5,417 research outputs found
Noise generation in the solid Earth, oceans, and atmosphere, from non-linear interacting surface gravity waves in finite depth
Oceanic pressure measurements, even in very deep water, and atmospheric
pressure or seismic records, from anywhere on Earth, contain noise with
dominant periods between 3 and 10 seconds, that is believed to be excited by
ocean surface gravity waves. Most of this noise is explained by a nonlinear
wave-wave interaction mechanism, and takes the form of surface gravity waves,
acoustic or seismic waves. Previous theoretical works on seismic noise focused
on surface (Rayleigh) waves, and did not consider finite depth effects on the
generating wave kinematics. These finite depth effects are introduced here,
which requires the consideration of the direct wave-induced pressure at the
ocean bottom, a contribution previously overlooked in the context of seismic
noise. That contribution can lead to a considerable reduction of the seismic
noise source, which is particularly relevant for noise periods larger than 10
s. The theory is applied to acoustic waves in the atmosphere, extending
previous theories that were limited to vertical propagation only. Finally, the
noise generation theory is also extended beyond the domain of Rayleigh waves,
giving the first quantitative expression for sources of seismic body waves. In
the limit of slow phase speeds in the ocean wave forcing, the known and
well-verified gravity wave result is obtained, which was previously derived for
an incompressible ocean. The noise source of acoustic, acoustic-gravity and
seismic modes are given by a mode-specific amplification of the same
wave-induced pressure field near the zero wavenumber.Comment: Paper accepted for publication in the Journal of Fluid Mechanic
Heating rates for an atom in a far-detuned optical lattice
We calculate single atom heating rates in a far detuned optical lattice, in
connection with recent experiments. We first derive a master equation,
including a realistic atomic internal structure and a quantum treatment of the
atomic motion in the lattice. The experimental feature that optical lattices
are obtained by superimposing laser standing waves of different frequencies is
also included, which leads to a micromotional correction to the light shift
that we evaluate. We then calculate, and compare to experimental results, two
heating rates, the "total" heating rate (corresponding to the increase of the
total mechanical energy of the atom in the lattice), and the ground bande
heating rate (corresponding to the increase of energy within the ground energy
band of the lattice).Comment: 11 pages, 3 figures, 1 tabl
Resonant control of spin dynamics in ultracold quantum gases by microwave dressing
We study experimentally interaction-driven spin oscillations in optical
lattices in the presence of an off-resonant microwave field. We show that the
energy shift induced by this microwave field can be used to control the spin
oscillations by tuning the system either into resonance to achieve near-unity
contrast or far away from resonance to suppress the oscillations. Finally, we
propose a scheme based on this technique to create a flat sample with either
singly- or doubly-occupied sites, starting from an inhomogeneous Mott
insulator, where singly- and doubly-occupied sites coexist.Comment: 4 pages, 5 figure
Surface oxide on thin films of yttrium hydride studied by neutron reflectometry
The applicability of standard methods for compositional analysis is limited
for H-containing films. Neutron reflectometry is a powerful, non-destructive
method that is especially suitable for these systems due to the large negative
scattering length of H. In this work we demonstrate how neutron reflectometry
can be used to investigate thin films of yttrium hydride. Neutron reflectometry
gives a strong contrast between the film and the surface oxide layer, enabling
us to estimate the oxide thickness and oxygen penetration depths. A surface
oxide layer of 5-10 nm thickness was found for unprotected yttrium hydride
films
Statistics of eigenfunctions in open chaotic systems: a perturbative approach
We investigate the statistical properties of the complexness parameter which
characterizes uniquely complexness (biorthogonality) of resonance eigenstates
of open chaotic systems. Specifying to the regime of isolated resonances, we
apply the random matrix theory to the effective Hamiltonian formalism and
derive analytically the probability distribution of the complexness parameter
for two statistical ensembles describing the systems invariant under time
reversal. For those with rigid spectra, we consider a Hamiltonian characterized
by a picket-fence spectrum without spectral fluctuations. Then, in the more
realistic case of a Hamiltonian described by the Gaussian Orthogonal Ensemble,
we reveal and discuss the r\^ole of spectral fluctuations
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