Buckled nano rod – a two state system and quantum effects on its dynamics using system plus reservoir model

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 the fundamental vibrational mode drops continuously to zero (first buckling instability). As one tunes the separation between the ends of a rod, the system remains stable beyond the instability and develops a double-well potential for transverse motion. The two minima in the 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 the other. The saddle point for the change from one state to the 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 until then, becomes a hill top and two new saddles are generated. The new saddles have bent configurations and as the rod goes through further instabilities, they remain stable and the rate calculated according to harmonic approximation around a saddle point remains finite. In our earlier paper, a classical rate calculation including friction was carried out [J. Comput. Theor. Nanosci. 4, 1 (2007)], 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.

Keywords:

• buckled
• nano rod
• quantum
• classical
• transition state theory

Acknowledgements

The author is greatful to Prof. K.L. Sebastian for his continuous guidance and for suggesting this very interesting problem. The author also thanks Prof. Eli Pollak for useful comments and suggestions on the manuscript.