Results 1 - 10 of 103
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[en] Magnetic induction profiles across the width of a Bi2Sr2CaCu2O8+δ crystal were measured while the external magnetic field was ramped down from above the vortex solid-solid phase transition field down to zero. These profiles indicate dynamic coexistence of two vortex phases: a disordered phase in the sample interior and a quasi-ordered phase near the sample edges. The border between these two phases, marked by an abrupt change in the slope of the profiles, progresses towards the sample center and appears at the same induction, Bf, independent of the location. By increasing the sweep rate of the external field, Bf can be monotonically decreased to values well below the transition field. These results emphasize the role of the electric field in the vortex solid-solid phase transition, and question the interpretation of such ''supercooling'' experiments as signifying the first order nature of this transition
[en] Local magnetic measurements as a function of temperature (mH(T)) in Nd1.85Ce0.15CuO4-d (NCCO) and untwinned YBa2Cu3O7-δ (YBCO) crystals reveal an abrupt increase in the local magnetization at a field dependent temperature, in a certain field range. Both crystals also exhibit a pronounced kink in their local magnetization vs field (mT(H)) curves. However, while in YBCO the anomalies in mT(H) and mH(T) curves occur along the same line in the field-temperature plane, in NCCO the (B, T) locations of the anomalies do not coincide. In both crystals the anomalies in mT(H) are identified as signifying vortex solid-solid disorder induced transition. However, they show that the anomalies in mH(T), although associated with the vortex solid-solid transition, do not necessarily indicate the location of the transition
[en] We study the effect of magnetic fields on the resistance R of a superconducting La1.84Sr0.16CuO4 film patterned into a 'double' network comprising nanosized square loops having their vertexes linked by relatively long wires. The results are compared with those obtained in a regular network of square loops of the same size. Both networks exhibit periodic dependence of R on the ratio Φ/Φ0 between the flux penetrating a loop and the superconducting flux quantum. However, while the regular network exhibit features characteristics of collective behavior of the loops, the double network exhibits a single-loop behavior. This observation indicates uncorrelated arrangements of fluxoids in the double network, in agreement with a recent theoretical prediction.
[en] Time evolution of the induction profile across a Bi2Sr2CaCu2O8+δ crystal was magneto-optically recorded in two experiments: during ramping the external magnetic field or after a sudden application of the field. The results of these experiments reveal a front-like propagation or retreat of a transient disordered vortex state, respectively. Analysis of the front velocity shows that while the retreat is continuously decelerated, the propagation is accelerated, approaching a constant velocity when the front induction reaches the vortex order-disorder phase transition field Bod. The latter observation suggests a method for determining the value of Bod, eliminating transient state effects
[en] The vortex order-disorder phase transition line in (La0.937Sr0.063)2CuO4 exhibits a steep concave decrease throughout the whole temperature range. This unusual behavior is explained postulating that in (La0.937Sr0.063)2CuO4, both thermal and disorder-induced fluctuations take part in destabilizing the vortex lattice. Irradiation of the samples with electrons causes a significant decrease in both the magnitude of the transition field and the curvature of the transition line. These results are interpreted as caused by the enhanced role of disorder-induced fluctuations as compared with thermal fluctuations
[en] Time relaxation of the local persistent current in a Bi2Sr2CaCu2O8+δ crystal was magneto-optically recorded after suddenly exposing the crystal to a constant magnetic field between 140 and 840 G. A remarkable behavior of the relaxation is revealed for fields below the order-disorder vortex phase transition: The relaxation starts at a slow rate, it then accelerates for a short period of time, after which a slow relaxation rate is resumed. We show that this acceleration marks a local transition of a transient disordered vortex state to a thermodynamically favored quasi-ordered vortex phase. The slow relaxation rates, before and after the transition, reflect ordinary thermally activated flux creep process in the disordered and quasi-ordered vortex phases, respectively
[en] A high temporal resolution magneto-optical system was employed to image the induction distribution on the surface of a Bi2Sr2CaCu2O8+δ crystal while the external magnetic field was ramped up at a constant rate. These data reveal coexistence of quasi-ordered and disordered vortex phases, near the order-disorder phase transition line. The coexistence region in the B-T phase diagram narrows down with increasing temperature or decreasing sweep rate. These observations clarify previous interpretations of phenomena associated with the fishtail, e.g. the shift of the fishtail onset to higher inductions for slower sweep rates and the absence of a fishtail at low temperatures
[en] Measurements on nanoscale structures constructed from high-temperature superconductors are expected to shed light on the origin of superconductivity in these materials. To date, loops made from these compounds have had sizes of the order of hundreds of nanometeres. Here, we report the results of measurements on loops of La1.84Sr0.16CuO4, a high-temperature superconductor that loses its resistance to electric currents when cooled below ∼38 K, with dimensions down to tens of nanometres. We observe oscillations in the resistance of the loops as a function of the magnetic flux through the loops. The oscillations have a period of h/2e, and their amplitude is much larger than the amplitude of the resistance oscillations expected from the Little-Parks effect. Moreover, unlike Little-Parks oscillations, which are caused by periodic changes in the superconducting transition temperature, the oscillations we observe are caused by periodic changes in the interaction between thermally excited moving vortices and the oscillating persistent current induced in the loops. However, despite the enhanced amplitude of these oscillations, we have not detected oscillations with a period of h/e, as recently predicted for nanoscale loops of superconductors with d-wave symmetry or with a period of h/4e, as predicted for superconductors that exhibit stripes.
[en] 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. [copyright] 2001 American Institute of Physics
[en] 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