Shapiro-step spectrum and phase-velocity coherence inNbSe3in a uniform temperature gradient

We have measured the Shapiro-step spectrum of the charge-density-wave conductor ${\mathrm{NbSe}}_3$, under conditions of combined ac and dc electric fields and an applied uniform temperature gradient. Increasing the temperature gradient from zero results in a well-defined breakup of the Shapiro-step spectrum. Under fixed temperature gradient, the isothermal Shapiro-step spectrum can be effectively recovered by application of a sufficiently-large-amplitude ac driving field. We identify different interference peaks of the broken spectrum with different macroscopically phase-velocity-coherent regions of the crystal, and we explicitly determine the volumes of these dynamically coherent domains.