D. Giller

Nucleation and growth of the quasi-ordered vortex phase in Bi2Sr2CaCu2O8+δ

Abstract The time evolution of the vortex structure in a Bi2Sr2CaCu2O8+δ crystal is magneto-optically recorded after a sudden change of the external magnetic field applied to the sample. The magneto-optical images reveal dynamic coexistence of a quasi-ordered vortex phase and a transient disordered vortex phase. The front of the quasi-ordered state moves with time toward the sample edges (center) when the field is suddenly increased (decreased), until a quasi-ordered state is established throughout the entire sample, as dictated by thermodynamics. Effects of magnetic relaxation on the growth rate of the quasi-ordered state are discussed.

Investigation of flux creep in high-Tc superconductors using Hall-sensor array

The details of a new method for studying thermally activated flux creep in thin superconducting samples is described. The method employs a linear array of microscopic Hall sensors to measure the time and spatial dependence of the magnetic induction across the sample. These data are analyzed on the basis of the local rate equation for thermally activated flux motion, taking into account both the in-plane and out-of-plane components of the induction field. Following this analysis, flux creep parameters, such as the flux-line current density, flux-line average velocity, and the activation energy for flux creep, can be directly determined as a function of position and time. New experimental data in a superconducting Nd1.85Ce0.15CuO4−δ crystal demonstrate this method.

Magneto-optical imaging of transient vortex states in superconductors

A high temporal resolution magneto-optical system is employed to follow the crystallization process of the quasiordered vortex state in Bi2Sr2CaCu2O8+δ crystals following a sudden change in the applied magnetic field. Two types of experiments are performed. In the first one the sample is suddenly exposed to a steady magnetic field smaller than the vortex order–disorder transition field, Bod. In the second type of experiment the sample is initially exposed to an external field larger than Bod, and then the field is suddenly reduced. The two types of experiments reveal growth of the quasiordered state proceeding in opposite directions: from the sample center toward its edge in the first experiment, and from the sample edge toward the center in the second experiment. This motion enables tracing of the time evolution of the thermodynamic quasiordered vortex phase in the early stages of its formation.

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.

Determination of the microscopic pinning mechanism in high-temperature superconductors

Abstract We report on a new, relatively simple, magnetic method to determine the dominant pinning mechanism: Spatial fluctuations of the transition temperature Tc (“δTc-pinning”) or of the charge carrier mean free path l (“δl-pinning”). The method is based on measuring the magnetization curves in the range of the second anomalous peak, identifying the field Bss which signifies the vortex solid–solid transition. The qualitative temperature dependence of Bss determines the pinning mechanism: Bss increasing (decreasing) with T indicates δl-pinning (δTc-pinning). Using this method we have determined the pinning mechanism in YBa2Cu3O7−δ and Nd1.85Ce0.15CuO4−δ crystals to be δl-pinning and δTc-pinning, respectively.

SELF-ORGANIZATION OF VORTICES IN TYPE-II SUPERCONDUCTORS DURING MAGNETIC RELAXATION

We revise the applicability of the theory of self-organized criticality (SOC) to the process of magnetic relaxation in type-II superconductors. The driving parameter of self-organization of vortices is the energy barrier for flux creep and not the current density. The power spectrum of the magnetic noise due to vortex avalanches is calculated and is predicted to vary with time during relaxation.

Possibility of Kauzmann points in the vortex matter phase diagram of single crystal YBa2Cu3O7-δ

Abstract 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 (YBCO), 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 YBCO 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 non-monotonic 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.

Dynamics of the second magnetization peak in Bi2Sr2CaCu2O8+δ

Abstract Local magnetization curves at different times, extracted from high-temporal resolution magneto-optical measurements in Bi 2 Sr 2 CaCu 2 O 8+ δ , demonstrate the absence of the second magnetization peak (SMP) at short times, its appearance at longer times, and the movement of its onset toward higher induction fields approaching the limit of the thermodynamic order–disorder transition field. We utilize theoretical analysis, based on the Landau–Khalatnikov dynamic equation adopted for the vortex matter order parameter, to simulate the measured time evolution of the SMP. We show that the observed time evolution is a manifestation of a dynamic coexistence of transient-disordered and ordered vortex phases.

Extraction of current density distribution from local magnetic measurements

Abstract We describe a method to extract the current density distribution in a superconducting sample from measurements of the time dependence of the magnetic induction profiles across the sample. Application of this method is demonstrated for a Nd1.85Ce0.15CuO4−δ single crystal in the remanent state.

Magneto-optical imaging of transient vortex states in Bi2Sr2CaCu2O8+δ crystals

Abstract Transient vortex states, created after a sudden exposure of a Bi 2 Sr 2 CaCu 2 O 8+δ crystal to a steady magnetic field, are observed using a high-temporal resolution magneto-optical imaging system. The images reveal dynamic coexistence of a disordered phase near the sample edges and a quasi-ordered phase in the sample interior, prior to the establishment of a quasi-ordered state throughout the entire sample as dictated by the thermodynamic conditions. Different points on the borderline between these two phases move toward the sample edges at different rates, depending on the local induction.