Searcharxiv⌕ Search

arXiv subjects

E. Z. Kuchinskii

Publications and source records attributed to E. Z. Kuchinskii.

53 records · Page 3Linked to original sources
arXiv2005

Pseudogaps in Strongly Correlated Metals: A generalized dynamical mean-field theory approach

We generalize the dynamical-mean field (DMFT) approximation by including into the DMFT equations some length scale via a momentum dependent ``external'' self-energy S(k). This external self-energy describes non-local dynamical correlations induced by short-ranged collective SDW-like antiferromagnetic spin (or CDW-like charge) fluctuations. At high enough temperatures these fluctuations can be viewed as a quenched Gaussian random field with finite correlation length. This generalized DMFT+Sk approach is used for the numerical solution of the weakly doped one-band Hubbard model with repulsive Coulomb interaction on a square lattice with nearest and next nearest neighbour hopping. The effective single impurity problem in this generalized DMFT+Sk is solved by numerical renormalization group (NRG). Both types of strongly correlated metals, namely (i) doped Mott insulator and (ii) the case of bandwidth W~U (U - value of local Coulomb interaction) are considered. Densities of states, spectral functions and ARPES spectra calculated within DMFT+Sk show a pseudogap formation near the Fermi level of the quasiparticle band.

cond-mat.str-el↗
arXiv2005

"Destruction" of the fermi surface due to pseudogap fluctuations in strongly correlated systems

We generalize the dynamical - mean field theory (DMFT) by including into the DMFT equations dependence on correlation length of pseudogap fluctuations via additional (momentum dependent) self - energy. This self - energy describes non - local dynamical correlations induced by short - ranged collective SDW - like antiferromagnetic spin (or CDW - like charge) fluctuations. At high enough temperatures these fluctuations can be viewed as a quenched Gaussian random field with finite correlation length. This generalized DMFT+Σ_k approach is used for the numerical solution of the weakly doped one - band Hubbard model with repulsive Coulomb interaction on a square lattice with nearest and next nearest neighbour hopping. The effective single impurity problem is solved by numerical renormalization group (NRG). Both types of strongly correlated metals, namely (i) doped Mott insulator and (ii) the case of bandwidth W<U (U - value of local Coulomb interaction) are considered. Calculating profiles of spectral densities for different parameters of the model we demonstrate the qualitative picture of Fermi surface "destruction" and formation of "Fermi arcs" due to pseudogap fluctuations in qualitative agreement with ARPES experiments. "Blurring" of the Fermi surface is enhanced with the growth of the Coulomb interaction.

cond-mat.str-el↗
arXiv2005

Pseudogaps in Strongly Correlated Metals

We generalize the dynamical-mean field (DMFT) approximation by including into the DMFT equations some length scale via a (momentum dependent) ``external'' self-energy Σ_k. This external self-energy describes non-local dynamical correlations induced by short-ranged collective SDW-like antiferromagnetic spin (or CDW-like charge) fluctuations. At high enough temperatures these fluctuations can be viewed as a quenched Gaussian random field with finite correlation length. This generalized DMFT+Σ_k approach is used for the numerical solution of the weakly doped one--band Hubbard model with repulsive Coulomb interaction on a square lattice with nearest and next nearest neighbour hopping. The effective single impurity problem in this generalized DMFT+Σ_k is solved by numerical renormalization group (NRG). Both types of strongly correlated metals, namely (i) doped Mott insulator and (ii) the case of bandwidth W<=U (U - value of local Coulomb interaction) are considered. Densities of states, spectral functions and ARPES spectra calculated within DMFT+Σ_k show a pseudogap formation near the Fermi level of the quasiparticle band.

cond-mat.str-el↗
arXiv2004

Superconductivity in the "Hot Spots" Model of the Pseudogap State: Impurity Scattering and Phase Diagram

We analyze the anomalies of superconducting state (both s- and d-wave pairing) in the model of the pseudogap state induced by Heisenberg spin fluctuations of antiferromagnetic short-range order, and based on the scenario of strong scattering near the "hot spots" on the Fermi surface. We present microscopic derivation of Ginzburg-Landau expansion, taking into account all Feynman graphs of perturbation theory for electron interaction with fluctuations of short-range order and in the "ladder" approximation for electron scattering by normal (nonmagnetic) impurities. We determine the dependence of superconducting critical temperature T_c and other characteristics of a superconductor on the parameters of the pseudogap and impurity scattering. It is shown that within this model it is possible to explain the typical phase diagram of high- temperature superconductors.

