Mixed-order phase transition in a colloidal crystal

TL;DR

This paper reports the first experimental observation of a mixed-order phase transition in a colloidal crystal, combining a discontinuous change in structure with critical fluctuations, and provides analytical predictions and experimental validation.

Contribution

It is the first to experimentally realize and analyze a mixed-order phase transition in a colloidal system, confirming theoretical predictions.

Findings

Diverging fluctuations and correlation length at the transition point

Discontinuous solid-solid transition induced by magnetic field

Mean-field critical exponents consistent with theory

Abstract

Mixed-order phase transitions display a discontinuity in the order parameter like first-order transitions yet feature critical behavior like second-order transitions. Such transitions have been predicted for a broad range of equilibrium and nonequilibrium systems, but their experimental observation has remained elusive. Here, we analytically predict and experimentally realize a mixed-order equilibrium phase transition. Specifically, a discontinuous solid-solid transition in a two-dimensional crystal of paramagnetic colloidal particles is induced by a magnetic field $H$. At the transition field $H_{\text{s}}$, the energy landscape of the system becomes completely flat, which causes diverging fluctuations and correlation length $\xi\propto | H^2 - H_{\text{s}}^2|^{-1/2}$. Mean-field critical exponents are predicted, since the upper critical dimension of the transition is $d_{\text{u}}=2$. Our colloidal system provides an experimental test bed to probe the unconventional properties of mixed-order phase transitions.