143,722 research outputs found
Junctions of one-dimensional quantum wires - correlation effects in transport
We investigate transport of spinless fermions through a single site dot
junction of M one-dimensional quantum wires. The semi-infinite wires are
described by a tight-binding model. Each wire consists of two parts: the
non-interacting leads and a region of finite extent in which the fermions
interact via a nearest-neighbor interaction. The functional renormalization
group method is used to determine the flow of the linear conductance as a
function of a low-energy cutoff for a wide range of parameters. Several fixed
points are identified and their stability is analyzed. We determine the scaling
exponents governing the low-energy physics close to the fixed points. Some of
our results can already be derived using the non-self-consistent Hartree-Fock
approximation.Comment: version accepted for publication in Phys. Rev. B, 14 pages, 7 figures
include
Aharonov-Bohm effect in the chiral Luttinger liquid
Edge states of the quantum Hall fluid provide an almost unparalled
opportunity to study mesoscopic effects in a highly correlated electron system.
In this paper we develop a bosonization formalism for the finite-size edge
state, as described by chiral Luttinger liquid theory, and use it to study the
Aharonov-Bohm effect. The problem we address may be realized experimentally by
measuring the tunneling current between two edge states through a third edge
state formed around an antidot in the fractional quantum Hall effect regime. A
renormalization group analysis reveals the existence of a two-parameter
universal scaling function G(X,Y) that describes the Aharonov-Bohm resonances.
We also show that the strong renormalization of the tunneling amplitudes that
couple the antidot to the incident edge states, together with the nature of the
Aharonov-Bohm interference process in a chiral system, prevent the occurrence
of perfect resonances as the magnetic field is varied, even at zero
temperature.Comment: 16 pages, Revtex, 5 figures available from [email protected]
Universal transport signatures of Majorana fermions in superconductor-Luttinger liquid junctions
One of the most promising proposals for engineering topological
superconductivity and Majorana fermions employs a spin-orbit coupled nanowire
subjected to a magnetic field and proximate to an s-wave superconductor. When
only part of the wire's length contacts to the superconductor, the remaining
conducting portion serves as a natural lead that can be used to probe these
Majorana modes via tunneling. The enhanced role of interactions in one
dimension dictates that this configuration should be viewed as a
superconductor-Luttinger liquid junction. We investigate such junctions between
both helical and spinful Luttinger liquids, and topological as well as
non-topological superconductors. We determine the phase diagram for each case
and show that universal low-energy transport in these systems is governed by
fixed points describing either perfect normal reflection or perfect Andreev
reflection. In addition to capturing (in some instances) the familiar
Majorana-mediated `zero-bias anomaly' in a new framework, we show that
interactions yield dramatic consequences in certain regimes. Indeed, we
establish that strong repulsion removes this conductance anomaly altogether
while strong attraction produces dynamically generated effective Majorana modes
even in a junction with a trivial superconductor. Interactions further lead to
striking signatures in the local density of states and the line-shape of the
conductance peak at finite voltage, and also are essential for establishing
smoking-gun transport signatures of Majorana fermions in spinful Luttinger
liquid junctions.Comment: 25 pages, 6 figures, v
Interaction effects in superconductor/quantum spin Hall devices: universal transport signatures and fractional Coulomb blockade
Interfacing s-wave superconductors and quantum spin Hall edges produces
time-reversal-invariant topological superconductivity of a type that can not
arise in strictly 1D systems. With the aim of establishing sharp fingerprints
of this novel phase, we use renormalization group methods to extract universal
transport characteristics of superconductor/quantum spin Hall heterostructures
where the native edge states serve as leads. We determine scaling forms for the
conductance through a grounded superconductor and show that the results depend
sensitively on the interaction strength in the leads, the size of the
superconducting region, and the presence or absence of time-reversal-breaking
perturbations. We also study transport across a floating superconducting island
isolated by magnetic barriers. Here we predict e-periodic Coulomb-blockade
peaks, as recently observed in nanowire devices [Albrecht et al., Nature 531,
206 (2016)], with the added feature that the island can support fractional
charge tunable via the relative orientation of the barrier magnetizations. As
an interesting corollary, when the magnetic barriers arise from strong
interactions at the edge that spontaneously break time-reversal symmetry, the
Coulomb-blockade periodicity changes from e to e/2. These findings suggest
several future experiments that probe unique characteristics of topological
superconductivity at the quantum spin Hall edge.Comment: 18 pages, 7 figure
Correlation effects on electronic transport through dots and wires
We investigate how two-particle interactions affect the electronic transport
through meso- and nanoscopic systems of two different types: quantum dots with
local Coulomb correlations and quasi one-dimensional quantum wires of
interacting electrons. A recently developed functional renormalization group
scheme is used that allows to investigate systems of complex geometry.
