Conductance Quantization in Metallic Nanowires

Abstract

Recent investigations carried out in our group concerning the conductance quantization of metallic nanowires are reviewed. These include: i) The formation of metallic nanowires between macroscopic electrodes, including liquid metals, demonstrating that at the last stages of the contact breakage, a nanowire exists, independently of the initial contact size. ii) A statistical study of the conductance using thousands of consecutive contact breakage experiments, both at room and at liquid helium temperatures. These histograms, totally reproducible, present clear peaks close to integer values of the quantum of conductance G0=2e2/h for diamagnetic metals like Gold, Silver, Copper, Sodium, Platinum.... Ferromagnetic metals, Iron, Cobalt and Nickel, exhibit a flat conductance histogram. This effect is attributed to the combination of the lifting of the spin degeneracy in the ferromagnetic nanowires and the effect of geometry and disorder. The measured conductance histograms are basically independent of the temperature. iii) A discussion of the position and width of the observed peaks. Just geometrical effects can not explain the large conductance peak shifts observed experimentally, and disorder, behaving as a residual resistance, has to be invoked to explain them. iv) First realization of conductance quantization in Bi at 4K. Conductance plateaus lasting 20-100 nm electrode separation are presented; the histogram displays also clear peaks. v) A statistical study of the conductance plateau duration, demonstrating a broad distribution of this duration, 0.05-0.4 nm, with an average value that decreases as conductance increases. vi) A discussion of force and energy quantization within a resonant energy model of two reservoirs connected by a ballistic channel. vii) Experiments performed in ultra high vacuum, where we manage to stabilize the nanowires for hours and study switching and current voltage characteristics for different quantum conductance channels with remarkable accuracy. viii) Visualization inside scanning and transmission electron microscopes of the metallic contact between two macroscopic electrodes at the micron and nanometer scales. These experiments provide experimental evidence of the formation of a connective neck between the electrodes. ix) Experiments on light emission from breaking nanowires. A plausible explanation for this phenomenon is presented.