Cubic Defects: Comparing the Eight-State-System with its Two-Level-Approximation

TL;DR

This paper compares the full eight-state model of cubic defects with its two-level approximation, highlighting differences in thermodynamic predictions, responses to external forces, and coupled pair behaviors, with implications for experimental measurements.

Contribution

It provides a detailed analysis of the differences between the eight-state and two-level models for cubic defects, including experimental proposals and theoretical insights.

Findings

Different temperature dependence of specific heat and susceptibility predicted by models

Unique features in external force responses in the eight-state model

Distinct behaviors in strongly coupled defect pairs

Abstract

Substitutional defects in a cubic symmetry (such as a lithium defect in a KCl host crystal) can be modeled appropriately by an eight-state-system. Usually this tunneling degree of freedom is approximated by a two-level-system. We investigate the observable differences between the two models in three contexts. First we show that the two models predict different relations between the temperature dependence of specific heat and static susceptibility. Second we demonstrate that in the presence of external forces (pressure and electric field) the eight-state-system shows features that cannot be understood within the framework of the two-level-approximation. In this context we propose an experiment for measuring the parameter for tunneling along the face diagonal. Finally we discuss the differences between the models appearing for strongly coupled pairs. Geometric selection rules and particular forms of asymmetry lead to clear differences between the two models.