Conductance Of Nanocontacts Research Articles

Structural transformation between crystal and amorphous states and relating conductance variation in pure molybdenum nanocontacts

The amorphization and crystallization of the molybdenum (Mo) nanocontacts (NCs) via pulsed voltage application were observed in situ at the atomic resolution by transmission electron microscopy. Simultaneously, the variation in conductance of NCs during structural transformation was measured. The structural transformation could be controlled by the height of applied pulse waves. The amorphization was accelerated when NCs have longer constriction regions, which has not been found in NCs comprised of other metallic elements. The amorphized states of NCs were maintained during the observation at room temperature. The conductance ratio of amorphous Mo NCs to crystal Mo NCs was 0.62, which is lower than the ratio of liquid to solid Mo and is the lowest among those of other metallic NCs, leading to applications to advanced switching and memory functions.

Magnetic impurities in nanotubes: From density functional theory to Kondo many-body effects

Low-temperature electronic conductance in nanocontacts, scanning tunneling microscopy (STM), and metal break junctions involving magnetic atoms or molecules is a growing area with important unsolved theoretical problems. While the detailed relationship between contact geometry and electronic structure requires a quantitative ab initio approach such as density functional theory (DFT), the Kondo many-body effects ensuing from the coupling of the impurity spin with metal electrons are most properly addressed by formulating a generalized Anderson impurity model to be solved with, for example, the numerical renormalization group (NRG) method. Since there is at present no seamless scheme that can accurately carry out that program, we have in recent years designed a systematic method for semiquantitatively joining DFT and NRG. We apply this DFT-NRG scheme to the ideal conductance of single wall (4,4) and (8,8) nanotubes with magnetic adatoms (Co and Fe), both inside and outside the nanotube, and with a single carbon atom vacancy. A rich scenario emerges, with Kondo temperatures generally in the Kelvin range, and conductance anomalies ranging from a single channel maximum to destructive Fano interference with cancellation of two channels out of the total four. The configuration yielding the highest Kondo temperature (tens of Kelvins) and a measurable zero-bias anomaly is that of a Co or Fe impurity inside the narrowest nanotube. The single atom vacancy has a spin, but a very low Kondo temperature is predicted. The geometric, electronic, and symmetry factors influencing this variability are all accessible, which makes this approach methodologically instructive and highlights many delicate and difficult points in the first-principles modeling of the Kondo effect in nanocontacts.

Kondo Effect and Conductance of Nanocontacts with Magnetic Impurities

We study the impact of dynamical correlations on the electronic structure and coherent transport properties of Cu nanocontacts hosting a single magnetic impurity (Ni, Co, Fe) in the contact region. The strong dynamical correlations of the impurity 3d electrons are fully taken into account by combining density-functional calculations with a dynamical treatment of the impurity 3d shell in the one-crossing approximation. We find that dynamical correlations give rise to the Kondo effect and lead to Fano features in the coherent transport characteristics similar to those observed in related experiments.

Spin-flip conductance and magnetoresistance of magnetic nanocontacts

The quantized conductance of nanocontacts with atomic sizes is calculated with allowance made for the conduction-electron spin flip in terms of the quantum scattering theory. The exact solution of the Schrodinger equation describing the electron motion in a piecewise-smooth potential is used as the zeroth-order approximation of the perturbation theory. The probabilities of electron transmission (reflection) through a magnetic domain wall, as well as the spin-conserving and spin-flip conductances of the nanocontact, are calculated. It is demonstrated that the spin-flip conductance imposes the natural limitation on the formally infinite increase in the ballistic magnetoresistance of the nanocontact when its cross-sectional area tends to zero.

An undergraduate laboratory experiment on quantized conductance in nanocontacts

We describe an undergraduate laboratory experiment on conductance steps observed to occur near integer multiples of 2e2/h as nanocontacts form and break between gold wires in loose contact. This experimental arrangement was first described by Costa-Krämer et al. [Surf. Sci. 342, L1144 (1995)]. A simple op-amp circuit in conjunction with an inexpensive storage oscilloscope suffice to observe the steps. The experiment may be extended by interfacing to a computer, which accumulates a histogram of conductance values as the wires are brought into and out of contact many times. The histogram shows peaks near integer multiples of 2e2/h. We emphasize the pedagogical issues involved in bringing an experiment from forefront condensed-matter physics research into the undergraduate laboratory.