3,323 research outputs found
A New Era in High-energy Physics
In TeV-scale gravity, scattering of particles with center-of-mass energy of
the order of a few TeV can lead to the creation of nonperturbative, extended,
higher-dimensional gravitational objects: Branes. Neutral or charged, spinning
or spinless, Einsteinian or supersymmetric, low-energy branes could
dramatically change our picture of high-energy physics. Will we create branes
in future particle colliders, observe them from ultra high energy cosmic rays,
and discover them to be dark matter?Comment: 8 pages, 2 figures. Essay submitted on Mar 26, 2002 to the Gravity
Research Foundation. Awarded the third prize in the 2002 GRF competitio
Graphene as an electronic membrane
Experiments are finally revealing intricate facts about graphene which go
beyond the ideal picture of relativistic Dirac fermions in pristine two
dimensional (2D) space, two years after its first isolation. While observations
of rippling added another dimension to the richness of the physics of graphene,
scanning single electron transistor images displayed prevalent charge
inhomogeneity. The importance of understanding these non-ideal aspects cannot
be overstated both from the fundamental research interest since graphene is a
unique arena for their interplay, and from the device applications interest
since the quality control is a key to applications. We investigate the membrane
aspect of graphene and its impact on the electronic properties. We show that
curvature generates spatially varying electrochemical potential. Further we
show that the charge inhomogeneity in turn stabilizes ripple formation.Comment: 6 pages, 11 figures. Updated version with new results about the
re-hybridization of the electronic orbitals due to rippling of the graphene
sheet. The re-hybridization adds the next-to-nearest neighbor hopping effect
discussed in the previous version. New reference to recent STM experiments
that give support to our theor
Effect of magnetic order on the superfluid response of single-crystal ErNiBC: A penetration depth study
We report measurements of the in-plane magnetic penetration depth (T) in single crystals of ErNiBC down to 0.1 K using
a tunnel-diode based, self-inductive technique at 21 MHz. We observe four
features: (1) a slight dip in (T) at the Nel
temperature = 6.0 K, (2) a peak at = 2.3 K, where a weak
ferromagnetic component sets in, (3) another maximum at 0.45 K, and (4) a final
broad drop down to 0.1 K. Converting to superfluid density , we see
that the antiferromagnetic order at 6 K only slightly depresses
superconductivity. We seek to explain some of the above features in the context
of antiferromagnetic superconductors, where competition between the
antiferromagnetic molecular field and spin fluctuation scattering determines
increased or decreased pairbreaking. Superfluid density data show only a slight
decrease in pair density in the vicinity of the 2.3 K feature, thus supporting
other evidences against bulk ferromagnetism in this temperature range.Comment: 15 pages, 5 figure
Resolved Depletion Zones and Spatial Differentiation of N2H+ and N2D+
We present a study on the spatial distribution of N2D+ and N2H+ in thirteen
protostellar systems. Eight of thirteen objects observed with the IRAM 30m
telescope show relative offsets between the peak N2D+ (J=2-1) and N2H+ (J=1-0)
emission. We highlight the case of L1157 using interferometric observations
from the Submillimeter Array and Plateau de Bure Interferometer of the N2D+
(J=3-2) and N2H+ (J=1-0) transitions respectively. Depletion of N2D+ in L1157
is clearly observed inside a radius of ~2000 AU (7") and the N2H+ emission is
resolved into two peaks at radii of ~1000 AU (3.5"), inside the depletion
region of N2D+. Chemical models predict a depletion zone in N2H+ and N2D+ due
to destruction of H2D+ at T ~ 20 K and the evaporation of CO off dust grains at
the same temperature. However, the abundance offsets of 1000 AU between the two
species are not reproduced by chemical models, including a model that follows
the infall of the protostellar envelope. The average abundance ratios of N2D+
to N2H+ have been shown to decrease as protostars evolve by Emprechtinger et
al., but this is the first time depletion zones of N2D+ have been spatially
resolved. We suggest that the difference in depletion zone radii for N2H+ and
N2D+ is caused by either the CO evaporation temperature being above 20 K or an
H2 ortho-to-para ratio gradient in the inner envelope.Comment: Accepted to ApJ. 44 pages 13 Figure
Parkinson\u27s disease and multiple system atrophy have distinct α-synuclein seed characteristics
Auditory Cortex is Important in the Extinction of Two Different Tone-Based Conditioned Fear Memories in Rats
Extensive fear extinction research is guided by the view that there are structures in the brain that develop inhibitory control over the expression of conditioned fear memories. While the medial prefrontal cortex has recently captured attention as the locus of plasticity essential for extinction of conditioned fear, the auditory cortex is another plausible cortical area involved in extinction learning since it is considered a sufficient conditioned stimulus (CS) pathway in tone fear conditioning. We examined the role of auditory cortex in extinction of auditory-based fear memories with a standard tone-on conditioning, wherein a tone CS predicted a footshock unconditioned stimulus (US), or a novel tone-off conditioning, in which the tone was continually present and the offset of the tone was the CS predicting the US. Rats with bilateral auditory cortex lesions were trained in either paradigm and subsequently trained in extinction to the CS. Auditory cortex lesions had no effect on acquisition but impaired extinction to both CSs. These findings indicate that the auditory cortex contributes to extinction of wide-ranging auditory fear memories, as evidenced by deficits in both tone-on CS and tone-off CS extinction training
Quantum limit transport and destruction of the Weyl nodes in TaAs
Weyl fermions are a new ingredient for correlated states of electronic
matter. A key difficulty has been that real materials also contain non-Weyl
quasiparticles, and disentangling the experimental signatures has proven
challenging. We use magnetic fields up to 95 tesla to drive the Weyl semimetal
TaAs far into its quantum limit (QL), where only the purely chiral 0th Landau
levels (LLs) of the Weyl fermions are occupied. We find the electrical
resistivity to be nearly independent of magnetic field up to 50 tesla: unusual
for conventional metals but consistent with the chiral anomaly for Weyl
fermions. Above 50 tesla we observe a two-order-of-magnitude increase in
resistivity, indicating that a gap opens in the chiral LLs. Above 80 tesla we
observe strong ultrasonic attenuation below 2 kelvin, suggesting a
mesoscopically-textured state of matter. These results point the way to
inducing new correlated states of matter in the QL of Weyl semimetals
A self-tuning mechanism in (3+p)d gravity-scalar theory
We present a new type of self-tuning mechanism for ()d brane world
models in the framework of gravity-scalar theory. This new type of self-tuning
mechanism exhibits a remarkable feature. In the limit , being
the string coupling, the geometry of bulk spacetime remains virtually unchanged
by an introduction of the Standard Model(SM)-brane, and consequently it is
virtually unaffected by quantum fluctuations of SM fields with support on the
SM-brane. Such a feature can be obtained by introducing Neveu-Schwarz(NS)-brane
as a background brane on which our SM-brane is to be set. Indeed, field
equations naturally suggest the existence of the background NS-brane. Among the
given such models, of the most interest is the case with , where
represents the bulk cosmological constant. This model contains a pair
of coincident branes (of the SM- and the NS-branes), one of which is a
codimension-2 brane placed at the origin of 2d transverse space (), another a codimension-1 brane placed at the edge of .
These two branes are (anti) T-duals of each other, and one of them may be
identified as our SM-brane plus the background NS-brane. In the presence of the
background NS-brane (and in the absence of ), the 2d transverse space
becomes an orbifold with an appropriate deficit angle.
But this is only possible if the ()d Planck scale and the string
scale () are of the same order, which
accords with the hierarchy assumption \cite{1,2,3} that the electroweak scale
is the only short distance scale existing in nature
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