerate central peaks merge into a single peak at higher temperatures. In the insulating state, we obtain two peaks and a strongly temperature dependent low-energy peak corresponding to the creation of a core-hole on a thermally excited empty site. These results for the insulating state should be similar to those of the more general Hubbard model. Also, the strong correlations suggest that even without any additional broadening due to Auger like processes, the core-hole lifetime will be short.

cond-mat.str-el↗

arXiv2018

Stroboscopic Tests for Thermalization of Electrons in Pump/Probe Experiments

One of the goals of pump/probe spectroscopies is to determine how electrons relax after they have been driven out of equilibrium. It is challenging to determine how close electrons are to a thermal state solely by fitting their distribution to a Fermi-Dirac distribution. Instead, we propose that one compare the effective temperatures of both fermions and collective bosonic modes (derived from the fermions) to determine the distance from a thermal state. Measurements of effective fermionic and bosonic temperatures can be achieved directly via photoemission and nonresonant Raman scattering. Their difference quantifies the distance from thermal equilibrium.

cond-mat.str-el↗

arXiv2018

Nonlocal correlations in the optical conductivity spectra

Optical conductivity spectra are studied for the Falicov-Kimball model with correlated hopping on the Bethe lattice. An expression for the current-current correlation function is derived using dynamical mean field theory. In the metallic phase with small correlated hopping values, the shape of Drude peak deviates from the Debye relaxation peak, and an additional wide peak is observed on the optical conductivity spectra and on Nyquist plot when Fermi level is in the vicinity of the two particle resonance. At larger values of the correlated hopping parameter, the density of states contains three bands and the corresponding optical spectra and Nyquist plots display a more complicated shape with additional peaks. For strong local correlations, the correlated hopping reduces the width of the upper Hubbard band resulting in a decrease of the Drude peak spectral weight for the doped Mott insulator.

cond-mat.str-el↗

arXiv2017

Resonant enhancement of thermoelectric properties by correlated hopping for the Falicov-Kimball model on Bethe lattice

The effect of correlated hopping on the charge and heat transport of strongly correlated particles is studied for the Falicov-Kimball model on the Bethe lattice. Exact solutions for the one particle density of states (DOS) and two particle transport function (the "quasiparticle" scattering time) are derived using dynamical mean field theory. For a wide range of the correlated hopping, the transport function exhibits singularities due to the resonant two-particle contribution, whereas the one particle DOS does not show any anomalous features. By tuning the number of itinerant electrons, so as to bring the Fermi level close to the resonant frequency, we get a large increase of the electrical and thermal conductivities, and the thermoelectric power. When the hopping amplitude between the occupied sites is reduced sufficiently, the itinerant electrons localize in the clusters of sites occupied by $f$ electrons. This gives rise to an additional narrow band in the DOS between the lower and upper Hubbard bands, but has only a minor effect on the thermoelectric properties.

cond-mat.str-el↗

arXiv2016

Theoretical description of pump/probe experiments in electron mediated charge-density-wave insulators

In this review, we develop the formalism employed to describe charge-density-wave insulators in pump/probe experiments using ultra short driving pulses of light. The theory emphasizes exact results in the simplest model for a charge-density wave insulator (given by a noninteracting systems with two bands and a gap) and by employing nonequilibrium dynamical mean-field theory to solve the Falicov-Kimball model in its ordered phase. We show both how to develop the formalism and how the solutions behave. Care is taken to describe the details behind these calculations and to show how to verify their accuracy via sum-rule constraints.

cond-mat.str-el↗

arXiv2016

Spectral properties of four-time fermionic Green's functions

The spectral relations for the four-time fermionic Green's functions are derived in the most general case. The terms which correspond to the zero-frequency anomalies, known before only for the bosonic Green's functions, are separated and their connection with the second cumulants of the Boltzmann distribution function is elucidated. The high-frequency expansions of the four-time fermionic Green's functions are provided for different directions in the frequency space.

