arXiv subjects

A. Shaulov

Publications and source records attributed to A. Shaulov.

11 recordsLinked to original sources

arXiv2006

Flux-flow resistivity anisotropy in the instability regime in the a-b plane of epitaxial YBCO thin films

Measurements of the nonlinear flux-flow resistivity $ρ$ and the critical vortex velocity $\rm v^*_ϕ$ at high voltage bias close to the instability regime predicted by Larkin and Ovchinnikov \cite{LO} are reported along the node and antinode directions of the d-wave order parameter in the \textit{a-b} plane of epitaxial $YBa_2Cu_3O_{7-δ}$ films. In this pinning-free regime, $ρ$ and $\rm v^*_ϕ$ are found to be anisotropic with values in the node direction larger on average by 10% than in the antinode direction. The anisotropy of $ρ$ is almost independent of temperature and field. We attribute the observed results to the anisotropic quasiparticle distribution on the Fermi surface of $YBa_2Cu_3O_{7-δ}$.

cond-mat.supr-con↗

arXiv2003

Dynamics of transient disordered vortex states in BSCCO

The dynamics of transient disordered vortex states in BSCCO was magneto-optically traced in three experiments: (i) during continuous injection of transient vortex states while ramping up the external magnetic field, (ii) during annealing of injected transient states while keeping the external field constant, and (iii) during annealing of transient 'supercooled' disordered states while ramping down the external field. The results reveal front-like propagation (experiment i) or retreat (experiments ii and iii) of the transient vortex states, at a rate governed by the rate of change of the external field, the annealing time tau of the transient states and the creep rate. The experimental results are theoretically analyzed in terms of competition between generation and annealing of transient disordered vortex states. Extraction of the annealing time tau from the above three experiments, yields the same results for tau as a function of the induction, B, and temperature T. Knowledge of tau(B,T) allows for correct determination of the thermodynamic order-disorder vortex phase transition line.

cond-mat.supr-con↗

arXiv2002

Unified order-disorder vortex phase transition in high-Tc superconductors

The diversity of vortex melting and solid-solid transition lines measured in different high-T$_{c}$ superconductors is explained, postulating a unified order-disorder phase transition driven by both thermally- and disorder-induced fluctuations. The temperature dependence of the transition line and the nature of the disordered phase (solid, liquid, or pinned liquid) are determined by the relative contributions of these fluctuations and by the pinning mechanism. By varying the pinning mechanism and the pinning strength one obtains a spectrum of monotonic and non-monotonic transition lines similar to those measured in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$%_{8}$, YBa$_{2}$Cu$_{3}$O$_{7-δ}$, Nd$_{1.85}$Ce$_{0.15}$CuO$%_{4-δ}$, Bi$_{1.6}$Pb$_{0.4}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ and (La$% _{0.937}$Sr$_{0.063}$)$_{2}$CuO$_{4}$

cond-mat.supr-con↗

arXiv2001

Disorder and thermally driven vortex-lattice melting in La{2-x}Sr{x}CuO{4} crystals

Magnetization measurements in La{2-x}Sr{x}CuO{4} crystals indicate vortex order-disorder transition manifested by a sharp kink in the second magnetization peak. The transition field exhibits unique temperature dependence, namely a strong decrease with temperature in the entire measured range. This behavior rules out the conventional interpretation of a disorder-driven transition into an entangled vortex solid phase. It is shown that the transition in La{2-x}Sr{x}CuO{4} is driven by both thermally- and disorder-induced fluctuations, resulting in a pinned liquid state. We conclude that vortex solid-liquid, solid-solid and solid to pinned-liquid transitions are different manifestations of the same thermodynamic order-disorder transition, distinguished by the relative contributions of thermal and disorder-induced fluctuations.

cond-mat.supr-con↗

arXiv2001

Possibility of Kauzmann points in the vortex matter phase diagram of single crystal YBa$_2$Cu$_3$O$_{7-δ}$

