Chaotic response of NbSe3: Evidence for a new charge-density-wave phase

We have measured the charge-density-wave response characteristics of Nb${\mathrm{Se}}_3$ for combined ac and dc driving fields. In the switching regime a clear period-doubling route to chaos is observed, where the bifurcation cascade is periodic in applied dc bias and ac amplitude. These observations provide strong evidence for a new charge-density-wave phase which, in terms of classical dynamics, behaves underdamped.