Transitions between Phases with Equal Wave Numbers in a Double Ising Spin Model. Application to Betaine Calcium Chloride Dihydrate

TL;DR

This paper investigates phase transitions between equal wave number phases in a Double Ising Spin model, using theoretical and simulation methods, and explains dielectric anomalies in BCCD.

Contribution

It introduces a detailed analysis of internal phase transitions with equal wave numbers in a Double Ising model, combining mean field, transfer matrix, and Monte Carlo approaches.

Findings

Characterization of structural changes via Fourier harmonics.

Explanation of dielectric anomalies in BCCD.

Identification of different pseudo spin configurations during transitions.

Abstract

A Double Ising Spin model for uniaxially structurally modulated materials exhibits as a special feature phase transitions between phases with equal wave numbers but different pseudo spin configurations. The character of these `internal' transitions is investigated in mean field approximation, with the mean field transfer matrix method, and in Monte Carlo simulations. The structural changes at the transitions are characterized by different strengths of harmonics in a Fourier analysis of the spatial modulation. A dielectric anomaly in the phase diagram of betaine calcium chloride dihydrate (BCCD) and seemingly contradictory structure analyses are explained.