1,873 research outputs found
Multiple Particle Scattering in Quantum Point Contacts
Recent experiments performed on weakly pinched quantum point contacts, have
shown a resistance that tend to decrease at low source drain voltage. We show
that enhanced Coulomb interactions, prompt by the presence of the point
contact, may lead to anomalously large multiple-particle scattering at finite
bias voltage. These processes tend to decrease at low voltage, and thus may
account for the observed reduction of the resistance. We concentrate on the
case of a normal point contact, and model it by a spinfull interacting
Tomonaga-Luttinger liquid, with a single impurity, connected to non interacting
leads. We find that sufficiently strong Coulomb interactions enhance
two-electron scattering, so as these dominate the conductance. Our calculation
shows that the effective charge, probed by the shot noise of such a system,
approaches a value proportional to e* = 2e at sufficiently large backscattering
current. This distinctive hallmark may be tested experimentally. We discuss
possible applications of this model to experiments conducted on Hall bars.Comment: 5 pages, 2 figure
Helical liquids and Majorana bound states in quantum wires
We show that the combination of spin-orbit coupling with a Zeeman field or
strong interactions may lead to the formation of a helical liquid in
single-channel quantum wires. In a helical liquid, electrons with opposite
velocities have opposite spin precession. We argue that zero-energy Majorana
bound states are formed in various situations when the wire is situated in
proximity to a conventional s-wave superconductor. This occurs when the
external magnetic field, the superconducting gap, or, in particular, the
chemical potential vary along the wire. We discuss experimental consequences of
the formation of the helical liquid and the Majorana bound states.Comment: 4+epsilon page
MFV SUSY: A Natural Theory for R-Parity Violation
We present an alternative approach to low-energy supersymmetry. Instead of
imposing R-parity we apply the minimal flavor violation (MFV) hypothesis to the
R-parity violating MSSM. In this framework, which we call MFV SUSY, squarks can
be light and the proton long lived without producing missing energy signals at
the LHC. Our approach differs from that of Nikolidakis and Smith in that we
impose holomorphy on the MFV spurions. The resulting model is highly
constrained and R-parity emerges as an accidental approximate symmetry of the
low-energy Lagrangian. The size of the small R-parity violating terms is
determined by the flavor parameters, and in the absence of neutrino masses
there is only one renormalizable R-parity violating interaction: the
baryon-number violating superpotential term. Low energy
observables (proton decay, dinucleon decay and oscillation) pose
only mild constraints on the parameter space. LHC phenomenology will depend on
whether the LSP is a squark, neutralino, chargino or slepton. If the LSP is a
squark it will have prompt decays, explaining the non-observation of events
with missing transverse energy at the LHC.Comment: 41 pages, 14 figures; v3: minor corrections, matches published
versio
Charge oscillations in Quantum Dots: Renormalization group and Hartree method calculations
We analyze the local level occupation of a spinless, interacting two-level
quantum dot coupled to two leads by means of Wilson's numerical renormalization
group method. A gate voltage sweep, causing a rearrangement of the charge such
that the system's energy is minimized, leads to oscillations, and sometimes
even inversions, in the level occupations. These charge oscillations can be
understood qualitatively by a simple Hartree analysis. By allowing a relative
sign in one tunneling matrix element between dot and leads, we extend our
findings to more generic models.Comment: 6 pages, 7 figure
Controlled dephasing of a quantum dot in the Kondo regime
Kondo correlation in a spin polarized quantum dot (QD) results from the
dynamical formation of a spin singlet between the dot's net spin and a Kondo
cloud of electrons in the leads, leading to enhanced coherent transport through
the QD. We demonstrate here significant dephasing of such transport by coupling
the QD and its leads to potential fluctuations in a near by 'potential
detector'. The qualitative dephasing is similar to that of a QD in the Coulomb
Blockade regime in spite of the fact that the mechanism of transport is quite
different. A much stronger than expected suppression of coherent transport is
measured, suggesting that dephasing is induced mostly in the 'Kondo cloud' of
electrons within the leads and not in the QD.Comment: to be published in PR
