Table of contents

We examine quantum properties of mesoscopic, Josephson coupled superconducting dots, in the limit that charging effects and quantization of energy levels within the dots are negligible, but quasi-particle transmission into the weak link is not. We demonstrate that quasi-particle resonances lead to current-phase relations, which deviate markedly from those of weak links connecting macroscopic superconductors. Results for the steady state DC Josephson current of two coupled dots are presented.

The temperature dependence of saturation magnetization has been observed for an unstable FCC Fe₅₀Cu₅₀ solid-solution sample prepared by mechanical alloying. The unstable Fee Fe₅₀Cu₅₀ solid solution has a Curie temperature of 505+or-5 K. At low temperatures the saturation magnetization is well approximated by the relationship M(T)=M(0)(1-BT³2/), where M(0)=108.7 emu g^-1 and B=4.21*10^-5 K^-32/. This holds over a large temperature range (T/T_c<0.4) indicating the presence of long-wavelength spin-wave excitations and disordered atomic structure. On annealing the as-milled sample at annealing temperatures above about 500 K, decomposition of the metastable FCC Fe₅₀Cu₅₀ solid solution was observed. The magnetic moment of Fe atoms in the FCC Fe_100-xCu_x Phases remains at about 2.35 mu _B for 50<x<61, and drops rapidly to 0.544 mu _B for x=70.7. This results in the 40% reduction of the saturation magnetization at zero temperature for the sample annealed at 673 K for 1 h when compared to that for the as-milled sample. The Curie temperature of the FCC Fe_29.3Cu_70.7 solid solution falls to about 200 K.

Using the high-energy neutrons available at the ISIS facility, a deep inelastic neutron scattering study of polycrystalline sodium has been made. In this experiment the scattering function, S(Q, omega ), has been measured at four different temperatures (30 K, 100 K, 200 K, 300 K) in order to study the changes in the neutron Compton profiles. The data were analysed using the impulse approximation and the known density of states. Within the 4% accuracy of the experiment, the results are consistent with sodium behaving as a harmonic solid.

Films of holmium-yttrium alloys were grown by molecular beam epitaxy (MBE), and found to have a mosaic spread of about 0.2 degrees , a concentration uniformity of better than 0.5% and a concentration accuracy of better than 2%. Neutron scattering techniques at the Brookhaven HFBR were used to study the magnetic structures and phase transitions of four samples Ho_xY_1-x with x=1.0, 0.9, 0.7 and 0.5 in zero field, and one sample with x=0.7 as a function of applied magnetic field. In zero field the structures are all helical structures with the moments in the basal planes and the wavevectors of the helix along the hexagonal c-axis. At T_N, the wavevectors are 0.282c*, and decrease on cooling, until at low temperatures the structures become commensurate with wavevectors of 1/5c*(x=1), 2/9c*(x=0.9), 1/4c*(x=0.7) and 4/15c*(x=0.5). The critical exponent beta was measured for each of the films and is consistent for all of the materials with beta =0.50+or-0.05. There is some rounding at T_N which may be due to the second and long length scale that has been found in pure holmium. When a magnetic field between 0 and 6 T is applied in the basal plane, the phase diagram for x=0.7 has six phases. On cooling in a low field the sequence is paramagnetic, incommensurate helix, q=1/4 helix, while for fields above 4 T, the sequence is paramagnetic, incommensurate fan, q=1/4 fan. On increasing the field the helical phase goes directly to the fan phase, but on decreasing the field from the q=1/4 fan phase, the system shows a q=1/4 helifan phase between the fan and helical phases.

The dipolar nuclear magnetic relaxation rate associated with the hopping diffusion of atoms in a disordered solid is calculated by Monte Carlo methods. The model is intended to simulate the diffusion of hydrogen atoms trapped at interstitial positions in the matrix of metal atoms in amorphous alloys. The principal features of the model system are that the atoms hop on a spatially disordered array of traps and the trapping energy varies from trap to trap so that the diffusion of the hydrogen is characterized by a distribution of jump rates. The effective jump rate from a trap is assumed to have an Arrhenius dependence on temperature. Calculated at constant temperature, the characteristic peak in the relaxation rate, which occurs in ordered solids when the product of the average jump rate and the Larmor frequency is approximately unity, is found to be broadened and shifted in frequency, particularly when the occupancy of the traps is high. The long-range diffusion constant is also calculated and used to evaluate the effect of atom-vacancy correlations. It is found that the shifts in the relaxation peak cannot be accounted for solely by these correlation effects and it is suggested that multiple hopping of the more rapidly diffusing spins is a contributory factor. The shifts have a profound effect on the temperature dependence of the relaxation rate when the distribution of jump rates is also dependent on temperature. The adjustments to the peak in the relaxation caused by the distribution are small when the temperature dependence is taken into account, showing that experiments involving only the temperature variation of the relaxation are unlikely to be a sensitive method for detecting the presence of a jump-rate distribution. This aspect of the results of the computer model is illustrated by comparison with experimental data.

