21,021 research outputs found
Thermal fluctuations and anomalous elasticity of homogeneous nematic elastomers
We present a unified formulation of a rotationally invariant nonlinear
elasticity for a variety of spontaneously anisotropic phases, and use it to
study thermal fluctuations in nematic elastomers and spontaneously anisotropic
gels. We find that in a thermodynamic limit homogeneous nematic elastomers are
universally incompressible, are characterized by a universal ratio of shear
moduli, and exhibit an anomalous elasticity controlled by a nontrivial low
temperature fixed point perturbative in D=3-epsilon dimensions. In three
dimensions, we make predictions that are asymptotically exact.Comment: 4 RevTeX pgs,,submitted to Europhysics Letter
Quantum Hall Effect in Thin Films of Three-Dimensional Topological Insulators
We show that a thin film of a three-dimensional topological insulator (3DTI)
with an exchange field is a realization of the famous Haldane model for quantum
Hall effect (QHE) without Landau levels. The exchange field plays the role of
staggered fluxes on the honeycomb lattice, and the hybridization gap of the
surface states is equivalent to alternating on-site energies on the AB
sublattices. A peculiar phase diagram for the QHE is predicted in 3DTI thin
films under an applied magnetic field, which is quite different from that
either in traditional QHE systems or in graphene.Comment: 4 pages, 4 figure
A topological look at the quantum spin Hall state
We propose a topological understanding of the quantum spin Hall state without
considering any symmetries, and it follows from the gauge invariance that
either the energy gap or the spin spectrum gap needs to close on the system
edges, the former scenario generally resulting in counterpropagating gapless
edge states. Based upon the Kane-Mele model with a uniform exchange field and a
sublattice staggered confining potential near the sample boundaries, we
demonstrate the existence of such gapless edge states and their robust
properties in the presence of impurities. These gapless edge states are
protected by the band topology alone, rather than any symmetries.Comment: 5 pages, 4 figure
The effect of temperature anisotropy on observations of Doppler dimming and pumping in the inner corona
Recent observations of the spectral line profiles and intensity ratio of the
O VI 1032 {\AA} and 1037.6 {\AA} doublet by the Ultraviolet Coronagraph
Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO), made in
coronal holes below 3.5 , provide evidence for Doppler dimming of the O VI
1037.6 {\AA} line and pumping by the chromospheric C II 1037.0182 {\AA} line.
Evidence for a significant kinetic temperature anisotropy of O ions was
also derived from these observations. We show in this Letter how the component
of the kinetic temperature in the direction perpendicular to the magnetic
field, for both isotropic and anisotropic temperature distributions, affects
both the amount of Doppler dimming and pumping. Taking this component into
account, we further show that the observation that the O VI doublet intensity
ratio is less than unity can be accounted for only if pumping by C II 1036.3367
{\AA} in addition to C II 1037.0182 {\AA} is in effect. The inclusion of the C
II 1036.3367 {\AA} pumping implies that the speed of the O ions can
reach 400 km/s around 3 which is significantly higher than the reported
UVCS values for atomic hydrogen in polar coronal holes. These results imply
that oxygen ions flow much faster than protons at that heliocentric distance.Comment: 9 pages, 3 figure
Universal Conductance Fluctuations in Mesoscopic Systems with Superconducting Leads: Beyond the Andreev Approximation
We report our investigation of the sample to sample fluctuation in transport
properties of phase coherent normal metal-superconductor hybrid systems.
Extensive numerical simulations were carried out for quasi-one dimensional and
two dimensional systems in both square lattice (Fermi electron) as well as
honeycomb lattice (Dirac electron). Our results show that when the Fermi energy
is within the superconducting energy gap , the Andreev conductance
fluctuation exhibits a universal value (UCF) which is approximately two times
larger than that in the normal systems. According to the random matrix theory,
the electron-hole degeneracy (ehD) in the Andreev reflections (AR) plays an
important role in classifying UCF. Our results confirm this. We found that in
the diffusive regime there are two UCF plateaus, one corresponds to the
complete electron-hole symmetry (with ehD) class and the other to conventional
electron-hole conversion (ehD broken). In addition, we have studied the Andreev
conductance distribution and found that for the fixed average conductance
the Andreev conductance distribution is a universal function that depends only
on the ehD. In the localized regime, our results show that ehD continues to
serve as an indicator for different universal classes. Finally, if normal
transport is present, i.e., Fermi energy is beyond energy gap , the AR
is suppressed drastically in the localized regime by the disorder and the ehD
becomes irrelevant. As a result, the conductance distribution is that same as
that of normal systems
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