Static and dynamic phases of vortices in the high temperature superconductor YBa_2Cu_3O_7_-_#delta#

Transport measurements have been used to probe static and dynamic vortex states within clean single crystalline samples of the high temperature superconductor YBa_2Cu_3O_7_-_#delta#. A bespoke software tool has been designed to execute complex series of experimental instructions, facilitating the systematic investigation of a range of different history effects within the vortex system. A new approach has been developed for controlling the interaction of vortices with the underlying pinning potential, by means of the driving current modulation form and magnitude. A wide range of evidence from both transport and ac-susceptibility techniques is presented in support of the existence of a finite width transition region (TR) in the vicinity of the melting line, over which vortex liquid and solid phases coexist. Dynamic behaviour in this TR has been explained in terms of the percolation of vortex liquid between vortex solid domains. Furthermore, within the TR we have observed current induced switching effects from low to high resistivity states and attributed this behaviour to a redistribution of vortex solid domains. A novel 'history dependence' technique has been developed for probing relaxation processes across the phase diagram. We have demonstrated that the relaxation properties of the system change abruptly across the TR but that there is no relaxation over experimental time-scales at any point within this region. This is contrary to expectations for a model of glassy relaxation but entirely consistent with the notion of a region of coexistent vortex liquid and solid phases. We have found that the dynamics of this vortex solid are crucially dependent upon the modulation form of the driving current. In particular, we have demonstrated that the driven vortex system has the tendency to stabilise in two particular dynamic phases, one ordered the other relatively disordered, dependent on the nature of the applied drive. Transient responses observed on switching between drive modulation-forms were interpreted in terms of changeovers between these phases. In response to oscillatory drives that were asymmetric either with respect to their amplitude or to the positive and negative part of their period, we observed a highly unusual dynamic state. This state manifested itself in terms of large-amplitude oscillations of the voltage amplitude which were independent of the drive modulation- frequency but highly dependent on the asymmetry of the drive. This anomalous response has been explained in terms of distinct regions of ordered and disordered dynamic phases coexisting within the sample, their relative proportions changing periodically with time. From such observations we have 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. (author)Available from British Library Document Supply Centre-DSC:DXN040248 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo