SPIN-LATTICE RELAXATION-RATE ANOMALY AT STRUCTURAL PHASE-TRANSITIONS

The theory of spin-lattice relaxation (SLR)-rate anomaly at structural phase transitions proposed about 30 years ago is reconsidered taking into account that knowledge about the relevant lattice response functi ons has changed considerably. We use both the results of previous auth ors and perform original calculations of the response functions when i t is necessary. We consider displacive systems and use the perturbatio n theory to treat the lattice anharmonicities in a broad temperature r egion whenever possible. Some comments about the order-disorder system s are made as well. The possibility of Linear coupling of the order pa rameter and the resonance frequency is always assumed. It is found tha t in the symmetrical phase the anomaly is due to the one-phonon proces ses, the anomalous part being proportional to either (T-T-c)(-1) or (T -T-c)(-1/2) depending on some condition on the soft-mode dispersion. I n both cases the value of the SLR rate at the boundary of applicabity of the theory (close to the phase transition) is estimated to be 10(2) -10(3) times more than the typical value of the SLR rate in an ideal c rystal. An essential specific feature of the nonsymmetrical phase is a ppearance of third-order anharmonicities that are well known to lead t o a low-frequency dispersion of the order-parameter damping constant. We have found that this constant exhibits, in addition, a strong wave- vector dispersion, so that the damping constant determing the SLR rate is quite different from that at zero wave vector. In the case of two- component order parameter the damping constant for the component with nonzero equilibrium value is different from that for the other compone nt, the difference is of the same order of magnitude as the damping co nstants themselves. In the case of the incommensurate phase a part of the mentioned third-order anharmonicity is responsible for longitudina l-transversal interaction that is well known to influence the static l ongitudinal response function. We calculate as well the dynamic respon se function to find that for the SLR calculations the imaginary part i s of main importance. Due to this interaction the longitudinal SLR rat e acquires a dependence on the Larmor frequency. This dependence is ho wever, fairly weak: a logarithmic one. The implications of the obtaine d results for interpretation of the experimental data on SLR in incomm ensurate phase are discussed as well.