Table of contents

Using ab initio calculations, we find that the calculated energy barrier for exchange diffusion of Ni adatoms on Ni(100) surfaces shows a surprisingly large dependence on the size of the surface unit cell. It decreases from 1.39 to 0.78 eV when the cell size changes from (2×2) to (6×6). This is due to the long-ranged strain field created by the transition state for atomic exchange, which needs a larger cell to relax. The hopping diffusion energy, on the other hand, shows only a very small size effect and remains approximately constant at 0.82-0.86 eV, independently of the cell size. Our results indicate that Ni diffusion on Ni(100) occurs by the exchange mechanism and this is consistent with recent experiments. Previous results obtained using (3×3) or (4×4) unit cells did not converge sufficiently well to yield correct conclusions.

The electrical resistivity of the magnetically ordered CeCu₅Au has been investigated under pressure up to 8.5 GPa. In the magnetically ordered region (p<3.4 GPa) the residual resistivity ρ₀ shows a pronounced maximum as a function of pressure. Even in the non-magnetic region ρ₀decreases monotonically by more than a factor of three. These two effects can be qualitatively explained in terms of the interplay of pressure, magnetism and disorder in a strongly correlated electron system with weak disorder.

We report the observation of heavy-fermion superconductivity in CeCoIn₅ at T_c = 2.3 K. When compared to the pressure-induced T_c of its cubic relative CeIn₃ (T_c~200 mK), the T_c of CeCoIn₅ is remarkably high. We suggest that this difference may arise from magnetically mediated superconductivity in the layered crystal structure of CeCoIn₅.

The temperature dependences of magnetic susceptibility, the specific heat and the thermal expansion of β-Mn_1-xOs_x alloys have been investigated. The electronic specific heat coefficient γ is significantly large and drastically decreases with increasing x. The specific heat peak associated with the Néel temperature T_N is clearly observed in high Os concentration regions. On the other hand, in low Os concentration regions, the peak is not clear and a linear relation between γ and T_N^3/4 is observed. Therefore, it is concluded that β-Mn_1-xOs_x alloys with low x are weak itinerant-electron antiferromagnets. The thermal expansion coefficient α in the paramagnetic state is remarkably large in the low Os concentration regions and becomes smaller with increasing x; that is, the larger the electronic specific heat coefficient γ, the larger the value of α in the paramagnetic regions.

We present ab initio calculations of the Fermi surfaces and of the electronic transport properties of the 1/1 and 2/1 approximants to icosahedral AlPdRe and AlPdMn quasicrystals. Our investigations are based on realistic structural models produced by a cut-and-projection method, a self-consistent tight-binding linearized muffin-tin orbital (TB-LMTO) Hamiltonian and Bloch-Boltzmann as well as Kubo-Greenwood theories for the electronic transport properties. Our results are analysed in the light of the current discussions of the character of electronic eigenstates in quasicrystals in general and of the very exotic transport properties of icosahedral AlPdRe in particular.

The temperature dependence of the conductance of a quantum point contact has been measured. The conductance as a function of the Fermi energy shows temperature-independent fixed points, located at roughly multiple integers of e²/h. Around the first fixed point at e²/h, the experimental data for different temperatures can be scaled onto a single curve. For pure thermal smearing of the conductance steps, a scaling parameter of one is expected. The measured scaling parameter, however, is significantly larger than 1. The deviations are interpreted as a signature of the potential landscape of the quantum point contact, and of the source-drain bias voltage. We relate our results phenomenologically to the metal-insulator transition in two dimensions.

The current fluctuations in ferromagnet/superconductor (FM/SC) junctions are studied for s- and d-wave pairing by taking into account the roughness of the interfacial barrier and exchange splitting in the FM. It is shown that the ferromagnetic exchange splitting gives rise to a decrease in the average current and the shot noise power; the noise power-to-current ratio is increased for eV<Δ₀ but decreased for eV>Δ₀ in FM/s-wave SC junctions (V being the bias voltage and Δ₀ the energy gap), while the ratio is increased rapidly with the exchange splitting at low voltages and tends towards the same value for high voltages in FM/d-wave SC junctions. The interface roughness is found to lead to a decrease in the average current and an increase in the noise power-to-current ratio.

