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Yu. N. Ovchinnikov

Publications and source records attributed to Yu. N. Ovchinnikov.

18 recordsLinked to original sources
arXiv2015

How the Phase Slips in a Current-Biased Narrow Superconducting Stripe?

The theory of current transport in a narrow superconducting channel accounting for thermal fluctuations is revisited. The value of voltage appearing in the sample is found as the function of temperature (close to transition temperature $T-T_{\mathrm{c}}$ $\ll T_{\mathrm{c}}$) and bias current $J<J_{\mathrm{c}}$ ( $J_{\mathrm{c}}$ is a value of critical current calculated in the framework of the BCS approximation, neglecting thermal fluctuations). It is shown that the careful analysis of vortex crossing of the stripe results in considerable increase of the activation energy.

cond-mat.supr-con↗
arXiv2014

Flat Thomas-Fermi artificial atoms

We consider two-dimensional (2D) "artificial atoms" confined by an axially symmetric potential $V(ρ)$. Such configurations arise in circular quantum dots and other systems effectively restricted to a 2D layer. Using the semiclassical method, we present the first fully self-consistent and analytic solution yielding equations describing the density distribution, energy, and other quantities for any form of $V(ρ)$ and an arbitrary number of confined particles. An essential and nontrivial aspect of the problem is that the 2D density of states must be properly combined with 3D electrostatics. The solution turns out to have a universal form, with scaling parameters $ρ^2/R^2$ and $R/a_B^*$ ($R$ is the dot radius and $a_B^*$ is the effective Bohr radius).

cond-mat.mes-hall↗
arXiv2011

On Spectra of Linearized Operators for Keller-Segel Models of Chemotaxis

We consider the phenomenon of collapse in the critical Keller-Segel equation (KS) which models chemotactic aggregation of micro-organisms underlying many social activities, e.g. fruiting body development and biofilm formation. Also KS describes the collapse of a gas of self-gravitating Brownian particles. We find the fluctuation spectrum around the collapsing family of steady states for these equations, which is instrumental in derivation of the critical collapse law. To this end we develop a rigorous version of the method of matched asymptotics for the spectral analysis of a class of second order differential operators containing the linearized Keller-Segel operators (and as we argue linearized operators appearing in nonlinear evolution problems). We explain how the results we obtain are used to derive the critical collapse law, as well as for proving its stability.

nlin.PS↗
arXiv2009

Fluctuation-Dissipative Phenomena in a Narrow Superconducting Channel Carrying Current Below Critical

The theory of current transport in a narrow superconducting channel accounting for thermal fluctuations is developed. These fluctuations result in the appearance of small but finite dissipation in the sample. The value of corresponding voltage is found as the function of temperature (close to transition temperature) and arbitrary bias current. It is demonstrated that the value of the activation energy (exponential factor in the Arrenius law) when current approaches to the critical one is proportional to (1-J/Jc)^(5/4). This result is in concordance with the one for the affine phenomenon of the Josephson current decay due to the thermal phase fluctuations, where the activation energy proportional (1-J/J_c)^(3/2)(the difference in the exponents is related to the additional current dependence of the order parameter). Found dependence of the activation energy on current explains the enormous discrepancy between the theoretically predicted before and the experimentally observed broadening of the resistive transition.

cond-mat.supr-con↗
arXiv2009

On collapse of wave maps

We derive the universal collapse law of degree 1 equivariant wave maps (solutions of the sigma-model) from the 2+1 Minkowski space-time,to the 2-sphere. To this end we introduce a nonlinear transformation from original variables to blowup ones. Our formal derivations are confirmed by numerical simulations.

