Tsuyoshi Tamegai

The Anisotropic Upper Critical Field of Single Crystal YBa2Cu3Ox

We report a study of anisotropic upper critical field of single crystal samples of orthorhombic YBa2Cu3Ox. The critical field shows anisotropy characteristic of a quasi-two-dimensional superconductor, and is highest when the field is oriented perpendicular to the c-axis. For this field direction, the zero resistance state persists up to 86 K in a magnetic field of 90 kOe, which is lower by only 5 K than the zero resistance temperature in the absence of magnetic field. The anisotropy Hc2⊥/Hc2// is about 2 at 90 K, 5 at 86.5 K and tends to increase with decreasing temperature.

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.

Absence of current direction dependence of the resistive state of high temperature superconductors in magnetic fields

Resistive transition of Bi 2 Sr 2 CaCu 2 O 8+ y film in magnetic fields is studied in the light of the currently proposed flux creep model. Measurements with precise angular alignment have clarified that the resistive transition does not depend on the relative direction between magnetic field and transport current. This calls for a significant revision of the Lorentz-force-driven flux creep picture. A fluctuation induced phase-slip phenomenon as an alternative model is also discussed.

Metal-Insulator Transition in the Bi2Sr2Ca1-xYxCu2O8+y System

Metal-insulator transition driven by the change of the carrier concentration was investigated in the 80 K superconductor Bi2Sr2CaCu2O8+y. The carrier concentration was controlled by the substitution of yttrium for calcium. Experimental results from structural, transport, and magnetic studies are discussed in comparison with other high-temperature superconductor systems, La2-xSrxCuO4 and YBa2Cu3O7-y.

The anisotropic superconductivity of RBa2Cu3O7−x (R : Y, Gd and Ho) single crystals

Abstract Our study of high temperature superconductivity in RBa 2 Cu 3 O 7 − x (R:Y, Gd and Ho) using single crystal samples is reviewed with emphases on the anisotropic transport properties in the superconducting phase as well as the normal phase. Recent transport results obtained by use of a new technique for attaching a number of electrical leads on a small single crystal are also presented.

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.

Hall effect in high temperature superconductors near Tc

Abstract The diagonal and Hall conductivities of thin film samples of ErBa2Cu3O7−y and Bi2Sr2CaCu2O8+y have been studied in the superconducting transition region. Superconducting fluctuations above Tc give rise to a negative contribution to the Hall conductivity. The excess Hall conductivity, Δσxy, exhibits a stronger singularity than the excess diagonal conductivity, Δσxy, in agreement with Fukuyama et al.s theory. In the magnetic-field-induced resistive state below Tc, the Hall angle exhibits complicated temperature- and field-dependent behavior, including sign change. The Hall effect behaviors is discussed in the light of the flux dynamics and the nature of the resistive state.

Possible Superconductivity above 25 K in Single-Crystalline Co-Doped BaFe2As2

We present the superconducting properties of single-crystalline Ba(Fe 0.9 Co 0.1 ) 2 As 2 by measuring its magnetization, resistivity, upper critical field, Hall coefficient, and magneto-optical images. The magnetization measurements reveal a fish-tail hysteresis loop at high temperatures and a relatively high critical current density above J c = 10 5 A/cm 2 at low temperatures. The upper critical field determined by resistive transition is anisotropic with an anisotropic parameter of ∼3.5. Hall effect measurements indicate that Ba(Fe 0.9 Co 0.1 ) 2 As 2 is a multiband system and that the mobility of electrons is dominant. Magneto-optical imaging reveals a prominent Bean-like penetration of vortices, although there is a slight inhomogeneity in a sample. Moreover, we observe distinct superconductivity above 25 K, which leads us to speculate that a higher transition temperature can be realized by fine-tuning the Co-doping level.

The Strontium Content Dependence of Pressure Effect in (La1-xSrx)2CuO4

The enhancement of the superconducting transition temperature by pressure was studied up to ~1.5 GPa for samples of (La1-xSrx)2CuO4 with varying strontium content x. Simultaneously measured Hall coefficients were found to be independent of pressure in the present range, ruling out a pressure induced carrier density change as an origin of the Tc enhancement. The pressure coefficient of Tc was found roughly the same for the two regimes. i.e. for x<0.08, where Tc increases with x. and for 0.08<x<0.15 where it decreases with x. These results are discussed in the context of the current controversy over the nature of the electronic state in this class of materials.

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.