Niobium hyperfine structure in crystal calcium tungstate

A study of the niobium hyperfine structure in single crystal calcium tungstate was made by the combination of the technique of electron paramagnetic resonance and electron nuclear double resonance (EPR/ENDOR). The microwave frequency was about 9.4 GHz and the radio frequency from 20MHz to 70 MHz. The rare earth ions Nd(3+), U(3+), or Tm(3+) were added as the charge compensator for Nb(5+). To create niobium paramagnetic centers, the sample was irradiated at 77 deg K with a 10 thousand curie Co-60 gamma source for 1 to 2 hours at a dose rate of 200 K rads per hour and then transferred quickly into the cavity. In a general direction of magnetic field, the spectra showed 4 sets of 10 main lines corresponding to 4 nonequivalent sites of niobium with I = 9/2. These 4 sets of lines coalesced into 2 sets of 10 in the ab-plane and into a single set of 10 along the c-axis. This symmetry suggested that the tungsten ions are substituted by the niobium ions in the crystal

Micromagnetic Simulation of Non Uniform Nanodots with Perpendicular Anisotropy

Multilayered Pt/[Co/Pt]/sub 5/ films were fabricated into patterned dots with diameters of 120 and 200 nm by laser interference lithography. Anomalous Hall effect (AHE) measurements were used on an array of dots to measure the switching field and the effects of reversal of an area with reduced anisotropy in the dot were also investigated by micromagnetic simulation. It was shown that a reduction in switching field is caused by an area of reduced anisotropy. The effect of various combinations of a reduction in anisotropy and/or exchange constant, and the size and shape of the area were also discussed in this paper

Destruction of superconductivity in disordered materials : a dimensional crossover

The disorder-induced Superconductor-to-Insulator Transition in amorphous Nb$_{x}$Si$_{1-x}$ two-dimensional thin films is studied for different niobium compositions $x$ through a variation of the sample thickness $d$. We show that the critical thickness $d_c$, separating a superconducting regime from an insulating one, increases strongly with diminishing $x$, thus attaining values of over 100 {\AA}. The corresponding phase diagram in the $(d, x)$ plane is inferred and related to the three-dimensional situation. The two-dimensional Superconductor-to-Insulator Transition well connects with the three-dimensional Superconductor-to-Metal Transition

Constraints on unparticle long range forces from big bang nucleosynthesis bounds on the variation of the gravitational coupling

We use big bang nucleosynthesis bounds on the variation of the gravitational coupling to derive constraints on the strength of the deviation from the gravitational inverse-square law due to tensor and vector unparticle exchange.Comment: 4 pages, 3 figures, change in the title and other minor changes to match version published in Physical Review

Observation of thermally activated glassiness and memory dip in a-NbSi insulating thin films

We present electrical conductance measurements on amorphous NbSi insulating thin films. These films display out-of equilibrium electronic features that are markedly different from what has been reported so far in disordered insulators. Like in the most studied systems (indium oxide and granular Al films), a slow relaxation of the conductance is observed after a quench to liquid helium temperature which gives rise to the growth of a memory dip in MOSFET devices. But unlike in these systems, this memory dip and the related conductance relaxations are still visible up to room temperature, with clear signatures of a temperature dependent dynamics

Magnetic ordering of weakly coupled frustrated quantum spin chains

The ordering temperature of a quasi-one-dimensional system, consisting of weakly interacting quantum spin-1/2 chains with antiferromagnetic spin-frustrating couplings (or zig-zag ladder) is calculated. The results show that a quantum critical point between two phases of the one-dimensional subsystem plays a crucial role. If the one-dimensional subsystem is in the antiferromagnetic-like phase in the ground state, similar to the phase of a spin chain without frustration, weak couplings yield magnetic ordering of the Neel type. For intra-chain spin-frustrating interactions larger than the critical one (at which the quantum phase transition takes place), the quasi-one-dimensional spin system manifests a spiral magnetic incommensurate ordering. The obtained results of our quantum theory are compared with the quasi-classical approximations. The calculated features of magnetic ordering are expected to be generic for weakly coupled quantum spin chains with gapless excitations and spin-frustrating nearest and next-nearest neighbor interactions.Comment: 6 pages, 2 figure

Effect of annealing on the superconducting properties of a-Nb(x)Si(1-x) thin films

a-Nb(x)Si(1-x) thin films with thicknesses down to 25 {\AA} have been structurally characterized by TEM (Transmission Electron Microscopy) measurements. As-deposited or annealed films are shown to be continuous and homogeneous in composition and thickness, up to an annealing temperature of 500{\deg}C. We have carried out low temperature transport measurements on these films close to the superconductor-to-insulator transition (SIT), and shown a qualitative difference between the effect of annealing or composition, and a reduction of the film thickness on the superconducting properties of a-NbSi. These results question the pertinence of the sheet resistance R_square as the relevant parameter to describe the SIT.Comment: 9 pages, 12 figure

A length scale for the superconducting Nernst signal above Tc_{c} in Nb0.15_{0.15}Si0.85_{0.85}

We present a study of the Nernst effect in amorphous superconducting thin films of Nb$_{0.15}$Si$_{0.85}$. The field dependence of the Nernst coefficient above T$_{c}$ displays two distinct regimes separated by a field scale set by the Ginzburg-Landau correlation length. A single function $F(\xi)$, with the correlation length as its unique argument set either by the zero-field correlation length (in the low magnetic field limit) or by the magnetic length (in the opposite limit), describes the Nernst coefficient. We conclude that the Nernst signal observed on a wide temperature ($30 \times T_c$) and field ($4 \times B_{c2}$) range is exclusively generated by short-lived Cooper pairs.Comment: 4 pages, 4 figure

Magnetic field-induced quantum superconductor-insulator transition in Nb0.15Si0.85Nb_{0.15}Si_{0.85}

A study of magnetic-field tuned superconductor-insulator transitions in amorphous $Nb_{0.15}Si_{0.85}$ thin films shows that quantum superconductor-insulator transitions are characterized by an unambiguous signature -- a kink in the temperature profile of the critical magnetic field. Using this criterion, we show that the nature of the magnetic-field tuned superconductor-insulator transition depends on the orientation of the field with respect to the film. For perpendicular magnetic field, the transition is controlled by quantum fluctuations with indications for the existence of a Bose insulator; while for parallel magnetic field, the transition is classical, driven by the breaking of Cooper pairs at the temperature dependent critical field $H_{c2}$.Comment: 5 pages, 4 figure

Cluster variation - Pade` approximants method for the simple cubic Ising model

The cluster variation - Pade` approximant method is a recently proposed tool, based on the extrapolation of low/high temperature results obtained with the cluster variation method, for the determination of critical parameters in Ising-like models. Here the method is applied to the three-dimensional simple cubic Ising model, and new results, obtained with an 18-site basic cluster, are reported. Other techniques for extracting non-classical critical exponents are also applied and their results compared with those by the cluster variation - Pade` approximant method.Comment: 8 RevTeX pages, 3 PostScript figure