We highlight interesting thermomagnetic history effects across the transition line between the (quasi) ordered and disordered vortex states in single crystal YBa$_2$Cu$_3$O$_{7-δ}$, and argue that these features are indicative of the first order nature of the transition line. We suggest that the destruction of the ordered vortex state in YBa$_2$Cu$_3$O$_{7-δ}$ leading to vortex liquid (at high temperatures and low fields) and amorphous vortex solid (at low temperatures and high fields), takes place along a unified first-order transition line. The nonmonotonic behavior of this first order transition line gives rise to the possibility of more than one Kauzmann point where the entropies of the ordered and disordered vortex states are equal. In the high temperature region, one may order the vortex lattice by warming it, giving rise to an inverse melting effect.

cond-mat.supr-con↗

arXiv2001

Crystallization of the ordered vortex phase in high temperature superconductors

The Landau-Khalatnikov time-dependent equation is applied to describe the crystallization process of the ordered vortex lattice in high temperature superconductors after a sudden application of a magnetic field. Dynamic coexistence of a stable ordered phase and an unstable disordered phase, with a sharp interface between them, is demonstrated. The transformation to the equilibrium ordered state proceeds by movement of this interface from the sample center toward its edge. The theoretical analysis dictates specific conditions for the creation of a propagating interface, and provides the time scale for this process.

cond-mat.supr-con↗

arXiv2000

Fluctuations in $YBa_{2}Cu_{3}O_{6.5}$ single crystal: an evidence for 2D->3D crossover

Magnetization measurements as a function of temperature are reported for $YBa_{2}Cu_{3}O_{6.5}$ crystal (T_{c}=45.2 K) for fields between 0.2 to $3.5\ Tesla$. All isochamps for H>1 Tesla intersect at $T_{2D}^{\star}\simeq 42.8\ K$, implying fluctuations contribution to the magnetization. These curves collapse into a single curve when magnetization and temperature are scaled according to the predicted ''2D scaling'' in the fluctuation regime. Surprisingly, the low field curves also intersect, at $T_{3D}^{\star}\simeq 43.4 K$, and they obey a 3D scaling. We provide a theoretical picture of the magnetization in the fluctuation regime based on the Lawrence-Doniach model. Within this model we calculate the field and temperature dependence of the magnetization. The two intersection points and the $2D\to 3D$ crossover are consistent with the experimental observation.

cond-mat.supr-con↗

arXiv1998

Current density inhomogeneity throughout the thickness of superconducting films and its effect on their irreversible magnetic properties

We calculate the distribution of the current density $j$ in superconducting films along the direction of an external field applied perpendicular to the film plane. Our analysis reveals that in the presence of bulk pinning $j$ is inhomogeneous on a length scale of order the inter vortex distance. This inhomogeneity is significantly enhanced in the presence of surface pinning. We introduce new critical state model, which takes into account the current density variations throughout the film thickness, and show how these variations give rise to the experimentally observed thickness dependence of $% j$ and magnetic relaxation rate.

cond-mat.supr-con↗

arXiv1996

On the origin of the irreversibility line in thin YBaCuO7 films with and without columnar defects

We report on measurements of the angular dependence of the irreversibility temperature $T_{irr}(θ) $ in $YBa_2Cu_3O_{7-δ}$ thin films, defined by the onset of a third harmonic signal and measured by a miniature Hall probe. From the functional form of $T_{irr}(θ)$ we conclude that the origin of the irreversibility line in unirradiated films is a dynamic crossover from an unpinned to a pinned vortex liquid. In irradiated films the irreversibility temperature is determined by the trapping angle.

supr-con↗

arXiv1996

Plastic vortex-creep in YBa$_2$Cu$_3$O$_{7-x}$ crystals

Local magnetic relaxation measurements in YBa$_2$Cu$_3$O$_{7-x}$ crystals show evidence for plastic vortex-creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the ''fishtail'' magnetization. In this range the activation energy $U_{pl}$, which decreases with field, drops below the elastic (collective) creep activation energy, $U_{el}$, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa$_2$Cu$_3$O$_{7-x}$.

supr-con↗

arXiv1996

Irreversible magnetization in thin YBCO films rotated in external magnetic field

The magnetization M of a thin YBaCuO film is measured as a function of the angle $θ$ between the applied field H and the c-axis. For fields above the first critical field, but below the Bean's field for first penetration H*, M is symmetric with respect to $θ=π$ and the magnetization curves for forward and backward rotation coincide. For H>H* the curves are asymmetric and they do not coincide. These phenomena have a simple explanation in the framework of the Bean critical state model.

supr-con↗