Light scattering from tantalum diselenide
The temperature dependent light scattering properties of the 2Hpolytype of tantalum diselenide has been carefully studied. Below a 120 K phase transition, incommensurate charge density waves appear which introduce .additional structure into the Raman spectra. The charge density waves become commensurate below a 110 K phase transition, with a consequent tripling of the size of the unit cell. Several Raman-active modes are strongly coupled to the formation of the charge density waves, which can be discussed in terms of Landau or microsc6pic theories. Parameterization of these modes with damped Lorentzians does not show a mean field behavior for the oscillator frequency. This indicates the presence of critical fluctuations or mode coupling. A depolarized central peak is observed in close-in Raman scattering both with and without an iodine vapor cell filter. The intensity and width of this central peak both reach a maximum value at a temperature considerably below the charge density wave transition temperatures, but can be parameterized with the inclusion of a relaxing self-energy term in a coupled mode analysis. Resonance Raman effects on the single phonon excitations have indicated the. presence of a band gap in the yellow region of the spectrum. This enhancement has also been correlated with a luminescence observed with a large variety of laser excitation frequencies. The temperature dependence of this luminescence is observed to behave similarly to a band gap seen in thermoreflectance data. The optical constants have also been measured. A dip in the real part of the dielectric constant also occurs in the yellow part of the spectrum, and is related to the observed luminescence and resonance effects.
light scattering, tantalum diselenide, temperature dependent light scattering, charge density waves
University of Illinois - Urbana-Champaign
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