550 research outputs found
Mesoscopic BCS pairing in the repulsive 1d-Hubbard model
We study mesoscopic pairing in the one dimensional repulsive Hubbard model
and its interplay with the BCS model in the canonical ensemble. The key tool is
comparing the Bethe ansatz equations of the two models in the limit of small
Coulomb repulsion. For the ordinary Hubbard interaction the BCS Bethe equations
with infinite pairing coupling are recovered; a finite pairing is obtained by
considering a further density-dependent phase-correlation in the hopping
amplitude of the Hubbard model. We find that spin degrees of freedom in the
Hubbard ground state are arranged in a state of the BCS type, where the
Cooper-pairs form an un-condensed liquid on a ``lattice'' of single particle
energies provided by the Hubbard charge degrees of freedom; the condensation in
the BCS ground state corresponds to Hubbard excitations constituted by a sea of
spin singlets.Comment: 15 pages, 6 figures. To be published on Physical Review
Robustness of adiabatic passage trough a quantum phase transition
We analyze the crossing of a quantum critical point based on exact results
for the transverse XY model. In dependence of the change rate of the driving
field, the evolution of the ground state is studied while the transverse
magnetic field is tuned through the critical point with a linear ramping. The
excitation probability is obtained exactly and is compared to previous studies
and to the Landau-Zener formula, a long time solution for non-adiabatic
transitions in two-level systems. The exact time dependence of the excitations
density in the system allows to identify the adiabatic and diabatic regions
during the sweep and to study the mesoscopic fluctuations of the excitations.
The effect of white noise is investigated, where the critical point transmutes
into a non-hermitian ``degenerate region''. Besides an overall increase of the
excitations during and at the end of the sweep, the most destructive effect of
the noise is the decay of the state purity that is enhanced by the passage
through the degenerate region.Comment: 16 pages, 15 figure
Rescaling multipartite entanglement measures for mixed states
A relevant problem regarding entanglement measures is the following: Given an
arbitrary mixed state, how does a measure for multipartite entanglement change
if general local operations are applied to the state? This question is
nontrivial as the normalization of the states has to be taken into account.
Here we answer it for pure-state entanglement measures which are invariant
under determinant 1 local operations and homogeneous in the state coefficients,
and their convex-roof extension which quantifies mixed-state entanglement. Our
analysis allows to enlarge the set of mixed states for which these important
measures can be calculated exactly. In particular, our results hint at a
distinguished role of entanglement measures which have homogeneous degree 2 in
the state coefficients.Comment: Published version plus one important reference (Ref. [39]
One-dimensional anyons with competing -function and derivative -function potentials
We propose an exactly solvable model of one-dimensional anyons with competing
-function and derivative -function interaction potentials. The
Bethe ansatz equations are derived in terms of the -particle sector for the
quantum anyonic field model of the generalized derivative nonlinear
Schr\"{o}dinger equation. This more general anyon model exhibits richer physics
than that of the recently studied one-dimensional model of -function
interacting anyons. We show that the anyonic signature is inextricably related
to the velocities of the colliding particles and the pairwise dynamical
interaction between particles.Comment: 9 pages, 2 figures, minor changes, references update
Exact correlation functions of the BCS model in the canonical ensemble
We evaluate correlation functions of the BCS model for finite number of
particles. The integrability of the Hamiltonian relates it with the Gaudin
algebra . Therefore, a theorem that Sklyanin proved for the
Gaudin model, can be applied. Several diagonal and off-diagonal correlators are
calculated. The finite size scaling behavior of the pairing correlation
function is studied.Comment: 4 pages revtex; 2 figures .eps. Revised version to be published in
Phys. Rev. Let
VLT/ISAAC H-band spectroscopy of embedded massive YSOs
We have performed intermediate resolution (R = 5000), high signal-to-noise
H-band spectroscopy of a small, initial sample of three massive embedded young
stellar objects (YSOs), using VLT/ISAAC. The sample has been selected from
sources characterised in previous literature as being likely of OB spectral
type, to be unambiguously associated with bright (H < 14) single point sources
in the 2MASS database, and to have no optical counterparts. Of the targets
observed, one object shows a ~B3 spectrum, similar to a main sequence object of
the same spectral type. A second object exhibits weak HeI and H emission,
indicating an early-type source: we detect HeII absorption, which supports a
previous indirect derivation of the spectral type as mid-O. The third object
does not show absorption lines, so no spectral type can de derived. It does,
however, exhibit a rich spectrum of strong, broad emission lines and is likely
to be surrounded by dense circumstellar material and at a very early
evolutionary stage. Our results from this very small sample are in agreement
with those of Kaper et al. (2002), who also find spectra similar to optically
visible main sequence stars, together with emission line objects representing a
very early evolutionary phase, in their much larger sample of K-band spectra.Comment: 10 pages, 14 figures, A&A (accepted
Entanglement study of the 1D Ising model with Added Dzyaloshinsky-Moriya interaction
We have studied occurrence of quantum phase transition in the one-dimensional
spin-1/2 Ising model with added Dzyaloshinsky-Moriya (DM) interaction from bi-
partite and multi-partite entanglement point of view. Using exact numerical
solutions, we are able to study such systems up to 24 qubits. The minimum of
the entanglement ratio R \tau 2/\tau 1 < 1, as a novel estimator of
QPT, has been used to detect QPT and our calculations have shown that its
minimum took place at the critical point. We have also shown both the
global-entanglement (GE) and multipartite entanglement (ME) are maximal at the
critical point for the Ising chain with added DM interaction. Using matrix
product state approach, we have calculated the tangle and concurrence of the
model and it is able to capture and confirm our numerical experiment result.
Lack of inversion symmetry in the presence of DM interaction stimulated us to
study entanglement of three qubits in symmetric and antisymmetric way which
brings some surprising results.Comment: 18 pages, 9 figures, submitte
Entanglement Entropy of One-dimensional Gapped Spin Chains
We investigate the entanglement entropy (EE) of gapped S=1 and spin
chains with dimerization. We find that the effective boundary degrees of
freedom as edge states contribute significantly to the EE. For the
dimerized Heisenberg chain, the EE of the sufficiently long chain is
essentially explained by the localized effective spins on the
boundaries. As for S=1, the effective spins are also causing a Kennedy
triplet that yields a lower bound for the EE. In this case, the residual
entanglement reduces substantially by a continuous deformation of the
Heisenberg model to that of the AKLT Hamiltonian.Comment: 5 pages, 6 figure
Integrable model for interacting electrons in metallic grains
We find an integrable generalization of the BCS model with non-uniform
Coulomb and pairing interaction. The Hamiltonian is integrable by construction
since it is a functional of commuting operators; these operators, which
therefore are constants of motion of the model, contain the anisotropic Gaudin
Hamiltonians. The exact solution is obtained diagonalizing them by means of
Bethe Ansatz. Uniform pairing and Coulomb interaction are obtained as the
``isotropic limit'' of the Gaudin Hamiltonians. We discuss possible
applications of this model to a single grain and to a system of few interacting
grains.Comment: 4 pages, revtex. Revised version to be published in Phys. Rev. Let
Bethe Ansatz for 1D interacting anyons
This article gives a pedagogic derivation of the Bethe Ansatz solution for 1D
interacting anyons. This includes a demonstration of the subtle role of the
anyonic phases in the Bethe Ansatz arising from the anyonic commutation
relations. The thermodynamic Bethe Ansatz equations defining the temperature
dependent properties of the model are also derived, from which some groundstate
properties are obtained.Comment: 22 pages, two references added, small improvements to tex
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