On prethermal time crystals from semi-holography

We demonstrate the existence of a pair of almost dissipationless oscillating modes at low temperatures in both the shear and sound channels of a hybrid quantum system, comprised of a weakly self-interacting perturbative sector coupled to strongly self-interacting holographic degrees of freedom described by a black hole geometry. We argue that these modes realize prethermal time-crystal behavior in semiholographic systems without fine-tuning and can be observed by measuring operators that probe either the hard (perturbative) or the soft (holographic) sector. We also find novel Gregory Laflamme type instabilities that lead to the formation of inhomogeneities at higher temperatures. These results provide evidence that black holes with planar horizons and dynamical boundary conditions can develop both inhomogeneous and metastable time crystal phases over a wide range of temperatures set by an intermediate scale. Furthermore, they suggest that such phases can be realized without external driving in nonAbelian plasmas of asymptotically free gauge theories in the large-N limit.