6,759 research outputs found
Zero-Bias Anomalies in Narrow Tunnel Junctions in the Quantum Hall Regime
We report on the study of cleaved-edge-overgrown line junctions with a
serendipitously created narrow opening in an otherwise thin, precise line
barrier. Two sets of zero-bias anomalies are observed with an enhanced
conductance for filling factors and a strongly suppressed conductance
for . A transition between the two behaviors is found near . The zero-bias anomaly (ZBA) line shapes find explanation in
Luttinger liquid models of tunneling between quantum Hall edge states. The ZBA
for occurs from strong backscattering induced by suppression of
quasiparticle tunneling between the edge channels for the Landau
levels. The ZBA for arises from weak tunneling of quasiparticles
between the edge channels.Comment: version with edits for clarit
Energy Level Statistics of Quantum Dots
We investigate the charging energy level statistics of disordered interacting
electrons in quantum dots by numerical calculations using the Hartree
approximation. The aim is to obtain a global picture of the statistics as a
function of disorder and interaction strengths. We find Poisson statistics at
very strong disorder, Wigner- Dyson statistics for weak disorder and
interactions, and a Gaussian intermediate regime. These regimes are as expected
from previous studies and fundamental considerations, but we also find
interesting and rather broad crossover regimes. In particular, intermediate
between the Gaussian and Poisson regimes we find a two-sided exponential
distribution for the energy level spacings. In comparing with experiment, we
find that this distribution may be realized in some quantum dots.Comment: 21 pages 10 figure
Proximity effects and triplet correlations in Ferromagnet/Ferromagnet/Superconductor nanostructures
We report the results of a study of superconducting proximity effects in
clean Ferromagnet/Ferromagnet/Superconductor ()
heterostructures, where the pairing state in S is a conventional singlet
s-wave. We numerically find the self-consistent solutions of the Bogoliubov-de
Gennes (BdG) equations and use these solutions to calculate the relevant
physical quantities. By linearizing the BdG equations, we obtain the
superconducting transition temperatures as a function of the angle
between the exchange fields in and . We find that
the results for in systems are clearly different
from those in systems, where monotonically increases
with and is highest for antiparallel magnetizations. Here,
is in general a non-monotonic function, and often has a minimum
near . For certain values of the exchange field and
layer thicknesses, the system exhibits reentrant superconductivity with
: it transitions from superconducting to normal, and then returns to a
superconducting state again with increasing . This phenomenon is
substantiated by a calculation of the condensation energy. We compute, in
addition to the ordinary singlet pair amplitude, the induced odd triplet
pairing amplitudes. The results indicate a connection between equal-spin
triplet pairing and the singlet pairing state that characterizes . We find
also that the induced triplet amplitudes can be very long-ranged in both the S
and F sides and characterize their range. We discuss the average density of
states for both the magnetic and the S regions, and its relation to the pairing
amplitudes and . The local magnetization vector, which exhibits reverse
proximity effects, is also investigated.Comment: 14 pages including 11 figure
Multi-Lepton Collider Signatures of Heavy Dirac and Majorana Neutrinos
We discuss the possibility of observing multi-lepton signals at the Large
Hadron Collider (LHC) from the production and decay of heavy Standard Model
(SM) singlet neutrinos added in extensions of SM to explain the observed light
neutrino masses by seesaw mechanism. In particular, we analyze two `smoking
gun' signals depending on the Dirac or Majorana nature of the heavy neutrino:
(i) for Majorana case, the same-sign di-lepton signal which can be used as a
probe of lepton-number violation, and (ii) for Dirac case, the tri-lepton
signal which conserves lepton number but may violate lepton flavor. Within a
minimal Left-Right symmetric framework in which these additional neutrino
states arise naturally, we find that in both cases, the signals can be
identified with virtually no background beyond a TeV, and the heavy gauge boson
W_R can be discovered in this process. This analysis also provides a direct way
to probe the nature of seesaw physics involving the SM singlets at TeV scale,
and in particular, to distinguish type-I seesaw with purely Majorana heavy
neutrinos from inverse seesaw with pseudo-Dirac counterparts.Comment: 19 pages, 7 figures; typo in eq. 5 fixed; matches published versio
Divergence and Shannon information in genomes
Shannon information (SI) and its special case, divergence, are defined for a
DNA sequence in terms of probabilities of chemical words in the sequence and
are computed for a set of complete genomes highly diverse in length and
