D. T. Fuchs

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.

Surface barrier dominated transport in NbSe2

Transport current distribution in clean

TRANSPORT PROPERTIES OF BI2SR2CACU2O8 CRYSTALS WITH AND WITHOUT SURFACE BARRIERS

{2\mathrm{H}\ensuremath{-}\mathrm{N}\mathrm{b}\mathrm{S}\mathrm{e}}_{2}

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

crystals is studied by measuring the self-induced magnetic field across the sample. Below

Current-induced decoupling of vortices in Bi2Sr2CaCu2O8

{T}_{c}

Resistivity onset at the first-order vortex-lattice phase transition in Bi2Sr2CaCu2O8

most of the current flows at the edges of the crystals due to strong surface barriers, which are found to dominate the transport properties and the resistive transition. The measured critical current is determined by the critical current for vortex penetration through the surface barrier rather than by bulk pinning.

Effect of surface barriers on transport properties of Bi2Sr2CaCu2O8 single crystals using the Corbino disc configuration

Large BSCCO crystals with electrical contacts positioned far from the edges are studied by transport measurements, then cut into the common narrow strip geometry, and remeasured. Instead of showing larger resistance, the narrow strip samples display a dramatic drop in the resistance, enhanced activation energies, and nonlinear characteristics due to strong surface barriers. The surface barriers also dominate the resistive drop at the first-order phase transition. Because the surface barriers are avoided in large crystals, we are able to probe the solid phase and find good agreement with the recent predictions of Bragg glass theory.

Sublimation and hysteretic transition of the vortex-lattice in Bi2Sr2CaCu2O8

Simultaneous transport and magnetization studies in

Simultaneous resistivity onset and first-order vortex-lattice phase transition in Bi2Sr2CaCu2O8.

{\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}