Time evolution of the extremely diluted Blume-Emery-Griffiths neural network

The time evolution of the extremely diluted Blume-Emery-Griffiths neural network model is studied, and a detailed equilibrium phase diagram is obtained exhibiting pattern retrieval, fluctuation retrieval and self-sustained activity phases. It is shown that saddle-point solutions associated with fluctuation overlaps slow down considerably the flow of the network states towards the retrieval fixed points. A comparison of the performance with other three-state networks is also presented.Comment: 8 pages, 5 figure

Motional sidebands and direct measurement of the cooling rate in the resonance fluorescence of a single trapped ion

Resonance fluorescence of a single trapped ion is spectrally analyzed using a heterodyne technique. Motional sidebands due to the oscillation of the ion in the harmonic trap potential are observed in the fluorescence spectrum. From the width of the sidebands the cooling rate is obtained and found to be in agreement with the theoretical prediction.Comment: 4 pages, 4 figures. Final version after minor changes, 1 figure replaced; to be published in PRL, July 10, 200

Thermodynamics of Mesoscopic Vortex Systems in 1+1 Dimensions

The thermodynamics of a disordered planar vortex array is studied numerically using a new polynomial algorithm which circumvents slow glassy dynamics. Close to the glass transition, the anomalous vortex displacement is found to agree well with the prediction of the renormalization-group theory. Interesting behaviors such as the universal statistics of magnetic susceptibility variations are observed in both the dense and dilute regimes of this mesoscopic vortex system.Comment: 4 pages, REVTEX, 6 figures included. Comments and suggestions can be sent to [email protected]

Vortex wandering in a forest of splayed columnar defects

We investigate the scaling properties of single flux lines in a random pinning landscape consisting of splayed columnar defects. Such correlated defects can be injected into Type II superconductors by inducing nuclear fission or via direct heavy ion irradiation. The result is often very efficient pinning of the vortices which gives, e.g., a strongly enhanced critical current. The wandering exponent \zeta and the free energy exponent \omega of a single flux line in such a disordered environment are obtained analytically from scaling arguments combined with extreme-value statistics. In contrast to the case of point disorder, where these exponents are universal, we find a dependence of the exponents on details in the probability distribution of the low lying energies of the columnar defects. The analytical results show excellent agreement with numerical transfer matrix calculations in two and three dimensions.Comment: 11 pages, 9 figure

Interstitials, Vacancies, and Supersolid Order in Vortex Crystals

Interstitials and vacancies in the Abrikosov phase of clean Type II superconductors are line imperfections, which cannot extend across macroscopic equilibrated samples at low temperatures. We argue that the entropy associated with line wandering nevertheless can cause these defects to proliferate at a sharp transition which will exist if this occurs below the temperature at which the crystal actually melts. Vortices are both entangled and crystalline in the resulting ``supersolid'' phase, which in a dual ``boson'' analog system is closely related to a two-dimensional quantum crystal of He$^4$ with interstitials or vacancies in its ground state. The supersolid {\it must} occur for $B\gg B_\times$, where $B_\times$ is the decoupling field above which vortices begin to behave two-dimensionally. Numerical calculations show that interstitials, rather than vacancies, are the preferred defect for $B\gg \phi_0/\lambda_\perp^2$, and allow us to estimate whether proliferation also occurs for B\,\lot\,B_\times.The implications of the supersolid phase for transport measurements, dislocation configurations and neutron diffraction are discussed.Comment: 53 pages and 15 figures, available upon request, written in plain TE

