arXiv subjects

A. S. Mishchenko

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

At least 37 records · Page 2Linked to original sources

arXiv2012

Quantum Dynamics of the Hubbard-Holstein Model in Equilibrium and Non-Equilibrium: Application to Pump-Probe Phenomena

The spectral response and physical features of the 2D Hubbard-Holstein model are calculated both in equilibrium at zero and low chemical dopings, and after an ultra short powerful light pulse, in undoped systems. At equilibrium and at strong charge-lattice couplings, the optical conductivity reveals a 3-peak structure in agreement with experimental observations. After an ultra short pulse and at nonzero electron-phonon interaction, phonon and spin subsystems oscillate with the phonon period $T_{ph} \approx 80$ fs. The decay time of the phonon oscillations is about 150-200 fs, similar to the relaxation time of the charge system. We propose a criterion for observing these oscillations in high $T_c$ compounds: the time span of the pump light pulse $τ_{pump}$ has to be shorter than the phonon oscillation period $T_{ph}$.

cond-mat.str-el↗

arXiv2011

Bordisms of manifolds with proper action of a discrete group: signatures and descriptions of $G$-bundles

In this work the equivariant signature of a manifold with proper action of a discrete group is defined as an invariant of equivariant bordisms. It is shown that the computation of this signature can be reduced to its computation on fixed points sets equipped with their tubular neighborhoods. It is given a description of the equivariant vector bundles with action of a discrete group $G$ for the case when the action over the base is proper quasi-free, i.e. the stationary subgroup of any point is finite. The description is given in terms of some classifying space.

math.AT↗

arXiv2011

Characteristic classes of transitive Lie algebroids. Categorical point of view

The talk was done at the International Conference "Analysis, Topology and Applications", Harbin, China, 23.08.2011. Transitive Lie algebroids have specific properties that allow to look at the transitive Lie algebroid as an element of the object of a homotopy functor. Roughly speaking each transitive Lie algebroids can be described as a vector bundle over the tangent bundle of the manifold which is endowed with additional structures. Therefore transitive Lie algebroids admits a construction of inverse image generated by a smooth mapping of smooth manifolds. The intention of my talk is to use a homotopy classification of transitive Lie algebroids due to K.Mackenzie (2005) and on this basis to construct a classifying space. The realization of the intention allows to describe characteristic classes of transitive Lie algebroids form the point of view a natural transformation of functors similar to the classical abstract characteristic classes for vector bundles and to compare them with that derived from the Chern-Weil homomorphism by J.Kubarski (1991).

math.AT↗

arXiv2011

On the nature of the optical conductivity of polaron

We study theoretically a fundamental issue in solids: the evolution of the optical spectra of polaron as the electron-phonon coupling increases. By comparing the exact results obtained by diagrammatic Monte Carlo method and the data got through exact diagonalization within an appropriate subspace of the phononic wavefunctions, the physical nature of the crossover from weak to strong coupling is revealed. The optical spectra are well understood by the quantum mechanical superposition of states with light and heavy phonon clouds corresponding to large and small polarons, respectively. It is also found that the strong coupling Franck-Condon regime is not accessible at realistic values of the coupling constant.

cond-mat.str-el↗

arXiv2011

Polaronic metal in lightly doped high-Tc cuprates

We present a combined study of the angle-resolved-photoemission spectroscopy (ARPES) and quantum Monte Carlo simulations to propose a novel polaronic metallic state in underdoped cuprates. An approximation scheme is proposed to represent underdoped cuprates away from 1/2 filling, replacing the many-body Hamiltonian by that of a single polaron with effective electron-phonon interaction (EPI), that successfully explains many puzzles such as a large momentum-dependent dichotomy between nodal and anti-nodal directions, and an unconventional doping dependence of ARPES in the underdoped region.

cond-mat.str-el↗

arXiv2011

Sharp transition for single polarons in the one-dimensional Su-Schrieffer-Heeger model