math.AP↗
arXiv2008

The effect of Magnetic Field on MQT Escape Time in "Small" Josephson Junctions

We study the phenomenon of macroscopic quantum tunneling (MQT) in a finite size Josephson junction (JJ) with an externally applied magnetic field. As it is well known, the problem of MQT in a point-like JJ is reduced to the study of the under-barrier motion of a quantum particle in the washboard potential. In the case of a finite size JJ placed in magnetic field, this problem is considerably more complex since, besides the phase, the potential itself, depends on space variables. We find the general expressions both for the crossover temperature $T_{0}$ between thermally activated and macroscopic quantum tunneling regimes and the escape time $τ_{\mathrm{esc}}$. It turns out that in proximity of particular values of magnetic field the crossover temperature can vary non-monotonically.

cond-mat.supr-con↗
arXiv2007

Macroscopic quantum tunneling in "small" Josephson junctions in magnetic field

We study the phenomenon of macroscopic quantum tunneling (MQT) in small Josephson junctions (JJ) with an externally applied magnetic field. The latter results in the appearance of the Fraunhofer type modulation of the current density along the barrier. The problem of MQT for a point-like JJ is reduced to the motion of the quantum particle in the washboard potential. In the case of a finite size JJ under consideration, this problem corresponds to a MQT in potential which itself, besides the phase, depends on space variables. Finally, the general expression for the crossover temperature T_0 between thermally activated and macroscopic quantum tunneling regimes and the escaping time tau_esc have been calculated.

cond-mat.supr-con↗
arXiv2006

Resonance phenomena in Macroscopic Quantum Tunneling: the small viscosity limit

We present a new theoretical approach to describe the quantum behavior of a macroscopic system interacting with an external irradiation field, close to the resonant condition. Here we consider the extremely underdamped regime for a system described by a double well potential. The theory includes both: transitions from one well to the other and relaxation processes. We simulate resonant phenomena in a rf-SQUID, whose parameters lie in the range typically used in the experiments. The dependence of the transition probability W on the external drive of the system $ϕ_x$ shows three resonance peaks. One peak is connected with the resonant tunneling and the two others with the resonant pumping. The relative position of the two peaks correlated to the resonant pumping depends on the pumping frequency $ν$ and on the system parameters. The preliminary measurements on our devices show a low dissipation level and assure that they are good candidates in order to realize new experiments on the resonant phenomena in the presence of an external microwave irradiation of proper frequency.

quant-ph↗
arXiv2005

Fast electronic relaxation in metal nanoclusters via excitation of coherent shape deformations: Circumventing a bottleneck

Electron-phonon relaxation in size-quantized systems may become inhibited when the spacing of discrete electron energy levels exceeds the magnitude of the phonon frequency. We show, however, that nanoclusters can support a fast nonradiative relaxation channel which derives from their distinctive ability to undergo Jahn-Teller shape deformations. Such a deformation represents a collective and coherent vibrational excitation and enables electronic transitions to occur without a multiphonon bottleneck. We analyze this mechanism for a metal cluster within the analytical framework of a three-dimensional potential well undergoing a spheroidal distortion. An expression for the time evolution of the distortion parameter is derived, the electronic level crossing condition formulated, and the probability of electronic transition at a level crossing is evaluated. An application to electron-hole recombination in a closed-shell aluminum cluster with 40 electrons shows that the short (~250 fs) excitation lifetime observed in recent pump-probe experiments can be explained by the proposed mechanism.

physics.atm-clus↗
arXiv2004

Strong vortex-antivortex fluctuations in the type II superconducting film

The small size vortex-antivortex pairs proliferation in type II superconducting film is considered for the wide interval of temperatures below Tc. The corresponding contribution to free energy is calculated. It is shown that these fluctuations give the main contribution to the heat capacity of the film both at low temperatures and in the vicinity of transition.

cond-mat.supr-con↗
arXiv2003

Lift of degeneracy of Landau levels of 2D electron gas by point-like impurities

We study the density of states of two-dimensional electron gas in magnetic field with scattering on point-like impurities that are uniformly distributed along the sample. We show that the electron-impurity interaction completely lifts the degeneracy of the Landau levels even at low impurity concentration. This statement is in contradiction with the previous results obtained in the unphysical approximation of two-dimensional impurities. There is a large region of the electron energy $ω$, counted from the Landau level, where the density of states $ρ(ω)$ is inversely proportional to $|ω|$ and proportional to the impurity concentration. Our results are applicable to different 2D electron systems as heterostructures, inversion layers and interfaces.

