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Prof. Stefano Iubini

ISC-CNR, University of Florence

Tuesday, March 5th, 2013 at 02:30:00 PM  

Room 281, Physics and Astronomy Dept., via Sansone 1, Sesto Fiorentino

Published on-line at 06:22:45 PM on Thursday, February 28th, 2013

Discrete breathers and negative temperature states

Second seminar in the series "Sguardi sulla complessità" on complexity by CSDC, ISC and Physics Dept. of CNR and Università di Firenze.

Since the pioneering work of Onsager and Ramsey in the 40s and 50s, physical states at negative (absolute) temperatures have attracted the curiosity of researchers and shown how science can challenge common sense. In negative-temperature regimes, the temperature is above infinity and high-energy states are more populated than low-energy ones. After many years elapsed since the first experimental evidences of negative temperatures in quantum nuclear-spin systems, recent experiments have realized a negative temperature state in a system of ultracold bosons trapped in optical lattice, modeled by a Bose-Hubbard Hamiltonian.

I will discuss the statistical behavior of a semi-classical limit of the Bose-Hubbard model, namely the Discrete Nonlinear Schroedinger Equation. By monitoring the microcanonical temperature, it is possible to show that there exists a parameter region where the system evolves towards a state characterized by a finite density of spatially localized nonlinear excitations (discrete breathers) and a negative temperature. Such a state persists over very long (astronomical) times since the convergence to equilibrium becomes increasingly slower as a consequence of a coarsening process. I will also discuss possible mechanisms for the generation of negative-temperature states in experimental setups.