Buckled nano rod - a two state system and its dynamics using system plus reservoir model

TL;DR

This paper models a buckled nano rod as a two-state system, analyzing its dynamics and transition rates between buckled states using a system-plus-reservoir approach, including quantum effects and friction.

Contribution

It extends previous classical rate calculations to include quantum tunneling effects in the buckled nano rod system using a system-plus-reservoir model.

Findings

Quantum rate calculations show friction reduces transition rates.

Rate remains finite at higher buckling instabilities due to new saddle configurations.

Quantum effects influence the stability and transition dynamics of the buckled states.

Abstract

We consider a suspended elastic rod under longitudinal compression. The compression can be used to adjust potential energy for transverse displacements from harmonic to double well regime. As compressional strain is increased to the buckling instability, the frequency of fundamental vibrational mode drops continuously to zero (first buckling instability). As one tunes the separation between ends of a rod, the system remains stable beyond the instability and develops a double well potential for transverse motion. The two minima in potential energy curve describe two possible buckled states at a particular strain. From one buckled state it can go over to the other by thermal fluctuations or quantum tunnelling. Using a continuum approach and transition state theory (TST) one can calculate the rate of conversion from one state to other. Saddle point for the change from one state to other is the straight rod configuration. The rate, however, diverges at the second buckling instability. At this point, the straight rod configuration, which was a saddle till then, becomes hill top and two new saddles are generated. The new saddles have bent configurations and as rod goes through further instabilities, they remain stable and the rate calculated according to harmonic approximation around saddle point remains finite. In our earlier paper classical rate calculation including friction has been carried out [J. Comput. Theor. Nanosci. {\bf 4} (2007) {\it 1}], by assuming that each segment of the rod is coupled to its own collection of harmonic oscillators - our rate expression is well behaved through the second buckling instability. In this paper we have extended our method to calculate quantum rate using the same system plus reservoir model. We find that friction lowers the rate of conversion.