A. P. Rassau

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.

Metastability of the vortex solid in detwinned YBa2Cu3Oy

Abstract Pronounced history dependences have been observed in the transport characteristics of YBa 2 Cu 3 O y single crystals for temperatures and fields below the thermodynamic melting line. By using an annealing current to control the point at which the vortex system is exposed to the underlying disorder potential, we have been able to probe the competition between pinning, thermal and vortex–vortex interactions across the melting transition. In this way we have shown that in field cooled measurements, the majority of pinning in the vortex solid state arises due to processes occurring in a narrow region just below the melting temperature.

Dynamic stability of the vortex solid state of YBa2Cu3O7-δ single crystals

Abstract Investigations have been carried out on history dependences within the driven vortex solid state of YBa 2 Cu 3 O 7-δ single crystals. It has been demonstrated that the driven system has the tendency to stabilise in two particular dynamic phases: one ordered, the other relatively disordered. Transient responses observed on switching between drive modulation forms were interpreted in terms of changeovers between these phases. An unusual oscillatory response, arising on application of an asymmetric drive, has been explained in terms of distinct regions of such phases coexisting within the sample, their relative proportions changing periodically with time. Specifically, it has been demonstrated that the ordered dynamic state is inherently stable, in that it is easier to coherently drive an ordered-phase domain out of the sample than it is to break it apart.

Investigations of the zero-field (a,b)-plane conductivity of YBa2Cu3O7−δ near the critical temperature

Abstract The resistive transition R(T) and the current–voltage characteristics (I–V) in the range 10−9–10−6 V and close to the critical temperature (Tc) were measured in a single-grain of melt-textured YBa2Cu3O7−δ (YBCO) and in a detwinned YBCO single-crystal, with the transport current parallel to the (a,b) planes and in zero external magnetic field using a SQUID picovoltmeter. In the case of the melt-textured sample, we measured the I–V curves also at higher levels of dissipation. For T

Anomalous in-plane magneto-resistance of low-oxygenation YBa2Cu3Oy

Abstract We have observed very strong suppression of the transition to zero resistance by a magnetic field in underdoped YBa2Cu3Oy single crystals ( T c =60 K ). It follows from the comparison of transport and magnetic data that the main part of the resistivity tail corresponds to the vortex liquid state. We demonstrate that resistance of the vortex liquid cannot be described using a simple Bardeen–Stephen model.

Dynamic Transitions within the Vortex Solid of YBa2Cu3Oy Single Crystals

Transport measurements on YBa2Cu3Oyhave revealed strong history dependences in pinning properties of the vortex solid. By using an annealing current to control exposure of the vortex system to the disorder potential, we were able to study the development of such hysteretic responses, across the region of the vortex freezing transition. We have explained the observed behaviour in terms of coexistent vortex liquid and solid phases.

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.