6,806 research outputs found
Ultracold Quantum Gases: Beyond-Mean-Field Effects
Full text linkIn this thesis we present a detailed investigation of the role played by quantum and thermal fluctuations in ultracold Bose gases. We begin with a review of several important concepts and analytical tools within a functional integration formalism. We first focus on the so-called zero-range approximation for the interaction potential, by recovering the Bogoliubov results and the Landau two-fluid model from a field-theory perspective. In deriving the beyond-mean-field equation of state, we are going to show that a crucial point concerns the proper regularization of the divergent zero-point energy. Among the alternative approaches to investigate finite-temperatures Bose gases, we apply the kinetic theory to explain some recent results on the sound propagation in two-dimensional Bose gases.
We then move to consider the eventual corrections to the thermodynamics of Bose gases due to the finite-range character of the interaction potential. The coupling constants of the finite-range model are related to measurable scattering parameters through an effective-field-theory approach. The role of finite-range corrections is considered not only in three spatial dimensions but also in systems with lower dimensionalities. Our analytical predictions are in good agreement with available Monte Carlo simulations and consistent with other theoretical frames, as the Lieb-Lininger theory for one-dimensional systems.
In the third chapter, the relevance of fluctuations is investigated from an alternative point of view. Indeed, for a single-component Bose gas we have actually considered their effect as deviations of thermodynamic quantities from the mean-field and zero-range picture. In the case of collapsing Bose mixtures, we are going to show that zero-temperature fluctuations play a crucial stabilizing role against the collapse instability. Because of this peculiar mechanism, ultracold mixtures can display finite-density configurations also in free space. Inspired by recent experiments, we characterize this novel self-bound state by comparison with bright solitons, following a variational scheme. We also consider the case of binary mixtures where a coherent internal coupling is turned on.
In the last chapter, we move to deal with dipolar condensates. In particular, we are interested in beyond-mean-field effects leading to the formation of inhomogeneous ground states. In order to provide a reliable answer to the open issue of superfluid properties of these structures, we present our recent numerical investigation on the phase diagram of dipolar bosons
PPPs in health and social services: a performance measurement perspective
No full textThis paper addresses the question of public accountability in the context of public–private partnerships (PPPs) by unpacking the principles behind performance measurement systems for service-focused PPPs. The authors present a contingent-based approach to add to our understanding of PPPs and propose a performance measurement framework based on four dimensions and a set of key performance measures. The model described in the paper supplies a baseline on which public decision-makers can develop customized performance indicators
Thermal field theory of bosonic gases with finite-range effective interaction
No full textWe study a dilute and ultracold Bose gas of interacting atoms by using an effective field theory which takes into account the finite-range effects of the interatomic potential. Within the formalism of functional integration from the grand canonical partition function, we derive beyond-mean-field analytical results which depend on both the scattering length and the effective range of the interaction. In particular, we calculate the equation of state of the bosonic system as a function of these interaction parameters both at zero and finite temperature including one-loop Gaussian fluctuation. In the case of zero-range effective interaction, we explicitly show that, due to quantum fluctuations, the bosonic system is thermodynamically stable only for very small values of the gas parameter. We find that a positive effective range above a critical threshold is necessary to remove the thermodynamical instability of the uniform configuration. Remarkably, also for relatively large values of the gas parameter, our finite-range results are in quite good agreement with recent zero-temperature Monte Carlo calculations obtained with hard-sphere bosons
Collective modes across the soliton-droplet crossover in binary Bose mixtures
Full text linkWe study the collective modes of a binary Bose mixture across the soliton to droplet crossover in a quasi-one-dimensional waveguide with a beyond-mean-field equation of state and a variational Gaussian ansatz for the scalar bosonic field of the corresponding effective action. We observe a sharp difference in the collective modes in the two regimes. Within the soliton regime, modes vary smoothly upon the variation of particle number or interaction strength. On the droplet side, collective modes are inhibited by the emission of particles. This mechanism turns out to be dominant for a wide range of particle numbers and interactions. In a small window of particle number range and for intermediate interactions, we find that monopole frequency is likely to be observed. We focus on the spin-dipole modes for the case of equal intraspecies interactions and equal equilibrium particle numbers in the presence of a weak longitudinal confinement. We find that such modes might be unobservable in the real-time dynamics close to the equilibrium as their frequency is higher than the particle emission spectrum by at least one order of magnitude in the droplet phase. Our results are relevant for experiments with two-component Bose-Einstein condensates for which we provide realistic parameters
Action Functional for a Particle with Damping
Full text linkIn this brief report we discuss the action functional of a particle with damping, showing that it can be obtained from the dissipative equation of motion through a modification which makes the new dissipative equation invariant for time reversal symmetry. This action functional is exactly the effective action of Caldeira-Leggett model but, in our approach, it is derived without the assumption that the particle is weakly coupled to a bath of infinite harmonic oscillators
Le sperimentazioni gestionali nel SSN: rilevazione nazionale e analisi dei modelli emergenti
Full text linkIl presente capitolo ha quindi il triplice obiettivo di (i) mappare le SG attive e concluse nel SSN, in parallelo con un’analisi delle politiche regionali in materia; (ii) classificare le caratteristiche delle stesse SG; (iii) introdurre elementi di valutazione sulle loro configurazioni di governance. Si tratta quindi di un’analisi valutativa ex post, affinché il SSN e i suoi policy maker possano apprendere sia dalle esperienze di successo, sia dalle scelte rivelatesi inadeguate. Come noto, quest’ultima è una pratica tanto preziosa quanto politicamente sensibile e scarsamente
diffusa nel sistema.
