Transport spectroscopy in a time-modulated open quantum dot

We have investigated the time-modulated coherent quantum transport phenomena in a ballistic open quantum dot. The conductance $G$ and the electron dwell time in the dots are calculated by a time-dependent mode-matching method. Under high-frequency modulation, the traversing electrons are found to exhibit three types of resonant scatterings. They are intersideband scatterings: into quasibound states in the dots, into true bound states in the dots, and into quasibound states just beneath the subband threshold in the leads. Dip structures or fano structures in $G$ are their signatures. Our results show structures due to 2$\hbar\omega$ intersideband processes. At the above scattering resonances, we have estimated, according to our dwell time calculation, the number of round-trip scatterings that the traversing electrons undertake between the two dot openings.Comment: 8 pages, 5 figure

Phase transitions and noise crosscorrelations in a model of directed polymers in a disordered medium

We show that effective interactions mediated by disorder between two directed polymers can be modelled as the crosscorrelation of noises in the Kardar-Parisi-Zhang (KPZ) equations satisfied by the respective free energies of these polymers. When there are two polymers, disorder introduces attractive interactions between them. We analyze the phase diagram in details and show that these interactions lead to new phases in the phase diagram. We show that, even in dimension $d=1$, the two directed polymers see the attraction only if the strength of the disorder potential exceeds a threshold value. We extend our calculations to show that if there are $m$ polymers in the system then $m$-body interactions are generated in the disorder averaged effective free energy.Comment: To appear in Phys. Rev. E(2000

Spin Injection in a Ballistic Two-Dimensional Electron Gas

We explore electrically injected, spin polarized transport in a ballistic two-dimensional electron gas. We augment the Buettiker-Landauer picture with a simple, but realistic model for spin-selective contacts to describe multimode reservoir-to-reservoir transport of ballistic spin 1/2 particles. Clear and unambiguous signatures of spin transport are established in this regime, for the simplest measurement configuration that demonstrates them directly. These new effects originate from spin precession of ballistic carriers; they exhibit strong dependence upon device geometry and vanish in the diffusive limit. Our results have important implications for prospective ``spin transistor'' devices.Comment: Submitted to Phys. Rev. Let

Antecedent use of fluoroquinolones is associated with resistance to moxifloxacin in Clostridium difficile

ObjectiveMoxifloxacin is characterized by high activity against Gram-positive cocci and some Gram-positive and -negative anaerobes, including Clostridium difficile. This study investigates the role of prior quinolone use in relation to patterns of susceptibility of C. difficile to moxifloxacin.MethodsSixty-three clinical isolates of C. difficile were investigated for toxigenicity, susceptibility to moxifloxacin, and mutations in the DNA gyrase gene. The medical histories for 50 of these patients were available and used to identify previous fluoroquinolone use.ResultsThirty-three (52.4%) strains showed resistance to moxifloxacin (MICs ≥ 16 mg/L). All moxifloxacin-resistant strains harbored a mutation at amino acid codon Ser-83 of gyrA. Forty-five isolates (71.4%) were toxigenic; all moxifloxacin-resistant strains were in this group. Resistance to moxifloxacin was associated with prior use of fluoroquinolones (P-value 0.009, chi-square).ConclusionsAlthough the use of moxifloxacin to treat C. difficile-associated diarrhea is not likely to be common, these data show a relationship between antecedent fluoroquinolone use and resistance to moxifloxacin in C. difficile isolates, and raise questions regarding selection pressure for resistance placed on colonizing bacteria exposed to fluoroquinolones. Mutations in gyrA are involved in moxifloxacin resistance

Hard-core Yukawa model for two-dimensional charge stabilized colloids

The hyper-netted chain (HNC) and Percus-Yevick (PY) approximations are used to study the phase diagram of a simple hard-core Yukawa model of charge-stabilized colloidal particles in a two-dimensional system. We calculate the static structure factor and the pair distribution function over a wide range of parameters. Using the statics correlation functions we present an estimate for the liquid-solid phase diagram for the wide range of the parameters.Comment: 7 pages, 9figure

Spin alignments of vector mesons in deeply inelastic lepton-nucleon scattering

We extend the calculations of the spin alignments of vector mesons in $e^+e^-$ annihilation in a recent Rapid Communication to deeply inelastic lepton-nucleon scatterings. We present the results for different mesons in the current fragmentation regions of $\mu^- N$$\to$$\mu^- VX$ at high energies and $\nu_\mu N$$\to$$\mu^- VX$ at both high and low energies. We also present the predictions for $\nu_\mu N$$\to$$\mu^- VX$ at NOMAD energies in the target fragmentation region using a valence quark model.Comment: 4 pages, 6 figures; to appear in Phys. Rev.

