IS THERE AN UNUSUAL CONDENSATION IN QUANTUM PARAELECTRICS ?

Quantum paraelectrics are incipient ferroelectrics whose transition is hindered by the zero-point quantum motion. Recently however, electron-paramagnetic-resonance measurements uncovered a phase-transition-like anomaly in the quantum paraelectric SrTiO3 at T-q approximate to 37 K. The observed line shifts are accompanied by no measurable splitting or broadening, suggesting that the possible transition would be towards a dynamical state preserving the crystal symmetry. This new "Muller state" generated considerable interest. In this report, we concentrate on data obtained on oxide perovskites. Light and neutron scattering spectroscopies confirmed the existence of T-q in SrTiO3 and revealed several anomalies among which a new branch whose origin remains unclear. A new quasi-elastic doublet was also found, both in SrTiO3 and KTaO3, which is produced by second sound. Although some measurements were interpreted in terms of a coherent quantum state, there seems to be so far no direct evidence for such a state, while there are considerable theoretical arguments against it. Other possible interpretations will be ventilated.