739 research outputs found

### From semiconductors to superconductors: a simple model for pseudogaps

We consider a two dimensional semiconductor with a local attraction among the
carriers. We study the ground state of this system as a function of the
semiconductor gap. We find a direct transition from a superconducting to an
insulating phase for no doping at a critical value, the single particle
excitations being always gapped. For finite doping we find a smooth crossover.
We calculate the critical temperature due to both the particle excitations and
the Berezinkii-Kosterlitz-Thouless transition.Comment: 14 pages. Accepted for publication on Eur. Phys. Jour.

### Tunable Kondo screening in a quantum dot device

We consider electron transport along a single-mode channel which is in
contact, via tunnel junctions in its walls, with two quantum dots. Electron
tunneling to and from the dots contributes to the electron backscattering, and
thus modifies the channel conductance. If the dots carry spin, the channel
conductance becomes temperature dependent due to the Kondo effect. The two-dot
device geometry allows for a formation of S=1 localized spin due to the
indirect exchange interaction, called Ruderman-Kittel-Kasuya-Yosida
interaction. This device offers a possibility to study the crossover between
fully screened and under-screened Kondo impurity. We investigate the
manifestation of such crossover in the channel conductance

### Ground state fragmentation of repulsive BEC in double-trap potentials

The fragmentation of the ground state of a repulsive condensate immersed into
a double-trap potential is found to be a general and critical phenomenon. It
takes place for a given number of bosons if their scattering length is larger
than some critical value or for a given value of the scattering length if the
number of bosons is above some critical number. We demonstrate that the
geometry of the inner trap determines these critical parameters while the
number of the fragments and the fraction of bosons in the various fragments can
be manipulated by the outer trap. There is also a maximal number of bosons for
which the ground state is fragmented. If this number is exceeded, the
fragmented state becomes a very low-lying excited state of the condensate. This
maximal number of bosons can be substantially manipulated by varying the inner
and outer traps. To study three-fold fragmentation we have chosen a potential
well with two barriers as the inner trap and embedded by two types of outer
ones. A many-fold fragmentation is also addressed.Comment: 18 pages + 9 figure

### Steps and facets at the surface of soft crystals

We consider the shape of crystals which are soft in the sense that their
elastic modulus $\mu$ is small compared to their surface tension $\gamma$, more
precisely $\mu a < \gamma$ where $a$ is the lattice spacing. We show that
their surface steps penetrate inside the crystal as edge dislocations. As a
consequence, these steps are broad with a small energy which we calculate. We
also calculate the elastic interaction between steps a distance $d$ apart,
which is a $1/d^2$ repulsion. We finally calculate the roughening temperatures
of successive facets in order to compare with the remarkable shapes of
lyotropic crystals recently observed by P. Pieranski et al. Good agreement is
found.Comment: 8 Pages, 1 Figure. To appear on Eur. Phys. Journal.

### The generalized multi-channel Kondo model: Thermodynamics and fusion equations

The SU(N) generalization of the multi-channel Kondo model with arbitrary
rectangular impurity representations is considered by means of the Bethe
Ansatz. The thermodynamics of the model is analyzed by introducing modified
fusion equations for the impurity, leading to a simple description of the
different IR fixed points of the theory. The entropy at zero temperature is
discussed; in particular the overscreened case is explained in terms of quantum
group representation.Comment: 41 pages, 8 figures, harvma

### Magnetically Tunable Kondo - Aharonov-Bohm Effect in a Triangular Quantum Dot

The role of discrete orbital symmetry in mesoscopic physics is manifested in
a system consisting of three identical quantum dots forming an equilateral
triangle. Under a perpendicular magnetic field, this system demonstrates a
unique combination of Kondo and Aharonov-Bohm features due to an interplay
between continuous [spin-rotation SU(2)] and discrete (permutation C3v)
symmetries, as well as U(1) gauge invariance. The conductance as a function of
magnetic flux displays sharp enhancement or complete suppression depending on
contact setups.Comment: 4 pages, 3 .eps figure

### General variational many-body theory with complete self-consistency for trapped bosonic systems

In this work we develop a complete variational many-body theory for a system
of $N$ trapped bosons interacting via a general two-body potential. In this
theory both the many-body basis functions {\em and} the respective expansion
coefficients are treated as variational parameters. The optimal variational
parameters are obtained {\em self-consistently} by solving a coupled system of
non-eigenvalue -- generally integro-differential -- equations to get the
one-particle functions and by diagonalizing the secular matrix problem to find
the expansion coefficients. We call this theory multi-configurational Hartree
for bosons or MCHB(M), where M specifies explicitly the number of one-particle
functions used to construct the configurations. General rules for evaluating
the matrix elements of one- and two-particle operators are derived and applied
to construct the secular Hamiltonian matrix. We discuss properties of the
derived equations. It is demonstrated that for any practical computation where
the configurational space is restricted, the description of trapped bosonic
systems strongly depends on the choice of the many-body basis set used, i.e.,
self-consistency is of great relevance. As illustrative examples we consider
bosonic systems trapped in one- and two-dimensional symmetric and asymmetric
double-well potentials. We demonstrate that self-consistency has great impact
on the predicted physical properties of the ground and excited states and show
that the lack of self-consistency may lead to physically wrong predictions. The
convergence of the general MCHB(M) scheme with a growing number M is validated
in a specific case of two bosons trapped in a symmetric double-well.Comment: 53 pages, 8 figure

### Integer filling metal insulator transitions in the degenerate Hubbard model

We obtain exact numerical solutions of the degenerate Hubbard model in the
limit of large dimensions (or large lattice connectivity). Successive
Mott-Hubbard metal insulator transitions at integer fillings occur at
intermediate values of the interaction and low enough temperature in the
paramagnetic phase. The results are relevant for transition metal oxides with
partially filled narrow degenerate bands.Comment: 4 pages + 4 figures (in 5 ps-files), revte

### Lattice susceptibility for 2D Hubbard Model within dual fermion method

In this paper, we present details of the dual fermion (DF) method to study
the non-local correction to single site DMFT. The DMFT two-particle Green's
function is calculated using continuous time quantum monte carlo (CT-QMC)
method. The momentum dependence of the vertex function is analyzed and its
renormalization based on the Bethe-Salpeter equation is performed in
particle-hole channel. We found a magnetic instability in both the dual and the
lattice fermions. The lattice fermion susceptibility is calculated at finite
temperature in this method and also in another recently proposed method, namely
dynamical vertex approximation (D$\Gamma$A). The comparison between these two
methods are presented in both weak and strong coupling region. Compared to the
susceptibility from quantum monte carlo (QMC) simulation, both of them gave
satisfied results.Comment: 10 pages, 11 figure

### Quantum Monte Carlo modelling of the spherically averaged structure factor of a many-electron system

The interaction and exchange-correlation contributions to the ground-state
energy of an arbitrary many-electron system can be obtained from a spherical
average of the wavevector-dependent diagonal structure factor (SF). We model
the continuous-k spherically averaged SF using quantum Monte Carlo calculations
in finite simulation cells. We thus derive a method that allows to
substantially reduce the troublesome Coulomb finite-size errors that are
usually present in ground-state energy calculations. To demonstrate this, we
perform variational Monte Carlo calculations of the interaction energy of the
homogeneous electron gas. The method is, however, equally applicable to
arbitrary inhomogeneous systems.Comment: 4 pages, 5 figure

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