20,060 research outputs found
The Principle of Minimal Resistance in Non-Equilibrium Thermodynamics
Analytical models describing the motion of colloidal particles in given
velocity fields are presented. In addition to local approaches, leading to well
known master equations such as the Langevin and the Fokker-Planck equations, a
global description based on path integration is reviewed. This shows that under
very broad conditions, during its evolution a dissipative system tends to
minimize its energy dissipation in such a way to keep constant the Hamiltonian
time rate, equal to the difference between the flux-based and the force-based
Rayleigh dissipation functions. At steady state, the Hamiltonian time rate is
maximized, leading to a minimum resistance principle. In the unsteady case, we
consider the relaxation to equilibrium of harmonic oscillators and the motion
of a Brownian particle in shear flow, obtaining results that coincide with the
solution of the Fokker-Planck and the Langevin equations
On Localization and Regularization
Different regularizations are studied in localization of path integrals. We
discuss the effect of the choice of regularization by evaluating the partition
functions for the harmonic oscillator and the Weyl character for SU(2). In
particular, we solve the Weyl shift problem that arises in path integral
evaluation of the Weyl character by using the Atiyah-Patodi-Singer
-invariant and the Borel-Weil theory.Comment: 15 pages, LaTe
Search for a light dark sector particle at LHCb
A search is presented for a hidden-sector boson, , produced in the
decay , with and . The search is performed using a
-collision data sample collected at and 8 TeV with the LHCb
detector, corresponding to integrated luminosities of 1 and 2 fb
respectively. No significant signal is observed in the mass range MeV, and upper limits are placed on the branching fraction
product as a function of the mass and lifetime of the
boson. These limits place the most stringent constraints to date on many
theories that predict the existence of additional low-mass dark bosons.Comment: Presentation at the DPF 2015 Meeting of the American Physical Society
Division of Particles and Fields, Ann Arbor, Michigan, August 4-8, 201
Dielectric function and plasmons of doped three-dimensional Luttinger semimetals
Luttinger semimetals are three-dimensional electron systems with a parabolic
band touching and an effective total spin . In this paper, we present an
analytical theory of dielectric screening of inversion-symmetric Luttinger
semimetals with an arbitrary carrier density and conduction-valence effective
mass asymmetry. Assuming a spherical approximation for the single-particle
Luttinger Hamiltonian, we determine analytically the dielectric screening
function in the random phase approximation for arbitrary values of the wave
vector and frequency, the latter in the complex plane. We use this analytical
expression to calculate the dispersion relation and Landau damping of the
collective modes in the charge sector (i.e., plasmons).Comment: 17 pages, 5 figures, published versio
Electronic structure calculations and molecular dynamics simulations with linear system-size scaling
We present a method for total energy minimizations and molecular dynamics
simulations based either on tight-binding or on Kohn-Sham hamiltonians. The
method leads to an algorithm whose computational cost scales linearly with the
system size. The key features of our approach are
(i) an orbital formulation with single particle wavefunctions constrained to
be localized in given regions of space, and (ii) an energy functional which
does not require either explicit orthogonalization of the electronic orbitals,
or inversion of an overlap matrix.
The foundations and accuracy of the approach and the performances of the
algorithm are discussed, and illustrated with several numerical examples
including Kohn-Sham hamiltonians. In particular we present calculations with
tight-binding hamiltonians for diamond, graphite, a carbon linear chain and
liquid carbon at low pressure. Even for a complex case such as liquid carbon --
a disordered metallic system with differently coordinated atoms -- the
agreement between standard diagonalization schemes and our approach is very
good. Our results establish the accuracy and reliability of the method for a
wide class of systems and show that tight binding molecular dynamics
simulations with a few thousand atoms are feasible on small workstations
Electronic structure of heavily-doped graphene: the role of foreign atom states
Using density functional theory calculations we investigate the electronic
structure of graphene doped by deposition of foreign atoms. We demonstrate
that, as the charge transfer to the graphene layer increases, the band
structure of the pristine graphene sheet is substantially affected. This is
particularly relevant when Ca atoms are deposed on graphene at CaC
stoichiometry. Similarly to what happens in superconducting graphite
intercalated compounds, a Ca bands occurs at the Fermi level. Its hybridization
with the C states generates a strong non-linearity in one of the
bands below the Fermi level, at energies comparable to the graphene E
phonon frequency. This strong non-linearity, and not manybody effects as
previously proposed, explains the large and anisotropic values of the apparent
electron-phonon coupling measured in angular resolved photoemission.Comment: 4 pages, 2 figures, see also M. Calandra and F. Mauri,arXiv:0707.146
A helical jet model for OJ287
Context. OJ287 is a quasar with a quasi-periodic optical light curve, with
the periodicity observed for over 120 years. This has lead to a binary black
hole model as a common explanation of the quasar. The radio jet of OJ287 has
been observed for a shorter time of about 30 years. It has a complicated
structure that varies dramatically in a few years time scale. Aims. Here we
propose that this structure arises from a helical jet being observed from a
small and varying viewing angle. The viewing angle variation is taken to be in
tune with the binary orbital motion. Methods. We calculate the effect of the
secondary black hole on the inner edge of the accretion disk of the primary
using particle simulations. We presume that the axis of the helix is
perpendicular to the disk. We then follow the jet motion on its helical path
and project the jet to the sky plane. This projection is compared with
observations both at mm waves and cm waves. Results. We find that this model
reproduces the observations well if the changes in the axis of the conical
helix propagate outwards with a relativistic speed of about 0.85c. In
particular, this model explains at the same time the long-term optical
brightness variations as varying Doppler beaming in a component close to the
core, i.e. at parsec scale in real linear distance, while the mm and cm radio
jet observations are explained as being due to jet wobble at much larger (100
parsec scale) distances from the core.Comment: 5 pages, 7 figures, to be published in Astronomy & Astrophysic
Possibility of superconductivity in graphite intercalated with alkaline earths investigated with density functional theory
Using density functional theory we investigate the occurrence of
superconductivity in AC with A=Mg,Ca,Sr,Ba. We predict that at zero
pressure, Ba and Sr should be superconducting with critical temperatures
(T) 0.2 K and 3.0 K, respectively. We study the pressure dependence of
T assuming the same symmetry for the crystal structures at zero and finite
pressures. We find that the SrC and BaC critical temperatures should be
substantially enhanced by pressure. On the contrary, for CaC we find that
in the 0 to 5 GPa region, T weakly increases with pressure. The increase is
much smaller than what shown in several recent experiments. Thus we suggest
that in CaC a continous phase transformation, such as an increase in
staging, occurs at finite pressure. Finally we argue that, although MgC is
unstable, the synthesis of intercalated systems of the kind
MgCaC could lead to higher critical temperatures.Comment: 9 page
Charge density wave and superconducting dome in TiSe2 from electron-phonon interaction
At low temperature TiSe2 undergoes a charge density wave instability.
Superconductivity is stabilized either by pressure or by Cu intercalation. We
show that the pressure phase diagram of TiSe2 is well described by
first-principles calculations. At pressures smaller than 4 GPa charge density
wave ordering occurs, in agreement with experiments. At larger pressures the
disappearing of the charge density wave is due to a stiffening of the
short-range force-constants and not to the variation of nesting with pressure.
Finally we show that the behavior of Tc as a function of pressure is entirely
determined by the electron-phonon interaction without need of invoking
excitonic mechanisms. Our work demonstrates that phase-diagrams with competing
orders and a superconducting dome are also obtained in the framework of the
electron-phonon interaction.Comment: 4 pages, 7 picture
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