20,466 research outputs found

    Some remarks on 'superradiant' phase transitions in light-matter systems

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    In this paper we analyze properties of the phase transition that appears in a set of quantum optical models; Dicke, Tavis-Cummings, quantum Rabi, and finally the Jaynes-Cummings model. As the light-matter coupling is increased into the deep strong coupling regime, the ground state turns from vacuum to become a superradiant state characterized by both atomic and photonic excitations. It is pointed out that all four transitions are of the mean-field type, that quantum fluctuations are negligible, and hence these fluctuations cannot be responsible for the corresponding vacuum instability. In this respect, these are not quantum phase transitions. In the case of the Tavis-Cummings and Jaynes-Cummings models, the continuous symmetry of these models implies that quantum fluctuations are not only negligible, but strictly zero. However, all models possess a non-analyticity in the ground state in agreement with a continuous quantum phase transition. As such, it is a matter of taste whether the transitions should be termed quantum or not. In addition, we also consider the modifications of the transitions when photon losses are present. For the Dicke and Rabi models these non-equilibrium steady states remain critical, while the criticality for the open Tavis-Cummings and Jaynes-Cummings models is completely lost, i.e. in realistic settings one cannot expect a true critical behaviour for the two last models.Comment: 25 pages (single column), 6 figure

    Formation of the First Stars by Accretion

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    The process of star formation from metal-free gas is investigated by following the evolution of accreting protostars with emphasis on the properties of massive objects. The main aim is to establish the physical processes that determine the upper mass limit of the first stars. Although the consensus is that massive stars were commonly formed in the first cosmic structures, our calculations show that their actual formation depends sensitively on the mass accretion rate and its time variation. Even in the rather idealized case in which star formation is mainly determined by dot{M}acc, the characteristic mass scale of the first stars is rather uncertain. We find that there is a critical mass accretion rate dot{M}crit = 4 10^{-3} Msun/yr that separates solutions with dot{M}acc> 100 Msun can form, provided there is sufficient matter in the parent clouds, from others (dot{M}acc > dot{M}crit) where the maximum mass limit decreases as dot{M}acc increases. In the latter case, the protostellar luminosity reaches the Eddington limit before the onset of hydrogen burning at the center via the CN-cycle. This phase is followed by a rapid and dramatic expansion of the radius, possibly leading to reversal of the accretion flow when the stellar mass is about 100Msun. (abridged)Comment: 34 pages, 12 figures. ApJ, in pres

    Valuing a Beach Day with a Repeated Nested Logit Model of Participation, Site Choice, and Stochastic Time Value

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    Beach recreation values are often needed by policy-makers and resource managers to efficiently manage coastal resources, especially in popular coastal areas like Southern California. This article presents welfare values derived from random utility maximization-based recreation demand models that explain an individual’s decisions about whether or not to visit a beach and which beach to visit. The models utilize labor market decisions to reveal each individual’s opportunity cost of recreation time. The value of having access to the beach in San Diego County is estimated to be between 21and21 and 23 per day.Recreation demand, repeated nested logit, labor supply, opportunity cost of leisure, time, beach recreation., Demand and Price Analysis, Environmental Economics and Policy, Institutional and Behavioral Economics, Q26, J22, Q51.,


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    This study evaluated how farmers' perceptions of spatial yield variability are influenced by precision technologies. Farmer estimates from a mail survey were regressed on use of alternative information technologies and personal characteristics. Results indicate that farmers who adopted yield monitors with GPS for cotton perceived significantly higher spatial yield variability.Research and Development/Tech Change/Emerging Technologies,

    Matchings with externalities and attitudes

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    Two-sided matchings are an important theoretical tool used to model markets and social interactions. In many real-life problems the utility of an agent is influenced not only by their own choices, but also by the choices that other agents make. Such an influence is called an externality. Whereas fully expressive representations of externalities in matchings require exponential space, in this paper we propose a compact model of externalities, in which the influence of a match on each agent is computed additively. Under this framework, we analyze many-to-many matchings and one-to-one matchings where agents take different attitudes when reasoning about the actions of others. In particular, we study optimistic, neutral and pessimistic attitudes and provide both computational hardness results and polynomial-time algorithms for computing stable outcomes

    Dynamics of Dense Cores in the Perseus Molecular Cloud

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    We survey the kinematics of over one hundred and fifty candidate (and potentially star-forming) dense cores in the Perseus molecular cloud with pointed N2H+(1-0) and simultaneous C18O(2-1) observations. Our detection rate of N2H+ is 62%, rising to 84% for JCMT SCUBA-selected targets. In agreement with previous observations, we find that the dense N2H+ targets tend to display nearly thermal linewidths, particularly those which appear to be starless (using Spitzer data), indicating turbulent support on the small scales of molecular clouds is minimal. For those N2H+ targets which have an associated SCUBA dense core, we find their internal motions are more than sufficient to provide support against the gravitational force on the cores. Comparison of the N2H+ integrated intensity and SCUBA flux reveals fractional N2H+ abundances between 10^-10 and 10^-9. We demonstrate that the relative motion of the dense N2H+ gas and the surrounding C18O gas is less than the sound speed in the vast majority of cases (~90%). The point-to-point motions we observe within larger extinction regions appear to be insufficient to provide support against gravity, although we sparsely sample these regions.Comment: 49 pages, 20 figures. Accepted for publication in the Astrophysical Journa
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