Dynamic Density Response of Trapped Interacting Quantum Gases

An expression for the dynamic density response function has been obtained for an interacting quantum gas in Random Phase Approximation (RPA) including first order self and exchange contribution. It involves the single particle wave functions and eigen values. The expression simplifies when diagonal elements are considered. The diagonal elements of the imaginary part of Fourier transformed response function is relevant in the measurement of Bragg scattering cross-section and in several other applications.Comment: 2 pages, 0 figure, conferenc

Dynamics of Uniform Quantum Gases, I: Density and Current Correlations

A unified approach valid for any wavenumber, frequency, and temperature is presented for uniform ideal quantum gases allowing for a comprehensive study of number density and particle-current density response functions. Exact analytical expressions are obtained for spectral functions in terms of polylogarithms. Also, particle-number and particle-current static susceptibilities are presented which, for fugacity less than unity, additionally involve Kummer functions. The wavenumber and temperature dependent transverse-current static susceptibility is used to show explicitly that current correlations are of a long range in a Bose-condensed uniform ideal gas but for bosons above the critical temperature and for Fermi and Boltzmann gases at all temperatures these correlations are of short range. Contact repulsive interactions for systems of neutral quantum particles are considered within the random-phase approximation. The expressions for particle-number and transverse-current susceptibilities are utilized to discuss the existence or nonexistence of superfluidity in the systems under consideration

Analytical pair correlations in ideal quantum gases: Temperature-dependent bunching and antibunching

The fluctuation-dissipation theorem together with the exact density response spectrum for ideal quantum gases has been utilized to yield a new expression for the static structure factor, which we use to derive exact analytical expressions for the temperature{dependent pair distribution function g(r) of the ideal gases. The plots of bosonic and fermionic g(r) display "Bose pile" and "Fermi hole" typically akin to bunching and antibunching as observed experimentally for ultracold atomic gases. The behavior of spin-scaled pair correlation for fermions is almost featureless but bosons show a rich structure including long-range correlations near T_c. The coherent state at T=0 shows no correlation at all, just like single-mode lasers. The depicted decreasing trend in correlation with decrease in temperature for T < T_c should be observable in accurate experiments.Comment: 8 pages, 1 figure, minor revisio

Density excitations of a harmonically trapped ideal gas

The dynamic structure factor of a harmonically trapped Bose gas has been calculated well above the Bose-Einstein condensation temperature by treating the gas cloud as a canonical ensemble of noninteracting classical particles. The static structure factor is found to vanish as wavenumber squared in the long-wavelength limit. We also incorporate a relaxation mechanism phenomenologically by including a stochastic friction force to study the dynamic structure factor. A significant temperature dependence of the density-fluctuation spectra is found. The Debye-Waller factor has been calculated for the trapped thermal cloud as function of wavenumber and of particle number. A substantial difference is found between clouds of small and large particle number

Density Fluctuations in Uniform Quantum Gases

Analytical expressions are given for the static structure factor S(k) and the pair correlation function g(r) for uniform ideal Bose-Einstein and Fermi-Dirac gases for all temperatures. In the vicinity of Bose Einstein condensation (BEC) temperature, g(r) becomes long ranged and remains so in the condensed phase. In the dilute gas limit, g(r) of bosons & fermions do not coincide with Maxwell-Boltzmann gas but exhibit bunching & anti-bunching effect respectively. The width of these functions depends on the temperature and is scaled as \surd (inverse atomic mass). Our numerical results provide the precise quantitative values of suppression/increase (antibunching and bunching) of the density fluctuations at small distances in ideal quantum gases in qualitative agreement with the experimental observation for almost non-trapped dilute gases.Comment: 4 pages,8 figures,conferenc

Dynamic stability control in younger and older adults during stair descent.

The purpose of this study was to examine dynamic stability control in older and younger adults while descending stairs. Thirteen older (aged 64-77years) and 13 younger (aged 22-29years) adults descended a staircase at their preferred speed. A motion capture system and three force plates were used to determine locomotion mechanics. Dynamic stability was investigated by using the margin of stability, calculated as the instantaneous difference between anterior boundary of the base of support and extrapolated centre of mass. At the initiation of the single support phase, older adults demonstrated a more negative (p<.05) margin of stability value. The component responsible for the lower margin of stability in the elderly was the higher velocity of the centre of mass (p<.05). Before the initiation of the single support phase, the older adults showed a lower (p<.05) ankle and knee joint angular impulse compared to the younger ones. We found a significant correlation (r=.729, p<.05) between centre of mass velocity and joint angular impulse. These results indicate that older adults are at greater risk of falls while descending stairs potentially due to a reduced ability to generate adequate leg-extensor muscular output to safely control the motion of the body's centre of mass while stepping down

