Improved superconducting properties of melt-textured Nd123 by additional heat treatment

Abstract We have investigated the effect of additional heat-treatment on the superconducting transition and the flux pinning properties of Nd-Ba-Cu-O melt-textured in air. After the heat-treatment at high temperatures, ) 9008C, under low oxygen Ž . partial pressure, P O s 0.001 atm, the superconducting transition became sharper accompanied by an increase of J . c However, the increase of J was very small and the secondary peak effect commonly observed in Nd-Ba-Cu-O melt c Ž . textured in low P O could not be observed. Transmission electron microscopic observations and energy dispersive X-ray 2 analyses show that the spatial variation of the NdrBa ratio is reduced after high-temperature heat-treatment, which indicates that an improvement in T and J is attributed to a suppression of Nd substitution on the Ba site. q 1997 Elsevier Science c c B.V

Anisotropic Superconducting Properties of MgB2 Single Crystals

In-plane electrical transport properties of MgB2 single crystals grown under high pressure of 4-6 GPa and temperature of 1400-1700oC in Mg-B-N system have been measured. For all specimens we found sharp superconducting transition around 38.1-38.3K with transition width within 0.2-0.3K. Estimated resistivity value at 40K is about 1 mkOhmcm and resistivity ratio R(273K)/R(40K) of about 4.9. Results of measurements in magnetic field up to 5.5T perpendicular to Mg and B planes and up to 9T in parallel orientation show temperature dependent anisotropy of the upper critical field with anisotropy ratio increasing from 2.2 close to Tc up to about 3 below 30K. Strong deviation of the angular dependence of Hc2 from anisotropic mass model has been also found.Comment: 10pages, including 5 figures,submitted to Physica C (in press

Theory of plastic vortex creep

We develop a theory for plastic flux creep in a topologically disordered vortex solid phase in type-II superconductors. We propose a detailed description of the plastic vortex creep of the dislocated, amorphous vortex glass in terms of motion of dislocations driven by a transport current $j$. The {\em plastic barriers} $U_{pl}(j)\propto j^{-\mu}$ show power-law divergence at small drives with exponents $\mu=1$ for single dislocation creep and $\mu = 2/5$ for creep of dislocation bundles. The suppression of the creep rate is a hallmark of the transition from the topologically ordered vortex lattice to an amorphous vortex glass, reflecting a jump in $\mu$ from $\mu = 2/11$, characterizing creep in the topologically ordered vortex lattice near the transition, to its plastic values. The lower creep rates explain the observed increase in apparent critical currents in the dislocated vortex glass.Comment: 4 pages, 1 figur

Abrupt Change of Josephson Plasma Frequency at the Phase Boundary of the Bragg Glass in Bi_2Sr_2CaCu_2O_{8+\delta}

We report the first detailed and quantitative study of the Josephson coupling energy in the vortex liquid, Bragg glass and vortex glass phases of Bi_2Sr_2CaCu_2O_{8+\delta} by the Josephson plasma resonance. The measurements revealed distinct features in the T- and H-dependencies of the plasma frequency $\omega_{pl}$ for each of these three vortex phases. When going across either the Bragg-to-vortex glass or the Bragg-to-liquid transition line, $\omega_{pl}$ shows a dramatic change. We provide a quantitative discussion on the properties of these phase transitions, including the first order nature of the Bragg-to-vortex glass transition.Comment: 5pages, 4figure

Influence of random point defects introduced by proton irradiation on critical current density and vortex dynamics of Ba(Fe0.925Co0.075)2As2 single crystals

In this work we analyze the influence of random point defects introduced by 3 MeV proton irradiation on the critical current density (Jc) and vortex dynamics of a Ba(Fe0.925Co0.075)2As2 single crystal. The results show that at low temperatures (T) the irradiation produces an enhancement of Jc of up to 2.6 times. However the Jc (T) retention at different magnetic fields (H) in the elastic regime, estimated by the n exponent in Jc vs (1-(T/Tc)2)n, is poorer after the irradiations, due to the thermal softening of the pinning by the random point defects. We found that the elastic to plastic crossover and melting lines are only affected by the reduction of the superconducting critical temperature (Tc); they are exactly the same after rescaling the phase diagram by T/Tc. The pinning mechanisms in the single crystals can be associated with a mixed pinning landscape that produces a modulation in S(H,T) as a consequence of a fishtail or second peak in the magnetization.Fil: Haberkorn, Nestor Fabian. Los Alamos National Laboratory; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Maiorov, B.. Los Alamos National Laboratory; Estados UnidosFil: Usov, I. O.. Los Alamos National Laboratory; Estados UnidosFil: Weigand, M.. Los Alamos National Laboratory; Estados UnidosFil: Hirata, W.. Osaka University; JapónFil: Miyasaka, S.. Osaka University; JapónFil: Tajima, S.. Osaka University; JapónFil: Chikumoto, N.. International Superconductivity Technology Center. Superconductivity Research Laboratory; JapónFil: Tanabe, K.. International Superconductivity Technology Center. Superconductivity Research Laboratory; JapónFil: Civale, Leonardo. Los Alamos National Laboratory; Estados Unido

Josephson Plasma Resonance in Bi2Sr2CaCu2O8+yBi_2 Sr_2 Ca Cu_2 O_{8+y} with Spatially Dependent Interlayer-Phase Coherence

We study the Josephson plasma resonance (JPR) in Bi$_2$Sr$_2$CaCu$_2$O$_{8+y}$ (BSCCO) with spatially dependent interlayer-phase coherence (IPC). The half-irradiated BSCCO (HI-BSCCO), in which columnar defects are introduced only in a half of the sample, shows several resonance peaks, which are not simple superposition of the peaks in irradiated- and pristine-parts. JPR in HI-BSCCO changes its character from irradiated- to pristine-type at a crossover frequency ($\omega_{cr}$). We demonstrate that the one-dimensional \LSGE, which takes into account the spatial dependence of IPC, can reproduce most of the experimental findings including the presence of $\omega_{cr}$.Comment: 4 figure