No abstract

The thermopower and conductance of atomic-size metallic contacts have been simultaneously measured using a mechanically controllable break junction. For contacts approaching atomic dimensions, abrupt steps in the thermopower are observed which coincide with jumps in the conductance. The measured thermopower for a large number of atomic-size contacts is randomly distributed around the value for large contacts and can be either positive or negative in sign. However, it is suppressed at the quantum value of the conductance G_0 = 2e^2/h. We derive an expression that describes these results in terms of quantum interference of electrons backscattered in the banks.Comment: An error in Eq.(3) was corrected: the factor (1-cos(gamma)) should be taken under a square root. The estimate for the mean free path is now 5 nm, which is consistent with the value obtained from the series resistance of the contact

Recently it has been observed that the conductance fluctuations of atomic size gold contacts are suppressed when the conductance is equal to an integer multiple of the conductance quantum. The fact that these contacts tend to consist exclusively of fully open or closed modes has been argued to be the origin for this suppression. Here, the experiments have been extended to a wide range of metallic elements with different chemical valence and they provide new information about the relation between the mode composition and statistically preferred conductance values observed in conductance histograms.

The conductance of atomic size contacts has a small, random, voltage dependent component analogous to conductance fluctuations observed in diffusive wires (UCF). A new effect is observed in gold contacts, consisting of a marked suppression of these fluctuations when the conductance of the contact is close to integer multiples of the conductance quantum. Using a model based on the Landauer-Büttiker formalism we interpret this effect as evidence that the conductance tends to be built up from fully transmitted (i.e., saturated) channels plus a single, which is partially transmitted.PACS-numbers 73.23. Ad, 72.10.Fk, 73.40.Jn, 72.15.Lh Metallic contacts consisting of only a few atoms can be obtained using scanning tunneling microscopy (STM) or mechanically controllable break-junction [1] techniques. The electrical conductance through such contacts is described in terms of electronic wave modes by the Landauer-Büttiker formalism [2]. Each of the N modes forms a channel for the conductance, with a transmission probability T n between 0 and 1. The total conductance is given by the sum over these2 /h the quantum of conductance. By recording histograms of conductance values [3] for contacts of simple metals (Na, Au), a statistical preference was observed for conductances near integer values. This statistical preference was interpreted as an indication that transmitted modes in the most probable contacts are completely opened (T n = 1, i.e. saturation of channel transmission), in analogy with the conductance quantization observed in 2D electron gas devices [4]. Here, we test this interpretation by performing a new type of measurement giving access to the second moment of the distribution of the T n 's. We measure simultaneously, for a large number of gold atomic contacts obtained with the MCB technique, the conductance and its derivative with respect to the voltage. The atomic contacts are formed by breaking a gold wire at low temperatures, and then finely adjusting the size of the contact between the fresh fracture surfaces using a piezo-electric element [1]. Fig. 1 shows the differential conductance, ∂I/∂V measured as a function of bias voltage for three atomic size contacts with different conductance values, using a modulation voltage eV ≪ k B θ (with θ the temperature). For each contact, in order to illustrate reproducibility, both the curves for increasing and decreasing bias voltage are given. Measurements such as those of Fig. 1 suggest that the fluctuation pattern changes randomly between contact configurations, and that the amplitude of the fluctuations is suppressed for conductance values near G 0 . In order to establish such a relation it is necessary to statistically average over a large number of contacts. We do this by measuring the voltage dependence of the conductance (∂G/∂V = ∂ 2 I/∂V 2 ) and the conductance itself (G = ∂I/∂V ) by applying a voltage modulation, and 1

Single-atom junctions between superconducting niobium leads are produced using the mechanically controllable break junction technique. The current-voltage characteristics of these junctions are analyzed using an exact formulation for a superconducting quantum point contact. For tunneling between two single atoms with a sufficiently large vacuum barrier, it is found that a single channel dominates the current, and that the current-voltage characteristic is described by the theory, without adjustable parameters. For a contact of a single Nb atom it is shown that five conductance channels contribute to the conductance, in agreement with the number expected based on the number of valence orbitals for this d metal. For each of the channels the transmission probability is obtained from the fits, and the limits of accuracy of these numbers are discussed.