886 research outputs found
The Termination of the United States--Netherlands Antilles Income Tax Convention: A Failure of U.S. Tax Policy
RG transport theory for open quantum systems: Charge fluctuations in multilevel quantum dots in and out of equilibrium
We present the real-time renormalization group (RTRG) method as a method to
describe the stationary state current through generic multi-level quantum dots
with a complex setup in nonequilibrium. The employed approach consists of a
very rudiment approximation for the RG equations which neglects all vertex
corrections while it provides a means to compute the effective dot Liouvillian
self-consistently. Being based on a weak-coupling expansion in the tunneling
between dot and reservoirs, the RTRG approach turns out to reliably describe
charge fluctuations in and out of equilibrium for arbitrary coupling strength,
even at zero temperature. We confirm this in the linear response regime with a
benchmark against highly-accurate numerically renormalization group data in the
exemplary case of three-level quantum dots. For small to intermediate bias
voltages and weak Coulomb interactions, we find an excellent agreement between
RTRG and functional renormalization group data, which can be expected to be
accurate in this regime. As a consequence, we advertise the presented RTRG
approach as an efficient and versatile tool to describe charge fluctuations
theoretically in quantum dot systems
Establishing the nature of companion candidates to X-ray emitting late B-type stars
The most favored interpretation for the detection of X-ray emission from late
B-type stars is that these stars have a yet undiscovered late-type companion
(or an unbound nearby late-type star) that produces the X-rays. Several faint
IR objects at (sub)-arcsecond separation from B-type stars have been uncovered
in our earlier adaptive optics imaging observations, and some of them have been
followed up with the high spatial resolution of the Chandra X-ray observatory,
pinpointing the X-ray emitter. However, firm conclusions on their nature
requires a search for spectroscopic signatures of youth. Here we report on our
recent ISAAC observations carried out in low resolution spectroscopic mode.
Equivalent widths have been used to obtain information on spectral types of the
companions. All eight X-ray emitting systems with late B-type primaries studied
contain dwarf like companions with spectral types later than A7. The only
system in the sample where the companion turns out to be of early spectral type
is not an X-ray source. These results are consistent with the assumption that
the observed X-ray emission from late B-type stars is produced by an active
pre-main sequence companion star.Comment: 6 pages, 2 figures, 3 tables, accepted for publication in MNRA
Transport signature of pseudo-Jahn-Teller dynamics in a single-molecule transistor
We calculate the electronic transport through a molecular dimer, in which an
excess electron is delocalized over equivalent monomers, which can be locally
distorted. In this system the Born-Oppenheimer approximation breaks down
resulting in quantum entanglement of the mechanical and electronic motion. We
show that pseudo Jahn-Teller (pJT) dynamics of the molecule gives rise to
conductance peaks that indicate this violation. Their magnitude, sign and
position sharply depend on the electro-mechanical properties of the molecule,
which can be varied in recently developed three-terminal junctions with
mechanical control. The predicted effect depends crucially on the degree of
intramolecular delocalization of the excess electron, a parameter which is also
of fundamental importance in physical chemistry.Comment: 6 pages, 3 figure
Fermionic superoperators for zero-temperature non-linear transport: real-time perturbation theory and renormalization group for Anderson quantum dots
We study the transport through a strongly interacting Anderson quantum dot at
zero-temperature using the real-time renormalization group (RT-RG) in the
framework of a kinetic equation for the reduced density operator. We further
develop the general finite temperature real-time transport formalism by
introducing field superoperators that obey fermionic statistics. This direct
second quantization in Liouville-Fock space strongly simplifies the
construction of operators and superoperators which transform irreducibly under
the Anderson-model symmetry transformations. The fermionic field superoperators
naturally arise from the univalence (fermion-parity) superselection rule for
the total system. Expressed in these field superoperators, the causal structure
of the perturbation theory for the effective time-evolution
superoperator-kernel becomes explicit. The causal structure also implies the
existence of a fermion-parity protected eigenvector of the exact Liouvillian,
explaining a recently reported result on adiabatic driving [Phys. Rev. B 85,
075301 (2012)] and generalizing it to arbitrary order in the tunnel coupling.