We analyse heat transport phenomena in a one-dimensional monatomic lattice with harmonic and quartic interatomic potentials by means of the molecular dynamics technique, paying special attention to non-diffusive heat flow and local temperature profiles in steady nonequilibrium states. The non-diffusive heat current is attributed to modified KDV solitons, which decay through collisions with phonons with short wavelengths. Hence, the non-diffusive heat flow does not propagate beyond a critical distance I_cf. We confirm the anomalous diffusion of energy due to non-diffusive heat flow in steady non-equilibrium states for systems smaller than the critical length I_cr. The non-diffusive heat current does not contribute to the total heat flow for lattices larger than the critical length I_cr. The existence of Fourier's law is confirmed from first principles for the lattices. The temperature profiles become linear, and the resultant thermal conductivity is independent of the local temperatures of the lattice, in accordance with the temperatures profiles, so the local energy conservation law holds.

A method for calculating all-electron wavefunctions within the pseudopotential framework is presented. The phase-shift technique is used to construct valence wavefunctions with the proper node structure in a core region, matching the self-consistent partial pseudo-charge density that has been yielded by pseudopotential calculation in a solid. Making use of the total pseudo-charge density and the electrostatic potential of the pseudopotential calculation, the atomic-like all-electron calculation is performed.

The de Haas-van Alphen (DHVA) effect of the stage-1 AlCl₃ graphite intercalation compound has been studied in the temperature range from 1.5 K to 4.2 K with magnetic fields of 2.5 to 5 T. There is a dominant oscillation with a frequency of 1250 T with the magnetic field parallel to the c axis, which corresponds to an extremal cross sectional Fermi surface area of 0.119 AA^-2. The charge transfer per C atom is 0.032e⁺. The cyclotron mass is (0.261+or-0.001) m₀. The Fermi energy determined from the Holzwarth band model is -1.14 eV. No bearing of the dominant frequency is observed in the 2.5 to 5 T field range, which indicates little undulation of the Fermi surface along the c axis. The Dingle temperature T_D of one sample is (3.1+or-0.2) K, which corresponds to a mean free path l_D of (4.8+or-0.4)*10³ AA.

In this paper the distribution of linear statistics in random matrix models is evaluated in the large-N approximation. The distribution function is found to be a Gaussian, but with a mean and mode, which depend on the single-particle potential, that do not coincide, albeit with a vanishingly small discrepancy in the limit of a perfect conductor.

A summary of detailed measurements of field and temperature dependent magnetic and transport properties in F₁₀Pt₉₀ is presented, with particular emphasis on attempting to evaluate the influence of spin-orbit coupling. Analysis of the field dependent AC susceptibility near the Curie temperature T_c approximately=170 K enables effective exponent values to be estimated, although the presence of magnetic anisotropy arising from spin-orbit coupling prevents asymptotic exponent values from being found. The anisotropy of the magnetoresistance in this system also originates from spin-orbit coupling, and this behaviour is discussed in terms of both itinerant and localized models. A number of estimates for T_c are deduced from these different measurements and compared.

The saddle point van Hove singularity has a topological interpretation in terms of a switching of electron orbits from electron like to hole like. This orbital switching bears some resemblance to magnetic breakdown, but is clearly a distinct phenomenon. The tunnelling probability for orbital switching is calculated, and the results compared to magnetic breakdown. Just as in magnetic breakdown, the electronic orbits can form a quantum coherent network in the presence of finite orbit switching.

