Atomic-scale Conductors Research Articles

Structural Asymmetry of Metallic Single-Atom Contacts Detected by Current-Voltage Characteristics.

The complex behavior of the simplest atomic-scale conductors indicates that the electrode structure itself is significant in the design of future nanoscale devices. In this study, the structural asymmetry of metallic atomic contacts formed between two macroscopic Au electrodes at room temperature was investigated. Characteristic signatures of the structural asymmetries were detected by fast current-voltage (I-V) measurements with a time resolution of approximately 100 μs. Statistical analysis of more than 300,000 I-V curves obtained from more than 1000 contact-stretching processes demonstrates that the current rectification properties are correlated with the conductance of the nanocontacts. A substantial suppression of the variation in current rectification was observed for the atomic contacts with integer multiples of the conductance quantum. Statistical analysis of the time-resolved I-V curves revealed that the current rectification variations increased significantly from 500 μs onward before the breakage of the atomic contacts. Ab initio atomistic simulations of the stretching processes and corresponding I-V characteristics confirmed the magnitude of the rectification and related it to the structural asymmetries in the breakdown process of the junctions. Overall, we provide a better understanding of the interplay between geometric and electronic structures at atomically defined metal-metal interfaces by probing charge transport properties in extremely sensitive nanocontacts.

Physics insight and first-principles calculation of atomic conductance: implications for the future interconnects

There are still many unknowns and open questions concerning the properties of atomic wires, which can be very different from the bulk case. A physics-based compact model framework for analysis of atomic-scale conductors in the presence of point defects is described, and the results of the compact model are compared with First-principle DFT calculations. The calculations indicate that the typically more resistive bulk metals can emerge as lower-resistance options over Al and Cu at atomic dimensions. In the proposed model, the electrons are considered to be localized at their host atoms. The discussion topics include low-temperature resistance of atomic wires from different materials, and the influence of elementary point defects, such as atomic voids on electron transport. It is argued that the concept of resistivity should be used with care when applied to low-dimensional metals, for example the effective resistivity of small conductors is increased even in the absence of surface and grain-boundary scattering. The related concept of electron ‘mean free path’ needs also to be revisited. These questions could have significant implications in terms of the selection of most appropriate material systems for tera-scale metallization schemes.

Probing the orbital origin of conductance oscillations in atomic chains.

We investigate periodical oscillations in the conductance of suspended Au and Pt atomic chains during elongation under mechanical stress. Analysis of conductance and shot noise measurements reveals that the oscillations are mainly related to variations in a specific conduction channel as the chain undergoes transitions between zigzag and linear atomic configurations. The calculated local electronic structure shows that the oscillations originate from varying degrees of hybridization between the atomic orbitals along the chain as a function of the zigzag angle. These variations are highly dependent on the directionally and symmetry of the relevant orbitals, in agreement with the order-of-magnitude difference between the Pt and Au oscillation amplitudes observed in experiment. Our results demonstrate that the sensitivity of conductance to structural variations can be controlled by designing atomic-scale conductors in view of the directional interactions between atomic orbitals.

Experimental determination of conduction channels in atomic-scale conductors based on shot noise measurements

We demonstrate a general procedure for determining the conduction channels of quantum conductors from shot noise measurements. This numerical approach allows multichannel analysis which was previously limited to superconductors. Channel analysis of Ag and Au atomic contacts reveals a remarkable behavior in which the channels fully open one by one with increasing conductance. These results allow us to unambiguously distinguish between free-electron and tight-binding descriptions for the conductance of monovalent contacts. Furthermore, the channel resolution uncovers the presence of tunneling channels in parallel to the conductance through the main contact and provides a means for distinguishing between the contact conductance and tunneling contributions. Finally, unique channel distributions were found for Al and Pt contacts reflecting their distinct valence orbital structures.

Electron–phonon interaction in atomic-scale conductors: Einstein oscillators versus fullphonon modes

Two extreme pictures of electron–phonon interactions in nanoscale conductors arecompared: one in which the vibrations are treated as independent Einstein atomicoscillators, and one in which electrons are allowed to couple to the full, extended phononmodes of the conductor. It is shown that, under a broad range of conditions, the full-modepicture and the Einstein picture produce essentially the same net power at any given atomin the nanojunction. The two pictures begin to differ significantly in the limit of low latticetemperature and low applied voltages, where electron–phonon scattering is controlled bythe detailed phonon energy spectrum. As an illustration of the behaviour in this limit, westudy the competition between trapped vibrational modes and extended modes inshaping the inelastic current–voltage characteristics of one-dimensional atomic wires.

Shot noise in nanoscale conductors from first principles

We describe a field-theoretic approach to calculate quantum shot noise in nanoscale conductors from first principles. Our starting point is the second-quantization field operator to calculate shot noise in terms of single quasi-particle wavefunctions obtained self-consistently within density functional theory. The approach is valid in both linear and nonlinear response and is particularly suitable in studying shot noise in atomic-scale conductors. As an example we study shot noise in Si atomic wires between metal electrodes. We find that shot noise is strongly nonlinear as a function of bias and it is enhanced for one- and two-Si wires due to the large contribution from the metal electrodes. For longer wires it shows an oscillatory behavior for even and odd number of atoms with opposite trend with respect to the conductance, indicating that current fluctuations persist with increasing wire length.

Nanometer-scale metallic grains connected with atomic-scale conductors

We describe a technique for connecting a nanometer-scale gold grain to leads by atomic-scale gold point contacts. These devices differ from previous metallic quantum dots in that the conducting channels are relatively well transmitting. We investigate the dependence of the Coulomb blockade on contact resistance. The high-resistance devices display Coulomb blockade and the low-resistance devices display a zero-bias conductance dip, both in quantitative agreement with theory. We find that in the intermediate regime, where the sample resistance is close to h/e2, the I–V curve displays a Coulomb staircase with symmetric contact capacitances.

Current-induced forces in atomic-scale conductors

Abstract We present a self-consistent tight-binding formalism to calculate the forces on individual atoms due to the flow of electrical current in atomic-scale conductors. Simultaneously with the forces, the method yields the local current density and the local potential in the presence of current flow, allowing a direct comparison between these auantities. The method is applicable to structures of arbitrary atomie geometry and can be used to model current-induced mechanical effects in realistic nanoscale junctions and wires. The formalism is implemented within a simple ls tight-binding model and is applied to two model structurest atomie chains and a nanoscale wire containing a vacancy.

Current-induced forces in atomic-scale conductors

Current-induced forces in atomic-scale conductors

Spatial distribution of the electric current and field in atomic-scale conductors

Spatial distribution of the electric current and field in atomic-scale conductors

Spatial distribution of the electric current and field in atomic-scale conductors

Abstract This paper uses a tight-binding scattering formalism to calculate the spatial distribution of the electric current and field in small phase-coherent conductors. The calculation is simple and efficient, allowing one to obtain simultaneous pictures of the current density and the local potential and field in a wide range of atomic-scale structures. The use of the method is illustrated by two examples. The first shows the formation of a resistivity dipole across an extended obstacle. The second is conduction through a double constriction, where we compare the current and potential distributions at and off resonance. The current pattern at resonance is related to the wavefunction of the quasibound state responsible for the resonance. Off resonance, large internal closed currents are found inside the double constriction. At resonance the potential drops are concentrated at the entrances to the constriction, while off resonance essentially all of the potential drop occurs in the interior of the double c...