84 research outputs found

    Hypermetabolism and Coronavirus Disease 2019

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    Analytic results for Gaussian wave packets in four model systems: II. Autocorrelation functions

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    The autocorrelation function, A(t), measures the overlap (in Hilbert space) of a time-dependent quantum mechanical wave function, psi(x,t), with its initial value, psi(x,0). It finds extensive use in the theoretical analysis and experimental measurement of such phenomena as quantum wave packet revivals. We evaluate explicit expressions for the autocorrelation function for time-dependent Gaussian solutions of the Schrodinger equation corresponding to the cases of a free particle, a particle undergoing uniform acceleration, a particle in a harmonic oscillator potential, and a system corresponding to an unstable equilibrium (the so-called `inverted' oscillator.) We emphasize the importance of momentum-space methods where such calculations are often more straightforwardly realized, as well as stressing their role in providing complementary information to results obtained using position-space wavefunctions.Comment: 18 pages, RevTeX, to appear in Found. Phys. Lett, Vol. 17, Dec. 200

    Origin of the Canonical Ensemble: Thermalization with Decoherence

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    We solve the time-dependent Schrodinger equation for the combination of a spin system interacting with a spin bath environment. In particular, we focus on the time development of the reduced density matrix of the spin system. Under normal circumstances we show that the environment drives the reduced density matrix to a fully decoherent state, and furthermore the diagonal elements of the reduced density matrix approach those expected for the system in the canonical ensemble. We show one exception to the normal case is if the spin system cannot exchange energy with the spin bath. Our demonstration does not rely on time-averaging of observables nor does it assume that the coupling between system and bath is weak. Our findings show that the canonical ensemble is a state that may result from pure quantum dynamics, suggesting that quantum mechanics may be regarded as the foundation of quantum statistical mechanics.Comment: 12 pages, 4 figures, accepted for publication by J. Phys. Soc. Jp

    Long-Time Tails and Anomalous Slowing Down in the Relaxation of Spatially Inhomogeneous Excitations in Quantum Spin Chains

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    Exact analytic calculations in spin-1/2 XY chains, show the presence of long-time tails in the asymptotic dynamics of spatially inhomogeneous excitations. The decay of inhomogeneities, for tt\to \infty , is given in the form of a power law (t/τQ)νQ (t/\tau_{Q}) ^{-\nu_{Q}} where the relaxation time τQ\tau_{Q} and the exponent νQ\nu_{Q} depend on the wave vector QQ, characterizing the spatial modulation of the initial excitation. We consider several variants of the XY model (dimerized, with staggered magnetic field, with bond alternation, and with isotropic and uniform interactions), that are grouped into two families, whether the energy spectrum has a gap or not. Once the initial condition is given, the non-equilibrium problem for the magnetization is solved in closed form, without any other assumption. The long-time behavior for tt\to \infty can be obtained systematically in a form of an asymptotic series through the stationary phase method. We found that gapped models show critical behavior with respect to QQ, in the sense that there exist critical values QcQ_{c}, where the relaxation time τQ\tau_{Q} diverges and the exponent νQ\nu_{Q} changes discontinuously. At those points, a slowing down of the relaxation process is induced, similarly to phenomena occurring near phase transitions. Long-lived excitations are identified as incommensurate spin density waves that emerge in systems undergoing the Peierls transition. In contrast, gapless models do not present the above anomalies as a function of the wave vector QQ.Comment: 25 pages, 2 postscript figures. Manuscript submitted to Physical Review

    Post-operative Psychosocial Predictors of Outcome in Bariatric Surgery

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    Although there are several recent reviews of the pre-operative factors that influence treatment outcome for bariatric surgery, commensurate efforts to identify and review the predictive validity of post-operative variables are lacking. This review describes the post-operative psychosocial predictors of weight loss in bariatric surgery. Results suggest empirical support for post-operative binge eating, uncontrolled eating/grazing, and presence of a depressive disorder as negative predictors of weight loss outcomes; whereas, adherence to dietary and physical activity guidelines emerged as positive predictors of weight loss. With the exception of depression, psychological comorbidities were not consistently associated with weight loss outcomes. Results highlight the need for post-operative assessment of disordered eating and depressive disorder, further research on the predictive value of post-operative psychosocial factors, and development of targeted interventions

    Wishing for deburdening through a sustainable control after bariatric surgery

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    The aim of this study was an in-depth investigation of the change process experienced by patients undergoing bariatric surgery. A prospective interview study was performed prior to as well as 1 and 2 years after surgery. Data analyses of the transcribed interviews were performed by means of the Grounded Theory method. A core category was identified: Wishing for deburdening through a sustainable control over eating and weight, comprising three related categories: hoping for deburdening and control through surgery, feeling deburdened and practising control through physical restriction, and feeling deburdened and trying to maintain control by own willpower. Before surgery, the participants experienced little or no control in relation to food and eating and hoped that the bariatric procedure would be the first brick in the building of a foundation that would lead to control in this area. The control thus achieved in turn affected the participants' relationship to themselves, their roles in society, and the family as well as to health care. One year after surgery they reported established routines regarding eating as well as higher self-esteem due to weight loss. In family and society they set limits and in relation to health care staff they felt their concern and reported satisfaction with the surgery. After 2 years, fear of weight gain resurfaced and their self-image was modified to be more realistic. They were no longer totally self-confident about their condition, but realised that maintaining control was a matter of struggle to obtaining a foundation of sustainable control. Between 1 and 2 years after surgery, the physical control mechanism over eating habits started to more or less fade for all participants. An implication is that when this occurs, health care professionals need to provide interventions that help to maintain the weight loss in order to achieve a good long-term outcome
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