Enhancement of Superconductivity in Disordered Films by Parallel Magnetic Field

We show that the superconducting transition temperature T_c(H) of a very thin highly disordered film with strong spin-orbital scattering can be increased by parallel magnetic field H. This effect is due to polarization of magnetic impurity spins which reduces the full exchange scattering rate of electrons; the largest effect is predicted for spin-1/2 impurities. Moreover, for some range of magnetic impurity concentrations the phenomenon of {\it superconductivity induced by magnetic field} is predicted: superconducting transition temperature T_c(H) is found to be nonzero in the range of magnetic fields $0 < H^* <= H <= H_c$.Comment: 4 pages, 2 figure

Detecting z > 10 objects through carbon, nitrogen and oxygen emission lines

By redshift of 10, star formation in the first objects should have produced considerable amounts of Carbon, Nitrogen and Oxygen. The submillimeter lines of C, N and O redshift into the millimeter and centimeter bands (0.5 mm -- 1.2 cm), where they may be detectable. High spectral resolution observations could potentially detect inhomogeneities in C, N and O emission, and see the first objects forming at high redshift. We calculate expected intensity fluctuations and discuss frequency and angular resolution required to detect them. For CII emission, we estimate the intensity using two independent methods: the line emission coefficient argument and the luminosity density argument. We find they are in good agreement. At 1+z \sim 10, the typical protogalaxy has a velocity dispersion of 30 km s^{-1} and angular size of 1 arcsecond. If CII is the dominant coolant, then we estimate a characteristic line strength of \sim 0.1 K km s^{-1}. We also discuss other atomic lines and estimate their signal. Observations with angular resolution of 10^{-3} can detect moderately nonlinear fluctuations of amplitude 2 \cdot 10^{-5} times the microwave background. If the intensity fluctuations are detected, they will probe matter density inhomogeneity, chemical evolution and ionization history at high redshifts.Comment: 15 pages, 1 postscript figures included; Uses aaspp4.sty (AASTeX v4.0); Submitted to The Astrophysical Journa

Interface superconductivity in La1.48_{1.48}Nd0.4_{0.4}Sr0.12_{0.12}CuO4_{4}/La1.84_{1.84}Sr0.16_{0.16}CuO4_{4} bilayers

We identify a distinct superconducting phase at the interface of a La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$ (LNSCO)/La$_{1.84}$Sr$_{0.16}$CuO$_4$ (LSCO) epitaxial bilayer system using ac screening measurements. A model based on inter-diffusion of quasiparticles and condensate at the interface yields a thickness of $\sim$ 25 nm for the interfacial layer. Two-dimensional superconductivity of the interface layer appears to be governed by Kosterlitz-Thouless-Berezinskii transition. A parallel magnetic field suppresses the superconducting transition temperature of this layer with a pair breaking parameter $\alpha$ varying as $H^2$

Control structures for high speed processors

A special processor was designed to function as a Reed Solomon decoder with throughput data rate in the Mhz range. This data rate is significantly greater than is possible with conventional digital architectures. To achieve this rate, the processor design includes sequential, pipelined, distributed, and parallel processing. The processor was designed using a high level language register transfer language. The RTL can be used to describe how the different processes are implemented by the hardware. One problem of special interest was the development of dependent processes which are analogous to software subroutines. For greater flexibility, the RTL control structure was implemented in ROM. The special purpose hardware required approximately 1000 SSI and MSI components. The data rate throughput is 2.5 megabits/second. This data rate is achieved through the use of pipelined and distributed processing. This data rate can be compared with 800 kilobits/second in a recently proposed very large scale integration design of a Reed Solomon encoder

Threshold electric field in unconventional density waves

As it is well known most of charge density wave (CDW) and spin density wave (SDW) exhibit the nonlinear transport with well defined threshold electric field E_T. Here we study theoretically the threshold electric field of unconventional density waves. We find that the threshold field increases monotonically with temperature without divergent behaviour at T_c, unlike the one in conventional CDW. The present result in the 3D weak pinning limit appears to describe rather well the threshold electric field observed recently in the low-temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4.Comment: 4 pages, 2 figure