We study a single polaron in the Su-Schrieffer-Heeger (SSH) model using four different techniques (three numerical and one analytical). Polarons show a smooth crossover from weak to strong coupling, as a function of the electron-phonon coupling strength $λ$, in all models where this coupling depends only on phonon momentum $q$. In the SSH model the coupling also depends on the electron momentum $k$; we find it has a sharp transition, at a critical coupling strength $λ_c$, between states with zero and nonzero momentum of the ground state. All other properties of the polaron are also singular at $λ= λ_c$, except the average number of phonons in the polaronic cloud. This result is representative of all polarons with coupling depending on $k$ and $q$, and will have important experimental consequences (eg., in ARPES and conductivity experiments).

cond-mat.str-el↗

arXiv2011

Determination of Different Biological Factors on the Base of Dried Blood Spot Technology

It is well-known that distinct biological indices (analytes) have distinct variability. We try to use some mathematical algorithms to pick out a set of blood parameters which give an opportunity to retrieve the initial volume of the blood spotted, and use it to calculate exact concentrations of analyts interesting to a physician. For our analysis we used the database of biochemical blood parameters obtained in Russian Scientific Center of Roentgen-Radiology during 1995-2000, which includes more than 30000 of patients.

math.ST↗

arXiv2010

Transitive Lie algebroids - categorical point of view

The intention of this article is to make an attempt of classification of transitive Lie algebroids and on this basis to construct a classifying space. The realization of the intention allows to describe characteristic classes of transitive Lie algebroids form the point of view a natural transformation of functors similar to the classical abstract characteristic classes for vector bundles.

math.AT↗

arXiv2010

Interplay between charge-lattice interaction and strong electron correlations in cuprates: phonon anomaly and spectral kinks

We investigate the interplay between strong electron correlations and charge-lattice interaction in cuprates. The coupling between half breathing bond stretching phonons and doped holes in the t-t'-J model is studied by limited phonon basis exact diagonalization method. Nonadiabatic electron-phonon interaction leads to the splitting of the phonon spectral function at half-way to the zone boundary at $\vec{q}_s=\{(\pm π/ 2, 0), (0, \pm π/ 2) \}$ and to low energy kink feature in the electron dispersion, in agreement with experimental observations. Another kink due to strong electron correlation effects is observed at higher energy, depending on the strength of the charge-lattice coupling.

cond-mat.str-el↗

arXiv2008

Localization-delocalization transition of a polaron near an impurity

We solve the problem of polaron localization on an attractive impurity by means of direct-space Diagrammatic Monte Carlo implemented for the system in the thermodynamic limit. In particular we determine the ground state phase diagram in dependence on the electron-phonon coupling and impurity potential strength for the whole phonon frequency range. Including the quantum phonon dynamics we find and characterize a new phase which is missing in the zero phonon-frequency limit (adiabatic approximation), where self-trapped polarons are not localized at shallow impurities. We predict and show that in the vicinity of the localization transition a region with a mixture of weak- and strong-coupling spectral response is realized.

cond-mat.str-el↗

arXiv2008

Charge dynamics of doped holes in high Tc cuprates - A clue from optical conductivity

The charge dynamics in weakly hole doped high temperature superconductors is studied in terms of the accurate numerical solution to a model of a single hole interacting with a quantum lattice in an antiferromagnetic background, and accurate far-infrared ellipsometry measurements. The experimentally observed two electronic bands in the infrared spectrum can be identified in terms of the interplay between the electron correlation and electron-phonon interaction resolving the long standing mystery of the mid-infrared band.

cond-mat.str-el↗

arXiv2008

A possible mechanism of ultrafast amorphization in phase-change memory alloys: an ion slingshot from the crystalline to amorphous position

We propose that the driving force of an ultrafast crystalline-to-amorphous transition in phase-change memory alloys are strained bonds existing in the (metastable) crystalline phase. For the prototypical example of GST, we demonstrate that upon breaking of long Ge-Te bond by photoexcitation Ge ion shot from an octahedral crystalline to a tetrahedral amorphous position by the uncompensated force of strained short bonds. Subsequent lattice relaxation stabilizes the tetrahedral surroundings of the Ge atoms and ensures the long-term stability of the optically induced phase.