Tractable non-local correlation density functionals for flat surfaces and slabs

A systematic approach for the construction of a density functional for van der Waals interactions that also accounts for saturation effects is described, i.e. one that is applicable at short distances. A very efficient method to calculate the resulting expressions in the case of flat surfaces, a method leading to an order reduction in computational complexity, is presented. Results for the interaction of two parallel jellium slabs are shown to agree with those of a recent RPA calculation (J.F. Dobson and J. Wang, Phys. Rev. Lett. 82, 2123 1999). The method is easy to use; its input consists of the electron density of the system, and we show that it can be successfully approximated by the electron densities of the interacting fragments. Results for the surface correlation energy of jellium compare very well with those of other studies. The correlation-interaction energy between two parallel jellia is calculated for all separations d, and substantial saturation effects are predicted.Comment: 10 pages, 6 figure

Hidden dimers and the matrix maps: Fibonacci chains re-visited

The existence of cycles of the matrix maps in Fibonacci class of lattices is well established. We show that such cycles are intimately connected with the presence of interesting positional correlations among the constituent `atoms' in a one dimensional quasiperiodic lattice. We particularly address the transfer model of the classic golden mean Fibonacci chain where a six cycle of the full matrix map exists at the centre of the spectrum [Kohmoto et al, Phys. Rev. B 35, 1020 (1987)], and for which no simple physical picture has so far been provided, to the best of our knowledge. In addition, we show that our prescription leads to a determination of other energy values for a mixed model of the Fibonacci chain, for which the full matrix map may have similar cyclic behaviour. Apart from the standard transfer-model of a golden mean Fibonacci chain, we address a variant of it and the silver mean lattice, where the existence of four cycles of the matrix map is already known to exist. The underlying positional correlations for all such cases are discussed in details.Comment: 14 pages, 2 figures. Submitted to Physical Review

Random walks and polymers in the presence of quenched disorder

After a general introduction to the field, we describe some recent results concerning disorder effects on both `random walk models', where the random walk is a dynamical process generated by local transition rules, and on `polymer models', where each random walk trajectory representing the configuration of a polymer chain is associated to a global Boltzmann weight. For random walk models, we explain, on the specific examples of the Sinai model and of the trap model, how disorder induces anomalous diffusion, aging behaviours and Golosov localization, and how these properties can be understood via a strong disorder renormalization approach. For polymer models, we discuss the critical properties of various delocalization transitions involving random polymers. We first summarize some recent progresses in the general theory of random critical points : thermodynamic observables are not self-averaging at criticality whenever disorder is relevant, and this lack of self-averaging is directly related to the probability distribution of pseudo-critical temperatures $T_c(i,L)$ over the ensemble of samples $(i)$ of size $L$. We describe the results of this analysis for the bidimensional wetting and for the Poland-Scheraga model of DNA denaturation.Comment: 17 pages, Conference Proceedings "Mathematics and Physics", I.H.E.S., France, November 200

Divergences in Real-Time Classical Field Theories at Non-Zero Temperature

The classical approximation provides a non-perturbative approach to time-dependent problems in finite temperature field theory. We study the divergences in hot classical field theory perturbatively. At one-loop, we show that the linear divergences are completely determined by the classical equivalent of the hard thermal loops in hot quantum field theories, and that logarithmic divergences are absent. To deal with higher-loop diagrams, we present a general argument that the superficial degree of divergence of classical vertex functions decreases by one with each additional loop: one-loop contributions are superficially linearly divergent, two-loop contributions are superficially logarithmically divergent, and three- and higher-loop contributions are superficially finite. We verify this for two-loop SU(N) self-energy diagrams in Feynman and Coulomb gauges. We argue that hot, classical scalar field theory may be completely renormalized by local (mass) counterterms, and discuss renormalization of SU(N) gauge theories.Comment: 31 pages with 7 eps figure