Spin-state smectics in spin crossover materials

TL;DR

This paper introduces a two-dimensional model for spin crossover materials that exhibits spin-state smectic phases, characterized by one-dimensional order and unique thermodynamic properties, including residual entropy scaling.

Contribution

It identifies and characterizes spin-state smectic phases in a simple model, revealing their symmetry-breaking and thermodynamic properties, distinct from spin-state liquids and crystals.

Findings

Discovery of spin-state smectic phases with one-dimensional order.

Observation of plateaus in high-spin fraction.

Residual entropy scales with system size in smectic phases.

Abstract

We show that a simple two dimensional model of spin crossover materials gives rise to spin-state smectic phases where the pattern of high-spin (HS) and low-spin (LS) metal centers spontaneously breaks rotational symmetry and translational symmetry in one direction only. The spin-state smectics are distinct thermodynamic phases and give rise to plateaus in the fraction of HS metal centers. Smectic order leads to lines of Bragg peaks in the x-ray and neutron scattering structure factors. We identify two smectic phases and show that both are ordered in one direction, but disordered in the other, and hence that their residual entropy scales with the linear dimension of the system. This is intermediate to spin-state ices (examples of `spin-state liquids') where the residual entropy scales with the system volume, and antiferroeleastic ordered phases (examples of `spin-state crystals') where the residual entropy is independent of the size of the system.