1,700 research outputs found
Quantum Phase Transitions in the U(5)-O(6) Large N limit
The U(5)-O(6) transitional behavior of the Interacting Boson Model in the
large N limit is revisited. Some low-lying energy levels, overlaps of the
ground state wavefunctions, B(E2) transition rate for the decay of the first
excited energy level to the ground state, and the order parameters are
calculated for different total numbers of bosons. The results show that
critical behaviors of these quantities are greatly enhanced with increasing of
the total number of bosons N, especially fractional occupation probability for
d bosons in the ground state, the difference between the expectation value of
n_d in the first excited 0^+ state and the ground state, and another quantity
related to the isomer shift behave similarly in both the O(6)-U(5) large N and
U(5)-SU(3) phase transitions.Comment: 7 Pages LaTeX, 3 figure
Nuclear liquid-gas phase transition within the lattice gas model
We study the nuclear liquid-gas phase transition on the basis of a
two-component lattice gas model. A Metropolis type of sampling method is used
to generate microscopic states in the canonical ensemble. The effective
equation of state and fragment mass distributions are evaluated in a wide range
of temperatures and densities. A definition of the phase coexistence region
appropriate for mesoscopic systems is proposed. The caloric curve resulting
from different types of freeze-out conditions are presented.Comment: 13 pages including 4 figure
Quantum Critical Behavior of Two Coupled Bose-Einstein Condensates
The quantum critical behavior of the Bose-Hubbard model for a description of
two coupled Bose-Einstein condensates is studied within the framework of an
algebraic theory. Energy levels, wavefunction overlaps with those of the Rabi
and Fock regimes, and the entanglement are calculated exactly as functions of
the phase parameter and the number of bosons. The results show that the system
goes though a phase transition and that the critical behavior is enhanced in
the thermodynamic limit.Comment: 6 pages, LaTex, 3 figure
An Investigation of Orientational Symmetry-Breaking Mechanisms in High Landau Levels
The principal axes of the recently discovered anisotropic phases of 2D
electron systems at high Landau level occupancy are consistently oriented
relative to the crystal axes of the host semiconductor. The nature of the
native rotational symmetry breaking field responsible for this preferential
orientation remains unknown. Here we report on experiments designed to
investigate the origin and magnitude of this symmetry breaking field. Our
results suggest that neither micron-scale surface roughness features nor the
precise symmetry of the quantum well potential confining the 2D system are
important factors. By combining tilted field transport measurements with
detailed self-consistent calculations we estimate that the native anisotropy
energy, whatever its origin, is typically ~ 1 mK per electron.Comment: Reference added, minor notational changes; final published versio
Anisotropic transport in unidirectional lateral superlattice around half-filling of the second Landau level
We have observed marked transport anisotropy in short period (a=92 nm)
unidirectional lateral superlattices around filling factors nu=5/2 and 7/2:
magnetoresistance shows a sharp peak for current along the modulation grating
while a dip appears for current across the grating. By altering the ratio a/l
(with l=sqrt{hbar/eB_perp} the magnetic length) via changing the electron
density n_e, it is shown that the nu=5/2 anisotropic features appear in the
range 6.6 alt a/l alt 7.2 varying their intensities, becoming most conspicuous
at a/l simeq 6.7. The peak/dip broadens with temperature roughly preserving its
height/depth up to 250 mK. Tilt experiments reveal that the structures are
slightly enhanced by an in-plane magnetic field B_| perpendicular to the
grating but are almost completely destroyed by B_| parallel to the grating. The
observations suggest the stabilization of a unidirectional charge-density-wave
or stripe phase by weak external periodic modulation at the second Landau
level.Comment: REVTeX, 5 pages, 3 figures, Some minor revisions, Added notes and
reference
Analysis of the dynamic changes in the soft palate and uvula in obstructive sleep apnea-hypopnea using ultrafast magnetic resonance imaging