composition. We find the following: SI (but not divergence) is inversely
proportional to sequence length for a random sequence but is length-independent
for genomes; the genomic SI is always greater and, for shorter words and longer
sequences, hundreds to thousands times greater than the SI in a random sequence
whose length and composition match those of the genome; genomic SIs appear to
have word-length dependent universal values. The universality is inferred to be
an evolution footprint of a universal mode for genome growth.Comment: 4 pages, 3 tables, 2 figure
Relationship between Respiratory Load Perception and Perception of Nonrespiratory Sensory Modalities in Subjects with Life-Threatening Asthma
Subjects with life-threatening asthma (LTA) have reported decreased sensitivity to inspiratory resistive (R) loads. It is unknown if decreased sensitivity is specific for inspiratory R loads, other types of respiratory loads, or a general deficit affecting sensory modalities. This study hypothesized that impairment is specific to respiratory stimuli. This study tested perceptual sensitivity of LTA, asthmatic (A), and nonasthmatic (NA) subjects to 4 sensory modalities: respiratory, somatosensory, auditory, visual. Perceptual sensitivity was measured with magnitude estimation (ME): respiratory loads ME, determined using inspiratory R and pressure threshold (PT) loads; somatosensory ME, determined using weight ranges of 2–20 kg; auditory ME, determined using graded magnitudes of 1 kHz tones delivered for 3 seconds bilaterally; visual ME, determined using gray-to-white disk intensity gradations on black background. ME for inspiratory R loads lessened for LTA over A and NA subjects. There was no significant difference between the 3 groups in ME for PT inspiratory loads, weight, sound, and visual trials. These results demonstrate that LTA subjects are poor perceivers of inspiratory R loads. This deficit in respiratory perception is specific to inspiratory R loads and is not due to perceptual deficits in other types of inspiratory loads, somatosensory, auditory, or visual sensory modalities
Sum rule for the optical Hall angle
We consider the optical Hall conductivity of a general electronic medium and
prove that the optical Hall angle obeys a new sum rule. This sum rule governs
the response of an electronic fluid to a Lorentz electric field and can thought
of as the transverse counterpart to the f-sum rule in optical conductivity. The
physical meaning of this sum rule is discussed, giving a number of examples of
its application to a variety of of electronic media.Comment: Four pages. Latex file with two postscript figure
Dynamical mean-field equations for strongly interacting fermionic atoms in a potential trap
We derive a set of dynamical mean-field equations for strongly interacting
fermionic atoms in a potential trap across a Feshbach resonance. Our derivation
is based on a variational ansatz, which generalizes the crossover wavefunction
to the inhomogeneous case, and the assumption that the order parameter is
slowly varying over the size of the Cooper pairs. The equations reduce to a
generalized time-dependent Gross-Pitaevskii equation on the BEC side of the
resonance. We discuss an iterative method to solve these mean-field equations,
and present the solution for a harmonic trap as an illustrating example to
self-consistently verify the approximations made in our derivation.Comment: replaced with the published versio
The BCS-like gap in superconductor SmFeAsO_0.85F_0.15
Since the discovery of superconductivity in the cuprates two decades ago, it
has been firmly established that the CuO_2 plane is consequential for high T_C
superconductivity and a host of other very unusual properties. A new family of
superconductors with the general composition of LaFeAsO_(1-x)F_x has recently
been discovered but with the conspicuous lacking of the CuO_2 planes, thus
raising the tantalizing questions of the different pairing mechanisms in these
oxypnictide superconductors. Intimately related to pairing in a superconductor
are the superconducting gap, its value, structure, and temperature dependence.
Here we report the observation of a single gap in the superconductor
SmFeAsO_0.85F_0.15 with T_C = 42 K as measured by Andreev spectroscopy. The gap
value of 2Delta = 13.34+/-0.3 meV gives 2Delta/k_BT_C = 3.68, close to the BCS
prediction of 3.53. The gap decreases with temperature and vanishes at T_C in a
manner consistent with the Bardeen-Cooper-Schrieffer (BCS) prediction but
dramatically different from that of the pseudogap behavior in the cuprate
superconductors. Our results clearly indicate a nodeless gap order parameter,
which is nearly isotropic in size across different sections of the Fermi
surface, and are not compatible with models involving antiferromagnetic
fluctuations, strong correlations, t-J model, and the like, originally designed
for cuprates.Comment: 8 pages, 3 figure
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