Magnetoresistance of electrochemically produced nickel nanocontacts with quantized conductance

Nickel nanocontacts with quantized conductance have been obtained with the use of the electrochemical method of the formation of atomic-size contacts. The conductance jumps in the absence of the magnetic field are multiple of e ²/h in most experiments. The maximum magnetoresistance reaches 210%. The performed current-voltage investigations make it possible to determine the conditions necessary for forming nanocontacts with giant magnetoresistance, as well as a possible cause of the absence of magnetoresistance in some electrochemically formed nanocontacts.     

Metal-semiconductor nanocontacts: silicon nanowires

Strain evolution of coherent Ge islands on Si(001) is measured using a newly developed transmission electron microscopy technique based on two-beam dark-field strain imaging. The strain measurements show that a metastable Ge island shape is involved in the shape transition between pyramids and domes; this shape is more readily observed for growth at 550 than 600 degrees C because of the slower rate at which islands cross the kinetic barrier between shapes. The strain relaxation changes discontinuously between pyramids and domes, indicating that the underlying shape transition is first order.     

Multiplicative stochastic processes in statistical physics

For a large class of nonlinear stochastic processes with pure multiplicative fluctuations the corresponding time-dependent Fokker-Planck equation is solved exactly by means of analytic methods. We obtain a universal eigenvalue spectrum and the corresponding set of eigenfunctions.     

Magnetostriction and magnetoresistance in nanocontacts

Ni nanocontacts have been grown by electrodeposition using a self-terminating technique in a single electrolyte bath based on nickel sulfate, nickel chloride and boric acid. Resistance measurements performed on different samples presented two kinds of obviously different magnetoresistance effects. The analysis of the data sets showed that magnetostriction might play a key role in magnetoresistance of the electrodeposited Ni nanocontacts.     

Generalized statistical description of adsorption of polyatomics

Multisite-occupancy adsorption is described as a fractional statistic problem, based on Haldane’s statistics. Site exclusion is characterized by a statistical exclusion parameter, g, which relates to the molecular size and lattice geometry. A general adsorption isotherm is obtained and comparisons with experiments indicate that the spatial configuration of adsorbed molecules and lateral interactions may accurately be assessed from this theory. In addition, the theoretical framework developed in this paper allows to describe orientational transitions occurring in adlayers of polyatomics.     

Quantum conductance in semimetallic bismuth nanocontacts

Electronic transport properties of bismuth nanocontacts are analyzed using a low temperature scanning tunneling microscope. The subquantum steps observed in the conductance versus elongation curves give evidence of atomic rearrangements in the contact. The quantum nature of the conductance reveals itself through peaks in the conductance histograms. The shape of the curves at 77 K is described by a simple gliding mechanism for the contact evolution during elongation. The different behavior at 4 K suggests a transition from light to heavy charge carriers as the contact cross section is decreased.     

Current-driven vortex oscillations in metallic nanocontacts

We present experimental evidence of subgigahertz spin-transfer oscillations in metallic nanocontacts that are due to the translational motion of a magnetic vortex. The vortex is shown to execute large-amplitude orbital motion outside the contact region. Good agreement with analytical theory and micromagnetics simulations is found.     

Atomic description of elementary surface processes: diffusion and dynamics

A large fraction of processes which are at the foundation of our technological society involve physical and chemical properties of surfaces. Catalytic reactions and semiconductor devices production are two of the most important ones.

This paper describes a sample of some of the most relevant surface science experiments which have been recently performed, in order to understand elementary surface processes of model catalytic reactions and in semiconductor technology at the atomic level. The focus is on experiments performed with scanning tunneling microscopy and atomic force microscopy which have represented, in some cases, real breakthroughs in our understanding of these phenomena.

We then present an overview of possible experimental technique developments that can be foreseen for the future and that may give us a more in-depth understanding of the elementary processes which form the basis of important complex surface phenomena. Finally, some of the challenging tasks that lie ahead for surface scientists and the collateral opportunities are discussed.     

A new description of dissipative systems in quantum statistical mechanics

A method has been developed permitting to find the solution of the Liouville equation of quantum statistical mechanics. Canonical distributions are derived.     

Dynamical fluctuations in classical adiabatic processes: General description and their implications

Dynamical fluctuations in classical adiabatic processes are not considered by the conventional classical adiabatic theorem. In this work a general result is derived to describe the intrinsic dynamical fluctuations in classical adiabatic processes. Interesting implications of our general result are discussed via two subtopics, namely, an intriguing adiabatic geometric phase in a dynamical model with an adiabatically moving fixed-point solution, and the possible “pollution” to Hannay’s angle or to other adiabatic phase objects for adiabatic processes involving non-fixed-point solutions.     

The role of nickel in stimulated processes in NaCl

The influence of nickel impurity upon thermostimulated (ionic conductivity, TSE, TSL, TOB) processes and absorption, ESR and luminescence of sodium chloride doped with different concentrations of nickel and irradiated with different doses of X-rays is studied. It is shown that both the concentration of nickel and the dose of X-radiation play an important role. Further, the form of TSE-, TSL- and TOB-curves in NaCl-Ni changes, to a certain degree, very similarly as in case of calcium.The authors wish to express their appreciation to Dr. M.Lébl for preparing the crystals, Dr. I.Velická for the polarographic analysis, Ing. E.Mariani for the measurements of conductivity, to K.Dolealová, E.Linhartová and I.Sváková for careful measurements and elaboration of experimental results, to L.Berkes for the ionic conductivity measurements and to K.Raksányi for the purification of the fundamental NaCl material.     

Magnetism-induced ballistic conductance changes in palladium nanocontacts

We present first-principles calculations of the effects of magnetism on the ballistic conductance of a model Pd nanocontact, made of a short Pd monatomic stretched chain placed between two Pd leads, simulated by semi-infinite (100) slabs. The stretching makes the suspended Pd chain generally ferromagnetic. The spin-resolved ballistic conductance, calculated according to the Landauer-Büttiker formula is found to be 0.85G₀ for the spin-up and 1.15G₀ for the spin-down electrons (G₀ = 2e²/h is the conductance quantum). The total conductance ~2G₀ is lower, but still relatively close to that of the nonmagnetic Pd nanocontact with the same geometry, calculated to be 2.3G₀. To illustrate how magnetism and conductance depend on structural details, we change the three atom chain docking from the top to a hollow surface site, where at the same stress the Pd contact is nonmagnetic and the conductance decreases to 1.8G₀. Overall we find these calculated ballistic conductance values of very similar magnitude to the first histogram peak in the experimental data obtained for Pd at low temperature in mechanically controllable break junctions. We conclude that the 15% conductance changes caused by the onset or the demise of local magnetism, similar in magnitude to geometry-related conductance changes, are probably too small to be used as a diagnostic for the presence or absence of nanocontact magnetism.     

The Wigner phase-space description of collision processes

This year marks the 50th anniversary of the birth of the celebrated Wigner distribution function. Many advances made in various areas of science during the 50 year period can be attributed to the physical insights that the Wigner distribution function provides when applied to specific problems. In this paper the usefulness of the Wigner distribution function in collision theory is described.     

Nonschematic description of nuclear β-decay processes

The logft values for beta transitions in the deformed heavy rare earth nuclei are examined. The effects of the schematic approximation in the treatment of the residual interaction are discussed. It is found that this approach overestimates the beta transition rates by almost an order of magnitude.