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The electrical conductance G of mechanical break-junctions fabricated from the rare-earth metal dysprosium has been investigated at 4.2 K where Dy is in the ferromagnetic state. In addition to the usual variation of the conductance while breaking the wire mechanically, the conductance can be changed reproducibly by variation of the magnetic field H, due to the large magnetostriction of Dy. For a number of contacts, we observe discrete changes in G(H) in the range of several G(0) = 2e(2)/h. The behavior of G(H) and its angular dependence can be quantitatively understood by taking into account the magnetostrictive properties of Dy. This realization of a magnetostrictive few-atom switch demonstrates the possibility of reproducibly tuning the conductance of magnetic nanocontacts by a magnetic field.

The electrical resistance R of metallic nanocontacts subjected to controlled cyclic electromigration in ultra-high vacuum has been investigated in-situ as a function of applied voltage V . For sufficiently small contacts, i.e., large resistance, a decrease of R(V ) while increasing V is observed.This effect is tentatively attributed to the presence of contacts separated by thin vacuum barriers in parallel to ohmic nanocontacts. Simple model calculations indicate that both thermal activation or tunneling can lead to this unusual behavior. We describe our data by a tunneling model whose key parameter, i.e., the tunneling distance, changes because of thermal expansion due to Joule heating and/or electrostatic strain arising from the applied voltage. Oxygen exposure during electromigration prevents the formation of negative R(V ) slopes, and at the same time enhances the probability of uncontrolled melting, while other gases show little effects. In addition, indication for field emission has been observed in some samples.

We develop a microscopic model to calculate point-contact spectra between a metallic tip and a superconducting heavy-fermion system. We apply our tunneling model to the heavy fermion CeCoIn 5 , both in the normal and superconducting states. In point contact and scanning tunneling spectroscopy many heavy-fermion materials, such as CeCoIn 5 , exhibit an asymmetric differential conductance, dI / dV, combined with a strongly suppressed Andreev reflection signal in the superconducting state. We argue that both features may be explained in terms of a multichannel tunneling model in the presence of localized states near the interface. We find that it is not sufficient to tunnel into two itinerant bands of light and heavy electrons to explain the Fano line shape of the differential conductance. Localized states in the bulk or near the interface are an essential component for quantum interference to occur when an electron tunnels from the metallic tip of the point contact into the heavy-fermion system.

ABSTRACT:Stepped well-ordered semiconductor surfaces are important as nanotemplates for the fabrication of one-dimensional nanostructures. Therefore a detailed understanding of the underlying stepped substrates is crucial for advances in this field. Although measurements of step edges are challenging for scanning force microscopy (SFM), here we present simultaneous atomically resolved SFM and Kelvin probe force microscopy (KPFM) images of a silicon vicinal surface. We find that the local contact potential difference is large at the bottom of the steps and at the restatoms on the terraces, whereas it drops at the upper part of the steps and at the adatoms on the terraces. For the interpretation of the data we performed density functional theory (DFT) calculations of the surface dipole distribution. The DFT images accurately reproduce the experiments even without including the tip in the calculations. This underlines that the high-resolution KPFM images are closely related to intrinsic properties of the surface and not only to tip-surface interactions.Stepped well-ordered surfaces and, in particular vicinal semiconductor surfaces, are well suited for applications as nanotemplates for the fabrication of one-dimensional nanostructures. [1][2][3][4][5][6][7] Among such structures, monoatomic wires are candidates of interesting electronic properties such as Luttinger-liquid behavior. 8 The vicinal Si(111) surface with 10 • miscut towards the [1 1 1 1 2] direction is a popular stepped surface that can be used as a model system. This surface contains (7×7) reconstructed terraces oriented along the Si(111) direction, a well characterized and understood surface, separated by a stepped region. The presence of the (7 × 7) reconstructed areas makes this vicinal system an ideal testbed for surface characterization techniques and investigating its rich morphology and electronic features. Teys et al. proposed that this surface is oriented along the (7 7 10) direction. 2Within Teys model, the stepped part consists of a periodically ordered triple step with a height of 3 atomic layers and a width of 16 atomic rows, corresponding to a lateral periodicity of 5.2 nm.Scanning force microscopy (SFM), particularly in ultra-high vacuum (UHV) and its non-contact mode, has become one of the standard techniques for analyzing the topographic properties of flat surfaces at the atomic scale. 9On conducting surfaces, SFM provides complementary information to that obtained with scanning tunneling microscopy (STM), 10,11 in some cases with even higher spatial resolution. 12,13The atomic resolution capability of SFM arises from the short-range forces acting between an atomically sharp tip and a clean surface. 14 The differences between the work functions of the probing tip and the surface of the sample give rise to contact potential differences (CPD) that can be measured using Kelvin probe force microscopy (KPFM). 15-19The origin of atomic-scale KPFM contrast is still under discussion, since the work function is considered as a macroscopic concep...