cond-mat.stat-mech↗

arXiv2016

Time-domain pumping a quantum-critical charge-density-wave-ordered material

We determine the exact time-resolved photoemission spectroscopy for a nesting driven charge-density-wave (described by the spinless Falicov-Kimball model within dynamical mean-field theory). The pump-probe experiment involves two light pulses: the first is an ultrashort intense pump pulse that excites the system into nonequilibrium, and the second is a lower amplitude higher frequency probe pulse that photoexcites electrons. We examine three different cases: the strongly correlated metal, the quantum-critical charge density wave and the critical Mott insulator. Our results show that the quantum critical charge density wave has an ultra efficient relaxation channel that allows electrons to be de-excited during the pump pulse, resulting in little net excitation. In contrast, the metal and the Mott insulator show excitations that are closer to what one expects from these systems. In addition, the pump field produces spectral band narrowing, peak sharpening, and a spectral gap reduction, all of which rapidly return to their field free values after the pump is over.

cond-mat.str-el↗

arXiv2016

Directly characterizing the relative strength and momentum dependence of electron-phonon coupling using resonant inelastic x-ray scattering

The coupling between lattice and charge degrees of freedom in condensed matter materials is ubiquitous and can often result in interesting properties and ordered phases, including conventional superconductivity, charge density wave order, and metal-insulator transitions. Angle-resolved photoemission spectroscopy and both neutron and non-resonant x-ray scattering serve as effective probes for determining the behavior of appropriate, individual degrees of freedom -- the electronic structure and lattice excitation, or phonon dispersion, respectively. However, each provides less direct information about the mutual coupling between the degrees of freedom, usual through self-energy effects, which tend to renormalize and broaden spectral features precisely where the coupling is strong, impacting ones ability to quantitively characterize the coupling. Here we demonstrate that resonant inelastic x-ray scattering, or RIXS, can be an effective tool to directly determine the relative strength and momentum dependence of the electron-phonon coupling in condensed matter systems. Using a diagrammatic approach for an 8-band model of copper oxides, we study the contributions from the lowest order diagrams to the full RIXS intensity for a realistic scattering geometry, accounting for matrix element effects in the scattering cross-section as well as the momentum dependence of the electron-phonon coupling vertex. A detailed examination of these maps offers a unique perspective into the characteristics of electron-phonon coupling, which complements both neutron and non-resonant x-ray scattering, as well as Raman and infrared conductivity.

cond-mat.str-el↗

arXiv2015

Nonequilibrium response of an electron mediated charge-density-wave-ordered material to a large dc electric field

Using the Kadanoff-Baym-Keldysh formalism, we employ nonequilibrium dynamical mean-field theory to exactly solve for the nonlinear response of an electron-mediated charge-density-wave-ordered material. We examine both the dc current and the order parameter of the conduction electrons as the ordered system is driven by the electric field. Although the formalism we develop applies to all models, for concreteness, we examine the charge-density-wave phase of the Falicov-Kimball model, which displays a number of anomalous behaviors including the appearance of subgap density of states as the temperature increases. These subgap states should have a significant impact on transport properties, particularly the nonlinear response of the system to a large dc electric field.

cond-mat.str-el↗

arXiv2015

Nonequilibrium dynamical mean-field theory for the charge-density-wave phase of the Falicov-Kimball model

Nonequilibrium dynamical mean-field theory (DMFT) is developed for the case of the charge-density-wave ordered phase. We consider the spinless Falicov-Kimball model which can be solved exactly. This strongly correlated system is then placed in an uniform external dc electric field. We present a complete derivation for nonequilibrium dynamical mean-field theory Green's functions defined on the Keldysh-Schwinger time contour. We also discuss numerical issues involved in solving the coupled equations.

cond-mat.str-el↗

arXiv2014

Effect of correlated hopping on thermoelectric properties: Exact solutions for the Falicov-Kimball model

