arXiv subjects

V. M. Loktev

Publications and source records attributed to V. M. Loktev.

At least 55 records · Page 3Linked to original sources

arXiv2006

Impurity effects in a two--dimensional system with Dirac spectrum

It is demonstrated that in a two--band 2D system the resonance state is manifested close to the energy of the Dirac point in the electron spectrum for the sufficiently large impurity perturbation. With increasing the impurity concentration, the electron spectrum undergoes the rearrangement, which is characterized by the opening of the broad quasi--gap in the vicinity of the nodal point. If the critical concentration for the spectrum rearrangement is not reached, the domain of localized states remains exponentially small compared to the bandwidth.

cond-mat.dis-nn↗

arXiv2004

A possible scenario of metallization in boron doped diamond CB$_x$

Possibility for collectivization of acceptor states in a semiconductor, converting it to metal, is discussed within the scope of Anderson s-d hybride model. This model is generalized for multicomponent band structure and composite acceptor states, localized on pairs of neighbor dopants (impurity "dumbbells"), in order to describe boron doped diamond CB$_x$. The resulting parameters of band structure, in particular, position of the Fermi level, are compared to the recent experimental data on metallized and superconducting CB$_x$.

cond-mat.dis-nn↗

arXiv2004

On the theory of pseudogap anisotropy in the cuprate superconductors

We show by means of the theory of the order parameter phase fluctuations that the temperature of "closing" (or "opening") of the gap (and pseudogap) in the electron spectra of superconductors with anisotropic order parameter takes place within a finite temperature range. Every Fourier-component of the order parameter has its own critical temperature.

cond-mat.supr-con↗

arXiv2003

Group expansions for impurities in superconductors

A new method is proposed for practical calculation of the effective interaction between impurity scatterers in superconductors, based on algebraic properties of related Nambu matrices for Green functions. In particular, we show that the density of states within the s-wave gap can have a non-zero contribution (impossible either in Born and in T-matrix approximation) from non-magnetic impurities with concentration $c \ll 1$, beginning from $\sim c^{3}$ order.

cond-mat.supr-con↗

arXiv2003

Boundary Friction on Molecular Lubricants: Rolling Mode?

A theoretical model is proposed for low temperature friction between two smooth rigid solid surfaces separated by lubricant molecules, admitting their deformations and rotations. Appearance of different modes of energy dissipation (by ''rocking'' or ''rolling'' of lubricants) at slow relative displacement of the surfaces is shown to be accompanied by the stick-and-slip features and reveals a non-monotonic (mean) friction force {\it vs} external load

cond-mat.mtrl-sci↗

arXiv2003

Nodal quasiparticles in doped d-wave superconductors: self-consistent T-matrix approach

A comparative analysis has been done of the formerly established two self-consistent solutions for the density of quasiparticle states in doped d-wave superconductors, having strikingly different and disputed behavior near the Fermi energy. One of them (1) remains finite in this limit, while the other (2) tends to zero. To resolve this discrepancy, the known Ioffe-Regel criterion for band states, widely used for doped semiconductors, was applied to these solutions. It is shown that both them are valid in limited and different energy regions, where the corresponding quasiparticles are weakly damped. In particular, density of states of nodal quasiparticles near the Fermi level is provided by the (2) solution, while the (1) only applies far enough from this level.

cond-mat.supr-con↗

arXiv2003

Superconducting properties of the 2D models with different types of inter-particle coupling

The superconducting properties of the 2D fermion system with local and different types of the indirect boson-exchange attractions in the cases of s-wave and d-wave pairing are reviewed and analysed at T=0. In particular, the possibility of the crossover from the Bose-Einstein condensation regime to the BCS-like superconductivity with the carrier density and coupling changing in the case of different pairing channels is discussed. Gaussian fluctuations of the order parameter are taken into account and the carrier density dependence of the gap is studied in this case. The role of the interaction form in the physical behavior of the systems is also discussed.

cond-mat.supr-con↗

arXiv2003

Temperature-doping phase diagram of layered superconductors

The superconducting properties of a layered system are analyzed for the cases of zero- and non-zero angular momentum of the pairs. The effective thermodynamic potential for the quasi-2D XY-model for the gradients of the phase of the order parameter is derived from the microscopic superconducting Hamiltonian. The dependence of the superconducting critical temperature T_c on doping, or carrier density, is studied at different values of coupling and inter-layer hopping. It is shown that the critical temperature T_c of the layered system can be lower than the critical temperature of the two-dimensional Berezinskii-Kosterlitz-Thouless transition T_BKT at some values of the model parameters, contrary to the case when the parameters of the XY-model do not depend on the microscopic Hamiltonian parameters.

cond-mat.supr-con↗

arXiv2003

Fluctuating order parameter in doped cuprate superconductors

We discuss static fluctuations of the d-wave superconducting order parameter $Δ$ in CuO$_2$ planes, due to quasiparticle scattering by charged dopants. The analysis of two-particle anomalous Green functions at $T = 0$ permits to estimate the mean-square fluctuation $δ^2 = <Δ^2> - <Δ>^2$, averaged in random dopant configurations, to the lowest order in doping level $c$. Since $Δ$ is found to saturate with growing doping level while $δ$ remains to grow, this can explain the collapse of $T_c$ at overdoping. Also we consider the spatial correlations $<Δ(0)Δ({\bf R})>$ for order parameter in different points of the plane.