Single-crystal neutron-diffraction measurements on cobalt tungstate. CoWO₄, at 15 K and powder measurements in the temperature interval 5-55 K have confirmed that it orders antiferromagnetically below its Neel temperature of 55 K with a magnetic propagation vector ( 1/2 0 0). The chemical unit cell is monoclinic, space group P2/c, Z=2 with a=4.659(1), b=5.667(1), c=4.940(1) AA, beta =89.94(1) degrees at 15 K. The magnetic moments on the two equivalent Co2⁺ ions within the cell are coupled ferromagnetically and lie in the a-c plane at an angle 43.4(8) degrees from (001) towards -a. The magnetization density of the Co²⁺ ion is not spherically symmetric, but it can be modelled as a multipolar expansion on axes Z taken parallel to the moment direction and Y parallel to b. The moment on the Co²⁺ ion is 3.42(3) mu B with the largest coefficients, apart from Y₀₀, being Y₄₁₊=-0.20(2) and Y₄₂₊=-0.21(2). In addition, the radial form factor is expanded over that appropriate to a spin-only moment of a free ion as should be expected when there is a significant orbital moment. These observations are related to the ground-state wavefunction of the ion. There is also evidence for a small covalent transfer of moment to O amounting to 0.04(1) mu _B per ligand. The development of magnetic order is accompanied by magnetostriction, which mimics the magnitude of the ordered Co²⁺ moment and amounts to -3.4*10^-3 Delta V/V between 5 and 55 K.

A sample with an orientational magnetic texture was prepared by freezing a ferrofluid containing maghemite particles in a magnetic field. The degree of alignment of the easy directions obtained by field-cooling was examined by Mossbauer spectroscopy for varying strengths of the freezing field. We compare the results with the predictions of a simple model assuming that the intrinsic magnetic anisotropy in the particles is uniaxial and that the particles are non-interacting. Good agreement between the experimental results and the theoretical model is found. This allows a determination of the magnetic anisotropy energy constant of the particles, which is an important parameter in the model. The anisotropy energy constant is also determined from the reduction of the magnetic hyperfine field relative to the saturation value caused by collective magnetic excitations, as well as from a decay of remanence measurement. Good agreement between the estimates of the anisotropy energy constant by the three methods is found.

For pt.I. see ibid, vol.6, p.3081-90, (1994). The magnetic structure of ultrafine maghemite particles was studied by Mossbauer spectroscopy. We have compared the degree of spin alignment obtained in applied magnetic fields in particulate samples with different frozen-in orientational distributions of the easy directions of the magnetization. Full alignment of the spins was not obtained even in large applied fields (4.35 T). The degree of alignment with the applied field was found to be independent of the orientation of the easy directions in the sample when the applied field was larger than 0.75 T. This result shows that the incomplete alignment of the spins in ultrafine maghemite particles subjected to large applied fields is not due to incomplete alignment of the net particle magnetization due to large magnetic anisotropy, but rather stems from a canting of individual spins.

The magnetic structure of Nd₆Fe₁₃ Si has been studied by magnetization measurement, the singular point detection technique and neutron powder diffraction. It was found that the Nd₆Fe₁₃Si is ferrimagnetic with a compensation temperature of 357 K and a Curie temperature of 441 K. The spontaneous magnetization values are 12.9 J T^-1 kg^-1 at 5 K and 7.4 J T^-1 kg^-1 at 300 K. There are two values B₀^cr1 and B₀^cr2 of the magnetic phase transition field below 225 K: B₀^cr1=6.2 T and B₀^cr2=8.2 T at 77 K and B₀^cr=5.3 T at 300 K. The atomic magnetic moments are ordered in an antiparallel manner, and the two magnetosublattices are M_I up arrow (8f(Nd), 16l(Nd), 4d(Fe), 16l₂(Fe)) and Mn down arrow (16k(Fe), 16₁(Fe)).

A combined nuclear magnetic resonance (NMR) and Mossbauer effect study of YFe₁₀V₂ has been carried out. Based on the ⁵⁷Fe hyperfine fields (HFS) obtained from the Mossbauer results, the resonance lines of the NMR spectrum are assigned and the site preference of V atoms is determined. On the other hand, according to the site preference of vanadium atoms determined by NMR, together with neutron diffraction results, the ⁵⁷Fe Mossbauer spectrum is fitted. It can be deduced from the NMR results that the existence of one nearest-neighbour Fe atom leads to an increase in the V HF of about 0.35, and the core electron polarization contribution H_cp, the 4s conduction electron polarization contribution H_s and the effect of the second-nearest-neighbour Fe atoms on the HF sites cannot be negligible.

A high-pressure Raman scattering study on Li₂Ge₇O₁₅ up to 19 GPa reveals the occurrence of a phase transition from the crystalline to the amorphous state at 12.7 GPa. The amorphization is reversible below 15.5 GPa and irreversible above 15.5 GPa. This transformation occurs via polyhedral tilting. Crystallization processes of pressure-amorphized Li₂Ge₇O₁₅ are also investigated by Raman scattering. An intermediate phase ( alpha -quartz form GeO₂) is crystallized at 520 degrees C and the stable phase of Li₂Ge₇O₁₅ is formed at 620 degrees C.