Enhanced Coherence of Antinodal Quasiparticles in a Dirty d-wave Superconductor

Recent ARPES experiments show a narrow quasiparticle peak at the gap edge along the antinodal [1,0]-direction for the overdoped cuprate superconductors. We show that within weak coupling BCS theory for a d-wave superconductor the s-wave single-impurity scattering cross section vanishes for energies of the gap edge. This coherence effect occurs through multiple scattering off the impurity. For small impurity concentrations the spectral function has a pronounced increase of the (scattering) lifetime for antinodal quasiparticles but shows a very broad peak in the nodal direction, in qualitative agreement with experiment and in strong contrast to the behavior observed in underdoped cuprates.Comment: 4 pages, 3 figures, submitte

The upper critical field of filamentary Nb3Sn conductors

We have examined the upper critical field of a large and representative set of present multi-filamentary Nb3Sn wires and one bulk sample over a temperature range from 1.4 K up to the zero field critical temperature. Since all present wires use a solid-state diffusion reaction to form the A15 layers, inhomogeneities with respect to Sn content are inevitable, in contrast to some previously studied homogeneous samples. Our study emphasizes the effects that these inevitable inhomogeneities have on the field-temperature phase boundary. The property inhomogeneities are extracted from field-dependent resistive transitions which we find broaden with increasing inhomogeneity. The upper 90-99 % of the transitions clearly separates alloyed and binary wires but a pure, Cu-free binary bulk sample also exhibits a zero temperature critical field that is comparable to the ternary wires. The highest mu0Hc2 detected in the ternary wires are remarkably constant: The highest zero temperature upper critical fields and zero field critical temperatures fall within 29.5 +/- 0.3 T and 17.8 +/- 0.3 K respectively, independent of the wire layout. The complete field-temperature phase boundary can be described very well with the relatively simple Maki-DeGennes model using a two parameter fit, independent of composition, strain state, sample layout or applied critical state criterion.Comment: Accepted Journal of Applied Physics Few changes to shorten document, replaced eq. 7-

Thermal conductivity in B- and C- phase of UPt_3

Although the superconductivity in UPt_3 is one of the most well studied, there are still lingering questions about the nodal directions in the B and C phase in the presence of a magnetic field. Limiting ourselves to the low temperature regime (T<<Delta(0)), we study the magnetothermal conductivity with in semiclassical approximation using Volovik's approach. The angular dependence of the magnetothermal conductivity for an arbitrary field direction should clarify the nodal structure in UPt_3.Comment: 4 pages, 5 figure

Superconductivity in Geometrically Frustrated Pyrochlore RbOs2O6

We report the basic thermodynamic properties of the new geometrically frustrated beta-pyrochlore bulk superconductor RbOs2O6 with a critical temperature Tc = 6.4 K. Specific heat measurements are performed in magnetic fields up to 12 T. The electronic density of states at the Fermi level in the normal state results in gamma = (33.7 \pm 0.2) mJ/mol_f.u./K^2. In the superconducting state, the specific heat follows conventional BCS-type behavior down to 1 K, i.e. over three orders of magnitude in specific heat data. The upper critical field slope at Tc is 1.2 T/K, corresponding to a Maki-parameter alpha = 0.64 \pm 0.1. From the upper critical field mu0 Hc2 \approx 6 T at 0 K, we estimate a Ginzburg-Landau coherence length xi \approx 7.4 nm. RbOs2O6 is the second reported metallic AB2O6 type pyrochlore compound after KOs2O6, and one of only three pyrochlore superconductors in addition to Cd2Re2O7 and KOs2O6

Theory of the critical current in two-band superconductors with application to MgB2

Using a Green's function formulation of the superfluid current j_s, where a momentum q_s is applied to the Cooper pair, we have calculated j_s as a function of q_s, temperature, and impurity scattering for a two-band superconductor. We consider both renormalized BCS and full strong-coupling Eliashberg theory. There are two peaks in the current as a function of q_s due to the two energy scales for the gaps and this can give rise to non-standard behavior for the critical current. The critical current j_c, which is given as the maximum in j_s, can exhibit a kink as a function of temperature as the maximum is transferred from one peak to other. Other temperature variations are also possible and the universal BCS behavior is violated. The details depend on the material parameters of the system, such as the amount of coupling between the bands, the gap anisotropy, the Fermi velocities, and the density of states of each band. The Ginzburg-Landau relation between j_c, the penetration depth lambda_L and thermodynamic critical field H_c, is modified. Using Eliashberg theory with the electron-phonon spectral densities given from bandstructure calculations, we have applied our calculations for j_s and j_c to the case of MgB2 and find agreement with experiment.Comment: 13 pages, 7 figures, submitted to PR