Electronic Mach-Zehnder interferometer as a tool to probe fractional statistics
We study transport through an electronic Mach-Zehnder interferometer recently
devised at the Weizmann Institute. We show that this device can be used to
probe statistics of quasiparticles in the fractional quantum Hall regime. We
calculate the tunneling current through the interferometer as the function of
the Aharonov-Bohm flux, temperature and voltage bias, and demonstrate that its
flux-dependent component is strongly sensitive to the statistics of tunneling
quasiparticles. More specifically, the flux-dependent and flux-independent
contributions to the current are related by a power law, the exponent being a
function of the quasiparticle statistics.Comment: 22 pages; 8 figure
Quantifying selection in immune receptor repertoires
The efficient recognition of pathogens by the adaptive immune system relies
on the diversity of receptors displayed at the surface of immune cells. T-cell
receptor diversity results from an initial random DNA editing process, called
VDJ recombination, followed by functional selection of cells according to the
interaction of their surface receptors with self and foreign antigenic
peptides. To quantify the effect of selection on the highly variable elements
of the receptor, we apply a probabilistic maximum likelihood approach to the
analysis of high-throughput sequence data from the -chain of human
T-cell receptors. We quantify selection factors for V and J gene choice, and
for the length and amino-acid composition of the variable region. Our approach
is necessary to disentangle the effects of selection from biases inherent in
the recombination process. Inferred selection factors differ little between
donors, or between naive and memory repertoires. The number of sequences shared
between donors is well-predicted by the model, indicating a purely stochastic
origin of such "public" sequences. We find a significant correlation between
biases induced by VDJ recombination and our inferred selection factors,
together with a reduction of diversity during selection. Both effects suggest
that natural selection acting on the recombination process has anticipated the
selection pressures experienced during somatic evolution
Predicting the spectrum of TCR repertoire sharing with a data-driven model of recombination
Despite the extreme diversity of T cell repertoires, many identical T-cell
receptor (TCR) sequences are found in a large number of individual mice and
humans. These widely-shared sequences, often referred to as `public', have been
suggested to be over-represented due to their potential immune functionality or
their ease of generation by V(D)J recombination. Here we show that even for
large cohorts the observed degree of sharing of TCR sequences between
individuals is well predicted by a model accounting for by the known
quantitative statistical biases in the generation process, together with a
simple model of thymic selection. Whether a sequence is shared by many
individuals is predicted to depend on the number of queried individuals and the
sampling depth, as well as on the sequence itself, in agreement with the data.
We introduce the degree of publicness conditional on the queried cohort size
and the size of the sampled repertoires. Based on these observations we propose
a public/private sequence classifier, `PUBLIC' (Public Universal Binary
Likelihood Inference Classifier), based on the generation probability, which
performs very well even for small cohort sizes
Intersectional Border(ing)s
This special issue of Political Geography marks a contribution to the fields of feminist geopolitics and border studies by bringing together a series of papers, which use approaches based on Yuval-Davis’ ‘situated intersectionality’ (2015) to explore everyday bordering within and without contemporary Europe. The special issue is comprised of work undertaken by colleagues from across Europe and beyond as part of work package 9 ‘Borders, Intersectionality and the Everyday’ of the EUBorderscapes project (2012-2016). We term our approach to studying borders, borderscapes and bordering processes as ‘situated intersectional bordering’. The main contribution of this approach is that borders and borderings are understood as dialogical constructs and that if we are to understand how they are being made and re-made we must attempt to explore them through the situated gazes of differentially positioned social actors. We therefore suggest a holistic approach to understanding border(ing)s, which is embedded in everyday life. Through the study of the multi-layered complexities of everyday borderings we can ‘approach the truth’ (Hill-Collins, 1990)
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