Apnea and the respiratory cycle are dynamic processes in obstructive sleep apnea-hypopnea (OSAH), which occur only during sleep. Our study aimed to observe the dynamic changes in the soft palate and the uvula during wakefulness and sleep using ultrafast magnetic resonance imaging (UMRI) to provide reference data for the pathogenesis and treatment of OSAH. The dynamic changes in the soft palate and uvular tip of 15 male patients (average age: 50.43 ± 9.82 years) with OSAH were evaluated using UMRI of the upper airway while asleep and awake after 1 night of sleep deprivation. A series of midline sagittal images of the upper airway were obtained. The distance from the center of the soft palate to the x-axis (an extended line from the anterior nasal spine to the posterior nasal spine), from the uvular tip to the x-axis, from the center of the soft palate to the y-axis (a perpendicular line from the center of the pituitary to the x-axis), and from the uvular tip to the y-axis (designated as PX, UX, PY, and UY, respectively) were measured during sleep and wakefulness. The minimum PX, PY, UX, and UY were shorter during sleep than during wakefulness, whereas the maxima were longer during sleep (P < 0.01), the differences between the maximum and minimum PX, PY, UX, and UY were larger during sleep (P < 0.01). The upward, downward, forward, and backward ranges of movement of the soft palate and the uvular tip were larger during sleep in OSAH patients. This increased compliance may trigger each airway obstructive event
Dynamic alterations of the tongue in obstructive sleep apnea-hypopnea syndrome during sleep: analysis using ultrafast MRI
Patients with obstructive sleep apnea-hypopnea
syndrome (OSAHS) were evaluated using ultrafast magnetic
resonance imaging (UMRI) while asleep and awake to analyze
tongue changes. The upper airway of 21 OSAHS patients and 20
normal controls were examined during sleep using UMRI. A series of
midline sagittal images of the upper airway were obtained to measure
dynamic changes in tongue size and the distance from the tongue to
the x-axis (an extended line from the anterior nasal spine to posterior
nasal spine) and the y-axis (a perpendicular line from the center of
the pituitary to the x-axis). The maximum and minimum sagittal diameters of the tongue were shorter in the OSAHS group than in
the control group (P < 0.01) while awake, whereas the difference
between the maximum and minimum vertical diameters of the
tongue and the upper and central part of tongue between the posterior
border and the retropharyngeal wall were greater (P < 0.05). During
sleep, the maximum values and differences between the maximum
and minimum tongue sizes in the OSAHS group were larger than
in the control group (P < 0.05), whereas the minimum values were
lower than in the control group (P < 0.01). Tongue size significantly
differs between OSAHS patients and normal controls during sleep.
The tongue tends to move downward during OSAHS, which may be
attributed to increased upper airway resistance
Influence of the Coulomb Interaction on the Chemical Equilibrium of Nuclear Systems at Break-Up
The importance of a Coulomb correction to the formalism proposed by Albergo
et al. for determining the temperatures of nuclear systems at break-up and the
ensities of free nucleon gases is discussed. While the proposed correction has
no effect on the temperatures extracted based on double isotope ratios, it
becomes non-negligible when such temperatures or densities of free nucleon
gases are extracted based on multiplicities of heavier fragments of different
atomic numbers
Momentum--dependent nuclear mean fields and collective flow in heavy ion collisions
We use the Boltzmann-Uehling-Uhlenbeck model to simulate the dynamical
evolution of heavy ion collisions and to compare the effects of two
parametrizations of the momentum--dependent nuclear mean field that have
identical properties in cold nuclear matter. We compare with recent data on
nuclear flow, as characterized by transverse momentum distributions and flow
() variables for symmetric and asymmetric systems. We find that the precise
functional dependence of the nuclear mean field on the particle momentum is
important. With our approach, we also confirm that the difference between
symmetric and asymmetric systems can be used to pin down the density and
momentum dependence of the nuclear self consistent one--body potential,
independently. All the data can be reproduced very well with a
momentum--dependent interaction with compressibility K = 210 MeV.Comment: 15 pages in ReVTeX 3.0; 12 postscript figures uuencoded; McGill/94-1
New collective states of 2D electrons in high Landau levels
A brief summary of the emerging evidence for a new class of collective states
of two-dimensional electrons in partially occupied excited Landau levels is
presented. Among the most dramatic phenomena described are the large
anisotropies of the resistivity observed at very low temperatures near
half-filling of the third and higher Landau levels and the non-linear character
of the re-entrant integer quantized Hall states in the flanks of the same
levels. The degree to which these findings support recent theoretical
predictions of charge density wave ground states is discussed and a preliminary
comparison to recent transport theories is made.Comment: To be published in Physica E, as part of the proceedings of the 11th
International Winterschool on New Developments in Solid State Physics held in
Mauterndorf, Austria, February, 2000. 25 pages and 9 figures in a single pdf
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