4,350 research outputs found
Magnon transport and spin current switching through quantum dots
We study the nonequilibrium spin current through a quantum dot consisting of
two localized spin-1/2 coupled to two ferromagnetic insulators. The influence
of an intra-dot magnetic field and exchange coupling, different dot-reservoir
coupling configurations, and the influence of magnon chemical potential
differences vs. magnetic field gradients onto the spin current are examined. We
discuss various spin switching mechanisms and find that, in contrast to
electronic transport, the spin current is very sensitive to the specific
coupling configuration and band edges. In particular, we identify 1- and
2-magnon transport processes which can lead to resonances and antiresonances
for the spin current.Comment: 10 pages, 15 figure
Dynamical spin-spin correlation functions in the Kondo model out of equilibrium
We calculate the dynamical spin-spin correlation functions of a Kondo dot
coupled to two noninteracting leads held at different chemical potentials. To
this end we generalize a recently developed real-time renormalization group
method in frequency space (RTRG-FS) to allow the calculation of dynamical
correlation functions of arbitrary dot operators in systems describing spin
and/or orbital fluctuations. The resulting two-loop RG equations are
analytically solved in the weak-coupling regime. This implies that the method
can be applied provided either the voltage through the dot or the external
magnetic field are sufficiently large, , where the
Kondo temperature is the scale where the system enters the
strong-coupling regime. Explicitly, we calculate the longitudinal and
transverse spin-spin correlation and response functions as well as the
resulting fluctuation-dissipation ratios. The correlation functions in
real-frequency space can be calculated in Matsubara space without the need of
any analytical continuation. We obtain analytic results for the line-shape, the
small- and large-frequency limits and several other features like the height
and width of the peak in the transverse susceptibility at
, where denotes the reduced magnetic field.
Furthermore, we discuss how the developed method can be generalized to
calculate dynamical correlation functions of other operators involving
reservoir degrees of freedom as well.Comment: 30 page
Dissipative quantum mechanics beyond Bloch-Redfield: A consistent weak-coupling expansion of the ohmic spin boson model at arbitrary bias
We study the time dynamics of the ohmic spin boson model at arbitrary bias
and small coupling to the bosonic bath. Using perturbation
theory and the real-time renormalization group (RG) method we present a
consistent zero-temperature weak-coupling expansion for the time evolution of
the reduced density matrix one order beyond the Bloch-Redfield solution. We
develop a renormalized perturbation theory and present an analytical solution
covering the whole range from small to large times, including further results
for exponentially small or large times. Resumming all secular terms in all
orders of perturbation theory we find exponential decay for all terms of the
time evolution. We determine the preexponential functions and find slowly
varying logarithmic terms with the renormalized Rabi frequency as
energy scale together with strongly varying parts falling off asymptocially as
in leading order, in contrast to the unbiased case. Resumming all
logarithmic terms in all orders of perturbation theory via real-time RG we find
the correct renormalized tunneling and a power-law behaviour for the
oscillating modes with exponent crossing over from for exponentially
small times to a bias-dependent value for
exponentially large times. Furthermore, we present a degenerate perturbation
theory to calculate consistently the purely decaying mode one order beyond
Bloch-Redfield.Comment: 27 pages, 2 figure
Bosonization for Beginners --- Refermionization for Experts
This tutorial gives an elementary and self-contained review of abelian
bosonization in 1 dimension in a system of finite size , following and
simplifying Haldane's constructive approach. As a non-trivial application, we
rigorously resolve (following Furusaki) a recent controversy regarding the
tunneling density of states, , at the site of an impurity
in a Tomonaga-Luttinger liquid: we use finite-size refermionization to show
exactly that for g=1/2 its asymptotic low-energy behavior is
. This agrees with the results of Fabrizio &
Gogolin and of Furusaki, but not with those of Oreg and Finkel'stein (probably
because we capture effects not included in their mean-field treatment of the
