= Simon Braun
{c}
<Simon Braun> and collaborators prepared a state with <negative temperature> for motional degrees of freedom in an ultracold atomic system. They tailored a lattice model so the occupied high-energy states were compatible with the relevant upper-energy bound.<Negative Absolute Temperature for Motional Degrees of Freedom by Simon Braun and others>[Abstract: preparation and upper-band-edge momentum peaks]{ref}
In this regime $\beta=\partial(S/k_B)/\partial E<0$: increasing <energy> reduces the entropy of the constrained system. The state is not colder than absolute zero. Its population favours high-energy states, and its thermodynamic ordering differs from the positive-<temperature> branch. The experimental energy bounds and stability conditions are essential.
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