Furthermore, in the WBL the causal representation exponentially reduces the
number of diagrams for the time-evolution kernel. We perform a complete 2-loop
RG analysis at finite voltage and magnetic field, while systematically
accounting for the dependence on both the quantum dot and reservoir
frequencies. Using the second quantization in Liouville-space and symmetry
restrictions we obtain analytical RT-RG equations with an efficient numerical
solution and we extensively study the model parameter space, excluding the
Kondo regime. The incorporated renormalization effects result in an enhancement
of the inelastic cotunneling peak. Moreover, we find a tunnel-induced
non-linearity of the stability diagrams at finite voltage, both in the SET and
ICT regime.Comment: With respect to the version of 13.07.2012: Corrected typos. Fig.1 was
corrected. Right scales on Fig.6b were set. English grammatic improved. One
reference adde
Real-Time-RG Analysis of the Dynamics of the Spin-Boson Model
Using a real-time renormalization group method we determine the complete
dynamics of the spin-boson model with ohmic dissipation for coupling strengths
. We calculate the relaxation and dephasing time, the
static susceptibility and correlation functions. Our results are consistent
with quantum Monte Carlo simulations and the Shiba relation. We present for the
first time reliable results for finite cutoff and finite bias in a regime where
perturbation theory in or in tunneling breaks down. Furthermore, an
unambigious comparism to results from the Kondo model is achieved.Comment: 4 pages, 5 figures, 1 tabl
Full Frequency Back-Action Spectrum of a Single Electron Transistor during Qubit read-out
We calculate the spectral density of voltage fluctuations in a Single
Electron Transistor (SET), biased to operate in a transport mode where
tunneling events are correlated due to Coulomb interaction. The whole spectrum
from low frequency shot noise to quantum noise at frequencies comparable to the
SET charging energy is considered. We discuss the back-action
during read-out of a charge qubit and conclude that single-shot read-out is
possible using the Radio-Frequency SET.Comment: 4 pages, 5 figures, submitted to PR
Flavor fluctuations in 3-level quantum dots: Generic SU(3)-Kondo fixed point in equilibrium and non-Kondo fixed points in nonequilibrium
We study a -level quantum dot in the singly occupied cotunneling regime
coupled via a generic tunneling matrix to several multi-channel leads in
equilibrium or nonequilibrium. We derive an effective model where also each
reservoir has three channels labelled by the quark flavors , and
with an effective d.o.s. polarized w.r.t. an eight-dimensional -spin
corresponding to the eight generators of . In equilibrium we perform a
standard poor man scaling analysis and show that tunneling via virtual
intermediate states induces flavor fluctuations on the dot which become
-symmetric at a characteristic and exponentially small low-energy scale
. Using the numerical renormalization group (NRG) we study in detail the
linear conductance and confirm the -symmetric Kondo fixed point with
universal conductance for various tunneling setups by tuning the
level spacings on the dot. In contrast to the equilibrium case, we find in
nonequilibrium that the fixed point model is not -symmetric but
characterized by rotated -spins for each reservoir with total vanishing sum.
At large voltage we analyse the -spin magnetization and the current in
golden rule as function of a magnetic field for the isospin of the up/down
quark and the level spacing to the strange quark. As a smoking gun to detect
the nonequilibrium fixed point we find that the curve of zero -spin
magnetization has a particular shape on the dot parameters. We propose that our
findings can be generalized to the case of quantum dots with an arbitrary
number of levels.Comment: 24 pages, 8 figure
A renormalization-group analysis of the interacting resonant level model at finite bias: Generic analytic study of static properties and quench dynamics
Using a real-time renormalization group method we study the minimal model of
a quantum dot dominated by charge fluctuations, the two-lead interacting
resonant level model, at finite bias voltage. We develop a set of RG equations
to treat the case of weak and strong charge fluctuations, together with the
determination of power-law exponents up to second order in the Coulomb
interaction. We derive analytic expressions for the charge susceptibility, the
steady-state current and the conductance in the situation of arbitrary system
parameters, in particular away from the particle-hole symmetric point and for
asymmetric Coulomb interactions. In the generic asymmetric situation we find
that power laws can be observed for the current only as function of the level
position (gate voltage) but not as function of the voltage. Furthermore, we
study the quench dynamics after a sudden switch-on of the level-lead couplings.
The time evolution of the dot occupation and current is governed by exponential
relaxation accompanied by voltage-dependent oscillations and characteristic
algebraic decay.Comment: 24 pages, 13 figures; revised versio
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