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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

The conductance-voltage G(V) characteristics for contacts between electrodes of highly oriented pyrolytic graphite were studied in a wide range of G(0), bias voltages, and temperatures using the mechanically controllable break junction technique. Our results show a striking resemblance to corresponding data for conducting multiwall carbon nanotubes. For high-ohmic contacts between a single or a few graphene sheets, zero bias anomalies related to strong e–e Coulomb interactions and quantum interference of electrons is observed.

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 ${G}_{0}=2{e}^{2}/h$.

Magnetic particles of microscopic size can be created in the process of Ni, Fe and Co mechanically controllable break junctions fabrication and trapped between the electrodes by magnetic dipole forces. Tunneling between the protruding nanoparticle and the sample electrode shows clear distinctions from the usual junctions: heavy deviation of the current–distance I(z) dependence from the expected exponential behavior at electrode separations z below 4.0– 4.5 A and on numerous occasions a sudden jump-like decrease of the tunnel current at z≈1.5– 2.0 A . Possible mechanisms behind observed anomalies including the short-range magnetic exchange coupling are discussed.

The additional information provided by field emission resonance spectra significantly increases the potential of the mechanically controllable break junction technique. We have found that the pronounced three-dimensional nature of the electrodes results in an extremely high sensitivity of the field emission resonance spectra to the fine details of the surface geometry, electronic structure and electric field distribution at small electrode separations. At larger distances a quasiclassical approximation can be used for the determination of the metal work function, precise calibration of the electrode relative displacement and discrimination between “blunt” and “sharp” electrodes on the basis of the distance–voltage z ( V ) dependence.

Bulk resistivity and point contact spectra of ${\mathrm{Ni}}_{x}{\mathrm{Nb}}_{1\ensuremath{-}x}$ metallic glasses have been investigated as functions of temperature (0.3--300 K) and magnetic field (0--12 T). Metallic glasses in this family undergo a superconducting phase transition determined by the Nb concentration. When superconductivity was suppressed by a strong magnetic field, both the bulk sample $R(T)$ and the point contact differential resistance curves of ${\mathrm{Ni}}_{x}{\mathrm{Nb}}_{1\ensuremath{-}x}$ showed logarithmic behavior at low energies, which is explained by a strong electron-``two level system'' coupling. We studied the temperature, magnetic field, and contact resistance dependence of ${\mathrm{Ni}}_{44}{\mathrm{Nb}}_{56}$ point-contact spectra in the superconducting state and found telegraphlike fluctuations superimposed on superconducting characteristics. These $R(V)$ characteristics are extremely sensitive detectors for slow relaxing ``two level system'' motion.

This article presents an accurate method for calibration of the relative displacement Δz of mechanically controllable break junction electrodes and determination of the metal work function φ using oscillating behavior of tunnel conductance (known also as a Gundlach oscillation) in a field emission region. The precision of the proposed method is at least one order of magnitude better than the most reasonable estimations that have previously been possible. In combination with tunnel current-distance dependence I(z) measurements, the absolute vacuum gap can be evaluated with an accuracy ±0.25 A.

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.

The contributions by three different mechanisms of interactions between electrons and two-level fluctuators to the low-energy singularity present in point-contact spectra of metallic glasses have been studied by measuring rf response signals at 600 MHz and 60 GHz, and the low-frequency response at 1.85 kHz. The resulting curves indicate that a nonmagnetic Kondo-like interaction is the most important contribution, but, depending on the exact shape of the background signal due to electron-phonon and electron-electron interactions, elastic scattering on highly asymmetric two-level fluctuators may also be quite important.

In the conventional tunneling picture, the dependence of the tunnel resistance on the distance between two electrodes in vacuum is expected to be exponential. We have observed, however, clear deviations from this behavior over four to six orders of magnitude of the tunnel resistance at constant biases of 5–100 mV, using highly stable mechanically controllable break junctions of Al, Ag, Au, Pt, and Pdr. Previous theoretical studies indicate that deviations from exponential behavior can be caused by interactions between the two electrodes, as well as by certain electrode geometries. Since the first are predicted to influence the tunnel resistance over at most two orders of magnitude in the region close to contact, whereas the latter may extend over more than four orders of magnitude, we believe to have observed effects due to several different types of geometries.

