Stability of quantum and Sharvin constrictions

TL;DR

This study investigates the mechanical stability and deformation modes of gold atomic constrictions across quantum to semi-classical regimes, revealing fundamental deformation crossovers and size-dependent modulus enhancements.

Contribution

It provides new insights into atomic-scale deformation mechanisms and stability in gold constrictions, identifying size-dependent deformation crossovers and quantifying modulus enhancements at the nanoscale.

Findings

Discrete atomic displacements correspond to elementary slip distances in gold.

Two fundamental deformation crossovers are identified: from homogeneous shear to defect-mediated deformation, and from surface to volume-dominated deformation.

Size-dependent modulus enhancement, at least twice that of bulk gold, is observed near the Fermi wavelength.

Abstract

Previously, the authors reported direct evidence of channel saturation and conductance quantization in atomic-sized gold constrictions through mechanical perturbation studies, and also showed that peaks in conductance histograms are insufficient in evaluating their mechanical stability [Armstrong et al., Phys. Rev. B 82, 195416 (2010)]. In the present study, gold constrictions spanning the range from quantum to the semi-classical (Sharvin) conductance regimes are mechanically probed with pico-level resolution in applied force and deformation, along with simultaneous measurements of conductance. While reconfiguration from one constriction size to another is known to occur by apparently random discrete atomic displacements, results reveal a remarkable simplicity - the magnitude of discrete atomic displacements is limited to a small set of values that correspond to elementary slip distances in gold rather than Au-Au inter-atomic distance. Combined with measurements of spring constant of constrictions, results reveal two fundamental crossovers in deformation modes with increasing contact diameter - first, from homogeneous shear to defect mediated deformation at a diameter that is in close agreement with previous predictions (Sorensen et al., Phys. Rev. B 57, 3283 (1998)]; second, the discovery of another crossover marking surface to volume dominated deformation. A remarkable modulus enhancement is observed when the size of the constrictions approaches the Fermi wavelength of the electrons, and in the limit of a single-atom constriction it is at least 2 times that for bulk gold. Results provide atomistic insight into the stability of these constrictions and an evolutionary trace of deformation modes, beginning with a single-atom contact.