342,969 research outputs found
DC magnetic field generation in unmagnetized shear flows
The generation of DC magnetic fields in unmagnetized plasmas with velocity
shear is predicted for non relativistic and relativistic scenarios either due
to thermal effects or due to the onset of the Kelvin-Helmholtz instability
(KHI). A kinetic model describes the growth and the saturation of the DC field.
The predictions of the theory are confirmed by multidimensional
particle-in-cell simulations, demonstrating the formation of long lived
magnetic fields () along the full longitudinal
extent of the shear layer, with transverse width on the electron length scale
(), reaching magnitudes
Getting More from Pushing Less: Negative Specific Heat and Conductivity in Non-equilibrium Steady States
For students familiar with equilibrium statistical mechanics, the notion of a
positive specific heat, being intimately related to the idea of stability, is
both intuitively reasonable and mathematically provable. However, for system in
non-equilibrium stationary states, coupled to more than one energy reservoir
(e.g., thermal bath), negative specific heat is entirely possible. In this
paper, we present a ``minimal'' system displaying this phenomenon. Being in
contact with two thermal baths at different temperatures, the (internal) energy
of this system may increase when a thermostat is turned down. In another
context, a similar phenomenon is negative conductivity, where a current may
increase by decreasing the drive (e.g., an external electric field). The
counter-intuitive behavior in both processes may be described as `` getting
more from pushing less.'' The crucial ingredients for this phenomenon and the
elements needed for a ``minimal'' system are also presented.Comment: 14 pages, 3 figures, accepted for publication in American Journal of
Physic
Low temperature properties of the triangular-lattice antiferromagnet: a bosonic spinon theory
We study the low temperature properties of the triangular-lattice Heisenberg
antiferromagnet with a mean field Schwinger spin-1/2 boson scheme that
reproduces quantitatively the zero temperature energy spectrum derived
previously using series expansions. By analyzing the spin-spin and the boson
density-density dynamical structure factors, we identify the unphysical spin
excitations that come from the relaxation of the local constraint on bosons.
This allows us to reconstruct a free energy based on the physical excitations
only, whose predictions for entropy and uniform susceptibility seem to be
reliable within the temperature range $0< T <0.3J, which is difficult to access
by other methods. The high values of entropy, also found in high temperature
expansions studies, can be attributed to the roton-like narrowed dispersion at
finite temperatures.Comment: 16 pages, 5 figure
Electron-scale shear instabilities: magnetic field generation and particle acceleration in astrophysical jets
Strong shear flow regions found in astrophysical jets are shown to be
important dissipation regions, where the shear flow kinetic energy is converted
into electric and magnetic field energy via shear instabilities. The emergence
of these self-consistent fields make shear flows significant sites for
radiation emission and particle acceleration. We focus on electron-scale
instabilities, namely the collisionless, unmagnetized Kelvin-Helmholtz
instability (KHI) and a large-scale dc magnetic field generation mechanism on
the electron scales. We show that these processes are important candidates to
generate magnetic fields in the presence of strong velocity shears, which may
naturally originate in energetic matter outburst of active galactic nuclei and
gamma-ray bursters. We show that the KHI is robust to density jumps between
shearing flows, thus operating in various scenarios with different density
contrasts. Multidimensional particle-in-cell (PIC) simulations of the KHI,
performed with OSIRIS, reveal the emergence of a strong and large-scale dc
magnetic field component, which is not captured by the standard linear fluid
theory. This dc component arises from kinetic effects associated with the
thermal expansion of electrons of one flow into the other across the shear
layer, whilst ions remain unperturbed due to their inertia. The electron
expansion forms dc current sheets, which induce a dc magnetic field. Our
results indicate that most of the electromagnetic energy developed in the KHI
is stored in the dc component, reaching values of equipartition on the order of
in the electron time-scale, and persists longer than the proton
time-scale. Particle scattering/acceleration in the self generated fields of
these shear flow instabilities is also analyzed
Application of asymptotic expansions of maximum likelihood estimators errors to gravitational waves from binary mergers: the single interferometer case
In this paper we describe a new methodology to calculate analytically the
error for a maximum likelihood estimate (MLE) for physical parameters from
Gravitational wave signals. All the existing litterature focuses on the usage
