Stability of quantum and Sharvin constrictions
  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. 
Jason N. Armstrong, Susan Z. Hua, and Harsh Deep Chopra
cond-mat.mes-hall