Raman Study of Charge-Density-Wave Phase Transitions in 4Hb-TaSe₂

Abstract

Temperature dependence of Raman spectra in 4Hb-TaSe₂ was studied. The high-frequency spectra of 4Hb-TaSe₂ consist of 1T-TaSe₂- and 2H-TaSe₂-like modes. In relation to the two charge-density-wave phase transitions occurring in the octahedral (OH) and the trigonal-prismatic (TP) layers, two kinds of soft modes were observed. Mode repulsion was observed in the A_g and E_2g soft modes which are related to the phase transition of the OH layer. The two-phonon overtone scattering of the Kohn-anomaly modes of the TP layer was observed. The frequency of the lower edge of this mode corresponds to twice the minimum frequency of the Kohn-anomaly mode. From this frequency, temperature dependence of the electronic susceptibility of the TP layer was estimated. An E_2g rigid-layer mode was observed at 16.5 cm⁻¹.