5,285 research outputs found
Two-dimensional dilute Bose gas in the normal phase
We consider a two-dimensional dilute Bose gas above its superfluid transition temperature. We
show that the t-matrix approximation corresponds to the leading set of diagrams in the dilute limit,
provided the temperature is suciently larger than the superfluid transition temperature. Within this
approximation, we give an explicit expression for the wave-vector and frequency dependence of the selfenergy,
and calculate the corrections to the chemical potential and the eective mass arising from the
interaction. We also argue that the breakdown of diagrammatic classication scheme for the dilute Bose
gas, which occurs upon lowering the temperature, provides a criterion to estimate an upper bound for the
superfluid critical temperature. The upper bound to the critical temperature identied by this criterion
turns out to coincide with earlier results for the critical temperature obtained by Popov and by Fisher and
Hohenberg using dierent methods. Extension of this procedure to the three-dimensional case gives good
agreement with recent Monte Carlo data
Stability of Sarma phases in density imbalanced electron-hole bilayer systems
We study excitonic condensation in an electron-hole bilayer system with
unequal layer densities at zero temperature. Using mean-field theory we solve
the BCS gap equations numerically and investigate the effects of intra-layer
interactions. We analyze the stability of the Sarma phase with \bk,-\bk
pairing by calculating the superfluid mass density and also by checking the
compressibility matrix. We find that with bare Coulomb interactions the
superfluid density is always positive in the Sarma phase, due to a peculiar
momentum structure of the gap function originating from the singular behavior
of the Coulomb potential at zero momentum and the presence of a sharp Fermi
surface. Introducing a simple model for screening, we find that the superfluid
density becomes negative in some regions of the phase diagram, corresponding to
an instability towards a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) type superfluid
phase. Thus, intra-layer interaction and screening together can lead to a rich
phase diagram in the BCS-BEC crossover regime in electron-hole bilayer systems
Effects of density imbalance on the BCS-BEC crossover in semiconductor electron-hole bilayers
We study the occurrence of excitonic superfluidity in electron-hole bilayers
at zero temperature. We not only identify the crossover in the phase diagram
from the BCS limit of overlapping pairs to the BEC limit of non-overlapping
tightly-bound pairs but also, by varying the electron and hole densities
independently, we can analyze a number of phases that occur mainly in the
crossover region. With different electron and hole effective masses, the phase
diagram is asymmetric with respect to excess electron or hole densities. We
propose as the criterion for the onset of superfluidity, the jump of the
electron and hole chemical potentials when their densities cross.Comment: 4 pages, 3 figure
Synergistic sequential emission of fractional 10.600 and 1540 nm lasers for skin resurfacing. an ex vivo histological evaluation
Background: Fractional ablative and non-ablative lasers are useful treatments for skin rejuvenation. A procedure that provides the sequential application of fractional ablative followed by non-ablative laser treatment may reduce patients’ downtime and deliver better cosmetic results than with either laser alone. Objective: The purpose of the current study was to demonstrate the ameliorative and therapeutic effects in skin remodeling of the synergistic use of the two laser wavelengths (fractional ablative CO2 and non-ablative 1540 nm) with three different types of pulse shapes, S-Pulse (SP), D-Pulse (DP) and H-Pulse (HP), through which the CO2 laser can emit, performing an ex vivo histological evaluation. Methods: In this prospective study, ex vivo sheep inner thigh skin was chosen due to its similarity to human skin tissue, and a histological evaluation was performed. Three irradiation conditions, using all of the three CO2 pulse shapes (alone or averaged), were investigated: (1) 10.600 nm alone, the sequential irradiation of the two wavelengths in the same perfectly controlled energy pulses (DOT) for the entire scan area; ((2) 10.600 nm followed immediately by 1540 nm; and (3) 1540 nm followed immediately by 10.600 nm). Results: When comparing ablative to sequential irradiations, the synergy of the two wavelengths did not alter the typical ablative pulse shape of the 10.600 nm laser alone. With the same CO2 pulse shape, the lesion depth did not vary with the synergy of the two wavelengths, while thermal lesion width increased compared to CO2 alone. The ablation rate was achieved, while the total thermal lesion coverage in the scanning area of CO2 − 1540 lasers was greater than when using CO2 alone and then the other sequential irradiation. Conclusions: This study provides important preclinical data for new and early uses of the novel 10.600/1540 nm dual-wavelength non-ablative fractional laser. The synergy of the two wavelengths enhanced all the benefits already available when using CO2 laser systems both in terms of tone strengthening, thanks to a greater shrinking effect, and in terms of stimulation and collagen remodeling thanks to a greater volumetric thermal effect
Long-term low-dose dehydroepiandrosterone replacement therapy in aging males with partial androgen deficiency.
Dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulfate (DHEAS) age-related withdrawal is very likely to be involved in the aging process and the onset of age-related diseases, giving rise to the question of whether preventing or compensating the decline of these steroids may have endocrine and clinical benefits. The aim of the present trial was to evaluate the endocrine, neuroendocrine and clinical consequences of a long-term (1 year), low-dose (25 mg/day) replacement therapy in a group of aging men who presented the clinical characteristics of partial androgen deficiency (PADAM). Circulating DHEA, DHEAS, androstenedione, total testosterone and free testosterone, dihydrotestosterone (DHT), progesterone, 17-hydroxyprogesterone, allopregnanolone, estrone, estradiol, sex hormone binding globulin (SHBG), cortisol, follicle stimulating hormone (FSH), luteinizing hormone (LH), growth hormone (GH) and insulin-like growth factor 1 (IGF-1) levels were evaluated monthly to assess the endocrine effects of the therapy, while beta-endorphin values were used as a marker of the neuroendocrine effects. A Kupperman questionnaire was performed to evaluate the subjective symptoms before and after treatment. The results showed a great modification of the endocrine profile; with the exception of cortisol levels, which remained unchanged, DHEA, DHEAS, androstenedione, total and free testosterone, DHT, progesterone, 17-hydroxyprogesterone, estrone, estradiol, GH, IGF-1 and beta-endorphin levels increased significantly with respect to baseline values, while FSH, LH and SHBG levels showed a significant decrease. The Kupperman score indicated a progressive improvement in mood, fatigue and joint pain. In conclusion, the present study demonstrates that 25 mg/day of DHEA is able to cause significant changes in the hormonal profile and clinical symptoms and can counteract the age-related decline of endocrine and neuroendocrine functions. Restoring DHEA levels to young adult values seems to benefit the age-related decline in physiological functions but, however promising, placebo-controlled trials are required to confirm these preliminary results
Weakly Interacting, Dilute Bose Gases in 2D
This article surveys a number of theoretical problems and open questions in
the field of two-dimensional dilute Bose gases with weak repulsive
interactions. In contrast to three dimensions, in two dimensions the formation
of long-range order is prohibited by the Bogoliubov-Hohenberg theorem, and
Bose-Einstein condensation is not expected to be realized. Nevertheless, first
experimental indications supporting the formation of the condensate in low
dimensional systems have been recently obtained. This unexpected behaviour
appears to be due to the non-uniformity, introduced into a system by the
external trapping potential. Theoretical predictions, made for homogeneous
systems, require therefore careful reexamination.
We survey a number of popular theoretical treatments of the dilute weakly
interacting Bose gas and discuss their regions of applicability. The
possibility of Bose-Einstein condensation in a two-dimensional gas, the
validity of perturbative t-matrix approximation and diluteness condition are
issues that we discuss in detail.Comment: Survey, 25 pages RMP style, revised version, refs added, some changes
made, accepted for publication in Rev. Mod. Phy
Integer-spin Heisenberg Chains in a Staggered Magnetic Field. A Nonlinear -Model Approach
We present here a nonlinear sigma-model (NLM) study of a spin-1
antiferromagnetic Heisenberg chain in an external commensurate staggered
magnetic field. We find, already at the mean-field level, excellent agreement
with recent and very accurate Density Matrix Renormalization Group (DMRG)
studies, and that up to the highest values of the field for which a comparison
is possible, for the staggered magnetization and the transverse spin gap.
Qualitative but not quantitative agreement is found between the NLM
predictions for the longitudinal spin gap and the DMRG results. The origin of
the discrepancies is traced and discussed. Our results allow for extensions to
higher-spin chains that have not yet been studied numerically, and the
predictions for a spin-2 chain are presented and discussed. Comparison is also
made with previous theoretical approaches that led instead to predictions in
disagreement with the DMRG results.Comment: RevTex, 5 pages + 3 postscript figure
Selective organic functionalization of polycrystalline silicon-germanium for bioMEMS applications
AbstractWe selectively immobilized organofunctional silanes on top of polycrystalline silicon-germanium (poly-SiGe) layers, as a first step towards the fabrication of poly-SiGe-based bioMEMS (biomedical MicroElectroMechanicalSystems) by means of standard UV photolithography. 3-aminopropyl-dimethyl-ethoxysilane (APDMES) and 3-aminopropyl-triethoxysilane (APTES) molecules were immobilized onto resist-patterned poly-SiGe surfaces. The protocols for surface hydroxylation and silane immobilization were designed to be CMOS-compatible and to avoid damage to photoresist. Silanized surfaces were investigated both by means of fluorescence microscopy, and by FEG-SEM observation after labeling with 30 nm-diameter gold nanoparticles (NPs). We report the silanization protocols, together with the results indicating successful organic functionalization of the samples
A New Approach to Searching for Dark Matter Signals in Fermi-LAT Gamma Rays
Several cosmic ray experiments have measured excesses in electrons and
positrons, relative to standard backgrounds, for energies from ~ 10 GeV - 1
TeV. These excesses could be due to new astrophysical sources, but an
explanation in which the electrons and positrons are dark matter annihilation
or decay products is also consistent. Fortunately, the Fermi-LAT diffuse gamma
ray measurements can further test these models, since the electrons and
positrons produce gamma rays in their interactions in the interstellar medium.
Although the dark matter gamma ray signal consistent with the local electron
and positron measurements should be quite large, as we review, there are
substantial uncertainties in the modeling of diffuse backgrounds and,
additionally, experimental uncertainties that make it difficult to claim a dark
matter discovery. In this paper, we introduce an alternative method for
understanding the diffuse gamma ray spectrum in which we take the intensity
ratio in each energy bin of two different regions of the sky, thereby canceling
common systematic uncertainties. For many spectra, this ratio fits well to a
power law with a single break in energy. The two measured exponent indices are
a robust discriminant between candidate models, and we demonstrate that dark
matter annihilation scenarios can predict index values that require "extreme"
parameters for background-only explanations.Comment: v1: 11 pages, 7 figures, 1 table, revtex4; v2: 13 pages, 8 figures, 1
table, revtex4, Figure 4 added, minor additions made to text, references
added, conclusions unchanged, published versio
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