S. Ooi

Imaging the vortex-lattice melting process in the presence of disorder

General arguments suggest that first-order phase transitions become less sharp in the presence of weak disorder, while extensive disorder can transform them into second-order transitions; but the atomic level details of this process are not clear. The vortex lattice in superconductors provides a unique system in which to study the first-order transition on an inter-particle scale, as well as over a wide range of particle densities. Here we use a differential magneto-optical technique to obtain direct experimental visualization of the melting process in a disordered superconductor. The images reveal complex behaviour in nucleation, pattern formation, and solid–liquid interface coarsening and pinning. Although the local melting is found to be first-order, a global rounding of the transition is observed; this results from a disorder-induced broad distribution of local melting temperatures, at scales down to the mesoscopic level. We also resolve local hysteretic supercooling of microscopic liquid domains, a non-equilibrium process that occurs only at selected sites where the disorder-modified melting temperature has a local maximum. By revealing the nucleation process, we are able to experimentally evaluate the solid–liquid surface tension, which we find to be extremely small.

POSSIBLE NEW VORTEX MATTER PHASES IN BI2SR2CACU2O8

The vortex matter phase diagram of BSCCO crystals is analyzed by investigating vortex penetration through the surface barrier in the presence of a transport current. The strength of the effective surface barrier, its nonlinearity, and asymmetry are used to identify a possible new ordered phase above the first-order transition. This technique also allows sensitive determination of the depinning temperature. The solid phase below the first-order transition is apparently subdivided into two phases by a vertical line extending from the multicritical point.

Transport properties governed by surface barriers in Bi2Sr2CaCu2O8

One of the most common investigation techniques of type-II superconductors is the transport measurement, in which an electrical current is applied to a sample and the corresponding resistance is measured as a function of temperature and magnetic field. At temperatures well below the critical temperature, Tc, the resistance of a superconductor is usually immeasurably low. But at elevated temperatures and fields, in the so-called vortex liquid phase, a substantial linear resistance is observed. In this dissipative state, which in anisotropic high-temperature superconductors like Bi2Sr2CaCu2O8 may occupy most of the mixed-state phase diagram, the transport current is usually assumed to flow uniformly across the sample as in a normal metal. To test this assumption, we have devised a measurement approach which allows determination of the flow pattern of the transport current across the sample. The surprising result is that, in Bi2Sr2CaCu2O8 crystals, most of the current flows at the edges of the sample rather than in the bulk, even in the highly resistive state, due to the presence of strong surface barriers. This finding has significant implications for the interpretation of existing resistivity data and may be of importance for the development of high-temperature superconducting wires and tapes.A new measurement technique for investigation of vortex dynamics is introduced. The distribution of the transport current across a crystal is derived by a sensitive measurement of the self-induced magnetic field of the transport current. We are able to clearly mark where the flow of the transport current is characterized by bulk pinning, surface barrier, or a uniform current distribution. One of the novel results is that in BSCCO crystals most of the vortex liquid phase is affected by surface barriers resulting in a thermally activated apparent resistivity. As a result the standard transport measurements in BSCCO do not probe the dynamics of vortices in the bulk, but rather measure surface barrier properties.

Evolution of vortex phase diagram with oxygen-doping in Bi2Sr2CaCu2O8+y single crystals

Abstract We studied the vortex phase diagram in Bi 2 Sr 2 CaCu 2 O 8+ y with oxygen-doping by using micro-Hall-probes. The field B s , where the magnetization steps occur, increases with oxygen concentrations. In addition to this, we found that oxygen doping makes the entropy change smaller. The magnetization steps were completely suppressed by further doping, while the magnetization peak was observed at high temperatures, whose temperature dependence is similar to B s ( T ). Moreover, the peak field B p at low temperatures shows pronounced re-entrant behavior in highly overdoped samples. All these features in the evolution of phase diagram can be explained by taking account of disorders as well as the decrease in the anisotropy due to oxygen-doping.

EVIDENCE FOR FIELD-INDUCED EXCITATIONS IN LOW-TEMPERATURE THERMAL CONDUCTIVITY OF BI2SR2CACU2O8

The thermal conductivity ,

Vortex matter transition inBi2Sr2CaCu2O8+yunder tilted fields

\kappa

TRANSPORT PROPERTIES OF BI2SR2CACU2O8 CRYSTALS WITH AND WITHOUT SURFACE BARRIERS

, of Bi_2Sr_2CaCu_2O_8 was studied as a function of magnetic field. Above 5 K, after an initial decrease,

Shear-induced vortex decoupling in Bi 2 Sr 2 CaCu 2 O 8 crystals

\kappa(H)

Anomalous temperature dependence of the peak effect in iodine-intercalated Bi2Sr2CaCu2O8+y single crystals

presents a kink followed by a plateau, as recently reported by Krishana et al.. By contrast, below 1K, the thermal conductivity was found to \emph{increase} with increasing field. This behavior is indicative of a finite density of states and is not compatible with the existence of a field-induced fully gapped

Pairing and vortex states in Sr2RuO4 studied by Hall probe magnetometry

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