1,721,083 research outputs found
The Casimir effect: From quantum to critical fluctuations
No full textThe Casimir effect in quantum electrodynamics (QED) is perhaps the best-known example of fluctuation-induced long-ranged force acting on objects (conducting plates) immersed in a fluctuating medium (quantum electromagnetic field in vacuum). A similar effect emerges in statistical physics, where the force acting, e.g., on colloidal particles immersed in a binary liquid mixture is affected by the classical thermal fluctuations occurring in the surrounding medium. The resulting Casimir-like force acquires universal features upon approaching a critical point of the medium and becomes long-ranged at criticality. In turn, this universality allows one to investigate theoretically the temperature dependence of the force via representative models and to stringently test the corresponding predictions in experiments. In contrast to QED, the Casimir force resulting from critical fluctuations can be easily tuned with respect to strength and sign by surface treatments and temperature control. We present some recent advances in the theoretical study of the universal properties of the critical Casimir force arising in thin films. The corresponding predictions compare very well with the experimental results obtained for wetting layers of various fluids. We discuss how the Casimir force between a colloidal particle and a planar wall immersed in a binary liquid mixture has been measured with femto-Newton accuracy, comparing these experimental results with the corresponding theoretical predictions
Enrico Fermi in Pisa
No full textI discuss the early work of Enrico Fermi (1901-1954) as a student in Pisa at the Scuola Normale Superiore and at the University of Pisa (1918-1922), paying particular attention to the four papers he published during those years and to his licenza and doctoral theses
Dynamics of large deviations in the hydrodynamic limit: Noninteracting systems
No full textWe study the dynamics of the statistics of the energy transferred across a point along a quantum chain which is prepared in the inhomogeneous initial state obtained by joining two identical semi-infinite parts thermalized at two different temperatures. In particular, we consider the transverse field Ising and harmonic chains as prototypical models of noninteracting fermionic and bosonic excitations, respectively. Within the so-called hydrodynamic limit of large space-time scales we first discuss the mean values of the energy density and current, and then, aiming at the statistics of fluctuations, we calculate exactly the scaled cumulant generating function of the transferred energy. From the latter, the evolution of the associated large deviation function is
obtained. A natural interpretation of our results is provided in terms of a semiclassical picture of quasiparticles moving ballistically along classical trajectories Similarities and differences between the transferred energy scaled cumulant and the large deviation functions in the cases of noninteracting fermions and bosons are discussed
Large deviations and universality in quantum quenches
No full textWe study the large deviation statistics of the intensive work done by globally changing a control parameter in a thermally isolated quantum many-body system. We show that, upon approaching a critical point, large deviations well below the mean work display universal features related to the critical Casimir effect in the corresponding classical system. Large deviations well above the mean are, instead, of quantum nature and not captured by the quantum-to-classical correspondence. For a bosonic system we show that in this latter regime a transition from exponential to power-law statistics, analogous to the equilibrium Bose-Einstein condensation, may occur depending on the parameters of the quench and on the spatial dimensionality
Slow dynamics at critical points: the field-theoretical perspective
No full textThe dynamics at a critical point provides a simple instance of slow collective evolution, characterised by aging phenomena and by a violation of the fluctuation-dissipation relation even for long times. By virtue of the universality in critical phenomena it is possible to provide quantitative predictions for some aspects of these behaviours by field-theoretical methods. We review some of the theoretical results that have been obtained in recent years for the relevant (universal) quantities, such as the fluctuation-dissipation ratio, associated with the non-equilibrium critical dynamics
Relaxation phenomena at criticality
No full text64.60.Ht Dynamic critical phenomena, 64.60.an Finite-size systems,
Nonequilibrium relaxation of a trapped particle in a near-critical Gaussian field
No full textWe study the nonequilibrium relaxational dynamics of a probe particle
linearly coupled to a thermally fluctuating scalar field and subject to
a harmonic potential, which provides a cartoon for an optically trapped
colloid immersed in a fluid close to its bulk critical point. The
average position of the particle initially displaced from the position
of mechanical equilibrium is shown to feature long-time algebraic tails
as the critical point of the field is approached, the universal
exponents of which are determined in arbitrary spatial dimensions. As
expected, this behavior cannot be captured by adiabatic approaches which
assumes fast field relaxation. The predictions of the analytic,
perturbative approach are qualitatively confirmed by numerical
simulations
Ballistic front dynamics after joining two semi-infinite quantum Ising chains
No full textWe consider two semi-infinite quantum Ising chains initially at thermal equilibrium at two different
temperatures and subsequently joined by an interaction between their end points. Transport properties such
as the heat current are determined by the dynamics of the left- and right-moving fermionic quasiparticles which
characterize the ensuing unitary dynamics.Within the so-called semiclassical space-time scaling limit we extend
known results by determining the full space and time dependence of the density and current of energy and of
fermionic quasiparticles. Upon approaching the edge of the propagating front, these quantities as well as the
two-point correlation function display qualitatively different behaviors depending on the transverse field of the
chain being critical or not.While in the latter case corrections to the leading behavior are described, as expected,
by the Airy kernel, in the former a novel scaling form emerges with universal features
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