The effect of correlated hopping on the charge and heat transport is investigated for the Falicov-Kimball model. Exact solutions for the electrical and thermal conductivities and thermoelectric power are obtained within the dynamical mean field theory. The temperature dependences of the transport coefficients are analysed for particular values of correlated hopping which correspond to the significant reconstruction of the density of states and transport function. The cases with strong enhancement of thermoelectric properties are elucidated.

cond-mat.str-el↗

arXiv2012

Exact solution of a variety of X-ray probes in the Falicov-Kimball model with dynamical mean-field theory

We examine core-level X-ray photoemission spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES) and X-ray emission spectroscopy (XES) in the Falicov-Kimball model by using the exact solution from dynamical mean-field theory. XPS measures the core-hole propagator, XANES measures the absorption of X-rays when the core electron is excited to an unoccupied electronic state of the solid, and not emitted, and XES measures the spectra of light emitted as electrons fill the core-hole state created via some form of X-ray excitation. These three spectra are closely related to one another and display orthogonality catastrophe behavior at T=0. We illustrate how to evaluate these spectra efficiently at finite temperature, with a primary focus on the details of XANES.

cond-mat.str-el↗

arXiv2011

Resonant inelastic X-ray scattering in a Mott insulator

We calculate the resonant inelastic X-ray scattering (RIXS) response in a Mott insulator which is described by the Falicov-Kimball model. The model can be solved exactly within the single site dynamical mean-field theory (DMFT) approximation and the calculated RIXS response is accurate up to a local background correction. We find that on resonance the RIXS response is greatly enhanced over various other non-resonant background effects and the response systematically evolves from a single peak structure, arising due to relaxation processes within the lower Hubbard band, to a two peak structure, arising due to relaxation processes within the upper Hubbard band as well as across the Mott gap into the lower Hubbard band, as we vary the incident photon frequency to access states from the bottom of the lower Hubbard band to the top of the upper Hubbard band. The charge transfer excitations are found to disperse monotonically outwards (as a function of transfered energy) as we go from the center of the Brillouin zone towards the zone corner. These correlation induced features have been observed by Hasan {\it et. al.} (Science {\bf 288}, 1811 (2000)) and many other experimentalists in RIXS measurements over various transition metal oxide compounds and are found to be robust and survive even for large Auger lifetime broadening effects. As a comparison, we also calculate the dynamic structure factor for this model, which is proportional to the nonresonant part of the response, and does not show these specific signatures.

cond-mat.str-el↗

arXiv2010

Dynamical mean-field theory of correlated hopping: A rigorous local approach

A general approach for the description of correlated hopping in infinite dimensions, which is based on an expansion over electron hopping around the atomic limit, is developed. Such an approach keeps the dynamical mean-field theory local ideology and allows one to calculate the thermodynamical functions. A grand-canonical potential functional and a $Φ$-derivatible theory that does not introduce the self-energy is proposed. As limiting cases the Falicov--Kimball model with correlated hopping and the model with broken bonds ("diluted" conductor) are investigated, and the connection with the Blackman-Esterling-Berk coherent potential approximation approach in the theory of the binary alloy with off-diagonal disorder is considered.

cond-mat.str-el↗

arXiv2010

Resonant Raman scattering effects in a nesting driven charge-density-wave insulator: exact analysis of the spinless Falicov-Kimball model with dynamical mean-field theory

We calculate the total electronic Raman scattering spectrum for a system with a charge density wave on an infinite-dimensional hypercubic lattice. The problem is solved exactly for the spinless Falicov-Kimball model with dynamical mean-field theory. We include the nonresonant, mixed, and resonant contributions in three common experimental polarizations, and analyze the response functions for representative values of the energy of the incident photons. The complicated scattering response can be understood from the significant temperature dependence of the many-body density of states, and includes a huge enhancement for photon frequencies near the charge-density-wave gap energy.

cond-mat.str-el↗