EEG-based classification of video quality perception using steady state visual evoked potentials (SSVEPs)

Objective. Recent studies exploit the neural signal recorded via electroencephalography (EEG) to get a more objective measurement of perceived video quality. Most of these studies capitalize on the event-related potential component P3. We follow an alternative approach to the measurement problem investigating steady state visual evoked potentials (SSVEPs) as EEG correlates of quality changes. Unlike the P3, SSVEPs are directly linked to the sensory processing of the stimuli and do not require long experimental sessions to get a sufficient signal-to-noise ratio. Furthermore, we investigate the correlation of the EEG-based measures with the outcome of the standard behavioral assessment. Approach. As stimulus material, we used six gray-level natural images in six levels of degradation that were created by coding the images with the HM10.0 test model of the high efficiency video coding (H.265/MPEG-HEVC) using six different compression rates. The degraded images were presented in rapid alternation with the original images. In this setting, the presence of SSVEPs is a neural marker that objectively indicates the neural processing of the quality changes that are induced by the video coding. We tested two different machine learning methods to classify such potentials based on the modulation of the brain rhythm and on time-locked components, respectively. Main results. Results show high accuracies in classification of the neural signal over the threshold of the perception of the quality changes. Accuracies significantly correlate with the mean opinion scores given by the participants in the standardized degradation category rating quality assessment of the same group of images. Significance. The results show that neural assessment of video quality based on SSVEPs is a viable complement of the behavioral one and a significantly fast alternative to methods based on the P3 component.BMBF, 01GQ0850, Bernstein Fokus Neurotechnologie - Nichtinvasive Neurotechnologie für Mensch-Maschine Interaktio

Psychophysiology-based QoE assessment : a survey

We present a survey of psychophysiology-based assessment for quality of experience (QoE) in advanced multimedia technologies. We provide a classification of methods relevant to QoE and describe related psychological processes, experimental design considerations, and signal analysis techniques. We summarize multimodal techniques and discuss several important aspects of psychophysiology-based QoE assessment, including the synergies with psychophysical assessment and the need for standardized experimental design. This survey is not considered to be exhaustive but serves as a guideline for those interested to further explore this emerging field of research

Virtual reality for children with special needs

While virtual reality technology is already widely used in business and culture for immersion in new worlds of experience, virtual reality (VR) in the field of special and inclusive education is still not widespread. Students with intellectual and developmental disabilities (IDD) are often externally determined to various degrees in their lives. To cope with everyday life independently, practicing action skills is necessary. In a real-world physical environment, this is not always easy. Virtual reality offers a possibility to acquire skills without restrictive conditions.Within the project “Virtual Reality for Children with Special Needs” the potentials of virtual reality for students with IDD are explored. This research and development (R&D) project is a collaborative effort of the University of Applied Sciences (ZHAW Winterthur), the University of Teacher Education in Special Needs (HfH Zürich), and Vivala as the Foundation which supports persons with intellectual and developmental disabilities

Incidence and risk factors of severe adverse events with nevirapine-based antiretroviral therapy in HIV-infected women. MTCT-Plus program, Abidjan, Côte d'Ivoire

<p>Abstract</p> <p>Background</p> <p>In resource-limited settings where nevirapine-containing regimen is the preferred regimen in women, data on severe adverse events (SAEs) according to CD4 cell count are limited. We estimated the incidence of SAEs according to CD4 cell count and identify their risk factors in nevirapine-treated women.</p> <p>Methods</p> <p>All HIV-infected women who initiated nevirapine-containing regimen in the MTCT-Plus operational program in Abidjan, Côte d'Ivoire, were eligible for this study. Laboratory and clinical (rash) SAEs were classified as grade 3 and 4. Cox models were used to identify factors associated with the occurrence of SAEs.</p> <p>Results</p> <p>From August 2003 to October 2006, 290 women initiated a nevirapine-containing regimen at a median CD4 cell count of 186 cells/mm³(IQR 124-266). During a median follow-up on treatment of 25 months, the incidence of all SAEs was 19.5/100 patient-years. The 24-month probability of occurrence of hepatotoxicity or rash was not different between women with a CD4 cell count >250 cells/mm³and women with a CD4 cell count ≤250 cells/mm³(8.3% <it>vs</it>. 9.9%, Log-rank test: p = 0.75). In a multivariate proportional hazard model, neither CD4 cell count >250 cells/mm³at treatment initiation nor initiation NVP-based regimen initiated during pregnancy were associated with the occurrence of SAEs.</p> <p>Conclusion</p> <p>CD4 cell count >250 cells/mm³was not associated with a higher risk of severe hepatotoxicity and/or rash, as well as initiation of ART during pregnancy. Pharmacovogilance data as well as meta-analysis on women receiving NVP in these settings are needed for better information about NVP toxicity.</p