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Negative temperature states as exact equilibrium solutions of the Wave Kinetic equation for one dimensional lattices

We predict negative temperature states in the Discrete Nonlinear Schödinger equation as exact solutions of the associated Wave Kinetic equation. Those solutions are consistent with the classical thermodynamics formalism. Explicit calculation of the entropy as a function of the energy and number of particles is performed analytically. Direct numerical simulations of the DNLS equation are in agreement with theoretical results. We show that the key ingredient for observing negative temperatures in lattices is the boundedness of the dispersion relation in its domain. States with negative temperatures are characterized by an accumulation of particles and energy at wavenumber $k=π$.