863 research outputs found
Review and analysis of the DNW/Model 360 rotor acoustic data base
A comprehensive model rotor aeroacoustic data base was collected in a large anechoic wind tunnel in 1986. Twenty-six microphones were positioned around the azimuth to collect acoustic data for approximately 150 different test conditions. A dynamically scaled, blade-pressure-instrumented model of the forward rotor of the BH360 helicopter simultaneously provided blade pressures for correlation with the acoustic data. High-speed impulsive noise, blade-vortex interaction noise, low-frequency noise, and broadband noise were all captured in this extensive data base. Trends are presentes for each noise source, with important parametric variations. The purpose of this paper is to introduce this data base and illustrate its potential for predictive code validation
Realizing time crystals in discrete quantum few-body systems
The exotic phenomenon of time translation symmetry breaking under periodic
driving - the time crystal - has been shown to occur in many-body systems even
in clean setups where disorder is absent. In this work, we propose the
realization of time-crystals in few-body systems, both in the context of
trapped cold atoms with strong interactions and of a circuit of superconducting
qubits. We show how these two models can be treated in a fairly similar way by
adopting an effective spin chain description, to which we apply a simple
driving protocol. We focus on the response of the magnetization in the presence
of imperfect pulses and interactions, and show how the results can be
interpreted, in the cold atomic case, in the context of experiments with
trapped bosons and fermions. Furthermore, we provide a set of realistic
parameters for the implementation of the superconducting circuit.Comment: 6 pages, 4 figure
Bound States and Universality in Layers of Cold Polar Molecules
The recent experimental realization of cold polar molecules in the rotational
and vibrational ground state opens the door to the study of a wealth of
phenomena involving long-range interactions. By applying an optical lattice to
a gas of cold polar molecules one can create a layered system of planar traps.
Due to the long-range dipole-dipole interaction one expects a rich structure of
bound complexes in this geometry. We study the bilayer case and determine the
two-body bound state properties as a function of the interaction strength. The
results clearly show that a least one bound state will always be present in the
system. In addition, bound states at zero energy show universal behavior and
extend to very large radii. These results suggest that non-trivial bound
complexes of more than two particles are likely in the bilayer and in more
complicated chain structures in multi-layer systems.Comment: 6 pages, 5 figures. Revised version to be publishe
Bound states of Dipolar Bosons in One-dimensional Systems
We consider one-dimensional tubes containing bosonic polar molecules. The
long-range dipole-dipole interactions act both within a single tube and between
different tubes. We consider arbitrary values of the externally aligned dipole
moments with respect to the symmetry axis of the tubes. The few-body structures
in this geometry are determined as function of polarization angles and dipole
strength by using both essentially exact stochastic variational methods and the
harmonic approximation. The main focus is on the three, four, and five-body
problems in two or more tubes. Our results indicate that in the weakly-coupled
limit the inter-tube interaction is similar to a zero-range term with a
suitable rescaled strength. This allows us to address the corresponding
many-body physics of the system by constructing a model where bound chains with
one molecule in each tube are the effective degrees of freedom. This model can
be mapped onto one-dimensional Hamiltonians for which exact solutions are
known.Comment: 22 pages, 7 figures, revised versio
Weakly bound states of polar molecules in bilayers
We investigate a system of two polarized molecules in a layered trap. The
molecules reside in adjacent layers and interact purely via the dipole-dipole
interaction. We determine the properties of the ground state of the system as a
function of the dipole moment and polarization angle. A bound state is always
present in the system and in the weak binding limit the bound state extends to
a very large distance and shows universal behavior.Comment: Presented at the 21st European Conference on Few-Body Problems in
Physics, Salamanca, Spain, 30 August - 3 September 201
Generation of spin currents by a temperature gradient in a two-terminal device
Theoretical and experimental studies of the interaction between spins and
temperature are vital for the development of spin caloritronics, as they
dictate the design of future devices. In this work, we propose a two-terminal
cold-atom simulator to study that interaction. The proposed quantum simulator
consists of strongly interacting atoms that occupy two temperature reservoirs
connected by a one-dimensional link. First, we argue that the dynamics in the
link can be described using an inhomogeneous Heisenberg spin chain whose
couplings are defined by the local temperature. Second, we show the existence
of a spin current in a system with a temperature difference by studying the
dynamics that follows the spin-flip of an atom in the link. A temperature
gradient accelerates the impurity in one direction more than in the other,
leading to an overall spin current similar to the spin Seebeck effect.Comment: 33 pages, 6 figure
R-process nucleosynthesis calculations with complete nuclear physics input
The r-process constitutes one of the major challenges in nuclear
astrophysics. Its astrophysical site has not yet been identified but there is
observational evidence suggesting that at least two possible sites should
contribute to the solar system abundance of r-process elements and that the
r-process responsible for the production of elements heavier than Z=56 operates
quite robustly producing always the same relative abundances. From the
nuclear-physics point of view the r-process requires the knowledge of a large
number of reaction rates involving exotic nuclei. These include neutron capture
rates, beta-decays and fission rates, the latter for the heavier nuclei
produced in the r-process. We have developed for the first time a complete
database of reaction rates that in addition to neutron-capture rates and
beta-decay half-lives includes all possible reactions that can induce fission
(neutron-capture, beta-decay and spontaneous fission) and the corresponding
fission yields. In addition, we have implemented these reaction rates in a
fully implicit reaction network. We have performed r-process calculations for
the neutrino-driven wind scenario to explore whether or not fission can
contribute to provide a robust r-process pattern
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