Temperature enhanced persistent currents and "ϕ0/2\phi_0/2 periodicity"

We predict a non-monotonous temperature dependence of the persistent currents in a ballistic ring coupled strongly to a stub in the grand canonical as well as in the canonical case. We also show that such a non-monotonous temperature dependence can naturally lead to a $\phi_0/2$ periodicity of the persistent currents, where $\phi_0$=h/e. There is a crossover temperature $T^*$, below which persistent currents increase in amplitude with temperature while they decrease above this temperature. This is in contrast to persistent currents in rings being monotonously affected by temperature. $T^*$ is parameter-dependent but of the order of $\Delta_u/\pi^2k_B$, where $\Delta_u$ is the level spacing of the isolated ring. For the grand-canonical case $T^*$ is half of that for the canonical case.Comment: some typos correcte

Persistent currents in coupled mesoscopic rings

We have analysed the nature of persistent currents in open coupled mesoscopic rings. Our system is comprised of two ideal loops connected to an electron reservoir. We have obtained analytical expressions for the persistent current densities in two rings in the presence of a magnetic field. We show that the known even-odd parity effects in isolated single loops have to be generalised for the case of coupled rings. We also show that when the two rings have unequal circumferences, it is possible to observe opposite currents (diamagnetic or paramagnetic) in the two rings for a given Fermi level.Comment: Submitted to PRB. 9 figures availabel on reques

Persistent Currents in the Presence of a Transport Current

We have considered a system of a metallic ring coupled to two electron reservoirs. We show that in the presence of a transport current, the persistent current can flow in a ring, even in the absence of magnetic field. This is purely a quantum effect and is related to the current magnification in the loop. These persistent currents can be observed if one tunes the Fermi energy near the antiresonances of the total transmission coefficient or the two port conductance.Comment: To appear in Phys. Rev. B. Three figures available on reques

Paramagnetic Meissner effect in superconductors from self-consistent solutions of Ginzburg-Landau equations

The paramagnetic Meissner effect (PME) is observed in small superconducting samples, and a number of controversial explanations of this effect are proposed, but there is as yet no clear understanding of its nature. In the present paper PME is considered on the base of the Ginzburg-Landau theory (GL). The one-dimensional solutions are obtained in a model case of a long superconducting cylinder for different cylinder radii R, the GL-parameters \kappa and vorticities m. Acording to GL-theory, PME is caused by the presence of vortices inside the sample. The superconducting current flows around the vortex to screeen the vortex own field from the bulk of the sample. Another current flows at the boundary to screen the external field H from entering the sample. These screening currents flow in opposite directions and contribute with opposite signs to the total magnetic moment (or magnetization) of the sample. Depending on H, the total magnetization M may be either negative (diamagnetism), or positive (paramagnetism). A very complicated saw-like dependence M(H) (and other characteristics), which are obtained on the base of self-consistent solutions of the GL-equations, are discussed.Comment: 6 pages, 5 figures, RevTex, submitted to Phys. Rev.

Effect of gas flow on electronic transport in a DNA-decorated carbon nanotube

We calculate the two-time current correlation function using the experimental data of the current-time characteristics of the Gas-DNA-decorated carbon nanotube field effect transistor. The pattern of the correlation function is a measure of the sensitivity and selectivity of the sensors and suggest that these gas flow sensors may also be used as DNA sequence detectors. The system is modelled by a one-dimensional tight-binding Hamiltonian and we present analytical calculations of quantum electronic transport for the system using the time-dependent nonequilibrium Green's function formalism and the adiabatic expansion. The zeroth and first order contributions to the current $I^{(0)}(\bar{t})$ and $I^{(1)}(\bar{t})$ are calculated, where $I^{(0)} (\bar{t})$ is the Landauer formula. The formula for the time-dependent current is then used to compare the theoretical results with the experiment.Comment: 14 pages, 5 figures and 2 table

An effective lowest Landau level treatment of demagnetization in superconducting mesoscopic disks

Demagnetization, which is inherently present in the magnetic response of small finite-size superconductors, can be accounted for by an effective $\kappa$ within a two-dimensional lowest Landau level approximation of the Ginzburg-Landau functional. We show this by comparing the equilibrium magnetization of superconducting mesoscopic disks obtained from the numerical solution of the three-dimensional Ginzburg-Landau equations with that obtained in the ``effective'' LLL approximation.Comment: 5 pages, 4 figures, submitted to Phys. Rev.

Friedel phases and phases of transmission amplitudes in quantum scattering systems

We illustrate the relation between the scattering phase appearing in the Friedel sum rule and the phase of the transmission amplitude for quantum scatterers connected to two one-dimensional leads. Transmission zero points cause abrupt phase changes $\pm\pi$ of the phase of the transmission amplitude. In contrast the Friedel phase is a continuous function of energy. We investigate these scattering phases for simple scattering problems and illustrate the behavior of these models by following the path of the transmission amplitude in the complex plane as a function of energy. We verify the Friedel sum rule for these models by direct calculation of the scattering phases and by direct calculation of the density of states.Comment: 12 pages, 12 figure

Vortex matter in superconducting mesoscopic disks: Structure, magnetization, and phase transitions

The dense vortex matter structure and associated magnetization are calculated for type-II superconducting mesoscopic disks. The magnetization exhibits generically first-order phase transitions as the number of vortices changes by one and presents two well-defined regimes: A non-monotonous evolution of the magnitude of the magnetization jumps signals the presence of a vortex glass structure which is separated by a second-order phase transition at $H_{c2}$ from a condensed state of vortices (giant vortex) where the magnitude of the jumps changes monotonously. We compare our results with Hall magnetometry measurements by Geim et al. (Nature 390, 259 (1997)) and claim that the magnetization exhibits clear traces of the presence of these vortex glass states.Comment: 4 pages, 3 figure

Manejo da mancha angular (Xanthomonas campestris pv. Mangiferae indica) na produção integrada de manga.

bitstream/CPATSA/33055/1/INT63.pd

On subgroups in division rings of type 22

Let $D$ be a division ring with center $F$. We say that $D$ is a {\em division ring of type $2$} if for every two elements $x, y\in D,$ the division subring $F(x, y)$ is a finite dimensional vector space over $F$. In this paper we investigate multiplicative subgroups in such a ring.Comment: 10 pages, 0 figure