Quantum suppression of shot noise in atom-size metallic contacts

The transmission of conductance modes in atom-size gold contacts is investigated by simultaneously measuring conductance and shot noise. The results give unambiguous evidence that the current in the smallest gold contacts is mostly carried by nearly fully transmitted modes. In particular, for a single-atom contact the contribution of additional modes is only a few percent. In contrast, the trivalent metal aluminum does not show this property.Comment: Fig. 2 replaced, small errors correcte

Effect of bonding of a CO molecule on the conductance of atomic metal wires

We have measured the effect of bonding of a CO molecule on the conductance of Au, Cu, Pt, and Ni atomic contacts at 4.2 K. When CO gas is admitted to the metal nano contacts, a conductance feature appears in the conductance histogram near 0.5 of the quantum unit of conductance, for all metals. For Au, the intensity of this fractional conductance feature can be tuned with the bias voltage, and it disappears at high bias voltage (above $\sim$ 200 mV). The bonding of CO to Au appears to be weakest, and associated with monotomic Au wire formation.Comment: 6 figure

Observation of electronic and atomic shell effects in gold nanowires

The formation of gold nanowires in vacuum at room temperature reveals a periodic spectrum of exceptionally stable diameters. This is identified as shell structure similar to that which was recently discovered for alkali metals at low temperatures. The gold nanowires present two competing `magic' series of stable diameters, one governed by electronic structure and the other by the atomic packing.Comment: 4 pages, 4 figure

Observation of shell structure in sodium nanowires

The quantum states of a system of particles in a finite spatial domain in general consist of a set of discrete energy eigenvalues; these are usually grouped into bunches of degenerate or close-lying levels, called shells. In fermionic systems, this gives rise to a local minimum in the total energy when all the states of a given shell are occupied. In particular, the closed-shell electronic configuration of the noble gases produces their exceptional stability. Shell effects have previously been observed for protons and neutrons in nuclei and for clusters of metal atoms. Here we report the observation of shell effects in an open system - a sodium metal nanowire connecting two bulk sodium metal electrodes, which are progressively pulled apart. We measure oscillations in the statistical distribution of conductance values, for contact cross-sections containing up to a hundred atoms or more. The period follows the law expected from shell-closure effects, similar to the abundance peaks at `magic numbers' of atoms in metal clusters.Comment: The argumentation in favour of shell structure owing to the fluctuations in the free energy of the nanowires has been strengthened. Further improvements in the presentation include the plot of the radius of the wires versus shell number in Fig.

Quantum interference effects in a system of two tunnel point-contacts in the presence of single scatterer: simulation of a double-tip STM experiment

The conductance of systems containing two tunnel point-contacts and a single subsurface scatterer is investigated theoretically. The problem is solved in the approximation of s-wave scattering giving analytical expressions for the wave functions and for the conductance of the system. Conductance oscillations resulting from the interference of electron waves passing through different contacts and their interference with the waves scattered by the defect are analyzed. The prospect for determining the depth of the impurity below the metal surface by using the dependence of the conductance as a function of the distance between the contacts is discussed. It is shown that the application of an external magnetic field results in Aharonov-Bohm type oscillations in the conductance, the period of which allows detection of the depth of the defect in a double tip STM experiment.Comment: 11 pages, 4 figures, to be published in Fiz. Nizk. Temp. (Low Temp. Phys.), V.37, No.1 (2011) corrected figure

Geometry and Conductance of Al Wires Suspended between Semi-Infinite Crystalline Electrodes

We present a first-principles study of a coherent relationship between the optimized geometry and conductance of a three-aluminum-atom wire during its elongation process. Our simulation employs the most definite model including semi-infinite crystalline electrodes using the overbridging boundary-matching method [Phys. Rev. B {\bf 67}, 195315 (2003)] extended to incorporate nonlocal pseudopotentials. The results that the conductance of the wire is $\sim$ 1 G$_0$ and the conductance trace as a function of electrode spacing shows a convex downward curve before breaking are in agreement with experimental data.Comment: 5 pages and 3 figure

Effect of disorder on the conductance of a Cu atomic point contact

We present a systematic study of the effect of the disorder in copper point contacts. We show that peaks in the conductance histogram of copper point contacts shift upon addition of nickel impurities. The shift increases initially linerarly with the nickel concentration, thus confirming that it is due to disorder in the nanowire, in accordance with predictions. In general, this shift is modelled as a resistance R_s which is placed in series with the contact resistance R_c. However, we obtain different R_s values for the two peaks in the histogram, R_s being larger for the peak at higher conductance.Comment: 6 pages, 4 figure

Absence of magnetically-induced fractional quantization in atomic contacts

Using the mechanically controlled break junction technique at low temperatures and under cryogenic vacuum conditions we have studied atomic contacts of several magnetic (Fe, Co and Ni) and non-magnetic (Pt) metals, which recently were claimed to show fractional conductance quantization. In the case of pure metals we see no quantization of the conductance nor half-quantization, even when high magnetic fields are applied. On the other hand, features in the conductance similar to (fractional) quantization are observed when the contact is exposed to gas molecules. Furthermore, the absence of fractional quantization when the contact is bridged by H_2 indicates the current is never fully polarized for the metals studied here. Our results are in agreement with recent model calculations.Comment: 4 pages, 3 figure