Friction Anomalies at First-Order Transition Spinodals: 1T-TaS$_2$

TL;DR

This paper demonstrates that nanofriction measurements can detect spinodal first-order phase transitions in 1T-TaS$_2$, revealing mechanical anomalies associated with the transformation triggered by AFM tip perturbations.

Contribution

It introduces a novel method to observe first-order spinodal transitions through nanofriction, previously unexplored for such phase changes.

Findings

Friction anomalies occur near spinodal points during phase transitions.

AFM tip perturbations can locally trigger and reveal spinodal transformations.

The method is demonstrated on the 1T-TaS$_2$ compound.

Abstract

Revealing phase transitions of solids through mechanical anomalies in the friction of nanotips sliding on their surfaces is an unconventional and instructive tool for continuous transitions, unexplored for first-order ones. Owing to slow nucleation, first-order structural transformations generally do not occur at the precise crossing of free energies, but hysteretically, near the spinodal temperatures where, below and above the thermodynamic transition temperature, one or the other metastable free energy branches terminates. The spinodal transformation, a collective one-shot event with no heat capacity anomaly, is easy to trigger by a weak external perturbations. Here we propose that even the gossamer mechanical action of an AFM tip may locally act as a surface trigger, narrowly preempting the spontaneous spinodal transformation, and making it observable as a nanofrictional anomaly. Confirming this expectation, the CCDW-NCCDW first-order transition of the important layer compound 1T-TaS$_2$ is shown to provide a demonstration of this effect.