770 research outputs found
Quantum transitions induced by the third cumulant of current fluctuations
We investigate the transitions induced by external current fluctuations on a
small probe quantum system. The rates for the transitions between the energy
states are calculated using the real-time Keldysh formalism for the density
matrix evolution. We especially detail the effects of the third cumulant of
current fluctuations inductively coupled to a quantum bit and propose a setup
for detecting the frequency-dependent third cumulant through the transitions it
induces.Comment: 4 pages, 3 figure
Methods for suspensions of passive and active filaments
Flexible filaments and fibres are essential components of important complex
fluids that appear in many biological and industrial settings. Direct
simulations of these systems that capture the motion and deformation of many
immersed filaments in suspension remain a formidable computational challenge
due to the complex, coupled fluid--structure interactions of all filaments, the
numerical stiffness associated with filament bending, and the various
constraints that must be maintained as the filaments deform. In this paper, we
address these challenges by describing filament kinematics using quaternions to
resolve both bending and twisting, applying implicit time-integration to
alleviate numerical stiffness, and using quasi-Newton methods to obtain
solutions to the resulting system of nonlinear equations. In particular, we
employ geometric time integration to ensure that the quaternions remain unit as
the filaments move. We also show that our framework can be used with a variety
of models and methods, including matrix-free fast methods, that resolve low
Reynolds number hydrodynamic interactions. We provide a series of tests and
example simulations to demonstrate the performance and possible applications of
our method. Finally, we provide a link to a MATLAB/Octave implementation of our
framework that can be used to learn more about our approach and as a tool for
filament simulation
Multilevel coherences in quantum dots
We study transport through strongly interacting quantum dots with energy
levels that are weakly coupled to generic multi-channel metallic leads. In the
regime of coherent sequential tunneling, where level spacing and broadening are
of the same order but small compared to temperature, we present a unified,
-invariant form of the kinetic equation for the reduced density matrix
of the dot and the tunneling current. This is achieved by introducing the
concept of flavor polarization for the dot and the reservoirs, and splitting
the kinetic equation in terms of flavor accumulation, anisotropic flavor
relaxation, as well as exchange-field- and detuning-induced flavor rotation. In
particular, we identify the exchange field as the cause of negative
differential conductance at off-resonance bias voltages appearing in generic
quantum-dot models. To illustrate the notion of flavor polarization, we analyze
the non-linear current through a triple quantum-dot device.Comment: 9 pages, 3 figure
Nonperturbative analysis of coupled quantum dots in a phonon bath
Transport through coupled quantum dots in a phonon bath is studied using the
recently developed real-time renormalization-group method. Thereby, the problem
can be treated beyond perturbation theory regarding the complete interaction. A
reliable solution for the stationary tunnel current is obtained for the case of
moderately strong couplings of the dots to the leads and to the phonon bath.
Any other parameter is arbitrary, and the complete electron-phonon interaction
is taken into account. Experimental results are quantitatively reproduced by
taking into account a finite extension of the wavefunctions within the dots.
Its dependence on the energy difference between the dots is derived.Comment: 8 pages, 6 figure
Resonant Tunneling through Multi-Level and Double Quantum Dots
We study resonant tunneling through quantum-dot systems in the presence of
strong Coulomb repulsion and coupling to the metallic leads. Motivated by
recent experiments we concentrate on (i) a single dot with two energy levels
and (ii) a double dot with one level in each dot. Each level is twofold
spin-degenerate. Depending on the level spacing these systems are physical
realizations of different Kondo-type models. Using a real-time diagrammatic
formulation we evaluate the spectral density and the non-linear conductance.
The latter shows a novel triple-peak resonant structure.Comment: 4 pages, ReVTeX, 4 Postscript figure
First measurement of the magnetic field on FK Com and its relation to the contemporaneous starspot locations
In this study we present simultaneous low-resolution longitudinal magnetic
field measurements and high-resolution spectroscopic observations of the cool
single giant FK Com. The variation of the magnetic field over the rotational
period of 2.4 days is compared with the starspot location obtained using
Doppler imaging techniques, V-band photometry and V-I colours. The
chromospheric activity is studied simultaneously with the photospheric activity
using high resolution observations of the Halpha, Hbeta and Hgamma line
profiles. Both the maximum (272 +/- 24 G) and minimum (60 +/- 17 G) in the mean
longitudinal magnetic field, , are detected close to the phases where cool
spots appear on the stellar surface. A possible explanation for such a
behaviour is that the active regions at the two longitudes separated by 0.2 in
phase have opposite polarities.Comment: 10 Pages, 11 figures (quality of Figures 7,8 and 10 reduced),
accepted for publication in MNRA
Interference and interaction effects in multi-level quantum dots
Using renormalization group techniques, we study spectral and transport
properties of a spinless interacting quantum dot consisting of two levels
coupled to metallic reservoirs. For strong Coulomb repulsion and an applied
Aharonov-Bohm phase , we find a large direct tunnel splitting
between the levels of
the order of the level broadening . As a consequence we discover a
many-body resonance in the spectral density that can be measured via the
absorption power. Furthermore, for , we show that the system can be
tuned into an effective Anderson model with spin-dependent tunneling.Comment: 5 pages, 4 figures included, typos correcte
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
Fermionic renormalization group methods for transport through inhomogeneous Luttinger liquids
We compare two fermionic renormalization group methods which have been used
to investigate the electronic transport properties of one-dimensional metals
with two-particle interaction (Luttinger liquids) and local inhomogeneities.
The first one is a poor man's method setup to resum ``leading-log'' divergences
of the effective transmission at the Fermi momentum. Generically the resulting
equations can be solved analytically. The second approach is based on the
functional renormalization group method and leads to a set of differential
equations which can only for certain setups and in limiting cases be solved
analytically, while in general it must be integrated numerically. Both methods
are claimed to be applicable for inhomogeneities of arbitrary strength and to
capture effects of the two-particle interaction, such as interaction dependent
exponents, up to leading order. We critically review this for the simplest case
of a single impurity. While on first glance the poor man's approach seems to
describe the crossover from the ``perfect'' to the ``open chain fixed point''
we collect evidence that difficulties may arise close to the ``perfect chain
fixed point''. Due to a subtle relation between the scaling dimensions of the
two fixed points this becomes apparent only in a detailed analysis. In the
functional renormalization group method the coupling of the different
scattering channels is kept which leads to a better description of the
underlying physics.Comment: 25 pages, accepted for publication in NJP, remarks added on the poor
man's RG treatment of the Y-junction and the Breit-Wigner line shape
- …