Phase Behavior of Type-II Superconductors with Quenched Point Pinning Disorder: A Phenomenological Proposal

A general phenomenology for phase behaviour in the mixed phase of type-II superconductors with weak point pinning disorder is outlined. We propose that the ``Bragg glass'' phase generically transforms via two separate thermodynamic phase transitions into a disordered liquid on increasing the temperature. The first transition is into a glassy phase, topologically disordered at the largest length scales; current evidence suggests that it lacks the long-ranged phase correlations expected of a ``vortex glass''. This phase has a significant degree of short-ranged translational order, unlike the disordered liquid, but no quasi-long range order, in contrast to the Bragg glass. This glassy phase, which we call a ``multi-domain glass'', is confined to a narrow sliver at intermediate fields, but broadens out both for much larger and much smaller field values. The multi-domain glass may be a ``hexatic glass''; alternatively, its glassy properties may originate in the replica symmetry breaking envisaged in recent theories of the structural glass transition. Estimates for translational correlation lengths in the multi-domain glass indicate that they can be far larger than the interline spacing for weak disorder, suggesting a plausible mechanism by which signals of a two-step transition can be obscured. Calculations of the Bragg glass-multi-domain glass and the multi-domain glass-disordered liquid phase boundaries are presented and compared to experimental data. We argue that these proposals provide a unified picture of the available experimental data on both high-T$_c$ and low-T$_c$ materials, simulations and current theoretical understanding.Comment: 70 pages, 9 postscript figures, modified title and minor changes in published versio

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

Investigations of the zero-field (a,b)-plane conductivity of YBa₂Cu₃O_7-δ near the critical temperature

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<Tc, we interpreted the lower part of the I–V curves in the framework of the Jensen–Minnhagen model of current-induced unbinding of thermally-created vortex–antivortex pairs, which leads to V ∝ I(I-Ic1)a-1. Our results confirm a recently proposed current–temperature (I–T) phase diagram by S.W. Pierson. In addition, we determined for the first time the dependence of the characteristic (unbinding) current Ic1 with t = 1-T/Tc for temperatures very close to Tc and low transport currents