362 research outputs found
Spectral properties of locally correlated electrons in a BCS superconductor
We present a detailed study of the spectral properties of a locally
correlated site embedded in a BCS superconducting medium. To this end the
Anderson impurity model with superconducting bath is analysed by numerical
renormalisation group (NRG) calculations. We calculate one and two-particle
dynamic response function to elucidate the spectral excitation and the nature
of the ground state for different parameter regimes with and without
particle-hole symmetry. The position and weight of the Andreev bound states is
given for all relevant parameters. We also present phase diagrams for the
different ground state parameter regimes. This work is also relevant for
dynamical mean field theory extensions with superconducting symmetry breaking.Comment: 22 pages, 12 figure
A spin-dependent local moment approach to the Anderson impurity model
We present an extension of the local moment approach to the Anderson impurity
model with spin-dependent hybridization. By employing the two-self-energy
description, as originally proposed by Logan and co-workers, we applied the
symmetry restoration condition for the case with spin-dependent hybridization.
Self-consistent ground states were determined through variational minimization
of the ground state energy. The results obtained with our spin-dependent local
moment approach applied to a quantum dot system coupled to ferromagnetic leads
are in good agreement with those obtained from previous work using numerical
renormalization group calculations
Antiferromagnetic Order of Strongly Interacting Fermions in a Trap: Real-Space Dynamical Mean-Field Analysis
We apply Dynamical Mean-Field Theory to strongly interacting fermions in an
inhomogeneous environment. With the help of this Real-Space Dynamical
Mean-Field Theory (R-DMFT) we investigate antiferromagnetic states of
repulsively interacting fermions with spin 1/2 in a harmonic potential. Within
R-DMFT, antiferromagnetic order is found to be stable in spatial regions with
total particle density close to one, but persists also in parts of the system
where the local density significantly deviates from half filling. In systems
with spin imbalance, we find that antiferromagnetism is gradually suppressed
and phase separation emerges beyond a critical value of the spin imbalance.Comment: 4 pages 5 figures, published versio
Numerical renormalization group calculation of near-gap peaks in spectral functions of the Anderson model with superconducting leads
We use the numerical renormalization group method (NRG) to investigate a
single-impurity Anderson model with a coupling of the impurity to a
superconducting host. Analysis of the energy flow shows, in contrast to
previous belief, that NRG iterations can be performed up to a large number of
sites, corresponding to energy differences far below the superconducting gap.
This allows us to calculate the impurity spectral function very accurately for
frequencies near the gap edge, and to resolve, in a certain parameter regime,
sharp peaks in the spectral function close to the gap edge.Comment: 18 pages, 7 figures, accepted for publication in Journal of Physics:
Condensed Matte
Bound states in straight quantum waveguides with combined boundary conditions
We investigate the discrete spectrum of the Hamiltonian describing a quantum
particle living in the two-dimensional straight strip. We impose the combined
Dirichlet and Neumann boundary conditions on different parts of the boundary.
Several statements on the existence or the absence of the discrete spectrum are
proven for two models with combined boundary conditions. Examples of
eigenfunctions and eigenvalues are computed numerically.Comment: 24 pages, LaTeX 2e with 4 eps figure
Andreev Bound States in the Kondo Quantum Dots Coupled to Superconducting Leads
We have studied the Kondo quantum dot coupled to two superconducting leads
and investigated the subgap Andreev states using the NRG method. Contrary to
the recent NCA results [Clerk and Ambegaokar, Phys. Rev. B 61, 9109 (2000);
Sellier et al., Phys. Rev. B 72, 174502 (2005)], we observe Andreev states both
below and above the Fermi level.Comment: 5 pages, 5 figure
Optimising the observation of optical kilonovae with medium size telescopes
We consider the optimisation of the observing strategy (cadence, exposure
time and filter choice) using medium size (2-m class) optical telescopes in the
follow-up of kilonovae localised with arcminute accuracy to be able to
distinguish among various kilonova models and viewing angles. To develop an
efficient observation plan, we made use of the synthetic light curves obtained
with the Monte Carlo radiative transfer code POSSIS for different kilonova
models and as a function of different viewing angles and distances. By adding
the appropriate photon counting noise to the synthetic light curves, we
analysed four alternative sequences having the same total time exposure of 8
hours, with different time windows (0.5, 1, 2, 4 h), each with , , and
filters, to determine the observing sequence that maximises the chance of a
correct identification of the model parameters. We suggest to avoid filter
and to avoid the use of colour curves. We also found that, if the error on
distance is 2%, 0.5, 1, 2-hour time window sequences are equivalent, so
we suggest to use 2-hour one, because it has 1 day cadence, so it can be easily
realised. When the distance of the source is unknown, 0.5 h time window
sequence is preferable.Comment: 9 pages, 8 figures, published in MNRA
Geometric coupling thresholds in a two-dimensional strip
We consider the Laplacian in a strip with the
boundary condition which is Dirichlet except at the segment of a length of
one of the boundaries where it is switched to Neumann. This operator is known
to have a non-empty and simple discrete spectrum for any . There is a
sequence of critical values at which new eigenvalues emerge
from the continuum when the Neumann window expands. We find the asymptotic
behavior of these eigenvalues around the thresholds showing that the gap is in
the leading order proportional to with an explicit coefficient
expressed in terms of the corresponding threshold-energy resonance
eigenfunction
Low energy fixed points of the sigma-tau and the O(3) symmetric Anderson models
We study the single channel (compactified) models, the sigma-tau model and
the O(3) symmetric Anderson model, which were introduced by Coleman et al., and
Coleman and Schofield, as a simplified way to understand the low energy
behaviour of the isotropic and anisotropic two channel Kondo systems. These
models display both Fermi liquid and marginal Fermi liquid behaviour and an
understanding of the nature of their low energy fixed points may give some
general insights into the low energy behaviour of other strongly correlated
systems. We calculate the excitation spectrum at the non-Fermi liquid fixed
point of the sigma-tau model using conformal field theory, and show that the
results are in agreement with those obtained in recent numerical
renormalization group (NRG) calculations. For the O(3) Anderson model we find
further logarithmic corrections in the weak coupling perturbation expansion to
those obtained in earlier calculations, such that the renormalized interaction
term now becomes marginally stable rather than marginally unstable. We derive a
Ward identity and a renormalized form of the perturbation theory that
encompasses both the weak and strong coupling regimes and show that the
chi/gamma ratio is 8/3 (chi is the total susceptibility, spin plus isospin),
independent of the interaction U and in agreement with the NRG calculations.Comment: 23 pages, LaTeX, 11 figures includes as eps-files, submitted to Phys.
Rev.
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