The ac magnetic response of mesoscopic type II superconductors

The response of mesoscopic superconductors to an ac magnetic field is numerically investigated on the basis of the time-dependent Ginzburg-Landau equations (TDGL). We study the dependence with frequency $\omega$ and dc magnetic field $H_{dc}$ of the linear ac susceptibility $\chi(H_{dc}, \omega)$ in square samples with dimensions of the order of the London penetration depth. At $H_{dc}=0$ the behavior of $\chi$ as a function of $\omega$ agrees very well with the two fluid model, and the imaginary part of the ac susceptibility, $\chi"(\omega)$, shows a dissipative a maximum at the frequency $\nu_o=c^2/(4\pi \sigma\lambda^2)$. In the presence of a magnetic field a second dissipation maximum appears at a frequency $\omega_p\ll\nu_0$. The most interesting behavior of mesoscopic superconductors can be observed in the $\chi(H_{dc})$ curves obtained at a fixed frequency. At a fixed number of vortices, $\chi"(H_{dc})$ continuously increases with increasing $H_{dc}$. We observe that the dissipation reaches a maximum for magnetic fields right below the vortex penetration fields. Then, after each vortex penetration event, there is a sudden suppression of the ac losses, showing discontinuities in $\chi"(H_{dc})$ at several values of $H_{dc}$. We show that these discontinuities are typical of the mesoscopic scale and disappear in macroscopic samples, which have a continuos behavior of $\chi(H_{dc})$. We argue that these discontinuities in $\chi(H_{dc})$ are due to the effect of {\it nascent vortices} which cause a large variation of the amplitude of the order parameter near the surface before the entrance of vortices.Comment: 12 pages, 9 figures, RevTex

Orientation of Vortices in a Superconducting Thin-Film: Quantitative Comparison of Spin-Polarized Neutron Reflectivity and Magnetization

We present a quantitative comparison of the magnetization measured by spin-polarized neutron reflectivity (SPNR) and DC magnetometry on a 1370 \AA\ -thick Nb superconducting film. As a function of magnetic field applied in the film plane, SPNR exhibits reversible behavior whereas the DC magnetization shows substantial hysteresis. The difference between these measurements is attributed to a rotation of vortex magnetic field out of the film plane as the applied field is reduced. Since SPNR measures only the magnetization parallel to the film plane whereas DC magnetization is strongly influenced by the perpendicular component of magnetization when there is a slight sample tilt, combining the two techniques allows one to distinguish two components of magnetization in a thin film.Comment: 12 pages, 8 figures, It will be printed in PRB, Oct. 200

Effects of gap anisotropy upon the electronic structure around a superconducting vortex

An isolated single vortex is considered within the framework of the quasiclassical theory. The local density of states around a vortex is calculated in a clean type II superconductor with an anisotropy. The anisotropy of a superconducting energy gap is crucial for bound states around a vortex. A characteristic structure of the local density of states, observed in the layered hexagonal superconductor 2H-NbSe2 by scanning tunneling microscopy (STM), is well reproduced if one assumes an anisotropic s-wave gap in the hexagonal plane. The local density of states (or the bound states) around the vortex is interpreted in terms of quasiparticle trajectories to facilitate an understanding of the rich electronic structure observed in STM experiments. It is pointed out that further fine structures and extra peaks in the local density of states should be observed by STM.Comment: 11 pages, REVTeX; 20 PostScript figures; An Animated GIFS file for the star-shaped vortex bound states is available at http://mp.okayama-u.ac.jp/~hayashi/vortex.htm

Multiple Traits for People Identification

Present biometric systems mostly rely on a single physical or behavioral feature for either identification or verification. However, day to day use of single biometries in massive or uncontrolled scenarios still has several shortcomings. These can be due to complex or unstable hardware settings, to changing environmental conditions or even to immature software procedures: some classification problems are intrinsically hard to solve. Possible spoofing of single biometric features is an additional issue. Last but not least, some features may occasionally lack the requisite of universality. As a consequence, biometric systems based on a single feature often have poor reliability, especially in applications where high security is needed. Multimodal systems, i.e., systems that concurrently exploit multiple features, are a possible way to achieve improved effectiveness and reliability. There are several issues that must be addressed when designing such a system, including the choice of the set of biometric features, the normalization method, the integration schema and the fusion process, and the use of a measure of reliability for each subsystem on a single response basis. This chapter describes the state of the art regarding such issues and sketches some suggestions for future work