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Cold metal working, which leads to strengthening and changes in various physical properties of metals on a macroscopic level, can have a significant effect on the conductance of nanowires drawn from these materials when they break. This effect is studied for Ag, Cu, Pt, and a number of other metals. Molecular hydrogen is found to affect the characteristics of breaking in silver nanowires.

The increase of the bias voltage V b above 0.4–0.5 V results in the emerging of new peaks in the conductance histograms of W, Mo and Ta. Only for Ta and for some of the Mo contacts these peaks are reproducible and related to stable atomic arrangements. For W the additional peaks are always contact specific. The shape of the conductance histogram at elevated V b depends also on the bias voltage and is changing in the process of measurements for the majority of Mo contacts. Possible reasons for this non-typical behavior of conductance of molybdenum and tungsten nanocontacts are discussed.

Mechanically Controllable Break-Junctions were used to study the physical adsorption of hydrogen molecules on the surfaces of W, Mo and Ta as well as their subsequent chemisorption and dissociation and diffusion of protons into the bulk of the material. Dissolving of hydrogen into Ta is accompanied by 1/f α resistance noise with 1.2

Contains fulltext : 58476.pdf (Publisher’s version ) (Open Access) We used the mechanically controllable break junction technique to discriminate between the electric field and temperature effects in the process of surface modification. The electric field strength at the surface of electrodes was accurately determined using the field emission resonance spectra and was gradually raised to the point where surface modification starts. We found that only a limited number of metals with large values of the work function, greater than or similar to4.5-5 eV, and evaporation fields, less than or similar to1.5-2 V/Angstrom, can be modified in this way. Adsorption of He on the surface drastically increases the local work function of material and enables the field induced nanostructuring of the electrode surface practically for all metals. (C) 2004 American Institute of Physics.

Interaction of the physically adsorbed molecular hydrogen with a breaking gold nanowire results in additional stable atomic configurations in few atom contacts and appearance of fractional peaks in the conductance histogram. This effect is explained by peculiar dynamic evolution of the hydrogen-embedded nanoconstriction due to competition between tensile and capillary forces. Dimerization within the atomic wire and hydrogen-assisted stabilization of gold dimers results in preferable atomic arrangements with conductivity close to 0.5 and 1.5 of quantum conductance unit ${G}_{0}=2{e}^{2}/h$.

The tunnel resistance of highly stable, mechanically controlled break junctions of Al, Au, Cu, Pb, Ni, Pt, and Pt-Ir, have been recorded as a function of the electrode spacing over 6-7 decades. Clear deviations from the expected exponential behavior have been observed. Comparison with previous experimental and theoretical studies indicate that the discussed deviations in some cases are most probably due to the shape of, rather than to interactions between, the two electrodes.

The tunnel resistance between two metallic electrodes of a mechanically controllable break junction, measured as a function of electrode separation, displays a clear deviation from exponential behavior when He is adsorbed at the electrodes at low temperatures. At T=1.2 K, two-level resistance fluctuations occur in a limited resistance range which are attributed to transfers of He atoms between adsorption potential minima. The transfer rate varies with the applied bias voltage to a power p, which is between 1.3 and 1.7 for 4He, and about 2 for 3He. This bias dependence may be due to inelastic interactions between tunneling electrons and the adsorbed He.

Physisorption of hydrogen molecules on the surface of gold and other coinage metals has been studied using distance tunneling spectroscopy. We have observed that the distance dependence of the tunnel current (resistance) displays a strong N-shaped deviation from exponential behavior. Such deviations are difficult to explain within the Tersoff–Hamann approximation. We suggest the scattering of tunneling electrons by H2 molecules as an origin for the observed effect. We have found that this phenomenon is also common for strongly adsorbed organic molecules with a single anchoring group. Pulling Au, Cu and Pt nanowires at 22 K in hydrogen environment shows that the break-junction electrodes are still connected through hydrogen–metal monoatomic chains down to very low conductance values of 10−4–10−6 G0.

