Spectral density of an interacting dot coupled indirectly to conducting leads

We study the spectral density of electrons rho in an interacting quantum dot (QD) with a hybridization lambda to a non-interacting QD, which in turn is coupled to a non-interacting conduction band. The system corresponds to an impurity Anderson model in which the conduction band has a Lorentzian density of states of width Delta2. We solved the model using perturbation theory in the Coulomb repulsion U (PTU) up to second order and a slave-boson mean-field approximation (SBMFA). The PTU works surprisingly well near the exactly solvable limit Delta2 -> 0. For fixed U and large enough lambda or small enough Delta2, the Kondo peak in rho(omega) splits into two peaks. This splitting can be understood in terms of weakly interacting quasiparticles. Before the splitting takes place the universal properties of the model in the Kondo regime are lost. Using the SBMFA, simple analytical expressions for the occurrence of split peaks are obtained. For small or moderate Delta2, the side bands of rho(omega) have the form of narrow resonances, that were missed in previous studies using the numerical renormalization group. This technique also has shortcomings for describing properly the split Kondo peaks. As the temperature is increased, the intensity of the split Kondo peaks decreases, but it is not completely suppressed at high temperatures.Comment: 13 pages, 13 figures, accepted in Phys. Rev.

Explicit minimal Scherk saddle towers of arbitrary even genera in R3\R^3

Starting from works by Scherk (1835) and by Enneper-Weierstra\ss \ (1863), new minimal surfaces with Scherk ends were found only in 1988 by Karcher (see \cite{Karcher1,Karcher}). In the singly periodic case, Karcher's examples of positive genera had been unique until Traizet obtained new ones in 1996 (see \cite{Traizet}). However, Traizet's construction is implicit and excludes {\it towers}, namely the desingularisation of more than two concurrent planes. Then, new explicit towers were found only in 2006 by Martin and Ramos Batista (see \cite{Martin}), all of them with genus one. For genus two, the first such towers were constructed in 2010 (see \cite{Valerio2}). Back to 2009, implicit towers of arbitrary genera were found in \cite{HMM}. In our present work we obtain {\it explicit} minimal Scherk saddle towers, for any given genus $2k$, $k\ge3$

Manipulating Majorana Fermions in Quantum Nanowires with Broken Inversion Symmetry

We study a Majorana-carrying quantum wire, driven into a trivial phase by breaking the spatial inversion symmetry with a tilted external magnetic field. Interestingly, we predict that a supercurrent applied in the proximate superconductor is able to restore the topological phase and therefore the Majorana end-states. Using Abelian bosonization, we further confirm this result in the presence of electron-electron interactions and show a profound connection of this phenomenon to the physics of a one-dimensional doped Mott-insulator. The present results have important applications in e.g., realizing a supercurrent assisted braiding of Majorana fermions, which proves highly useful in topological quantum computation with realistic Majorana networks.Comment: 5 pages, 3 figures, Supplementary Material is adde

Conductance through an array of quantum dots

We propose a simple approach to study the conductance through an array of $N$ interacting quantum dots, weakly coupled to metallic leads. Using a mapping to an effective site which describes the low-lying excitations and a slave-boson representation in the saddle-point approximation, we calculated the conductance through the system. Explicit results are presented for N=1 and N=3: a linear array and an isosceles triangle. For N=1 in the Kondo limit, the results are in very good agreement with previous results obtained with numerical renormalization group (NRG). In the case of the linear trimer for odd $N$, when the parameters are such that electron-hole symmetry is induced, we obtain perfect conductance $G_0=2e^2/h$. The validity of the approach is discussed in detail.Comment: to appear in Phys. Rev.

