307 research outputs found
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
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 ,
Conductance through an array of quantum dots
We propose a simple approach to study the conductance through an array of
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 , when
the parameters are such that electron-hole symmetry is induced, we obtain
perfect conductance . The validity of the approach is discussed in
detail.Comment: to appear in Phys. Rev.
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
Dissipative phase-fluctuations in superconducting wires capacitively coupled to diffusive metals
We study the screening of the Coulomb interaction in a quasi one-dimensional
superconductor given by the presence of either a one- or a two-dimensional
non-interacting electron gas. To that end, we derive an effective low-energy
phase-only action, which amounts to treating the Coulomb and superconducting
correlations in the random-phase approximation. We concentrate on the study of
dissipation effects in the superconductor, induced by the effect of Coulomb
coupling to the diffusive density-modes in the metal, and study its
consequences on the static and dynamic conductivity. Our results point towards
the importance of the dimensionality of the screening metal in the behavior of
the superconducting plasma mode of the wire at low energies. In absence of
topological defects, and when the screening is given by a one-dimensional
electron gas, the superconducting plasma mode is completely damped in the limit
, and consequently superconductivity is lost in the wire. In contrast,
we recover a Drude-response in the conductivity when the screening is provided
by a two-dimensional electron gas.Comment: 16 pages, 8 figures, 1 table, 2 appendice
Microcavity exciton-polariton mediated Raman scattering: Experiments and theory
We studied the intensity of resonant Raman scattering due to optical phonons
in a planar II-VI-type semiconductor microcavity in the regime of strong
coupling between light and matter. Two different sets of independent
experiments were performed at near outgoing resonance with the middle polariton
(MP)branch of the cavity. In the first, the Stokes-shifted photons were kept at
exact resonance with the MP, varying the photonic or excitonic character of the
polariton. In the second, only the incoming light wavelength was varied, and
the resonant profile of the inelastic scattered intensity was studied when the
system was tuned out of the resonant condition. Taking some matrix elements as
free parameters, both independent experiments are quantitatively described by a
model which incorporates lifetime effects in both excitons and photons, and the
coupling of the cavity photons to the electron-hole continuum. The model is
solved using a Green's function approach which treats the exciton-photon
coupling nonperturbatively.Comment: 10 pages, 6 figure
Variation in Patterns of Metal Accumulation in Thallus Parts of Lessonia trabeculata (Laminariales; Phaeophyceae): Implications for Biomonitoring
Seaweeds are well known to concentrate metals from seawater and have been
employed as monitors of metal pollution in coastal waters and estuaries.
However, research showing that various intrinsic and extrinsic factors can
influence metal accumulation, raises doubts about the basis for using seaweeds
in biomonitoring programmes. The thallus of brown seaweeds of the order
Laminariales (kelps) is morphologically complex but there is limited
information about the variation in metal accumulation between the different
parts, which might result in erroneous conclusions being drawn if not accounted
for in the biomonitoring protocol. To assess patterns of individual metals in
the differentiated parts of the thallus (blade, stipe, holdfast),
concentrations of a wide range of essential and non-essential metals (Fe, Cr,
Cu, Zn, Mn, Pb, Cd, Ni and Al) were measured in the kelp Lessonia trabeculata.
Seaweeds were collected from three sampling stations located at 5, 30 and 60 m
from an illegal sewage outfall close to Ventanas, Chile and from a pristine
location at Faro Curaumilla. For the majority of metals the highest
concentrations in bottom sediment and seaweed samples were found at the site
closest to the outfall, with concentrations decreasing with distance from the
outfall and at control stations; the exception was Cd, concentrations of which
were higher at control stations. The patterns of metal concentrations in
different thallus parts were metal specific and independent of sampling
station. These results and the available literature suggest that biomonitoring
of metals using seaweeds must take account of differences in the accumulation
of metals in thallus parts of complex seaweedsComment: Research articl
Crystal-field effects in the mixed-valence compounds Yb2M3Ga9 (M= Rh, Ir)
Magnetic susceptibility, heat capacity, and electrical resistivity
measurements have been carried out on single crystals of the intermediate
valence compounds Yb2Rh3Ga9 and Yb2Ir3Ga9. These measurements reveal a large
anisotropy due apparently to an interplay between crystalline electric field
(CEF) and Kondo effects. The temperature dependence of magnetic susceptibility
can be modelled using the Anderson impurity model including CEF within an
approach based on the Non-Crossing Approximation.Comment: Accepted to Phys. Rev.
One- and many-body effects on mirages in quantum corrals
Recent interesting experiments used scanning tunneling microscopy to study
systems involving Kondo impurities in quantum corrals assembled on Cu or noble
metal surfaces. The solution of the two-dimensional one-particle Schrodinger
equation in a hard wall corral without impurity is useful to predict the
conditions under which the Kondo effect can be projected to a remote location
(the quantum mirage). To model a soft circular corral, we solve this equation
under the potential W*delta(r-r0), where r is the distance to the center of the
corral and r0 its radius. We expand the Green's function of electron surface
states Gs0 for r<r0 as a discrete sum of contributions from single poles at
energies epsilon_i-I*delta_i. The imaginary part delta_i is the half-width of
the resonance produced by the soft confining potential, and turns out to be a
simple increasing function of epsilon_i. In presence of an impurity, we solve
the Anderson model at arbitrary temperatures using the resulting expression for
Gs0 and perturbation theory up to second order in the Coulomb repulsion U. We
calculate the resulting change in the differential conductance Delta dI/dV as a
function of voltage and space, in circular and elliptical corrals, for
different conditions, including those corresponding to recent experiments. The
main features are reproduced. The role of the direct hybridization between
impurity and bulk, the confinement potential, the size of the corral and
temperature on the intensity of the mirage are analyzed. We also calculate
spin-spin correlation functions.Comment: 13 pages, 12 figures, accepted for publication in Phys. Rev. B.
Calculations of spin correlations within an additional approximation adde
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