Plasmon behavior at the charge-density-wave onset in2H-TaSe2

We have studied experimentally and theoretically the thermoreflectance of $2H$-Ta${\mathrm{Se}}_2$ in the neighborhood of the plasmon resonance near 1 eV, as a function of temperature. An extensive red shift and change of width of the plasmon (a narrowing at first, then a broadening) occur as one heats up through the 90-120-K range where charge-density waves (CDW) disappear. From a close inspection of the dielectric response as a function of frequency and temperature we find that optical transitions disappear in the 0.2-0.8-eV region in correspondence with the weakening of CDW upon heating, while new intrabrand (1 eV) transitions replace them. This transfer of oscillator strength, entirely expected on the basis of a model CDW band structure, accounts qualitatively for both shift and narrowing (broadening) of the plasmon. A theory of the plasmon shift caused by the new CDW periodicity is developed, and applied to the present case, by means of a model umklapp response. The results show that the shift is chiefly due to the presence of the new CDW transitions in the 3 × 3 band structure. The temperature-induced change of optical response at the CDW onset is also found to be smooth and broad with temperature, unlike that of other properties, like specific heat, which exhibit a clear critical behavior. This indicates a "slow" change in the energy-band structure, for $T\sim 120$ K, probably mostly a change of electron lifetime, whose rate of increase seems maximum around 110 K.