5,690 research outputs found
Zero Temperature Thermodynamics of Asymmetric Fermi Gases at Unitarity
The equation of state of a dilute two-component asymmetric Fermi gas at
unitarity is subject to strong constraints, which affect the spatial density
profiles in atomic traps. These constraints require the existence of at least
one non-trivial partially polarized (asymmetric) phase. We determine the
relation between the structure of the spatial density profiles and the T=0
equation of state, based on the most accurate theoretical predictions
available. We also show how the equation of state can be determined from
experimental observations.Comment: 10 pages and 7 figures. (Minor changes to correspond with published
version.
Electron-hole pair condensation at the semimetal-semiconductor transition: a BCS-BEC crossover scenario
We act on the suggestion that an excitonic insulator state might
separate---at very low temperatures---a semimetal from a semiconductor and ask
for the nature of these transitions. Based on the analysis of electron-hole
pairing in the extended Falicov-Kimball model, we show that tuning the Coulomb
attraction between both species, a continuous crossover between a BCS-like
transition of Cooper-type pairs and a Bose-Einstein condensation of preformed
tightly-bound excitons might be achieved in a solid-state system. The precursor
of this crossover in the normal state might cause the transport anomalies
observed in several strongly correlated mixed-valence compounds.Comment: 5 pages, 5 figures, substantially revised versio
Potential-energy (BCS) to kinetic-energy (BEC)-driven pairing in the attractive Hubbard model
The BCS-BEC crossover within the two-dimensional attractive Hubbard model is
studied by using the Cellular Dynamical Mean-Field Theory both in the normal
and superconducting ground states. Short-range spatial correlations
incorporated in this theory remove the normal-state quasiparticle peak and the
first-order transition found in the Dynamical Mean-Field Theory, rendering the
normal state crossover smooth. For smaller than the bandwidth, pairing is
driven by the potential energy, while in the opposite case it is driven by the
kinetic energy, resembling a recent optical conductivity experiment in
cuprates. Phase coherence leads to the appearance of a collective Bogoliubov
mode in the density-density correlation function and to the sharpening of the
spectral function.Comment: 5 pages, 4 figure
Superconductor-insulator transition in Coulomb disorder
Superconductor-insulator transition driven by the decreasing concentration of
electrons is studied in the case of the disorder potential created by
randomly positioned charged impurities. Electrons and Cooper pairs (formed by
an non-Coulomb attraction) nonlinearly screen the random potential of
impurities. Both electrons and Cooper pairs can be delocalized or localized in
the resulting self-consistent potential. The border separating the
superconductor and insulator phases in the plane of the concentration of
electrons and the length of the Cooper pair is found. For a strong disorder the
central segment of this border follows the BEC-BCS crossover line defined for a
clean sample.Comment: 4.5 pages, introduction rewritten, a dozen of references added, 2D
case adde
Macroscopic entanglement of many-magnon states
We study macroscopic entanglement of various pure states of a one-dimensional
N-spin system with N>>1. Here, a quantum state is said to be macroscopically
entangled if it is a superposition of macroscopically distinct states. To judge
whether such superposition is hidden in a general state, we use an essentially
unique index p: A pure state is macroscopically entangled if p=2, whereas it
may be entangled but not macroscopically if p<2. This index is directly related
to the stability of the state. We calculate the index p for various states in
which magnons are excited with various densities and wavenumbers. We find
macroscopically entangled states (p=2) as well as states with p=1. The former
states are unstable in the sense that they are unstable against some local
measurements. On the other hand, the latter states are stable in the senses
that they are stable against local measurements and that their decoherence
rates never exceed O(N) in any weak classical noises. For comparison, we also
calculate the von Neumann entropy S(N) of a subsystem composed of N/2 spins as
a measure of bipartite entanglement. We find that S(N) of some states with p=1
is of the same order of magnitude as the maximum value N/2. On the other hand,
S(N) of the macroscopically entangled states with p=2 is as small as O(log N)<<
N/2. Therefore, larger S(N) does not mean more instability. We also point out
that these results are analogous to those for interacting many bosons.
