Stefano Iubini - ISC-CNR Firenze #
Discrete Breathers and Negative Temperature States #
Since the pioneering work of Onsager and Ramsey in the 1940s 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.
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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.
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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.