cond-mat.supr-con↗

arXiv2002

Magnetic effect from non-magnetic impurity in superconducting CuO_{2} plane

We propose a new model for impurity center formed by a cathion substitute for Cu, like Zn, in CuO_{2} planes. Its main effect on superconducting electrons is due to the non-zero exchange field on O sites, neighbors to the (non-magnetic) impurity. We discuss a strong suppression of d-wave order parameter, a zero- energy resonance in local density of states, and spin polarization of charge carriers, which can be related to the experimentally observed effects in Zn-doped copper oxides. These results are obtained \emph{without} using the unitary scattering limit.

cond-mat.supr-con↗

arXiv2002

Crossover from superfluidity to superconductivity in a system with doping dependent attraction

Zero temperature crossover from superfluidity to superconductivity with carrier density increasing is studied for a two-dimensional system in the s-wave and d-wave pairing channels. It was assumed that the particle attraction correlation length depends on carrier density $n_f$ as $r_0\sim1/\sqrt{n_f}$. Such a dependence was found experimentally for the radius of magnetic correlations in $La_{2-x}Sr_xCuO_4$. The short range Coulomb repulsion was also taken into account. It is shown that the behaviour of the system with doping is fundamentally different from the case with $r_0(n_f)=const$. In particular, similarly to the d-wave case, the crossover in the s-channel takes place only if the coupling is larger of some minimal value, otherwise the Cooper pairing scenario takes place at any small carrier density. The relevance of the model to high-temperature superconductors is discussed.

cond-mat.supr-con↗

arXiv2001

Quantum Phase Fluctuations Responsible for Pseudogap

The effect of ordering field phase fluctuations on the normal and superconducting properties of a simple 2D model with a local four-fermion attraction is studied. Neglecting the coupling between the spin and charge degrees of freedom an analytical expression has been obtained for the fermion spectral function as a single integral over a simple function. From this we show that, as the temperature increases through the 2D critical temperature and a nontrivial damping for a phase correlator develops, quantum fluctuations fill the gap in the quasiparticle spectrum. Simultaneously the quasiparticle peaks broaden significantly above the critical temperature, resembling the observed pseudogap behavior in high-T_c superconductors.

cond-mat.supr-con↗

arXiv2001

Formation of d-wave superconducting order in a randomly doped lattice

We consider the interplay between superconducting coupling and dopant impurity scattering of charge carriers in planar square lattice systems and examine physical conditions (doping level, temperature, local symmetry of coupling and scattering potentials) necessary in this model system to obtain a d-wave superconducting order, like that observed in real doped cuprate HTSC materials. Using the Lifshitz model for the disorder introduced into system by dopants, we analyze also the non-uniform structure of such d-wave parameter, including both its magnitude and phase variation. The results indicate that d-wave superconductivity turns possible in a doped metal until it can be destroyed at too high doping levels.

cond-mat.supr-con↗

arXiv2000

Green function of fermions in 2D superconducting Frohlich model with inhomogeneous order parameter

The fermion Green function and spectral characteristics for the 2D Frohlich model of superconductivity at static fluctuations in the phase of the order parameter are calculated. The results demonstrate strongly non-Fermi-liquid properties of the system at finite temperatures and relate with the pseudogap behavior of high-$T_{c}$ superconductors at relatively small charge carrier densities.

cond-mat.supr-con↗

arXiv1998

On temperature versus doping phase diagram of high critiical temperature superconductors

The attempt to describe the bell-shape dependence of the critical temperature of high-$T_{c}$ superconductors on charge carriers density is made. Its linear increase in the region of small densities (underdoped regime) is proposed to explain by the role of the order parameter phase 2D fluctuations which become less at this density growth. The critical temperature suppression in the region of large carrier densities (overdoped regime) is connected with the appearance (because of doping) of the essential damping of long-wave bosons which in the frame of the model proposed define the mechanism of indirect inter-fermion attraction.

cond-mat.supr-con↗

arXiv1997

Superconducting Condensate Formation in Metallic Systems with Arbitrary Carrier Density (review)

This article gives a contemporary and to some extent pedagogical review of the current theoretical understanding of the formation of the superconducting state in metallic systems with a variable density of carriers. We make an attempt to describe the crossover from the Bose-Einstein condensation type (small densities) to the Bardeen-Cooper-Schrieffer one (large densities). The functional methods are used throughout the treatment. The most of the results are considered in a review form for the first time. Some of them (in particular, the possible opening of a pseudogap) are used to explain the experimental data avaliable for high-temperatures superconductors.

cond-mat.supr-con↗

arXiv1997

On the Theory of the Pseudogap Formation in 2D Attracting Fermion Systems

Two-dimensional system of the fermions with the indirect Einstein phonon-exchange attraction and added local four-fermion interaction is considered. It is shown that in such a system at resulting attraction between particles a new nonsuperconducting phase arises along with the normal and superconducting phases. In this, called "abnormal normal", or pseudogap, phase the absolute value of the order parameter is finite but its phase is a random quantity. It is important that the new phase really exists at low carrier density only, i.e. it shrinks with doping increasing in the case of phonon attraction. The relevance of the results for high-temperature superconductors is speculated. Key words: 2D metal, arbitrary carrier density, normal phase, abnormal normal phase, pseudogap, suderconducting phase, Berezinskii-Kosterlitz-Thouless phase, electron-electron and electron-hole pairing

cond-mat.supr-con↗

arXiv1997

Nonlinear dislocation dynamics and crystal sonoluminescence

The sonoluminescence of ionic semiconductors were studied. The main attention is paid to threshold phenomena which accompany the light irradiation, namely --- point defect creation and nonlinear ultrasound wave attenuation. The model for description of processes under investigation which connects the sonoluminescence excitation with the onset of point defects (vacancies and intersticials) generation by moving under ultrasound action screw dislocation with a jog. The attempt is made to estimate the parameters of crystals which define the jog motion in its cristal relief.

cond-mat.mtrl-sci↗