Harmonic and subharmonic shapiro steps in orthorhombic Tas3

Observation of harmonic and subharmonic Shapiro steps in orthorhombic TaS₃ in the presence of combined rf and dc drive is reported, demonstrating that both the harmonic and subharmonic steps are a general characteristic of charge-density-wave response. Compared to NbSe₃, the noise spectrum is typically much broader in TaS₃, leading to poorly defined steps at small rf voltages. The steps sharpen with larger amplitudes of rf. Increased rf amplitude is also accompanied by an increase in the CDW current density at a given step, but this increase approaches a limiting value for large rf levels. The results are interpreted as evidence for increased coherence of the CDW response in the presence of large rf driving fields.