13,389 research outputs found
A heuristic approach to the weakly interacting Bose gas
Some thermodynamic properties of weakly interacting Bose systems are derived
from dimensional and heuristic arguments and thermodynamic relations, without
resorting to statistical mechanics
Temperature-dependent resistivity of suspended graphene
In this paper we investigate the electron-phonon contribution to the
resistivity of suspended single layer graphene. In-plane as well as flexural
phonons are addressed in different temperature regimes. We focus on the
intrinsic electron-phonon coupling due to the interaction of electrons with
elastic deformations in the graphene membrane. The competition between screened
deformation potential vs fictitious gauge field coupling is discussed, together
with the role of tension in the suspended flake. In the absence of tension,
flexural phonons dominate the phonon contribution to the resistivity at any
temperature with a and dependence at low and high
temperatures, respectively. Sample-specific tension suppresses the contribution
due to flexural phonons, yielding a linear temperature dependence due to
in-plane modes. We compare our results with recent experiments.Comment: 11 pages, 3 figure
The bound on viscosity and the generalized second law of thermodynamics
We describe a new paradox for ideal fluids. It arises in the accretion of an
\textit{ideal} fluid onto a black hole, where, under suitable boundary
conditions, the flow can violate the generalized second law of thermodynamics.
The paradox indicates that there is in fact a lower bound to the correlation
length of any \textit{real} fluid, the value of which is determined by the
thermodynamic properties of that fluid. We observe that the universal bound on
entropy, itself suggested by the generalized second law, puts a lower bound on
the correlation length of any fluid in terms of its specific entropy. With the
help of a new, efficient estimate for the viscosity of liquids, we argue that
this also means that viscosity is bounded from below in a way reminiscent of
the conjectured Kovtun-Son-Starinets lower bound on the ratio of viscosity to
entropy density. We conclude that much light may be shed on the
Kovtun-Son-Starinets bound by suitable arguments based on the generalized
second law.Comment: 11 pages, 1 figure, published versio
Fermi-liquid effects in the gapless state of marginally thin superconducting films
We present low temperature tunneling density-of-states measurements in Al
films in high parallel magnetic fields. The thickness range of the films, t=6-9
nm, was chosen so that the orbital and Zeeman contributions to their parallel
critical fields were comparable. In this quasi-spin paramagnetically limited
configuration, the field produces a significant suppression of the gap, and at
high fields the gapless state is reached. By comparing measured and calculated
tunneling spectra we are able to extract the value of the antisymmetric
Fermi-liquid parameter G^0 and thereby deduce the quasiparticle density
dependence of the effective parameter G^0_{eff} across the gapless state.Comment: 6 pages, 4 figure
Causality-based criteria for a negative refractive index must be used with care
Using the principle of causality as expressed in the Kramers-Kronig
relations, we derive a generalized criterion for a negative refractive index
that admits imperfect transparency at an observation frequency . It
also allows us to relate the global properties of the loss (i.e. its frequency
response) to its local behaviour at . However, causality-based criteria
rely the on the group velocity, not the Poynting vector. Since the two are not
equivalent, we provide some simple examples to compare the two criteria.Comment: slightly longer version of published PR
Simulation of Cosmic Ray neutrinos Interactions in Water
The program CORSIKA, usually used to simulate extensive cosmic ray air
showers, has been adapted to a water medium in order to study the acoustic
detection of ultra high energy neutrinos. Showers in water from incident
protons and from neutrinos have been generated and their properties are
described. The results obtained from CORSIKA are compared to those from other
available simulation programs such as Geant4.Comment: Talk presented on behalf of the ACoRNE Collaboration at the ARENA
Workshop 200
A note on the analogy between superfluids and cosmology
A new analogy between superfluid systems and cosmology is here presented,
which relies strongly on the following ingredient: the back-reaction of the
vacuum to the quanta of sound waves. We show how the presence of thermal
phonons, the excitations above the quantum vacuum for , enable us to
deduce an hydrodynamical equation formally similar to the one obtained for a
perfect fluid in a Universe obeying the Friedmann-Robertson-Walker metric.Comment: Accepted for publication in Modern Physics Letters
Consistency of a Causal Theory of Radiative Reaction with the Optical Theorem
The Abraham-Lorentz-Dirac equation for a point electron, while suffering from
runaway solutions and an acausal response to external forces, is compatible
with the optical theorem. We show that a theory of radiative reaction that
allows for a finite charge distribution is not only causal and free of runaway
solutions, but is also consistent with the optical theorem and the standard
formula for the Rayleigh scattering cross section.Comment: 4 pages, 2 figure
The effect of a velocity barrier on the ballistic transport of Dirac fermions
We propose a novel way to manipulate the transport properties of massless
Dirac fermions by using velocity barriers, defining the region in which the
Fermi velocity, , has a value that differs from the one in the
surrounding background. The idea is based on the fact that when waves travel
accross different media, there are boundary conditions that must be satisfied,
giving rise to Snell's-like laws. We find that the transmission through a
velocity barrier is highly anisotropic, and that perfect transmission always
occurs at normal incidence. When in the barrier is larger that the
velocity outside the barrier, we find that a critical transmission angle
exists, a Brewster-like angle for massless Dirac electrons.Comment: 4.3 pages, 5 figure
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