Surprises from quenches in long-range-interacting systems: temperature inversion and cooling

What happenswhen one of the parameters governing the dynamics of a long-range interacting system of particles in thermal equilibrium is abruptly changed (quenched) to a different value?While a shortrange system, under the same conditions, will relax in time to a new thermal equilibriumwith a uniform temperature across the system, a long-range system shows a fast relaxation to a nonequilibrium quasistationary state (QSS). The lifetime of such an off-equilibrium state diverges with the system size, and the temperature is non-uniform across the system.Quite surprisingly, the density profile in theQSS obtained after the quench is anticorrelatedwith the temperature profile in space, thus exhibiting the phenomenon of temperature inversion: denser regions are colder than sparser ones. We illustrate with extensivemolecular dynamics simulations the ubiquity of this scenario in a prototypical long-range interacting system subject to a variety of quenching protocols, and in amodel thatmimics an experimental setup of atoms interacting with light in an optical cavity.We further demonstrate how a procedure of iterative quenching combinedwithfiltering out the high-energy particles in the systemmay be employed to cool the system. Temperature inversion is observed in nature in some astrophysical settings; our results imply that such a phenomenon should be observable, and could even be exploitable to advantage, also in controlled laboratory experiments.