arXiv subjects

A. E. Koshelev

Publications and source records attributed to A. E. Koshelev.

84 records · Page 5Linked to original sources

arXiv1999

Plasma resonance at low magnetic fields as a probe of vortex line meandering in layered superconductors

We consider the magnetic field dependence of the plasma resonance frequency in pristine and in irradiated Bi$_2$Sr$_2$CaCu$_2$O$_8$ crystals near $T_c$. At low magnetic fields we relate linear in field corrections to the plasma frequency to the average distance between the pancake vortices in the neighboring layers (wandering length). We calculate the wandering length in the case of thermal wiggling of vortex lines, taking into account both Josephson and magnetic interlayer coupling of pancakes. Analyzing experimental data, we found that (i) the wandering length becomes comparable with the London penetration depth near T$_{c}$ and (ii) at small melting fields ($< 20$ G) the wandering length does not change much at the melting transition. This shows existence of the line liquid phase in this field range. We also found that pinning by columnar defects affects weakly the field dependence of the plasma resonance frequency near $T_c$.

cond-mat.supr-con↗

arXiv1999

Evaporation of the pancake-vortex lattice in weakly-coupled layered superconductors

We calculate the melting line of the pancake-vortex system in a layered superconductor, interpolating between two-dimensional (2D) melting at high fields and the zero-field limit of single-stack evaporation. Long-range interactions between pancake vortices in different layers permit a mean-field approach, the ``substrate model'', where each 2D crystal fluctuates in a substrate potential due to the vortices in other layers. We find the thermal stability limit of the 3D solid, and compare the free energy to a 2D liquid to determine the first-order melting transition and its jump in entropy.

cond-mat.supr-con↗

arXiv1999

Interlayer Quasiparticle Transport in the Vortex State of Josephson Coupled Superconductors

We calculate the dependence of the interlayer quasiparticle conductivity, $σ_q$, in a Josephson coupled d-wave superconductor on the magnetic field B||c and the temperature T. We consider a clean superconductor with resonant impurity scattering and a dominant coherent interlayer tunneling. When pancake vortices in adjacent layers are weakly correlated at low T the conductivity increases sharply with B before reaching an extended region of slow linear growth, while at high T it initially decreases and then reaches the same linear regime. For correlated pancakes $σ_q$ increases much more strongly with the applied field.

cond-mat.supr-con↗

arXiv1999

Fluctuation broadening of plasma resonance line in the vortex liquid state of layered superconductors

The Josephson plasma resonance (JPR) provides a sensitive probe of vortex states in layered superconductors. We demonstrate that in the case of weak damping in the liquid phase, broadening of the JPR line is caused mainly by random Josephson coupling arising from the density fluctuations of pancake vortices. In this case the JPR line has the universal shape, which is determined only by parameters of the superconductors and temperature. This mechanism gives a natural explanation for the experimentally observed asymmetric lineshape. The tail at high frequencies arises due to mixing of the propagating plasma modes by random Josephson coupling, while the tail at small frequencies is caused by the localized plasma modes originating from a rare fluctuation suppression of the Josephson coupling in large areas.

cond-mat.supr-con↗

arXiv1999

Crossing lattices, vortex chains, and angular dependence of melting line in layered superconductors

We investigate vortex structure and melting transition in very anisotropic layered superconductors at fields tilted with respect to the c-axis. We show that even a small in-plane field does not homogeneously tilt the vortex lattice but, instead, penetrates inside the superconductor in the form of Josephson vortices (JVs) similar to the Meissner state. At high c-axis magnetic field the phase field of the JV is built up from the phase perturbations created by displacements of pancake vortices. The crossing-lattices ground state leads to linear dependencies of the melting field and melting temperature on the in-plane field, in agreement with recent experimental observations. At small fields stacks of JVs accumulate additional pancake strings creating vortex rows with enhanced density. This mechanism explains the mixed chains-lattice state observed by Bitter decorations.

cond-mat.supr-con↗

arXiv1999

Angular Dependence of C-Axis Magnetoresistance in Bi-2212 Single Crystals With Columnar Defects