Furthermore, the origin of the huge entanglement, as measured either by p or
S(N), is discussed to be due to the spatial propagation of magnons.Comment: 30 pages, 5 figures. The manuscript has been shortened and typos have
been fixed. Data points of figures have been made larger in order to make
them clearly visibl
One-dimensional superfluid Bose-Fermi mixture: mixing, demixing and bright solitons
We study a ultra-cold and dilute superfluid Bose-Fermi mixture confined in a
strictly one-dimensional atomic waveguide by using a set of coupled nonlinear
mean-field equations obtained from the Lieb-Liniger energy density for bosons
and the Gaudin-Yang energy density for fermions. We consider a finite
Bose-Fermi inter-atomic strength g_{bf} and both periodic and open boundary
conditions. We find that with periodic boundary conditions, i.e. in a quasi-1D
ring, a uniform Bose-Fermi mixture is stable only with a large fermionic
density. We predict that at small fermionic densities the ground state of the
system displays demixing if g_{bf}>0 and may become a localized Bose-Fermi
bright soliton for g_{bf}<0. Finally, we show, using variational and numerical
solution of the mean-field equations, that with open boundary conditions, i.e.
in a quasi-1D cylinder, the Bose-Fermi bright soliton is the unique ground
state of the system with a finite number of particles, which could exhibit a
partial mixing-demixing transition. In this case the bright solitons are
demonstrated to be dynamically stable. The experimental realization of these
Bose-Fermi bright solitons seems possible with present setups.Comment: 11 pages, 11 figure
Detection of Macroscopic Entanglement by Correlation of Local Observables
We propose a correlation of local observables on many sites in macroscopic
quantum systems. By measuring the correlation one can detect, if any,
superposition of macroscopically distinct states, which we call macroscopic
entanglement, in arbitrary quantum states that are (effectively) homogeneous.
Using this property, we also propose an index of macroscopic entanglement.Comment: Although the index q was proposed for mixed states, it is also
applicable to pure states, on which we fix minor bugs (that will be reported
in PRL as erratum). The conclusions of the paper remain unchanged. (4 pages,
no figures.
Evidence for Quantized Displacement in Macroscopic Nanomechanical Oscillators
We report the observation of discrete displacement of nanomechanical
oscillators with gigahertz-range resonance frequencies at millikelvin
temperatures. The oscillators are nanomachined single-crystal structures of
silicon, designed to provide two distinct sets of coupled elements with very
low and very high frequencies. With this novel design, femtometer-level
displacement of the frequency-determining element is amplified into collective
motion of the entire micron-sized structure. The observed discrete response
possibly results from energy quantization at the onset of the quantum regime in
these macroscopic nanomechanical oscillators.Comment: 4 pages, two-column format. Related papers available at
http://nano.bu.edu
Adiabatic Landau-Zener-St\"uckelberg transition with or without dissipation in low spin molecular system V15
The spin one half molecular system V15 shows no barrier against spin
reversal. This makes possible direct phonon activation between the two levels.
By tuning the field sweeping rate and the thermal coupling between sample and
thermal reservoir we have control over the phonon-bottleneck phenomena
previously reported in this system. We demonstrate adiabatic motion of molecule
spins in time dependent magnetic fields and with different thermal coupling to
the cryostat bath. We also discuss the origin of the zero-field tunneling
splitting for a half-integer spin.Comment: to appear in Phys. Rev. B - Rapid Communication
Aircraft digital flight control technical review
The Aircraft Digital Flight Control Technical Review was initiated by two pilot induced oscillation (PIO) incidents in the spring and summer of 1992. Maj. Gen. Franklin (PEO) wondered why the Air Force development process for digital flight control systems was not preventing PIO problems. Consequently, a technical review team was formed to examine the development process and determine why PIO problems continued to occur. The team was also to identify the 'best practices' used in the various programs. The charter of the team was to focus on the PIO problem, assess the current development process, and document the 'best practices.' The team reviewed all major USAF aircraft programs with digital flight controls, specifically, the F-15E, F-16C/D, F-22, F-111, C-17, and B-2. The team interviewed contractor, System Program Office (SPO), and Combined Test Force (CTF) personnel on these programs. The team also went to NAS Patuxent River to interview USN personnel about the F/A-18 program. The team also reviewed experimental USAF and NASA systems with digital flight control systems: X-29, X-31, F-15 STOL and Maneuver Technology Demonstrator (SMTD), and the Variable In-Flight Stability Test Aircraft (VISTA). The team also discussed the problem with other experts in the field including Ralph Smith and personnel from Calspan. The major conclusions and recommendations from the review are presented
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