RESUMO
Polarized Raman, IR, and time-domain THz spectroscopy of orthorhombic lead zirconate single crystals have yielded a comprehensive picture of temperature-dependent quasiharmonic frequencies of its low-frequency phonon modes. It is argued that these modes primarily involve vibrations of Pb ions and librations of oxygen octahedra. Their relation to phonon modes of the parent cubic phase is proposed. Counts of the observed IR and Raman active modes belonging to distinct irreducible representations agree quite well with group-theory predictions. Analysis of the results yields insight into the phase transition mechanism, involving a soft ferroelectric branch coupled by a trilinear term to another two oxygen octahedra tilt modes.
RESUMO
We investigate the low-temperature equilibrium state of the p-state mean-field Potts glass. It is the presently believed that there is a temperature interval T(2) < T < T(c) within which the equilibrium state for p > p(*) approximately 2.82 is described by the cavity method corresponding to the first level of replica-symmetry breaking. We demonstrate that this locally stable solution is globally unstable and that true equilibrium in the region of instability of the replica-symmetric solution (T < T(c)), marked by the highest free energy, is a state displaying a continuous replica-symmetry breaking.