of the Cramer Rao Lower bounds (CRLB) as a mean to approximate the errors for
large signal to noise ratios. We show here how the variance and the bias of a
MLE estimate can be expressed instead in inverse powers of the signal to noise
ratios where the first order in the variance expansion is the CRLB. As an
application we compute the second order of the variance and bias for MLE of
physical parameters from the inspiral phase of binary mergers and for noises of
gravitational wave interferometers . We also compare the improved error
estimate with existing numerical estimates. The value of the second order of
the variance expansions allows to get error predictions closer to what is
observed in numerical simulations. It also predicts correctly the necessary SNR
to approximate the error with the CRLB and provides new insight on the
relationship between waveform properties SNR and estimation errors. For example
the timing match filtering becomes optimal only if the SNR is larger than the
kurtosis of the gravitational wave spectrum
Transverse electron-scale instability in relativistic shear flows
Electron-scale surface waves are shown to be unstable in the transverse plane
of a shear flow in an initially unmagnetized plasma, unlike in the
(magneto)hydrodynamics case. It is found that these unstable modes have a
higher growth rate than the closely related electron-scale Kelvin-Helmholtz
instability in relativistic shears. Multidimensional particle-in-cell
simulations verify the analytic results and further reveal the emergence of
mushroom-like electron density structures in the nonlinear phase of the
instability, similar to those observed in the Rayleigh Taylor instability
despite the great disparity in scales and different underlying physics.
Macroscopic () fields are shown to be generated by these
microscopic shear instabilities, which are relevant for particle acceleration,
radiation emission and to seed MHD processes at long time-scales
An optimal gap theorem
By solving the Cauchy problem for the Hodge-Laplace heat equation for
-closed, positive -forms, we prove an optimal gap theorem for
K\"ahler manifolds with nonnegative bisectional curvature which asserts that
the manifold is flat if the average of the scalar curvature over balls of
radius centered at any fixed point is a function of .
Furthermore via a relative monotonicity estimate we obtain a stronger
statement, namely a `positive mass' type result, asserting that if is
not flat, then for any
Off-Diagonal Long-Range Order in Bose Liquids: Irrotational Flow and Quantization of Circulation
On the basis of gauge invariance, it is proven in an elementary and
straightforward manner, but without invoking any {\it ad hoc} assumption, that
the existence of off-diagonal long-range order in one-particle reduced density
matrix in Bose liquids implies both the irrotational flow in a simply connected
region and the quantization of circulation in a multiply connected region, the
two fundamental properties of a Bose superfluid. The origin for both is the
phase coherence of condensate wave-functions. Some relevant issues are also
addressed.Comment: Revtex, 4 pages, no figure
Equilibrium Relativistic Mass Distribution for Indistinguishable Events
A manifestly covariant relativistic statistical mechanics of the system of
indistinguishable events with motion in space-time parametrized by an
invariant ``historical time'' is considered. The relativistic mass
distribution for such a system is obtained from the equilibrium solution of the
generalized relativistic Boltzmann equation by integration over angular and
hyperbolic angular variables. All the characteristic averages are calculated.
Expressions for the pressure and the density of events are found and the
relativistic equation of state is obtained. The Galilean limit is considered;
the theory is shown to pass over to the usual nonrelativistic statistical
mechanics of indistinguishable particles.Comment: TAUP-2115-9
Single Wall Nanotubes: Atomic Like Behaviour and Microscopic Approach
Recent experiments about the low temperature behaviour of a Single Wall
Carbon Nanotube (SWCNT) showed typical Coulomb Blockade (CB) peaks in the zero
bias conductance and allowed us to investigate the energy levels of interacting
electrons. Other experiments confirmed the theoretical prediction about the
crucial role which the long range nature of the Coulomb interaction plays in
the correlated electronic transport through a SWCNT with two intramolecular
tunneling barriers. In order to investigate the effects on low dimensional
electron systems due to the range of electron electron repulsion, we introduce
a model for the interaction which interpolates well between short and long
range regimes. Our results could be compared with experimental data obtained in
SWCNTs and with those obtained for an ideal vertical Quantum Dot (QD).
For a better understanding of some experimental results we also discuss how
defects and doping can break some symmetries of the bandstructure of a SWCNT.Comment: 8 pages, 4 figure
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