We used the mechanically controlled break-junction technique to observe the conductance through carbon monoxide molecules in several nonpolar liquids (toluene, heptane, cyclohexane). CO molecules bridge the electrodes prior to the break, causing the displacement of the first peak in the conductance histograms from ∼1.0 to 1.1-1.15 of the quantum conductance unit (G0 ) 2e 2 /h). After the break, these molecules remain attached to the electrodes and a peak related to the conductance through a CO molecule appears at 0.2-0.3 G0.

We used the mechanically controllable break-junction technique to study the consecutive stages of interaction of hydrogen molecules with surfaces of W, Mo, and Ta at 4.2--6 K. This includes the physical adsorption of ${\text{H}}_{2}$ molecules, their subsequent chemisorption, their dissociation, and the diffusion of protons into the bulk of the materials. The quantum diffusion is accompanied by $1/{f}^{\ensuremath{\alpha}}$ contact resistance noise. For tantalum hydride ${\text{TaH}}_{x}$, we found a 10% increase of the critical temperature ${T}_{c}$ with respect to pure Ta.

Conductance histograms of work-hardened Al show a series up to 11 equidistant peaks with a period of 1.15 +/- 0.02 of the quantum conductance unit G_0 = 2e^2/h. Assuming the peaks originate from atomic discreteness, this agrees with the value of 1.16 G_0 per atom obtained in numerical calculations by Hasmy et al., Comment: 4 pages, 4 figures

A strong deviation from an exponential dependence of the tunnel current on the distance between two electrodes was observed when I ( s ) characteristics of mechanically controllable break junctions of various materials were measured in 4 He and 3 He exchange gas. At 1.2 K, a two-level fluctuation of the tunnel resistance was observed in 4 He, which can be explained to be caused by the tunneling of a helium atom between the electrodes.

We apply the mechanically controllable break junctions technique to investigate the transition from tunneling to direct contact in tungsten. This transition is quite different from that of other metals and is determined by the local electronic properties of the tungsten surface and the relief of the electrodes at the point of their closest proximity. The conductance traces show a rich variety of patterns from the avalanche-like jump to a mesoscopic contact to the completely smooth transition between direct contact and tunneling. Due to the occasional absence of an adhesive jump the conductance of the contact can be continuously monitored at ultra-small electrode separations. The conductance histograms of tungsten are either featureless or show two distinct peaks related to the sequential opening of spatially separated groups of conductance channels. The role of surface states of tungsten and their contribution to the junction conductance at sub-Angstrom electrode separations are discussed., 6 pages, 6 figures

An adaptation of the mechanically controllable break junction technique using thin metallic films to create point contacts and tunnel junctions is presented. The junctions that are created using this very simple method are extremely stable, and can easily be adjusted from rather large point contacts (R≊0.1 Ω) to high‐resistance (1010 Ω) tunnel junctions. The electrodes can be kept very clean by breaking the sample in ultrahigh vacuum or in a very pure helium atmosphere at 1–4 K. Point‐contact spectra of good quality were measured for contacts of 0.1–500 Ω. For very small contacts, the expected steplike increase of junction resistance with decreasing contact size was observed, with a jump to tunneling behavior occurring at resistances of 12–14 kΩ.

Contains fulltext : 29709.pdf (Publisher’s version ) (Open Access)

Field emission resonance (FER) spectra measurements performed with the mechanically controllable break junction technique for Au, Be, Cu, Mg, Dy, Ni, Co, Fe, Pb, Pd and Pt have uncovered anomalously large increases in metal work functions caused by physisorption of He atoms. These results are in sharp contrast to the data for the other rare gases where the work functions decrease on a more moderate scale. At high coverage of He or for junctions measured directly in liquid helium the potential barrier height at the metal–helium interface is approximately twice as large as the work function of the metal under investigation.

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-H_2 junctions, and the possible microscopic realizations of the new configurations in palladium-hydrogen atomic-sized contacts are discussed., 4 pages, 4 figures

Al-insulator—Pb tunnel junctions with barriers produced in a glow discharge of CO2 or CO are studied. It is shown that for the first case a barrier is similar in its composition to those prepared using the plasma oxidation technique but contains molecules of the chemisorbed formic acid. HCOOH is found to be formed due to the organic synthesis in the glow discharge. In the second case there are junctions with barriers of 0.25 to 0.3 eV height. Certain assumptions of the barrier composition are made. [Russian Text Ignored]