22 research outputs found
Exact low temperature results for transport properties of the interacting resonant level model
Using conformal field theory and integrability ideas, we give a full
characterization of the low temperature regime of the anisotropic interacting
resonant level (IRLM) model. We determine the low temperature corrections to
the linear conductance exactly up to the 6th order. We show that the structure
displays 'Coulomb deblocking' at resonance, i.e., a strong impurity-wire
capacitive coupling enhances the conductance at low temperature.Comment: 4 pages, 2 figure
The Kondo Model with a Bulk Mass Term
We introduce two massive versions of the anisotropic spin 1/2 Kondo model and
discuss their integrability. The two models have the same bulk sine-Gordon
interactions, but differ in their boundary interactions. At the Toulouse free
fermion point each of the models can be understood as two decoupled Ising
models in boundary magnetic fields. Reflection S-matrices away from the free
fermion point are conjectured.Comment: 33 pages, Plain Te
Excited State TBA for the perturbed model
We examine some excited state energies in the non-unitary integrable quantum
field theory obtained from the perturbation of the minimal conformal field
theory model by its operator . Using the correspondence
of this IQFT to the scaling limit of the dilute lattice model (in a
particular regime) we derive the functional equations for the QFT commuting
transfer matrices. These functional equations can be transformed to a closed
set of TBA-like integral equations which determine the excited state energies
in the finite-size system. In particular, we explicitly construct these
equations for the ground state and two lowest excited states. Numerical results
for the associated energy gaps are compared with those obtained by the
truncated conformal space approach (TCSA).Comment: LaTeX, 32 pages, 6 figure
A note on the boundary spin XXZ chain
The open spin XXZ model with non-diagonal boundaries is considered.
Within the algebraic Bethe ansatz framework and in the spirit of earlier works
we derive suitable reference states. The derivation of the reference state is
the crucial point in this investigation, and it involves the solution of sets
of difference equations. For the spin representation, expressed in terms of
difference operators, the pseudo-vacuum is identified in terms of
-hypergeometric series. Having specified such states we then build the Bethe
states and also identify the spectrum of the model for generic values of the
anisotropy parameter .Comment: 12 pages, Late
Exact time-dependent density functional theory for impurity models
We employ the density matrix renormalization group to construct the exact
time-dependent exchange correlation potential for an impurity model with an
applied transport voltage. Even for short-ranged interaction we find an
infinitely long-ranged exchange correlation potential which is built up
{instantly} after switching on the voltage. Our result demonstrates the
fundamental difficulties of transport calculations based on time-dependent
density functional theory. While formally the approach works, important
information can be missing in the ground-state functionals and may be hidden in
the usually unknown non-equilibrium functionals
Bound States for a Magnetic Impurity in a Superconductor
We discuss a solvable model describing an Anderson like impurity in a BCS
superconductor. The model can be mapped onto an Ising field theory in a
boundary magnetic field, with the Ising fermions being the quasi-particles of
the Bogoliubov transformation in BCS theory. The reflection S-matrix exhibits
Andreev scattering, and the existence of bound states of the quasi-particles
with the impurity lying inside the superconducting gap.Comment: 7 pages, Plain Te
Low-Temperature Thermodynamics of and su(3)-invariant Spin Chains
We formulate the thermodynamic Bethe Ansatz (TBA) equations for the closed
(periodic boundary conditions) quantum spin chain in an external
magnetic field, in the (noncritical) regime where the anisotropy parameter
is real. In the limit , we recover the TBA equations of the
antiferromagnetic su(3)-invariant chain in the fundamental representation. We
solve these equations for low temperature and small field, and calculate the
specific heat and magnetic susceptibility.Comment: 31 pages, UMTG-16
Entanglement Entropy, decoherence, and quantum phase transition of a dissipative two-level system
The concept of entanglement entropy appears in multiple contexts, from black
hole physics to quantum information theory, where it measures the entanglement
of quantum states. We investigate the entanglement entropy in a simple model,
the spin-boson model, which describes a qubit (two-level system) interacting
with a collection of harmonic oscillators that models the environment
responsible for decoherence and dissipation. The entanglement entropy allows to
make a precise unification between entanglement of the spin with its
environment, decoherence, and quantum phase transitions. We derive exact
analytical results which are confirmed by Numerical Renormalization Group
arguments both for an ohmic and a subohmic bosonic bath. Those demonstrate that
the entanglement entropy obeys universal scalings. We make comparisons with
entanglement properties in the quantum Ising model and in the Dicke model. We
also emphasize the possibility of measuring this entanglement entropy using
charge qubits subject to electromagnetic noise; such measurements would provide
an empirical proof of the existence of entanglement entropy.Comment: 38 pages, 8 figures, related to cond-mat/0612095 and arXiv:0705.0957;
final version to appear in Annals of Physic
Thermodynamics of the 3-State Potts Spin Chain
We demonstrate the relation of the infrared anomaly of conformal field theory
with entropy considerations of finite temperature thermodynamics for the
3-state Potts chain. We compute the free energy and compute the low temperature
specific heat for both the ferromagnetic and anti-ferromagnetic spin chains,
and find the central charges for both.Comment: 18 pages, LaTex. Preprint # ITP-SB-92-60. References added and first
section expande
Transport in Quantum Dots from the Integrability of the Anderson Model
In this work we exploit the integrability of the two-lead Anderson model to
compute transport properties of a quantum dot, in and out of equilibrium. Our
method combines the properties of integrable scattering together with a
Landauer-Buttiker formalism. Although we use integrability, the nature of the
problem is such that our results are not generically exact, but must only be
considered as excellent approximations which nonetheless are valid all the way
through crossover regimes.
The key to our approach is to identify the excitations that correspond to
scattering states and then to compute their associated scattering amplitudes.
We are able to do so both in and out of equilibrium. In equilibrium and at zero
temperature, we reproduce the Friedel sum rule for an arbitrary magnetic field.
At finite temperature, we study the linear response conductance at the
symmetric point of the Anderson model, and reproduce Costi et al.'s numerical
renormalization group computation of this quantity. We then explore the
out-of-equilibrium conductance for a near-symmetric Anderson model, and arrive
at quantitative expressions for the differential conductance, both in and out
of a magnetic field. We find the expected splitting of the differential
conductance peak into two in a finite magnetic field, . We determine the
width, height, and position of these peaks. In particular we find for H >> T_k,
the Kondo temperature, the differential conductance has maxima of e^2/h
occuring for a bias V close to but smaller than H. The nature of our
construction of scattering states suggests that our results for the
differential magneto-conductance are not merely approximate but become exact in
the large field limit.Comment: 88 pages, 16 figures, uses harvmac.te