Nanowires of different nature have been shown to self-assemble as a function of stress at the contact between two macroscopic metallic leads. Here we demonstrate for Au wires that the balance between various metastable nanowire configurations is influenced by the microstructure of the starting materials and we discover a new set of periodic structures, which we interpret as due to the atomic discreteness of the contact size for the three principal crystal orientations.PACS numbers: 73.40. Jn, 61.46.+w, 68.65.La Metallic nanowires have shown rich properties in terms of self-organization phenomena that are controlled by a combination of the quantum nature of the conduction electrons and the atomic-scale surface energy (see review [1] and references therein). Here we report yet another surprising series of stable structures.At the very smallest scale the metals Au, Pt and Ir, spontaneously form into chains of atoms [2,3,4]. For slightly larger diameters further unusual arrangements referred to as 'weird wires' were predicted [5] and later observed in high-resolution transmission electron microscopy (TEM) [6,7,8]. The observed structures for Au have a helical arrangement in the form of concentric shells of atoms.In contrast to these atomic-packing driven structures, electronic shell filling has been shown to lead to an independent series of stable nanowire (NW) diameters for the free-electron-like alkali metals [9,10], and the noble metals [11,12,13]. These NWs were not imaged as in TEM, but their stability was inferred from frequently occurring stable conductance values during gentle breaking of the contacts (see below). The series of stable values has a characteristic period when plotted as a function of the square root of the conductance, √ G, which is a measure of the radius of the wires.Finally, regular bulk-packing of NWs has also been observed, where the surface energy is the driving force, leading to completing of flat facets of the wires. Such effects have been observed in TEM [14,15,16] as remarkably long and stable wires, mostly for Au along the [110] direction. This atomic shell filling series has also been observed in the conductance [17], again as a regular period in √ G. Exactly which of these types of NWs self-assembles appears to depend critically on the experimental conditions, which is not fully understood. We anticipate that the selection of local minima in the free energy is influenced by the dynamics of the wire formation and the boundary conditions imposed by the structure of the leads. Here, we present evidence that the microstructure of the starting material, whether work hardened or annealed, influences the appearance of a new series of stable NWs that are periodic in the conductance, G, as opposed to √ G for the NWs observed previously.Gold is the archetypal metal when investigating quantum transport phenomena in atomic-sized contacts [1]. The initial interest into these systems was in quantization of the conductance, in conjunction with the atomic discreteness of the contacts. The expe...
Results of an experimental study of palladium nanojunctions in hydrogen environment are presented. Two new hydrogen-related atomic configurations are found, which have a conductances of ∼ 0.5 and ∼ 1 quantum unit (2e 2 /h). Phonon spectrum measurements demonstrate that these configurations are situated between electrodes containing dissolved hydrogen. The crucial differences compared to the previously studied Pt-H2 junctions, and the possible microscopic realizations of the new configurations in palladium-hydrogen atomic-sized contacts are discussed.
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