2,056 research outputs found
A new look at Mourre's commutator theory
Mourre's commutator theory is a powerful tool to study the continuous
spectrum of self-adjoint operators and to develop scattering theory. We propose
a new approach of its main result, namely the derivation of the limiting
absorption principle from a so called Mourre estimate. We provide a new
interpretation of this result
Effects of Long Range Electronic Interactions on a One-dimensional Electron System
The effects of a long range electronic potential on a one dimensional chain
of spinless fermions are investigated by numerical techniques (Exact
Diagonalisation of rings with up to 30 sites complemented by finite size
analysis) and analytic calculations. Due to a competition between the
oscillations in the density and the (very slow) log divergence of the long
wavelength part of the Coulomb potential, the metallic character of the system
is enhanced at intermediate (and up to quite large) lengthscales. Despite some
similarities, we found that this quasi-metallic regime is not of the Luttinger
Liquid type, as evidenced by strong deviations from predictions of conformal
field theory and in agreement with the picture of a (very weakly pinned) Wigner
crystal. When the strength of the Coulomb interaction is substancially
increased, we observe a smooth cross-over to a strongly localized charge
density wave.Comment: Latex, Postscripts files include
Experimental study of parametric subharmonic instability for internal waves
Internal waves are believed to be of primary importance as they affect ocean
mixing and energy transport. Several processes can lead to the breaking of
internal waves and they usually involve non linear interactions between waves.
In this work, we study experimentally the parametric subharmonic instability
(PSI), which provides an efficient mechanism to transfer energy from large to
smaller scales. It corresponds to the destabilization of a primary plane wave
and the spontaneous emission of two secondary waves, of lower frequencies and
different wave vectors. Using a time-frequency analysis, we observe the time
evolution of the secondary waves, thus measuring the growth rate of the
instability. In addition, a Hilbert transform method allows the measurement of
the different wave vectors. We compare these measurements with theoretical
predictions, and study the dependence of the instability with primary wave
frequency and amplitude, revealing a possible effect of the confinement due to
the finite size of the beam, on the selection of the unstable mode
Realizing quantum Ising models in tunable two-dimensional arrays of single Rydberg atoms
Spin models are the prime example of simplified manybody Hamiltonians used to
model complex, real-world strongly correlated materials. However, despite their
simplified character, their dynamics often cannot be simulated exactly on
classical computers as soon as the number of particles exceeds a few tens. For
this reason, the quantum simulation of spin Hamiltonians using the tools of
atomic and molecular physics has become very active over the last years, using
ultracold atoms or molecules in optical lattices, or trapped ions. All of these
approaches have their own assets, but also limitations. Here, we report on a
novel platform for the study of spin systems, using individual atoms trapped in
two-dimensional arrays of optical microtraps with arbitrary geometries, where
filling fractions range from 60 to 100% with exact knowledge of the initial
configuration. When excited to Rydberg D-states, the atoms undergo strong
interactions whose anisotropic character opens exciting prospects for
simulating exotic matter. We illustrate the versatility of our system by
studying the dynamics of an Ising-like spin-1/2 system in a transverse field
with up to thirty spins, for a variety of geometries in one and two dimensions,
and for a wide range of interaction strengths. For geometries where the
anisotropy is expected to have small effects we find an excellent agreement
with ab-initio simulations of the spin-1/2 system, while for strongly
anisotropic situations the multilevel structure of the D-states has a
measurable influence. Our findings establish arrays of single Rydberg atoms as
a versatile platform for the study of quantum magnetism.Comment: This is the version of the manuscript as initially submitted to
Natur
High quality factor nitride-based optical cavities: microdisks with embedded GaN/Al(Ga)N quantum dots
We compare the quality factor values of the whispery gallery modes of
microdisks incorporating GaN quantum dots (QDs) grown on AlN and AlGaN barriers
by performing room temperature photoluminescence (PL) spectroscopy. The PL
measurements show a large number of high Q factor (Q) resonant modes on the
whole spectrum which allows us to identify the different radial mode families
and to compare them with simulations. We report a considerable improvement of
the Q factor which reflect the etching quality and the relatively low cavity
loss by inserting QDs into the cavity. GaN/AlN QDs based microdisks show very
high Q values (Q > 7000) whereas the Q factor is only up to 2000 in microdisks
embedding QDs grown on AlGaN barrier layer. We attribute this difference to the
lower absorption below bandgap for AlN barrier layers at the energies of our
experimental investigation
Experimental observation of a strong mean flow induced by internal gravity waves
We report the experimental observation of a robust horizontal mean flow
induced by internal gravity waves. A wave beam is forced at the lateral
boundary of a tank filled with a linearly stratified fluid initially at rest.
After a transient regime, a strong jet appears in the wave beam, with
horizontal recirculations outside the wave beam. We present a simple physical
mechanism predicting the growth rate of the mean flow and its initial spatial
structure. We find good agreement with experimental results
Measurement of the Angular Dependence of the Dipole-Dipole Interaction Between Two Individual Rydberg Atoms at a F\"orster Resonance
We measure the angular dependence of the resonant dipole-dipole interaction
between two individual Rydberg atoms with controlled relative positions. By
applying a combination of static electric and magnetic fields on the atoms, we
demonstrate the possibility to isolate a single interaction channel at a
F\"orster resonance, that shows a well-defined angular dependence. We first
identify spectroscopically the F\"orster resonance of choice and we then
perform a direct measurement of the interaction strength between the two atoms
as a function of the angle between the internuclear axis and the quantization
axis. Our results show good agreement with the expected angular dependence
, and represent an important step towards quantum
state engineering in two-dimensional arrays of individual Rydberg atoms.Comment: 5 pages, 4 figure
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