Soft Modes Associated with Incommensurate Phase Transitions in Biphenyl

Abstract

The Raman scattering technique has been employed to observe the temperature dependent modes associated with the incommensurate phase transitions in biphenyl. A new soft mode, whose frequency becomes zero as T₁=40.0 K is approached from below, has been found in the α_zz spectra. A temperature-dependent mode in the α_xy spectra, which had previously been assigned to the “soft” mode could be observed even above T₁. In addition, the anomalous increase in Rayleigh wing was observed in the vicinity of T₁. These features show that the latter mode with α_xy should be interpreted in terms of a precursor-order-induced hard-mode.