Critical dynamics at incommensurate phase transitions and NMR relaxation experiments

TL;DR

This paper analyzes the critical dynamics of incommensurate phase transitions in crystals, focusing on NMR relaxation experiments, and provides a comprehensive theoretical framework that matches experimental results.

Contribution

It introduces an extended renormalization scheme for analyzing critical dynamics in incommensurate phases, including coupling effects and Goldstone anomalies, with application to NMR data.

Findings

Excellent agreement with NMR relaxation measurements.

Characterization of the critical region width and crossover behavior.

Insights into Goldstone mode effects and phason gap relations.

Abstract

We study the critical dynamics of crystals which undergo a second-order phase transition from a high-temperature normal phase to a structurally incommensurate (IC) modulated phase. We give a comprehensive description of the critical dynamics of such systems, e.g. valid for crystals of the A_2BX_4 family. Using an extended renormalization scheme, we present a framework in which we analyze the phases above and below the critical temperature T_I. Above T_I, the crossover from the critical behavior to the mean-field regime is studied. Specifically, the resulting width of the critical region is investigated. In the IC modulated phase, we consider explicitly the coupling of the order parameter modes to one-loop order. Here the Goldstone anomalies and their effect on measurable quantities are investigated. We show their relation with the postulated phason gap. While the theory can be applied to a variety of experiments, we concentrate on quadrupole-perturbed nuclear magnetic resonance (NMR) experiments. We find excellent agreement with these dynamical measurements and provide answers for some questions that arose from recent results.