cond-mat.supr-con↗
arXiv2004

Superconductivity in the "Hot Spots" Model of the Pseudogap State

We analyze the anomalies of superconducting state in the model of pseudogap state induced by fluctuations of short - range order of "dielectric" (AFM(SDW) or CDW) type, and based on the scenario of "hot spots" formation on the Fermi surface, with the account of all Feynman graphs for electron interaction with pseudogap fluctuations, leading to strong scattering around the "hot spots". We determine the dependence of superconducting critical temperature T_c on the effective width of the pseudogap, correlation length of short - range order and concentration of nonmagnetic impurities. We also discuss possible connection of these results with the general form of the phase diagram of superconducting cuprates.

cond-mat.supr-con↗
arXiv2003

Supeconductivity in the Pseudogap State in "Hot - Spots" Model: Ginzburg - Landau Expansion

We analyze properties of superconducting state (for both s-wave and d-wave pairing), appearing on the "background" of the pseudogap state, induced by fluctuations of "dielectric" (AFM(SDW) or CDW) short -- range order in the model of the Fermi surface with "hot spots". We present microscopic derivation of Ginzburg - Landau expansion, taking into account all Feynman diagrams of perturbation theory over electron interaction with this short - range order fluctuations, leading to strong electronic scattering in the vicinity of "hot spots". We determine the dependence of superconducting critical temperature on the effective width of the pseudogap and on correlation length of short - range order fluctuations. We also find similar dependences of the main characteristics of such superconductor close to transition temperature. It is shown particularly, that specific heat discontinuity at the transition temperature is significantly decreased in the pseudogap region of the phase diagram.

cond-mat.supr-con↗
arXiv2001

Superconductivity in an Exactly Solvable Model of the Pseudogap State: Absence of Self Averaging

We analyze the anomalies of superconducting state within a simple exactly solvable model of the pseudogap state, induced by fluctuations of ``dielectric'' short range order, for the model of the Fermi surface with ``hot'' patches. The analysis is performed for the arbitrary values of the correlation length xi_{corr} of this short range order. It is shown that superconducting energy gap averaged over these fluctuations is non zero in a wide temperature range above T_c - the temperature of homogeneous superconducting transition. This follows from the absence of self averaging of the gap over the random field of fluctuations. For temperatures T>T_c superconductivity apparently appears in separate regions of space (``drops''). These effects become weaker for shorter correlation lengths xi_{corr} and the region of ``drops'' on the phase diagram becomes narrower and disappears for xi_{corr}-->0, however, for the finite values of xi_{corr} the complete self averaging is absent.

cond-mat.supr-con↗
arXiv2000

Superconductivity in the Pseudogap State due to Fluctuations of Short-Range Order

We analyze the anomalies of superconducting state (s and d-wave pairing) in a simple model of pseudogap state, induced by fluctuations of short - range order (e.g. antiferromagnetic), based on the model Fermi surface with "hot patches". We derive a system of recursion relations for Gorkov's equations which take into account all diagrams of perturbation theory for electron interaction with fluctuations of short-range order. Then we find superconducting transition temperature and gap behavior for different values of the pseudogap width and correlation lengths of short-range order fluctuations. In a similar approximation we derive the Ginzburg-Landau expansion and study the main physical characteristics of a superconductor close to the transition temperature, both as functions of the pseudogap width and correlation length of fluctuations. Results obtained are in qualitative agreement with a number of experiments on underdoped HTSC-cuprates.

cond-mat.supr-con↗
arXiv1999

Superconductivity in a Toy Model of the Pseudogap State

We analyze superconducting state (both s and d - wave) in a simple exactly solvable model of pseudogap state, induced by short - range order fluctuations (e.g. antiferromagnetic), which is based upon model Fermi - surface with "hot patches". It is shown that superconducting energy gap averaged over these fluctuations is non zero even for the temperatures larger than mean - field T_c of superconducting transition in a sample as a whole. For temperatures T>T_c superconductivity apparently exists within separate regions ("drops"). We study the spectral density and the density of states and demonstrate that superconductivity signals itself in these already for T>T_c, while at T_c itself nothing special happens from this point of view. These anomalies are in qualitative agreement with a number experiments on underdoped cuprates.

cond-mat.supr-con↗
arXiv1998

Models of the Pseudogap State of Two-Dimensional Systems

We analyze a number of ``nearly exactly'' solvable models of electronic spectrum of two-dimensional systems with well-developed fluctuations of short range order of ``dielectric'' (e.g. antiferromagnetic) or ``superconducting'' type, which lead to the formation of anisotropic pseudogap state on certain parts of the Fermi surface. We formulate a recurrence procedure to calculate one-electron Green's function which takes into account all Feynman diagrams in perturbation series and is based upon the approximate Ansatz for higher-order terms in this series. Detailed results for spectral densities and density of states are presented. We also discuss some important points concerning the justification of our Ansatz for higher-order contributions.