Per raggiungere gli obiettivi sopra riportati, il capitolo si articola in sei paragrafi. Il paragrafo successivo declina gli obiettivi della ricerca e la metodologia di analisi adottata. Il terzo paragrafo illustra le politiche regionali in tema di SG. Il quarto paragrafo presenta l’analisi aggregata e per cluster della popolazione organizzativa di SG. Il quinto paragrafo discute le implicazioni dell’analisi effettuata e propone 3 modelli possibili di Sperimentazione Gestionale sulla base di fattori istituzionali e di contesto. Il sesto paragrafo conclude il lavoro
Nonequilibrium statistical mechanics in one-dimensional bose gases
No full textWe study cold dilute gases made of bosonic atoms, showing that in the mean-field one-dimensional regime they support stable out-of-equilibrium states. Starting from the 3D Boltzmann-Vlasov equation with contact interaction, we derive an effective 1D Landau-Vlasov equation under the condition of a strong transverse harmonic confinement. We investigate the existence of out-of-equilibrium states, obtaining stability criteria similar to those of classical plasmas
Superfluid Filaments of Dipolar Bosons in Free Space
Full text linkWe systematically investigate the zero temperature phase diagram of bosons interacting via dipolar interactions in three dimensions in free space via path integral Monte Carlo simulations with a few hundreds of particles and periodic boundary conditions based on the worm algorithm. Upon increasing the strength of the dipolar interaction and at sufficiently high densities we find a wide region where filaments are stabilized along the direction of the external field. Most interestingly by computing the superfluid fraction we conclude that the superfluidity is anisotropic and is greatly suppressed along the orthogonal plane. Finally, we perform simulations at finite temperature confirming the stability of the filaments against thermal fluctuations and provide an estimate of the superfluid fraction in the weak coupling limit in the framework of the Landau two-fluid model
Superfluids, Fluctuations and Disorder
Full text linkWe present a field-theory description of ultracold bosonic atoms in the presence of a disordered external potential. By means of functional integration techniques, we aim to investigate and review the interplay between disordered energy landscapes and fluctuations, both thermal and quantum ones. Within the broken-symmetry phase, up to the Gaussian level of approximation, the disorder contribution crucially modifies both the condensate depletion and the superfluid response. Remarkably, it is found that the ordered (i.e., superfluid) phase can be destroyed also in regimes where the random external potential is suitable for a perturbative analysis. We analyze the simplest case of quenched disorder and then we move to present the implementation of the replica trick for ultracold bosonic systems. In both cases, we discuss strengths and limitations of the reviewed approach, paying specific attention to possible extensions and the most recent experimental outputs
Gli impatti del PPP nel ridisegno dell'offerta ospedaliera: esiti, determinanti, prospettive
Full text linkLo studio intende indagare qual è stata la porzione di rete ospedaliera radicalmente rinnovata grazie alle operazioni di PPP negli ultimi dieci anni, qual è stato l’impatto distintivo del ricorso allo strumento del PPP nel processo di ridisegno dell’offerta ospedaliera, quali sono i punti più critici del processo di costruzione e di gestione e quali suggerimenti di policy ne derivano
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