Considering simple setups we show that the method includes the essential
aspects of Luttinger liquid physics (one-dimensional wires) as well as of the
physics of local correlations, with the Kondo effect being an important
example. For more complex systems of coupled dots and Y-junctions of
interacting wires we find surprising new correlation effects.Comment: to appear in "Advances in Solid State Physics" Volume 46, Ed. R. Haug
(Springer, 2006
Galactos: Computing the Anisotropic 3-Point Correlation Function for 2 Billion Galaxies
The nature of dark energy and the complete theory of gravity are two central
questions currently facing cosmology. A vital tool for addressing them is the
3-point correlation function (3PCF), which probes deviations from a spatially
random distribution of galaxies. However, the 3PCF's formidable computational
expense has prevented its application to astronomical surveys comprising
millions to billions of galaxies. We present Galactos, a high-performance
implementation of a novel, O(N^2) algorithm that uses a load-balanced k-d tree
and spherical harmonic expansions to compute the anisotropic 3PCF. Our
implementation is optimized for the Intel Xeon Phi architecture, exploiting
SIMD parallelism, instruction and thread concurrency, and significant L1 and L2
cache reuse, reaching 39% of peak performance on a single node. Galactos scales
to the full Cori system, achieving 9.8PF (peak) and 5.06PF (sustained) across
9636 nodes, making the 3PCF easily computable for all galaxies in the
observable universe.Comment: 11 pages, 7 figures, accepted to SuperComputing 201
Enhancement of the Two-channel Kondo Effect in Single-Electron boxes
The charging of a quantum box, coupled to a lead by tunneling through a
single resonant level, is studied near the degeneracy points of the Coulomb
blockade. Combining Wilson's numerical renormalization-group method with
perturbative scaling approaches, the corresponding low-energy Hamiltonian is
solved for arbitrary temperatures, gate voltages, tunneling rates, and energies
of the impurity level. Similar to the case of a weak tunnel barrier, the shape
of the charge step is governed at low temperatures by the non-Fermi-liquid
fixed point of the two-channel Kondo effect. However, the associated Kondo
temperature TK is strongly modified. Most notably, TK is proportional to the
width of the level if the transmission through the impurity is close to unity
at the Fermi energy, and is no longer exponentially small in one over the
tunneling matrix element. Focusing on a particle-hole symmetric level, the
two-channel Kondo effect is found to be robust against the inclusion of an
on-site repulsion on the level. For a large on-site repulsion and a large
asymmetry in the tunneling rates to box and to the lead, there is a sequence of
Kondo effects: first the local magnetic moment that forms on the level
undergoes single-channel screening, followed by two-channel overscreening of
the charge fluctuations inside the box.Comment: 21 pages, 19 figure
The dynamics of quark-gluon plasma and AdS/CFT
In these pedagogical lectures, we present the techniques of the AdS/CFT
correspondence which can be applied to the study of real time dynamics of a
strongly coupled plasma system. These methods are based on solving
gravitational Einstein's equations on the string/gravity side of the AdS/CFT
correspondence. We illustrate these techniques with applications to the
boost-invariant expansion of a plasma system. We emphasize the common
underlying AdS/CFT description both in the large proper time regime where
hydrodynamic dynamics dominates, and in the small proper time regime where the
dynamics is far from equilibrium. These AdS/CFT methods provide a fascinating
arena interrelating General Relativity phenomenae with strongly coupled gauge
theory physics.Comment: 35 pages, 3 figures. Lectures at the 5th Aegean summer school, `From
gravity to thermal gauge theories: the AdS/CFT correspondence'. To appear in
the proceedings in `Lecture Notes in Physics
Optimisation of Quantum Trajectories Driven by Strong-field Waveforms
Quasi-free field-driven electron trajectories are a key element of
strong-field dynamics. Upon recollision with the parent ion, the energy
transferred from the field to the electron may be released as attosecond
duration XUV emission in the process of high harmonic generation (HHG). The
conventional sinusoidal driver fields set limitations on the maximum value of
this energy transfer, and it has been predicted that this limit can be
significantly exceeded by an appropriately ramped-up cycleshape. Here, we
present an experimental realization of such cycle-shaped waveforms and
demonstrate control of the HHG process on the single-atom quantum level via
attosecond steering of the electron trajectories. With our optimized optical
cycles, we boost the field-ionization launching the electron trajectories,
increase the subsequent field-to-electron energy transfer, and reduce the
trajectory duration. We demonstrate, in realistic experimental conditions, two
orders of magnitude enhancement of the generated XUV flux together with an
increased spectral cutoff. This application, which is only one example of what
can be achieved with cycle-shaped high-field light-waves, has farreaching
implications for attosecond spectroscopy and molecular self-probing
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