RAMAN-SCATTERING STUDY OF THE N = 4 INCOMMENSURATE PHASE OF BIPHENYL UNDER HYDROSTATIC-PRESSURE

We report results of a Raman-scattering study of the n = 4 (+/-q1, +/-q2) incommensurate phase (phase II) of biphenyl under hydrostatic pressure. We make use of a peculiar property of biphenyl, i.e., the stabilization under pressure of this incommensurate phase at very low temperatures. Thus, the low-frequency Raman spectra were studied as a function of pressure at 6 K, where the damping of modes is very small. Three specific Raman lines were observed. A theoretical analysis of the Raman activation of the excitations derived from the driving soft modes is developed. The single-q nature of the phase-II structure is confirmed. One Raman line is assigned to the homogeneous amplitude mode, whereas the two other lines are interpreted in terms of mixed modes combining basic structure normal coordinates of the upper soft branch at +/-q1 (or +/-q2) and of the lower soft branch at +/-3q1 (or +/-3q2). Intensity arguments show that the phason and amplitudon character subsists relatively far away from the satellite position.