The observation of, often subtle, effects due to interactions between the electrodes of a tunnel junction, requires a very stable configuration. This requirement is met by a mechanically controllable break (MCB) junction. Our measurements show clear indications of strong adhesion forces between the two electrodes. Using the MCB in an STM-like mode, we also found anomalously high corrugations on simple metal surfaces (e.g. Au).

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.

Peculiarities of point-contact spectra behavior of antimony are investigated in a magnetic field. It turns out that the singularities in the spectra can both decrease and increase depending not only on orientation of the contact axes with respect to the crystallographic axis of Sb and the magnetic field vector, but also on the ratio between the contact diameter, the Larmour radius of the charge carriers and their mean free path. The spectrum sign inversion for a dirty contact that is due to electron state delocalization as a result of electronphonon interaction is detected. It means that point-contact spectroscopy is possible in highly disordered materials when the quantum localization is significant.

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

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 gold 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., Comment: This version corrects an error in attributing the three observed periods, and includes a comparison with recent model calculations

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.

The conductance of breaking metallic nanojunctions shows plateaus alternated with sudden jumps, corresponding to the stretching of stable atomic configurations and atomic rearrangements, respectively. We investigate the structure of the conductance plateaus both by measuring the voltage dependence of the plateaus' slope on individual junctions and by a detailed statistical analysis on a large amount of contacts. Though the atomic discreteness of the junction plays a fundamental role in the evolution of the conductance, we find that the fine structure of the conductance plateaus is determined by quantum interference phenomenon to a great extent., 4 pages, 4 figures

Contains fulltext : 57617.pdf (Publisher’s version ) (Open Access) We fabricate iron nanolines by depositing an atomic beam of iron through a far-off resonant laser standing wave (SW) onto a glass-ceramic substrate. The laser SW is tuned 200 MHz above the D-5(4)-->F-5(5)o Fe-56 transition at a vacuum wavelength of 372.099 nm. The resulting nanolines exhibit a period of 186 nm, a height above the background of 8 nm and a full width at half maximum of 95 nm. These nanostructures cover a surface area of similar or equal to1.6x0.4 mm(2), corresponding to similar or equal to8600 iron lines with a length of similar or equal to400 mum. (C) 2004 American Institute of Physics.

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

Answer to the Comment on ``Point-Contact Study of Fast and Slow Two-Level Fluctuators in Metallic Glasses'' by Jan von Delft et al., 3 pages, no figures, accepted Phys. Rev. Letters

Contains fulltext : 28786.pdf (Publisher’s version ) (Open Access)

Contains fulltext : 29708.pdf (Publisher’s version ) (Open Access)

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

Mechanically controllable break junctions with nearly flat (at a lateral scale of ≃10 nm) electrodes and a greatly reduced number of surface defects were fabricated using a gentle procedure of surface modification. The modification process occurs at extremely close separations between the electrodes (≃1.5–2 A) where high electric fields and surface temperatures can be achieved at relatively low bias voltages. Experimental details, a possible physical mechanism and applications of the proposed technique are discussed.

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]

Dynamic evolution of the connective neck in Al and Pb mechanically controllable break junctions was studied during continuous approach of electrodes at bias voltages V_b up to a few hundred mV. A high level of power dissipation (10^-4 - 10^-3 W) and high current density (j > 10^10 A/cm^2) in the constriction lead to overheating of the contact area, electromigration and current-enhanced diffusion of atoms out of the "hot spot". At a low electrode approach rate (10 - 50 pm/s) the transverse dimension of the neck and the conductance of the junction depend on V_b and remain nearly constant over the approach distance of 10 - 30 nm. For V_b > 300 mV the connective neck consists of a few atoms only and the quantum nature of conductance manifests itself in abrupt steps and reversible jumps between two or more levels. These features are related to an ever changing number of individual conductance channels due to the continuous rearrangement in atomic configuration of the neck, the recurring motion of atoms between metastable states, the formation and breaking of isolated one-atom contacts and the switching between energetically preferable neck geometries., 21 pages 10 figures