Floquet Time Crystals
  We define what it means for time translation symmetry to be spontaneously broken in a quantum system, and show with analytical arguments and numerical simulations that this occurs in a large class of many-body-localized driven systems with discrete time-translation symmetry. 
Dominic V. Else and Bela Bauer and Chetan Nayak
cond-mat.dis-nn cond-mat.stat-mech cond-mat.str-el quant-ph