R. M. Langan

Comparative AC susceptibility and resistivity studies of the vortex lattice melting transition in detwinned YBa2Cu3O7-δ single crystals

Abstract We have studied the transition from the vortex liquid to the vortex solid state in clean, detwinned YBa2Cu3O7−δ single crystals in fields up to 12 T using AC susceptibility and resistivity measurements. For μ0H 1 , 2 ) was found to be accompanied by an AC field independent abrupt onset of both χ′ and χ″ components. The AC susceptibility data suggest that just below Tm there is a transition region where the liquid and solid phases coexist. The width of the transition region was found to be sample dependent and can be related to structural inhomogeneity. Above the multicritical field μ0Hmc=7 T simultaneous broadening of both the ρ(T) drop and the χ″(T) onset show that the first order melting transition is suppressed.

Organisation of vortex motion in detwinned YBa2Cu3O7−δ single crystals

Abstract The dynamic behaviour of a driven vortex lattice in the vicinity of the melting line has been investigated. We have shown that a square wave driving current with asymmetric period or amplitude can induce a slow periodic variation in the voltage response. We also show that this effect can be observed for current modulations with sinusoidal and triangular wave forms thereby demonstrating that these oscillations are not due to the sharp fronts of the probing current which are a feature of the square wave modulation.

Field induced suppression of the vortex lattice melting transition in twinned YBa2Cu3O7−δ

Abstract We present magneto-resistance data for a high quality, twinned YBa 2 Cu 3 O 7−δ crystal, taken with the current applied along the ab plane. The crystal was examined at a number of angles to an applied magnetic field, of up to 20T, in order to observe the influence of correlated and point-like disorder on the vortex dynamics. When the applied field was orientated at 15° to the crystalline c-axis (θ=15°), for fields below H * =12T, we observed the kink in ϱ(T) associated with vortex lattice melting. We found that when the field exceeded this value, there was a complete suppression of this kink and the ϱ(T) curves resembled those of crystals with extensive point disorder. This suppression in melting occurs abruptly between 11T and 12T. The melting transition is recovered when the angle between the c-axis and the field is increased. An analysis of features around 12T in ϱ(T) and ϱ(θ) has been performed for a number of fields and angles.

Effect of correlated and point-like disorder on transport properties of YBa2Cu3O7−δ single crystals

Pinning by point-like defects and twin boundaries in single-crystalline YBa2Cu3O7−δ has been investigated by measuring resistance and current-voltage characteristics in fields up to 5 T. A number of untwinned and twinned samples with different densities of point-like defects were studied. The experimental results are compared within the framework of vortex-entanglement.

Novel vortex dynamics in untwinned YBa2Cu3O7−δ single crystals

Magnetotransport measurements on a clean, untwinned YBa2Cu3O7−δ single crystal show that the vortex dynamics at temperatures just below the vortex lattice melting transition are highly dependent on the type of modulation of the probing current. While in the case of DC current the flux flow is disordered, the “shaking” of the vortex lattice by a square-wave current leads to a more uniform vortex motion. A small asymmetry (∼10%) in the durations of the positive and negative parts of the square-wave period induces periodical oscillations of the voltage response amplitude. The period of oscillations (∼100 s) is the same order of magnitude as the time needed for vortices to cross the sample (transit time). We relate the observed voltage oscillations to a periodically ordered vortex motion.

The peak effect as a phase boundary in detwinned YBa2Cu3O7−δ

Abstract We present magneto-resistance data from a high quality detwinned YBa 2 Cu 3 O 7−δ single crystal, taken using the conventional four-point method with the current applied along the ab plane and the field along the c-axis. Measurements were taken in fields of up to 12T. At temperatures just below the melting line, a sharp feature (a ‘peak effect’) appeared in the resistance when sufficiently high current densities were applied. We saw this feature as both a function of temperature, ϱ(T), and field, ϱ(H). At higher fields, above the multi-critical point H * =6T, the peak effect in ϱ(H) splits into two and spreads apart in field. This forms two branches in the H-T phase diagram, and their implications for the dynamics of the vortices are discussed.

a.c. susceptibility studies of the vortex lattice melting transition in clean, detwinned YBa2Cu3O6.92 single crystals

Abstract We present data from a.c. susceptibility and resistivity measurements in an applied d.c. magnetic field on clean detwinned YBa 2 Cu 3 O 6.92 single crystals. Our investigations have revealed an extremely sharp amplitude and frequency independent onset of both the in phase and out of phase a.c. susceptibility components. For fields below H * =6T the position of this onset in H-T plane agrees with the melting transition obtained from resistivity measurements on the same crystal. At H≥H * this transition becomes criteria dependent. We found that H * accurately correlates with the multi-critical point obtained from resistivity measurements.

Current-induced resistivity hysteresis in detwinned YBa2Cu3O7−δ single crystals

Abstract Resistivity-temperature and voltage-current characteristics of clean detwinned YBa 2 Cu 3 O 7−δ single crystals have been measured using a SQUID picovoltmeter in magnetic fields of up to 5 T. We have observed hysteretic behaviour near T m in the ϱ(T) dependence: the resistivity when warming the sample up (the “superheated” state) was lower than when cooling it down (the “supercooled” state). The V-I characteristics for the sample in the “superheated” and “supercooled” states were also found to be different. We have found that the “superheated” state is unstable and can be switched to the “supercooled” state by applying a comparatively low probing current density ( j ∼ 0.4A/cm 2 ). We relate the current-induced resistivity hysteresis to the rearrangement of the solid vortex domains due to the probing current.