Solid state ionic conductors are good candidates for the next generation of nonvolatile computer memory elements. Such devices have to show reproducible resistance switching at reasonable voltage and current values even if scaled down to the nanomete
r sizes. Here we study the switching characteristics of nanoscale junctions created between a tungsten tip and a silver film covered by a thin ionic conductor layer. Atomic-sized junctions show spectacular current induced switching characteristics, but both the magnitude of the switching voltage and the direction of the switching vary randomly for different junctions. In contrast, for somewhat larger junctions with diameters of a few nanometers a well defined, reproducible switching behavior is observed which is associated with the formation and destruction of nanoscale channels in the ionic conductor surface layer. Our results define a low size limit of 3 nm for reliable ionic nano-switches, which is well below the resolution of recent lithographic techniques.
The transport through a metal-superconductor interface is governed by a special charge conversion process, the Andreev reflection, where each incident electron drags another electron with itself to form a Cooper pair. At the normal side a hole is lef
t behind dressed by superconducting correlations. For a low transparency interface the simultaneous transfer of two charges is strongly suppressed leading to a reduced conductance. Here we demonstrate that this reduced conductance can be turned to an infinite one by tuning the nanoscale geometry. Creating variable size nanojunctions between a thin metallic film and a superconducting tip we study how multiple phase-coherent scatterings enhance the superconducting correlations at the normal side. By increasing the coherent volume of carriers initially the transmission through the interface is continuously enhanced. However, as the phase-coherent volume reaches the opposite surface of the thin film a resonator is formed, and a robust transition is induced due to Cooper pair condensation.
We present a novel statistical method for the study of stable atomic configurations in breaking nanowires based on the 2D cross-correlation analysis of conductance versus electrode separation traces. Applying this method, we can clearly resolve the t
ypical evolutions of the conductance staircase in some transition metal nanojunctions (Ni, Fe, V) up to high conductance values. In these metals our analysis demonstrates a very well ordered atomic narrowing of the nanowire, indicating a very regular, stepwise decrease of the number of atoms in the minimal cross section of the junction, in contrast to the majority of the metals. All these features are hidden in traditional conductance histograms.
Experimental results showing huge negative differential conductance in gold-hydrogen molecular nanojunctions are presented. The results are analyzed in terms of two-level system (TLS) models: it is shown that a simple TLS model cannot produce peaklik
e structures in the differential conductance curves, whereas an asymmetrically coupled TLS model gives perfect fit to the data. Our analysis implies that the excitation of a bound molecule to a large number of energetically similar loosely bound states is responsible for the peaklike structures. Recent experimental studies showing related features are discussed within the framework of our model.
In this paper the interaction of hydrogen molecules with atomic-sized superconducting nanojunctions is studied. It is demonstrated by conductance histogram measurements that the superconductors niobium, tantalum and aluminum show a strong interaction
with hydrogen, whereas for lead a slight interaction is observed, and for tin and indium no significant interaction is detectable. For Nb, Ta and Pb subgap method is applied to determine the transmission eigenvalues of the nanojunctions in hydrogen environment. It is shown, that in Nb and Ta the mechanical behavior of the junction is spectacularly influenced by hydrogen reflected by extremely long conductance traces, but the electronic properties based on the transmission eigenvalues are similar to those of pure junctions. Evidences for the formation of a single-molecule bridge between the electrodes -- as in recently studied platinum hydrogen system -- were not found.
Results of point contact Andreev reflection (PCAR) experiments on (In,Mn)Sb are presented and analyzed in terms of current models of charge conversion at a superconductor-ferromagnet interface. We investigate the influence of surface transparency, an
d study the crossover from ballistic to diffusive transport regime as contact size is varied. Application of a Nb tip to a (In,Mn)Sb sample with Curie temperature Tc of 5.4 K allowed the determination of spin-polarization when the ferromagnetic phase transition temperature is crossed. We find a striking difference between the temperature dependence of the local spin polarization and of the macroscopic magnetization, and demonstrate that nanoscale clusters with magnetization close to the saturated value are present even well above the magnetic phase transition temperature.