cond-mat.mes-hall↗
arXiv2000

Nonlinear fluctuation phenomena in the transport properties of superconductors

There exists a wide temperature region $({\rm Gi}T<T-T_c<T\sqrt{\rm Gi})$, where the influence of fluctuations on the thermodynamic properties of superconductors can be taken into account in the linear (Gaussian) approximation, while their influence on the kinetic properties is strongly nonlinear. Maki-Thompson contribution to conductivity saturates in this region. However, Aslamazov-Larkin contribution becomes even more singular. Such an enhancement is connected with the fact that nonlinear effects yield an increase in the lifetime of fluctuating pairs. Pairbreaking and energy relaxation processes can diminish the nonlinear effects.

cond-mat.supr-con↗
arXiv1999

Strong coupling in the Kondo problem in the low-temperature region

The magnetic field dependence of the average spin of a localized electron coupled to conduction electrons with an antiferromagnetic exchange interaction is found for the ground state. In the magnetic field range $μH\sim 0.5 T_c$ ($T_c$ is the Kondo temperature) there is an inflection point, and in the strong magnetic field range $μH\gg T_c$, the correction to the average spin is proportional to $(T_c/μH)^2$. In zero magnetic field, the interaction with conduction electrons also leads to the splitting of doubly degenerate spin impurity states.

cond-mat.str-el↗
arXiv1998

Effect of Magnetic Impurity Correlations on Josephson Tunneling

The ordering trend of magnetic impurities at low temperature results in the frustration of the pair-breaking effect and induces a ``recovery'' of superconducting properties. We show that this effect manifests itself in the deviation of the Josephson current amplitude from the values obtained within the Ambegaokar-Baratoff and the Abrikosov-Gor'kov models. We consider both weak and strong-coupling cases. The theory is applied to describe the experimental data obtained for the low-$T_c$ superconductor SmRh$_4$B$_4$. We further predict a ``recovery'' effect of the Josephson current in high-temperature superconductors.

cond-mat.supr-con↗
arXiv1997

Nuclear magnetic susceptibility of metals with magnetic impurities

We consider the contribution of magnetic impurities to the nuclear magnetic susceptibility $χ$ and to the specific heat $C$ of a metal. The impurity contribution to the magnetic susceptibility has a $1/T^2$ behaviour, and the impurity contribution to the specific heat has a $1/T$ behaviour, both in an extended region of temperatures $T$. In the case of a dirty metal the RKKY interaction of nuclear spins and impurity spins is suppressed for low temperatures and the main contribution to $C$ and $χ$ is given by their dipole-dipole interaction.

cond-mat.str-el↗
arXiv1997

Influence of magnetic impurities on the heat capacity of nuclear spins

It is found that in a wide range of temperatures and magnetic fields even a small concentration of magnetic impurities in a sample leads to a $T^{-1}$ temperature dependence of the nuclear heat capacity. This effect is related to a nuclear-spin polarization by the magnetic impurities. The parameter that controls the theory turns out not to be the impurity concentration $C_{imp}$ but instead the quantity $c_{imp} μ_e / μ_n$, where $μ_e$ and $μ_n$ are the magnetic moments of an electron and a nucleus, respectively. The ratio of $μ_e$ and $μ_n$ is of order of $10^3$.

cond-mat↗
arXiv1997

Resistance of Layered Superclean Superconductors at Low Temperatures

The low energy excitation spectrum is found for a layered superconductor vortex with a small number of impurities inside the vortex core. All levels are found to be correlated. This leads to the strong enhancement of conductivity in superclean layered superconductors.

cond-mat.supr-con↗