Stability of moving solitons in trans-polyacetylene in an electric field

In this work we study the dynamics and stability of charged solitons in trans-polyacetylene (tPA), and revisit the issue of the stability of these non-linear excitations under the effect of an external electric field applied parallel to the polymer. Using the formalism of the Su-Schrieffer-Heeger (SSH) model, we solve the coupled dynamical equations for electrons and classical nuclei at the mean-field level and in the regime of low external electric field $E$, where the dynamics of the moving soliton is adiabatic. Analyzing observable quantities in real space and frequency space, we identify the microscopic mechanisms triggering the dynamical instabilities of the soliton. In addition, we put forward the definition of a proper quantitative measure of its stability, an issue which to the best of our knowledge has remained an open question. Besides its intrinsic interest from the fundamental point of view, our work might be relevant for the design of novel organic electronic devices based on soliton-mediated transport.Comment: 14 pages, 9 figures, 1 appendi

Measurement of IEC Groups and Subgroups Using Advanced Spectrum Estimation Methods

The International Electrotechnical Commission (IEC) standards characterize the waveform distortions in power systems with the amplitudes of harmonic and interharmonic groups and subgroups. These groups/subgroups utilize the waveform spectral components obtained from a fixed frequency resolution discrete Fourier transform (DFT). Using the IEC standards allows for a compromise among the different goals, such as the needs for accuracy, simplification, and unification. In some cases, however, the power-system waveforms are characterized by spectral components that the DFT cannot capture with enough accuracy due to the fixed frequency resolution and/or the spectral leakage phenomenon. This paper investigates the possibility of a group/subgroup evaluation using the following advanced spectrum estimation methods: adaptive Prony, estimation of signal parameters via rotational invariance techniques, and root MUltiple-SIgnal Classification (MUSIC). These adaptive methods use variable lengths of time windows of analysis to ensure the best fit of the waveforms; they are not characterized by the fixed frequency resolution and do not suffer from the spectral leakage phenomenon. This paper also presents the results of the applications of these methods to three test waveforms, to current and voltage waveforms obtained from simulations of a real dc arc-furnace plant, and to waveforms measured at the point of common coupling of the low-voltage network supplying a high-performance laser printer

Dissipation-driven superconductor-insulator transition in linear arrays of Josephson junctions capacitively coupled to metallic films

We study the low-temperature properties of linear Josephson-junction arrays capacitively coupled to a proximate two-dimensional diffusive metal. Using bosonization techniques, we derive an effective model for the array and obtain its critical properties and phases at T = 0 using a renormalization group analysis and a variational approach. While static screening effects given by the presence of the metal can be absorbed in a renormalization of the parameters of the array, backscattering originated in the dynamically screened Coulomb interaction produces a non-trivial stabilization of the insulating groundstate and can drive a superconductor-insulator transition. We study the consequences for the transport properties in the low-temperature regime. In particular, we calculate the resisitivity as a function of the temperature and the parameters of the array, and obtain clear signatures of a superconductor-insulator transition that could be observed in experiments.Comment: 10 pages, 5 figures, submitted to Physical Review

Aislamiento de bacterias periodontopáticas desde hemocultivos y ateromas obtenidos de pacientes con aterosclerosis y periodontitis

Carlos Padilla E. Laboratorio de Investigación Microbiológica, Facultad de Ciencias de la Salud, Universidad de Talca. Talca. Chile. Fono: 71-200492. Fax: 71-200439. E mail: [email protected]: Periodontitis is a common oral disease produced by bacterial species that reside in the subgingival plaque. These microorganisms have been associated to atherosclerosis and it is suggested that periodontitis is a cardiovascular risk factor. Aim: To isolate periodontal bacteria from blood and atheroma samples, from patients with atherosclerosis and periodontitis. Material and methods: Twelve patients with periodontitis and a clinical diagnosis of atherosclerosis and 12 patients with periodontitis but without atherosclerosis were studied. Blood samples were obtained immediately before and after scaling and root planing. The samples were incubated in aerobic and anaerobic conditions. One week after scaling, atheromatous plaques were obtained during endarterectomy in the 12 patients with atherosclerosis. These were homogenized and cultured for aerobic and anaerobic bacteria. Microorganisms were identified by means ofPCR. Results: Five patients with and two without atherosclerosis, had bacteremia after scaling and root planing. Bacterial species isolated from blood samples were the same found in periodontic pockets. Four atheromatous plaques of patients with bacteremia yielded positive cultures. The isolated bacteria were the same found in blood samples and periodontal pockets. Conclusions: Bacteremia occurred in seven of 24 patients after scaling and root planing. In four patients, the same species found in periodontic pockets and blood cultures were detected in atherosclerotic plaques obtained one week after the dental procedure