Directional bonding explains the high conductance of atomic contacts in bcc metals

W Dednam and C Sabater and MR Calvo and C Untiedt and JJ Palacios and AE Botha and MJ Caturla, PHYSICAL REVIEW B, 101, 165417 (2020).

DOI: 10.1103/PhysRevB.101.165417

Atomic-sized contacts of iron, created in scanning tunneling microscope break junctions, present unusually high values of conductance compared to other metals. This result is counterintuitive since, at the nanoscale, body-centered-cubic metals are expected to exhibit lower coordination than face-centered-cubic metals. In this work we first perform classical molecular dynamics simulations of the contact rupture, using two different interatomic potentials. The first potential is isotropic, and produces mostly single-atom prerupture contacts. The second potential accounts for the directional bonding in the materials, and produces mostly highly coordinated prerupture structures, generally consisting of more than one atom in contact. To compare the two different types of structures with experiments, we use them as input to density functional theory electronic transport calculations of the conductance. We find that the highly coordinated structures, obtained from the anisotropic potential, yield higher conductances which are statistically in better agreement with those measured for body-centered- cubic iron. We thus conclude that the directional bonding plays an important role in body-centered-cubic metals.

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