We measured the angular dependence of the c-axis magnetoresistance rho_c(B) of Bi-2212 irradiated with heavy ions. At temperatures near 68 K the scaling of rho_c(B) with the c-axis magnetic field component B_perp is broken and the in-plane field, B_parallel, affects rho_c. At this temperature, at a specific field B_cr \approx B_Phi/2, magnetoresistance becomes independent of field orientation. This crossing point allows us to estimate the correlation length L of pancake positions along the c axis. We find L/s is about 100 at B=B_cr, where s is the interlayer spacing. This provides evidence of strong enhancement of pancake alignment in the vortex liquid in crystals with columnar defects.

cond-mat.supr-con↗

arXiv1999

Structure of vortex liquid phase in irradiated BSCCO(2212) crystals

The c-axis resistivity in irradiated and in pristine BSCCO(2212) crystals is measured as a function of the in-plane magnetic field component at fixed out-of-plane component B_\perp in the vortex liquid phase at T=67 K. From this data we extract the dependence of the phase difference correlation length inside layers on B_\perp and estimate the average length of pieces of vortex lines confined inside columnar defects as a function of the filling factor f=B_\perp / B_ϕ. The maximum length, about 15 interlayer distances, is reached near f=0.35.

cond-mat.supr-con↗

arXiv1998

Universal properties for linelike melting of the vortex lattice

Using numerical results obtained within two models describing vortex matter (interacting elastic lines (Bose model) and uniformly frustrated XY-model) we establish universal properties of the melting transition within the linelike regime. These properties, which are captured correctly by both models, include the scaling of the melting temperature with anisotropy and magnetic field, the effective line tension of vortices in the liquid regime, the latent heat, the entropy jump per entanglement length, and relative jump of Josephson energy at the transition as compared to the latent heat. The universal properties can serve as experimental fingerprints of the linelike regime of melting. Comparison of the models allows us to establish boundaries of the linelike regime in temperature and magnetic field.

cond-mat.supr-con↗

arXiv1998

Josephson Plasma Resonance as a Structural Probe of Vortex Liquid

Recent developments of the Josephson plasma resonance and transport c-axis measurements in layered high T$_{c}$ superconductors allow to probe Josephson coupling in a wide range of the vortex phase diagram. We derive a relation between the field dependent Josephson coupling energy and the density correlation function of the vortex liquid. This relation provides a unique opportunity to extract the density correlation function of pancake vortices from the dependence of the plasma resonance on the $ab$-component of the magnetic field at a fixed $c$-axis component.

cond-mat.supr-con↗

arXiv1998

Pinning-induced transition to disordered vortex phase in layered superconductors

Destruction of the vortex lattice by random point pinning is considered as a mechanism of the ``second peak'' transition observed experimentally in weakly coupled layered high temperature superconductors. The transition field separating the topologically ordered quasilattice from the amorphous vortex configuration is strongly influenced by the layered structure and by the nonlocal nature of the vortex tilt energy due to the magnetic interlayer coupling. We found three different regimes of transition depending on the relative strength of the Josephson and magnetic couplings. The regimes can be distinguished by the dependence of the transition field

cond-mat.supr-con↗

arXiv1996

Plasma resonance and remaining Josephson coupling in the ``decoupled vortex liquid phase'' in layered superconductors

We relate the frequency of the plasma resonance in layered superconductors with the frequency dependent superconducting response along the c-direction. We demonstrate that the sharp resonance can persist even when the global superconducting coherence in this direction is absent provided the resonance frequency is larger than the typical frequency of interlayer phase slips. In this situation the plasma frequency is determined by the average Josephson energy, which can be calculated using the high temperature expansion. We also find the temperature dependence of the average Josephson energy from the Monte Carlo simulations of the uniformly frustrated XY model and determine the applicability region of the high temperature expansion. The field and temperature scaling of the plasma frequency suggested by the high temperature expansion is consistent with experiment.

cond-mat↗

arXiv1996

Mechanism of thermally activated c-axis dissipation in layered High-T$_c$ superconductors at high fields

We propose a simple model which explains experimental behavior of $c$-axis resistivity in layered High-T$_c$ superconductors at high fields in a limited temperature range. It is generally accepted that the in-plane dissipation at low temperatures is caused by small concentration of mobile pancake vortices whose diffusive motion is thermally activated. We demonstrate that in such situation a finite conductivity appears also in $c$-direction due to the phase slips between the planes caused by the mobile pancakes. The model gives universal relation between the components of conductivity which is in good agreement with experimental data.

cond-mat↗