286 research outputs found
Pulsed laser deposition of SrTiO3/LaGaO3 and SrTiO3/LaAlO3: plasma plume effects
Pulsed laser deposition of SrTiO3/LaGaO3 and SrTiO3/LaAlO3 interfaces has
been analyzed with a focus on the kinetic energy of the ablated species. LaGaO3
and LaAlO3 plasma plumes were studied by fast photography and space-resolved
optical emission spectroscopy. Reflection high energy electron diffraction was
performed proving a layer-by-layer growth up to 10-1 mbar oxygen pressure. The
role of the energetic plasma plume on the two-dimensional growth and the
presence of interfacial defects at different oxygen growth pressure has been
discussed in view of the conducting properties developing at such
polar/non-polar interfaces
Zero sound in a single component fermion - Bose Einstein Condensate mixture
The resonant dynamics of mediated interactions supports zero-sound in a cold
atom degenerate mixture of a single component fermion gas and a Bose-Einstein
condensate (BEC). We characterize the onset of instability in the phase
separation of an unstable mixture and we find a rich collective mode structure
for stable mixtures with one undamped mode that exhibits an avoided crossing
and a Landau-damped mode that terminates.Comment: 4 pages, 2 figure
Subextensive singularity in the 2D Ising spin glass
The statistics of low energy states of the 2D Ising spin glass with +1 and -1
bonds are studied for square lattices with , and =
0.5, where is the fraction of negative bonds, using periodic and/or
antiperiodic boundary conditions. The behavior of the density of states near
the ground state energy is analyzed as a function of , in order to obtain
the low temperature behavior of the model. For large finite there is a
range of in which the heat capacity is proportional to .
The range of in which this behavior occurs scales slowly to as
increases. Similar results are found for = 0.25. Our results indicate that
this model probably obeys the ordinary hyperscaling relation , even though . The existence of the subextensive behavior is
attributed to long-range correlations between zero-energy domain walls, and
evidence of such correlations is presented.Comment: 13 pages, 7 figures; final version, to appear in J. Stat. Phy
Computing the Loewner driving process of random curves in the half plane
We simulate several models of random curves in the half plane and numerically
compute their stochastic driving process (as given by the Loewner equation).
Our models include models whose scaling limit is the Schramm-Loewner evolution
(SLE) and models for which it is not. We study several tests of whether the
driving process is Brownian motion. We find that just testing the normality of
the process at a fixed time is not effective at determining if the process is
Brownian motion. Tests that involve the independence of the increments of
Brownian motion are much more effective. We also study the zipper algorithm for
numerically computing the driving function of a simple curve. We give an
implementation of this algorithm which runs in a time O(N^1.35) rather than the
usual O(N^2), where N is the number of points on the curve.Comment: 20 pages, 4 figures. Changes to second version: added new paragraph
to conclusion section; improved figures cosmeticall
Growth methods of c-axis oriented MgB2 thin films by pulsed laser deposition
High quality MgB2 thin films have been obtained by pulsed laser deposition
both on MgO and on Al2O3 substrates using different methods. In the standard
two-step procedure, an amorphous precursor layer is deposited at room
temperature starting both from stoichiometric target and from boron target:
after this first step, it is annealed in magnesium atmosphere in order to
crystallize the superconducting phase. The so obtained films show a strong
c-axis orientation, evidenced by XRD analysis, a critical temperature up to 38
K and very high critical fields along the basal planes, up to 22T at 15K. Also
an in situ one step technique for the realization of superconducting MgB2 thin
films has been developed. In this case, the presence of an argon buffer gas
during deposition is crucial and we observe a strong dependence of the quality
of the deposited film on the background gas pressure. The influence of the Ar
atmosphere has been confirmed by time and space-resolved spectroscopy
measurements on the emission spectrum of the plume. The Ar pressure modifies
strongly the plasma kinetics by promoting excitation and ionization of the
plume species, especially of the most volatile Mg atoms, increasing their
internal energy.Comment: Paper presented at Boromag Workshop, Genoa 17-19 June 2002, in press
on SUS
Collective oscillations of an interacting trapped Fermi gas
We calculate the effects of two-body interactions on the low frequency
oscillations of a normal Fermi gas confined in a harmonic trap. The mean field
contribution to the collective frequencies is evaluated in the collisionless
regime using a sum rule approach. We also discuss the transition between the
collisionless and hydrodynamic regime with special emphasis to the spin dipole
mode in which two atomic clouds occupying different spin states oscillate in
opposite phase. The spin dipole mode is predicted to be overdamped in the
hydrodynamic regime. The relaxation time is calculated as a function of
temperature and the effects of Fermi statistics are explicitly pointed out.Comment: 4 pages, 1 figure include
Limits to Sympathetic Evaporative Cooling of a Two-Component Fermi Gas
We find a limit cycle in a quasi-equilibrium model of evaporative cooling of
a two-component fermion gas. The existence of such a limit cycle represents an
obstruction to reaching the quantum ground state evaporatively. We show that
evaporatively the \beta\mu ~ 1. We speculate that one may be able to cool an
atomic fermi gas further by photoassociating dimers near the bottom of the
fermi sea.Comment: Submitted to Phys. Rev
Collective excitations of degenerate Fermi gases in anisotropic parabolic traps
The hydrodynamic low-frequency oscillations of highly degenerate Fermi gases
trapped in anisotropic harmonic potentials are investigated. Despite the lack
of an obvious spatial symmetry the wave-equation turns out to be separable in
elliptical coordinates, similar to a corresponding result established earlier
for Bose-condensates. This result is used to give the analytical solution of
the anisotropic wave equation for the hydrodynamic modes.Comment: 11 pages, Revte
Finite-Size Scaling of the Domain Wall Entropy Distributions for the 2D Ising Spin Glass
The statistics of domain walls for ground states of the 2D Ising spin glass
with +1 and -1 bonds are studied for square lattices with , and = 0.5, where is the fraction of negative bonds, using periodic
and/or antiperiodic boundary conditions. When is even, almost all domain
walls have energy = 0 or 4. When is odd, most domain walls have
= 2. The probability distribution of the entropy, , is found
to depend strongly on . When , the probability distribution
of is approximately exponential. The variance of this distribution
is proportional to , in agreement with the results of Saul and Kardar. For
the distribution of is not symmetric about zero. In
these cases the variance still appears to be linear in , but the average of
grows faster than . This suggests a one-parameter scaling
form for the -dependence of the distributions of for .Comment: 13 page
Measurement of the two-photon absorption cross-section of liquid argon with a time projection chamber
This paper reports on laser-induced multiphoton ionization at 266 nm of
liquid argon in a time projection chamber (LAr TPC) detector. The electron
signal produced by the laser beam is a formidable tool for the calibration and
monitoring of next-generation large-mass LAr TPCs. The detector that we
designed and tested allowed us to measure the two-photon absorption
cross-section of LAr with unprecedented accuracy and precision:
sigma_ex=(1.24\pm 0.10stat \pm 0.30syst) 10^{-56} cm^4s{-1}.Comment: 15 pages, 9 figure
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