# Freericks group publishes research on quantum behavior of periodically driven interacting materials.

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Research completed with a visiting graduate student from Germany (Mona Kalthoff) and her advisor in Germany (Gotz Uhrig) has recently been published in the high-profile journal Physical Review Letters. The work studies a theoretical object called the interacting density of states. In equilibrium, without any periodic drive, a German physicist Harry Lehmann, proved that the density of states is always positive in 1954, allowing it to have a statistical interpretation involving probabilities. For driven systems, it is well known that the same interacting density of states is sometimes negative. In the recent PRL article, Freericks and collaborators have shown that if one averages the density of states over the driving period, then one recovers the Lehmann result guaranteeing positivity. This shows how the driven system still has some relationship with the equilibrium one that remains. Mona Kalthoff was a visiting student from Germany, supported by a fellowship from the German government for a six month visit to Georgetown in 2017-18. The research results are available at *Physical Review Letters* **122**, 130604 (2019).