Phase Transition and Thermal Order-by-Disorder in the Pyrochlore Quantum Antiferromagnet Er2Ti2O7: a High-Temperature Series Expansion Study

TL;DR

This study uses high-temperature series expansions to analyze the thermodynamic properties and phase transition of Er2Ti2O7, revealing thermal order-by-disorder effects and confirming experimental observations.

Contribution

It provides a detailed high-temperature series expansion analysis of Er2Ti2O7, linking theoretical predictions with experimental data and uncovering thermal order-by-disorder phenomena.

Findings

Model accurately describes thermodynamics of Er2Ti2O7

Identical susceptibilities for different order parameters up to 8th order

Thermal order-by-disorder selects the  state

Abstract

Several rare earth magnetic pyrochlore materials are well modeled by a spin-1/2 quantum Hamiltonian with anisotropic exchange parameters Js. For the Er2Ti2O7 material, the Js were recently determined from high-field inelastic neutron scattering measurements. Here, we perform high-temperature (T) series expansions to compute the thermodynamic properties of this material using these Js. Comparison with experimental data show that the model describes the material very well including the finite temperature phase transition to an ordered phase at Tc~1.2 K. We show that high temperature expansions give identical results for different q=0 xy order parameter susceptibilities up to 8th order in \beta=1/T (presumably to all orders in \beta). Conversely, a non-linear susceptibility related to the 6th power of the order parameter reveals a thermal order-by-disorder selection of the same non-colinear \psi_2 state as found in Er2Ti2O7.