An investigation of the Néel long-range order (NLRO) in the ground state of the antiferromagnetic Heisenberg spin system on the two-dimensional, uniform, bipartite lattice consisting of squares, hexagons and dodecagons is presented. On the basis of the analysis of the order parameter and the long-distance correlation function, NLRO is shown to occur in this system. Exact-diagonalization and variational (resonating-valence-bond) methods are applied.

The effect of additive Ga on the microstructure and magnetic properties of nanocomposite Pr₂(Fe,Co)₁₄B/α-(Fe, Co) ribbons has been investigated. One per cent Ga addition was found to improve significantly its magnetic properties. The remanence and maximum energy product increase from 1.14 T, 17 MG Oe for Ga-free samples to 1.22 T, 22.2 MG Oe for Ga-doped samples. The significant improvements of magnetic properties originate from the refinement of the grains of the samples by introducing Ga, which leads to a stronger exchange coupling between the magnetically hard and soft phases in comparison with that in Ga-free samples.

The demagnetization curves at different temperatures between 4 and 250 K were calculated for single-phase nanocrystalline Nd-Fe-B magnets by using the micromagnetic finite-element technique. The calculations were carried out for a model magnet comprised of 216 grains of dimension 20 nm, taking into account the anisotropic characteristics of the curve with respect to the field direction. The calculations simulate the experimental curves for the nanocrystalline Nd₁₃Fe₇₇B₁₀ magnet fairly well. The inter-grain exchange interaction in the magnet is estimated to be about 75% of the intra-grain exchange interaction.

Piezoelectric and dielectric measurements were performed on different batches of polarized NaNbO₃ ceramics. It has been established that, as a result of polarization, in these ceramics there appears a metastable ferroelectric phase existing for a long time. The presence of this phase manifests itself as a piezoelectric effect accompanied by a piezoresonance dispersion of the dielectric permittivity. The magnitudes and temperature dependencies of the piezoelectric coefficients d₃₃ of the NaNbO₃ ceramics were measured for the first time.

A generalized Cauchy dispersion formula applicable to both normal and anomalous dispersion in the refractive index of transparent materials is derived from the Kramers-Kronig relations. Several commonly used dispersion formulae are closely related to this result. The expansion coefficients, heretofore considered empirical, are simply the odd moments of the absorption spectrum. These ideas are illustrated for natural diamond, and high-purity silicon and germanium. Analysis of published data for these materials discloses extrinsic dispersive effects in the far infrared, even for `high-purity' samples. We attribute this dispersion to free-carrier or defect-induced absorptions at energies below the range of measurements.

The magnetic properties of the disordered alloy systems face-centred-cubic (fcc) Co_xRh_1-x and fcc Fe_xPd_1-x have been investigated by means of the spin-polarized relativistic version of the Korringa-Kohn-Rostoker method of band-structure calculation on the basis of the coherent potential approximation alloy theory. In particular, the spin and spin-orbit-induced orbital magnetic moments have been calculated. In addition, the magnetic circular x-ray dichroism (MCXD) for the x-ray absorption coefficient has been determined. The relationship of the corresponding MCXD spectra and the magnetic moments of the absorber atom is discussed, with most emphasis put on the 4d element, whose magnetism is induced by the alloy partner.

We present the results of soft x-ray emission measurements for undoped and protonated polyemeraldine. We show that the polaron-lattice band structure fully accounts for the observed x-ray transitions. A finite spectral intensity of C Kα and N Kα XES is observed at the Fermi level for protonated polyemeraldine, supporting the applicability of the polaronic-metal model for highly conducting polymers.