Coulomb gas that they obtained by an exact mapping; their treatment of
anti-commutation relations in this mapping is correct, however, contrary to
recent suggestions in the literature). --- The tutorial is addressed to readers
unfamiliar with bosonization, or for those interested in seeing ``all the
details'' explicitly; it requires knowledge of second quantization only, not of
field theory. At the same time, we hope that experts too might find useful our
explicit treatment of certain subtleties -- these include the proper treatment
of the so-called Klein factors that act as fermion-number ladder operators (and
also ensure the anti-commutation of different species of fermion fields), the
retention of terms of order 1/L, and a novel, rigorous formulation of
finite-size refermionization of both and the boson field itself.Comment: Revtex, 70 pages. Changes: Regarding the controversial tunneling
density of states at an impurity in a g=1/2 Luttinger liquid, we (1) give a
new, more explicit calculation, (2) show that contrary to recent suggestions
(including our own), Oreg and Finkel'stein treat fermionic anticommutation
relations CORRECTLY (see Appendix K), but (3) suggest that their MEAN-FIELD
treatment of their Coulomb gas may not be sufficiently accurat
Orbital and spin Kondo effects in a double quantum dot
Motivated by recent experiments, in which the Kondo effect has been observed
for the first time in a double quantum-dot structure, we study electron
transport through a system consisting of two ultrasmall, capacitively-coupled
dots with large level spacing and charging energy. Due to strong interdot
Coulomb correlations, the Kondo effect has two possible sources, the spin and
orbital degeneracies, and it is maximized when both occur simultaneously. The
large number of tunable parameters allows a range of manipulations of the Kondo
physics -- in particular, the Kondo effect in each dot is sensitive to changes
in the state of the other dot. For a thorough account of the system dynamics,
the linear and nonlinear conductance is calculated in perturbative and
non-perturbative approaches. In addition, the temperature dependence of the
resonant peak heights is evaluated in the framework of a renormalization group
analysis.Comment: 7 pages, 3 figures; submitted to Europhys. Let
Relaxation vs decoherence: Spin and current dynamics in the anisotropic Kondo model at finite bias and magnetic field
Using a nonequilibrium renormalization group method we study the real-time
evolution of spin and current in the anisotropic Kondo model (both
antiferromagnetic and ferromagnetic) at finite magnetic field and bias
voltage . We derive analytic expressions for all times in the weak-coupling
regime ( strong coupling scale). We find that
all observables decay both with the spin relaxation and decoherence rates
. Various -dependent logarithmic, oscillatory, and power-law
contributions are predicted. The low-energy cutoff of logarithmic terms is
generically identified by the difference of transport decay rates. For small
times , we obtain universal dynamics for spin and
current
Kondo model in nonequilibrium: Interplay between voltage, temperature, and crossover from weak to strong coupling
We consider an open quantum system in contact with fermionic metallic
reservoirs in a nonequilibrium setup. For the case of spin, orbital or
potential fluctuations, we present a systematic formulation of real-time
renormalization group at finite temperature, where the complex Fourier variable
of an effective Liouvillian is used as flow parameter. We derive a universal
set of differential equations free of divergencies written as a systematic
power series in terms of the frequency-independent two-point vertex only, and
solve it in different truncation orders by using a universal set of boundary
conditions. We apply the formalism to the description of the weak to strong
coupling crossover of the isotropic spin-1/2 nonequilibrium Kondo model at zero
magnetic field. From the temperature and voltage dependence of the conductance
in different energy regimes we determine various characteristic low-energy
scales and compare their universal ratio to known results. For a fixed finite
bias voltage larger than the Kondo temperature, we find that the
temperature-dependence of the differential conductance exhibits non-monotonic
behavior in the form of a peak structure. We show that the peak position and
peak width scale linearly with the applied voltage over many orders of
magnitude in units of the Kondo temperature. Finally, we compare our
calculations with recent experiments.Comment: 48 pages, 10 figure
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