73,529 research outputs found
Tunable Fano-Kondo resonance in side-coupled double quantum dot system
We study the interference between the Fano and Kondo effects in a
side-coupled double-quantum- dot system where one of the quantum dots couples
to conduction electron bath while the other dot only side-couples to the first
dot via antiferromagnetic (AF) spin exchange coupling. We apply both the
perturbative renormalization group (RG) and numerical renormalization group
(NRG) approaches to study the effect of AF coupling on the Fano lineshape in
the conduction leads. With particle-hole symmetry, the AF exchange coupling
competes with the Kondo effect and leads to a local spin-singlet ground state
for arbitrary small coupling, so called "two-stage Kondo effect". As a result,
via NRG we find the spectral properties of the Fano lineshape in the tunneling
density of states (TDOS) of conduction electron leads shows double dip-peak
features at the energy scale around the Kondo temperature and the one much
below it, corresponding to the two-stage Kondo effect; it also shows an
universal scaling behavior at very low energies. We find the qualitative
agreement between the NRG and the perturbative RG approach. Relevance of our
work to the experiments is discussed.Comment: 7 pages, 7 figure
Discovery of Counter-Rotating Gas in the Galaxies NGC1596 and NGC3203 and the Incidence of Gas Counter-Rotation in S0 Galaxies
We have identified two new galaxies with gas counter-rotation (NGC1596 and
NGC3203) and have confirmed similar behaviour in another one (NGC128), this
using results from separate studies of the ionized-gas and stellar kinematics
of a well-defined sample of 30 edge-on disc galaxies. Gas counter-rotators thus
represent 10+/-5% of our sample, but the fraction climbs to 21+/-11% when only
lenticular (S0) galaxies are considered and to 27+/-13% for S0s with detected
ionized-gas only. Those fractions are consistent with but slightly higher than
previous studies. A compilation from well-defined studies of S0s in the
literature yields fractions of 15+/-4% and 23+/-5%, respectively. Although
mainly based on circumstantial evidence, we argue that the counter-rotating gas
originates primarily from minor mergers and tidally-induced transfer of
material from nearby objects. Assuming isotropic accretion, twice those
fractions of objects must have undergone similar processes, underlining the
importance of (minor) accretion for galaxy evolution. Applications of gas
counter-rotators to barred galaxy dynamics are also discussed.Comment: 8 pages, including 1 table and 2 figures. Accepted for publication in
MNRAS. Version with full resolution figures available at
http://www-astro.physics.ox.ac.uk/~bureau/pub_list.htm
Common adversaries form alliances: modelling complex networks via anti-transitivity
Anti-transitivity captures the notion that enemies of enemies are friends,
and arises naturally in the study of adversaries in social networks and in the
study of conflicting nation states or organizations. We present a simplified,
evolutionary model for anti-transitivity influencing link formation in complex
networks, and analyze the model's network dynamics. The Iterated Local
Anti-Transitivity (or ILAT) model creates anti-clone nodes in each time-step,
and joins anti-clones to the parent node's non-neighbor set. The graphs
generated by ILAT exhibit familiar properties of complex networks such as
densification, short distances (bounded by absolute constants), and bad
spectral expansion. We determine the cop and domination number for graphs
generated by ILAT, and finish with an analysis of their clustering
coefficients. We interpret these results within the context of real-world
complex networks and present open problems
Quantum criticality in a double quantum-dot system
We discuss the realization of the quantum-critical non-Fermi liquid state,
originally discovered within the two-impurity Kondo model, in double
quantum-dot systems. Contrary to the common belief, the corresponding fixed
point is robust against particle-hole and various other asymmetries, and is
only unstable to charge transfer between the two dots. We propose an
experimental set-up where such charge transfer processes are suppressed,
allowing a controlled approach to the quantum critical state. We also discuss
transport and scaling properties in the vicinity of the critical point.Comment: 4 pages, 3 figs; (v2) final version as publishe
Quantum criticality out of equilibrium in the pseudogap Kondo model
We theoretically investigate the non-equilibrium quantum phase transition in
a generic setup: the pseudogap Kondo model where a quantum dot couples to
two-left (L) and right (R)-voltage-biased fermionic leads with power-law
density of states (DOS) with respect to their Fermi levels {\mu}_L/R,
{\rho}_c,L(R) ({\omega}) \propto |{\omega} - {\mu}_L(R) |r, and 0 < r < 1. In
equilibrium (zero bias voltage) and for 0 < r < 1/2, with increasing Kondo
correlations, in the presence of particle-hole symmetry this model exhibits a
quantum phase transition from a unscreened local moment (LM) phase to the Kondo
phase. Via a controlled frequency-dependent renormalization group (RG)
approach, we compute analytically and numerically the non-equilibrium
conductance, conduction electron T-matrix and local spin susceptibility at
finite bias voltages near criticality. The current-induced decoherence shows
distinct nonequilibrium scaling, leading to new universal non-equilibrium
quantum critical behaviors in the above observables. Relevance of our results
for the experiments is discussed.Comment: 4.1 pages, 2 figure
Minimal Theoretical Uncertainties in Inflationary Predictions
During inflation, primordial energy density fluctuations are created from
approximate de Sitter vacuum quantum fluctuations redshifted out of the horizon
after which they are frozen as perturbations in the background curvature. In
this paper we demonstrate that there exists an intrinsic theoretical
uncertainty in the inflationary predictions for the curvature perturbations due
to the failure of the well known prescriptions to specify the vacuum uniquely.
Specifically, we show that the two often used prescriptions for defining the
initial vacuum state -- the Bunch-Davies prescription and the adiabatic vacuum
prescription (even if the adiabaticity order to which the vacuum is specified
is infinity) -- fail to specify the vacuum uniquely in generic inflationary
spacetimes in which the total duration of inflation is finite. This conclusion
holds despite the absence of any trans-Planckian effects or effective field
theory cutoff related effects. We quantify the uncertainty which is applicable
to slow roll inflationary scenarios as well as for general FRW spacetimes and
find that the uncertainty is generically small. This uncertainty should be
treated as a minimal uncertainty that underlies all curvature perturbation
calculations.Comment: LaTeX file, 35 pages; some typos correcte
Random Feature Maps via a Layered Random Projection (LaRP) Framework for Object Classification
The approximation of nonlinear kernels via linear feature maps has recently
gained interest due to their applications in reducing the training and testing
time of kernel-based learning algorithms. Current random projection methods
avoid the curse of dimensionality by embedding the nonlinear feature space into
a low dimensional Euclidean space to create nonlinear kernels. We introduce a
Layered Random Projection (LaRP) framework, where we model the linear kernels
and nonlinearity separately for increased training efficiency. The proposed
LaRP framework was assessed using the MNIST hand-written digits database and
the COIL-100 object database, and showed notable improvement in object
classification performance relative to other state-of-the-art random projection
methods.Comment: 5 page
Thermalization and Quantum Correlations in Exactly Solvable Models
The generalized Gibbs ensemble introduced for describing few body
correlations in exactly solvable systems following a quantum quench is related
to the nonergodic way in which operators sample, in the limit of infinite time
after the quench, the quantum correlations present in the initial state. The
nonergodicity of the correlations is thus shown \emph{analytically} to imply
the equivalence with the generalized Gibbs ensemble for quantum Ising and
XX spin chains as well as for the Luttinger model the thermodynamic limit,
and for a broad class of initial states and correlation functions of both local
and nonlocal operators.Comment: 12 pages, 4 figures. Expanded in response to Referee criticis
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