cond-mat.supr-con↗
arXiv1997

Combinatorics of Feynman Diagrams for the Problems with Gaussian Random Field

The algorithm to calculate the generating function for the number of ``skeleton'' diagrams for the irreducible self-energy and vertex parts is derived for the problems with Gaussian random fields. We find an exact recurrence relation determining the number of diagrams for any given order of perturbation theory, as well as its asymptotics for the large order limit. These results are applied to the analysis of the problem of an electron in the Gaussian random field with the ``white-noise'' correlation function. Assuming the equality of all ``skeleton'' diagrams for the self-energy part in the given order of perturbation theory, we construct the closed integral equation for the one-particle Green's function, with its kernel defined by the previously introduced generating function. Our analysis demonstrate that this approximation gives the qualitatively correct form of the localized states ``tail'' in the density of states in the region of negative energies and is apparently quite satisfactory in the most interesting region of strong scattering close to the former band-edge, where we can derive the asymptotics of the Green's function and density of states in the limit of very strong scattering.

cond-mat.dis-nn↗
arXiv1996

Superconductivity Suppression Close to the Metal-Insulator Transition in Strongly Disordered Systems

On the basis of the recently proposed self-consistent theory of metal- insulator transition in strongly disordered systems, taking into account interaction effects, we study transition temperature $T_{c}$ suppression in disordered superconductors for the wide disorder interval --- from weakly disordered metal up to Anderson insulator, induced by "Coulomb pseudogap" formation in the density of states. It is shown that for a number of systems this theory provides rather satisfactory fit of experimental data.

cond-mat.supr-con↗
arXiv1994

Self-Consistent Theory of Metal Insulator Transitions in Disordered Systems

Self-consistent theory of electron localization in disordered systems is generalized for the case of interacting electrons. We propose and critically compare a number of possible self-consistency schemes which take into account the lowest perturbation theory contributions over the interaction. Depending on self-consistency scheme we can obtain either the continuous metal-insulator transition or that with the minimal metallic conductivity. Within the continuous transition approach we calculate the frequency dependence of generalized diffusion coefficient both for metallic and insulating phases. We also consider interaction renormalization of the single-electron density of states which demonstrates the growth of the effective pseudogap as system goes from metal to insulator.

cond-mat↗
arXiv1994

Ginzburg-Landau Expansion and Physical Properties of Odd-Gap Superconductors

Ginzburg-Landau expansion is derived for superconductors with the gap-function odd over $k-k_{F}$. It is shown that in this odd-gap case Ginzburg-Landau coefficients possess an additional dependence on the pairing coupling constant which leads to the appropriate dependence of the physical properties. Non-magnetic impurities influence the odd-gap pairing in a significant way and the mechanism of this influence is quite different from that predicted by the usual theory of "dirty" superconductors. In particular this is reflected in an anomalous behavior of the upper critical field $H_{c2}$ slope close to $T_{c}$. We also derive the complete temperature dependence of $H_{c2}$ for the odd-gap case.

cond-mat↗
arXiv1993

On the Theory of "Odd"-Pairing Superconductors

We consider the model of superconducting pairing with the energy gap function which is odd over $k-k_{F}$.\ In this case superconductivity is possible even in the presence of an arbitrarily large point-like repulsion between electrons,\ which is attractive for the theory of high-temperature superconductors.\ We discuss mainly a model pairing interaction for which the BCS equations can be solved exactly,\ allowing the complete analysis of the interplay of the usual (``even'') and ``odd'' pairing,\ depending on the coupling constant.\ We also show that the normal impurities (disorder) lead to a rapid degradation of the ``odd'' pairing,\ which is even more rapid than in case of magnetic impurities in traditional superconductors.

cond-mat↗
arXiv1993

Impurities in "Odd-Pairing" Superconductors

We present the results of theoretical analysis of normal impurities effects in superconductors with the gap being an odd function of k-k_{F}. This model proposed by Mila and Abrahams leads to the possibility of pairing in the presence of arbitrarily strong short-range repulsion between electrons and may be applied to high-Tc oxides.However, we demonstrate that normal impurities lead to rather strong suppression of this type of pairing, which is actually stronger than in the case of magnetic impurities in traditional superconductors. Relative stability of high-Tc cuprates to disordering makes this model a rather unlikely candidate for the pairing mechanism in these systems.

cond-mat↗