cond-mat.mtrl-sci↗

arXiv2007

Non-local composite spin-lattice polarons in high temperature superconductors

The non-local nature of the polaron formation in t-t'-t"-J model is studied in large lattices up to 64 sites by developing a new numerical method. We show that the effect of longer-range hoppings t' and t" is a large anisotropy of the electron-phonon interaction (EPI) leading to a completely different influence of EPI on the nodal and antinodal points in agreement with the experiments. Furthermore, nonlocal EPI preserves polaron's quantum motion, which destroys the antiferromagnetic order effectively, even at strong coupling regime, although the quasi-particle weight in angle-resolved-photoemission spectroscopy is strongly suppressed.

cond-mat.str-el↗

arXiv2007

Temperature dependence of the angle resolved photoemission spectra in the undoped cuprates: self-consistent approach to the t-J-Holstein model

We develop a novel self-consistent approach for studying the angle resolved photoemission spectra (ARPES) of a hole in the t-J-Holstein model giving perfect agreement with numerically exact Diagrammatic Monte Carlo data at zero temperature for all regimes of electron-phonon coupling. Generalizing the approach to finite temperatures we find that the anomalous temperature dependence of the ARPES in undoped cuprates is explained by cooperative interplay of coupling of the hole to magnetic fluctuations and strong electron-phonon interaction.

cond-mat.str-el↗

arXiv2007

Spectroscopic Properties of Polarons in Strongly Correlated Systems by Exact Diagrammatic Monte Carlo Method

We present recent advances in understanding of the ground and excited states of the electron-phonon coupled systems obtained by novel methods of Diagrammatic Monte Carlo and Stochastic Optimization, which enable the approximation-free calculation of Matsubara Green function in imaginary times and perform unbiased analytic continuation to real frequencies. We present exact numeric results on the ground state properties, Lehmann spectral function and optical conductivity of different strongly correlated systems: Frohlich polaron, Rashba-Pekar exciton-polaron, pseudo Jahn-Teller polaron, exciton, and interacting with phonons hole in the t-J model.

cond-mat.str-el↗

arXiv2006

Numerical study of the isotope effect in underdoped high-temperature superconductors: Calculation of the angle-resolved photoemission spectra

We present a numerical study of the isotope effect on the angle resolved photoemission spectra (ARPES) in the undoped cuprates. By the systematic-error-free Diagrammatic Monte Carlo method, the Lehman spectral function of a single hole in the tt't''-J model in the regime of intermediate and strong couplings to optical phonons is calculated for normal and isotope substituted systems. We found that the isotope effect is strongly energy-momentum dependent, and is anomalously enhanced in the intermediate coupling regime while it approaches to that of the localized hole model in the strong coupling regime. We predict the strengths of effect as well as the fine details of the ARPES lineshape change. Implications to the doped case are also discussed.

cond-mat.str-el↗

arXiv2006

On the validity of the Franck-Condon principle in the optical spectroscopy: optical conductivity of the Fröhlich polaron

The optical absorption of the Fröhlich polaron model is obtained by an approximation-free Diagrammatic Monte Carlo method and compared with two new approximate approaches that treat lattice relaxation effects in different ways. We show that: i) a strong coupling expansion, based on the the Franck-Condon principle, well describes the optical conductivity for large coupling strengths ($α>10$); ii) a Memory Function Formalism with phonon broadened levels reproduces the optical response for weak coupling strengths ($α<6$) taking the dynamic lattice relaxation into account. In the coupling regime $6<α<10$ the optical conductivity is a rapidly changing superposition of both Franck-Condon and dynamic contributions.

cond-mat.str-el↗

arXiv2004

Polaron in t-J model

We present numeric results for ground state and angle resolved photoemission spectra (ARPES) for single hole in t-J model coupled to optical phonons. The systematic-error free diagrammatic Monte Carlo is employed where the Feynman graphs for the Matsubara Green function in imaginary time are summed up completely with respect to phonons variables, while magnetic variables are subjected to non-crossing approximation. We obtain that at electron-phonon coupling constants relevant for high Tc cuprates the polaron undergoes self-trapping crossover to strong coupling limit and theoretical ARPES demonstrate features observed in experiment: a broad peak in the bottom of the spectra has momentum dependence which coincides with that of hole in pure t-J model.

cond-mat.str-el↗