The fabrication and physics of free-standing wires and short structures
Using electron beam lithography fine Au60Pd40 wire samples have been made, which were then used to investigate size effects in disordered systems. The wires made had widths from 140nm to 36nm, and lengths from 125nm to 100μm. The thickness was varied from 10nm to 50nm. Using gas etching techniques it was possible to make these wires free-standing with lengths of over 100μm and thicknesses of 10nm. At first some of the important electrical and mechanical properties of the free-standing wires were investigated. The tensile strength was found to be much larger than that found in bulk gold, and a simple model was developed to predict at what voltage the wires would melt. At low temperatures localisation and interaction contributions to the resistance were seen both in wires on a substrate and when free-standing. Electric field heating of the wires on a substrate was used to find the electron-phonon scattering time, which was seen to be proportional to T-2. The resistance rise for the free-standing wires was less than for the same wires on a substrate. Magnetoresistance measurements revealed this to be due to an increased negative localisation term in the presence of strong spin orbit scattering. The inelastic lengths were consistent with strong magnetic scattering which was reduced on being made free-standing. Electric field heating of the free-standing wires revealed that the thermal conduction of the wire is dominated by the electron contribution. At 1K the phonons in the wires should be one dimensional, but no contribution to the heat conduction was observed from these phonons. Below 800mK conductance fluctuations were observed in the magnetoresistance of the short samples. Below 125 K the amplitude of the fluctuations saturated which is consistent with both the interaction and the inelastic lengths being longer than the wire.