A new mechanism of structural transition in 2D Hertzian spheres in the presence of random pinning

TL;DR

This study uses molecular dynamics to show that random pinning significantly alters the phase transition mechanisms in 2D Hertzian spheres, introducing new intermediate phases and changing the nature of transitions.

Contribution

It reveals for the first time that random pinning induces new phases and changes the transition types in 2D Hertzian sphere systems.

Findings

Random pinning creates hexatic and tetratic phases.

Transitions include BKT-type and first-order.

Phase diagram is significantly altered.

Abstract

Using molecular dynamics simulation we have investigated the influence of random pinning on the phase diagram and melting scenarios of a two-dimensional (2D) system with the Hertz potential for $\alpha=5/2$. For the first time it has been shown that random pinning can cardinally change the mechanism of first-order transition between the different crystalline phases (triangular and square) by virtue of generating hexatic and tetratic phases: a triangular crystal to hexatic transition is of the continuous Berezinskii-Kosterlitz-Thouless (BKT) type, a hexatic to tetratic transition is of the first-order, and finally, a continuous BKT type transition from tetratic to the square crystal.