Dynamical scaling in Ising and vector spin glasses

We have studied numerically the dynamics of spin glasses with Ising and XY symmetry (gauge glass) in space dimensions 2, 3, and 4. The nonequilibrium spin-glass susceptibility and the nonequilibrium energy per spin of samples of large size L_b are measured as a function of anneal time t_w after a quench to temperatures T. The two observables are compared to the equilibrium spin-glass susceptibility and the equilibrium energy, respectively, measured as functions of temperature T and system size L for a range of system sizes. For any time and temperature a nonequilibrium time-dependent length scale L*(t_w,T) can be defined by comparing equilibrium and nonequilibrium quantities. Our analysis shows that for all systems studied, an "effective dynamical critical exponent" parametrization L*(t_w,T) = A(T) t^(1/z(T)) fits the data well at each temperature within the whole temperature range studied, which extends from well above the critical temperature to near T = 0 for dimension 2, or to well below the critical temperature for the other space dimensions studied. In addition, the data suggest that the dynamical critical exponent z varies smoothly when crossing the transition temperature.Comment: 14 pages, 13 figures, 9 table

Extended Scaling for the high dimension and square lattice Ising Ferromagnets

In the high dimension (mean field) limit the susceptibility and the second moment correlation length of the Ising ferromagnet depend on temperature as chi(T)=tau^{-1} and xi(T)=T^{-1/2}tau^{-1/2} exactly over the entire temperature range above the critical temperature T_c, with the scaling variable tau=(T-T_c)/T. For finite dimension ferromagnets temperature dependent effective exponents can be defined over all T using the same expressions. For the canonical two dimensional square lattice Ising ferromagnet it is shown that compact "extended scaling" expressions analogous to the high dimensional limit forms give accurate approximations to the true temperature dependencies, again over the entire temperature range from T_c to infinity. Within this approach there is no cross-over temperature in finite dimensions above which mean-field-like behavior sets in.Comment: 6 pages, 6 figure

A tale of one city: intra-institutional variations in migrating VLE platform

City University London committed in 2009 to make Moodle the Virtual Learning Environment (VLE) at the core of a new Strategic Learning Environment (SLE) comprised of VLE, externally facing website and related systems such as video streaming and virtual classrooms. Previously, the WebCT VLE had been separate from most of the other systems at the institution with very limited connections to other tools. Each of the schools within the institution was able to pursue their own strategy and timeframe for the migration and embedding of Moodle within their subject areas, within an absolute limit of 2 years. This paper outlines the approaches taken by the various schools, highlighting similarities and differences, and draws out common aspects from the project to make recommendations for institutions seeking to undertake similar migrations

Low temperature field-effect in crystalline organic material

Molecular organic materials offer the promise of novel electronic devices but also present challenges for understanding charge transport in narrow band systems. Low temperature studies elucidate fundamental transport processes. We report the lowest temperature field effect transport results on a crystalline oligomeric organic material, rubrene. We find field effect switching with on-off ratio up to 10^7 at temperatures down to 10 K. Gated transport shows a factor of ~10 suppression of the thermal activation energy in 10-50 K range and nearly temperature independent resistivity below 10 K.Comment: 5 pages, 4 figure

The Anomalous Hall effect in re-entrant AuFe alloys and the real space Berry phase

The Hall effect has been studied in a series of AuFe samples in the re-entrant concentration range, as well as in the spin glass range. The data demonstrate that the degree of canting of the local spins strongly modifies the anomalous Hall effect, in agreement with theoretical predictions associating canting, chirality and a real space Berry phase. The canonical parametrization of the Hall signal for magnetic conductors becomes inappropriate when local spins are canted.Comment: 4 pages, 1 eps figur

Spin-dependent Seebeck coefficients of Ni_{80}Fe_{20} and Co in nanopillar spin valves

We have experimentally determined the spin-dependent Seebeck coefficient of permalloy (Ni_{80}Fe_{20}) and cobalt (Co) using nanopillar spin valve devices. The devices were specifically designed to completely separate heat related effects from charge related effects. A pure heat current through the nanopillar spin valve, a stack of two ferromagnetic layers (F) separated by a non-magnetic layer (N), leads to a thermovoltage proportional to the spin-dependent Seebeck coefficient S_{S}=S_{\uparrow}-S_{\downarrow} of the ferromagnet, where S_{\uparrow} and S_{\downarrow} are the Seebeck coefficient for spin-up and spin-down electrons. By using a three-dimensional finite-element model (3D-FEM) based on spin-dependent thermoelectric theory, whose input material parameters were measured in separate devices, we were able to accurately determine a spin-dependent Seebeck coefficient of -1.8 microvolt/Kelvin and -4.5 microvolt/Kelvin for cobalt and permalloy, respectively corresponding to a Seebeck coefficient polarization P_{S}=S_{S}/S_{F} of 0.08 and 0.25, where S_{F} is the Seebeck coefficient of the ferromagnet. The results are in agreement with earlier theoretical work in Co/Cu multilayers and spin-dependent Seebeck and spin-dependent Peltier measurements in Ni_{80}Fe_{20}/Cu spin valve structures

Asymptotic behavior of the density of states on a random lattice

We study the diffusion of a particle on a random lattice with fluctuating local connectivity of average value q. This model is a basic description of relaxation processes in random media with geometrical defects. We analyze here the asymptotic behavior of the eigenvalue distribution for the Laplacian operator. We found that the localized states outside the mobility band and observed by Biroli and Monasson (1999, J. Phys. A: Math. Gen. 32 L255), in a previous numerical analysis, are described by saddle point solutions that breaks the rotational symmetry of the main action in the real space. The density of states is characterized asymptotically by a series of peaks with periodicity 1/q.Comment: 11 pages, 2 figure

Towards spin injection from silicon into topological insulators: Schottky barrier between Si and Bi2Se3

A scheme is proposed to electrically measure the spin-momentum coupling in the topological insulator surface state by injection of spin polarized electrons from silicon. As a first approach, devices were fabricated consisting of thin (<100nm) exfoliated crystals of Bi2Se3 on n-type silicon with independent electrical contacts to silicon and Bi2Se3. Analysis of the temperature dependence of thermionic emission in reverse bias indicates a barrier height of 0.34 eV at the Si-Bi2Se3 interface. This robust Schottky barrier opens the possibility of novel device designs based on sub-band gap internal photoemission from Bi2Se3 into Si