Comments on `Irreversibility in Response to Forces Acting on Graphene Sheets'

TL;DR

This paper critiques a previous study on the irreversibility of the compression-relaxation mechanism in graphene sheets, arguing that the mechanism is actually reversible and highlighting issues in the original analysis.

Contribution

It clarifies the reversibility of CRM in graphene sheets by addressing technical flaws and theoretical considerations in the prior work.

Findings

The original claim of irreversibility is invalid due to boundary stress issues.

The relation ΔA=<W> is valid only for infinitely slow processes, which was not properly accounted for.

The introduced rough state is actually a rippled state with boundary stress.

Abstract

In the letter "Phys. Rev. Lett 104, 196804 (2010)" the compression-relaxation mechanism (CRM) for a graphene sheet (GS) was reported to be irreversible and resulted in static ripples on GS, such that for T <Tc the free-energy of the rippled GS is smaller than that of roughened GS. We will point out several technical difficulties, such as the use of the relation Delta A=<W> for the free energy calculations and the definition of the rough state, with their simulations. We show that (at T <Tc) their introduced rough state suffer boundary stress, thus is a rippled state and their obtained inequality A_{ripple}<A_{rough} is no longer valid. Therefore the introduced mechanism is reversible. Furthermore, from theoretical point of view for an infinitely slow rate of CRM, the relation Delta A=<W> (in common non-equilibrium simulations) is allowed. In this case authors of Ref. [1] must report the used infinitely small rate and justify how it is in practice valid. We show Delta A <> <W> for the system that was used in [1].