Table of contents

It is shown that the theory of elasticity under hydrostatic pressure p at zero temperature is unified and simplified by the use of the Gibbs free energy G, rather than the energy E. The minima of G, but not of E, give the equilibrium structure; the second derivatives of G, but not of E, with respect to strains at the equilibrium structure give the zero-temperature elastic constants; the stability of a phase at p is then determined by the same Born stability conditions used at p = 0 when applied to the elastic constants from G. Examples are noted of mistakes due to use of E rather than G.

We report resistivity and calorimetric measurements on two single crystals of CePd₂Si₂ pressurized up to 7.4 GPa. A weak uniaxial stress induced in the pressure cell demonstrates the sensitivity of the physics to anisotropy. Stress applied along the c-axis extends the whole phase diagram to higher pressures and enhances the superconducting phase emerging around the magnetic instability, with a 40% increase of the maximum superconducting temperature, T_c, and a doubled pressure range. Calorimetric measurements demonstrate the bulk nature of the superconductivity.

Colossal magnetoresistance (CMR) (Δρ/ρ) and giant negative volume magnetostriction (ω) have been observed in the Curie temperature region of Sm_1−xSr_xMnO₃ manganites, for x = 0.33 compounds containing ferromagnetic (FM) and A-type antiferromagnetic (AFM) clusters and for x = 0.4, 0.45 compounds containing FM and both types of AFM clusters (A-type and charge-ordering (CO) type). For x = 0.33, the magnetization σ, Δρ/ρ and ω increase smoothly with magnetic field increase and saturation of Δρ/ρ and ω is not achieved. Isotherms of σ, Δρ/ρ and ω for x = 0.4 and 0.45 show another behaviour: sharp jumps of σ, Δρ/ρ and ω take place at H_C1 ≤H≤H_C2, and saturation is achieved at H≥H_C2. We consider that the reason for CMR and giant magnetostriction being observed in the compounds investigated is the increase of the FM phase volume under the action of the magnetic field. For x = 0.33 this increase is smooth because it arises from FM phase 'sprouting' on FM layers of A-type AFM phase. For x = 0.4 and 0.45 the increase of the volume of the FM part arises from CO clusters with CE-type AFM structure too. In this case, CO clusters are completely transformed to the FM state with a large saturation magnetization σ_s which is equal to ∼ 70 % of σ_s at T = 1.5 K. This transition is accompanied by crystal structure reconstruction that is manifested in both the temperature and magnetic field dependences of the anisotropic magnetostriction.

We report specific heat measurements on the heavy-fermion compound CePd₂Si₂ in magnetic fields up to 16 T and in the temperature range 1.4–16 K. A sharp peak in the specific heat signals the antiferromagnetic transition at T_N ∼ 9.3 K in zero field. The transition is found to shift to lower temperatures when a magnetic field is applied along the crystallographic a-axis, while a field applied parallel to the tetragonal c-axis does not affect the transition. The magnetic contribution to the specific heat below T_N is well described by a sum of a linear electronic term and an antiferromagnetic spin-wave contribution. Just below T_N, an additional positive curvature, especially at high fields, arises most probably due to thermal fluctuations. The field dependence of the coefficient of the low-temperature linear term, γ₀, extracted from the fits shows a maximum at about 6 T, at the point where an anomaly was detected in susceptibility measurements. The relative field dependences of both T_N and the magnetic entropy at T_N scale as [1 − (B/B₀)²] for B || a, suggesting the disappearance of antiferromagnetism at B₀ ∼ 42 T. The expected suppression of the antiferromagnetic transition temperature to zero makes the existence of a magnetic quantum critical point possible.

The structural and magnetic properties of the geometrically frustrated spinel system CdV₂O₄ with V³⁺ (S = 1), whose lattice constant and oxygen u-parameter differ significantly from those of isomorphous MgV₂O₄ and ZnV₂O₄, have been explored by means of x-ray diffraction and through measurements of the magnetization and nuclear magnetic resonance. CdV₂O₄ undergoes a cubic–tetragonal structural transition with c_t/a_t = 0.990 as well as a magnetic transition with a jump of the susceptibility at T_c1 = 97 K. Another magnetic anomaly appears at T_c2 = 35 K which may be a transition to the antiferromagnetic state. The analysis in terms of the high-temperature series expansion of up to eighth order precisely indicates the nearest-neighbour exchange-coupling constant for the cubic phase to be J = 44 K. In the vanadium spinel with S = 1, the spin-singlet V₄-tetramer model with the exchange coupling J_tet inside V₄, where J_tet = 2J, is applicable at temperatures above T ≈ J_tet: for CdV₂O₄ with J_tet ≤T_c1, all of the susceptibility data for the cubic phase are explained using that model, while for MgV₂O₄ and ZnV₂O₄ with J_tet ≥T_c1, it is difficult to account for the data for between J_tet and T_c1. In this sense, the present bound state may be regarded as pseudotetramers.

In this study, collective and tracer diffusion kinetics is addressed for the ternary random alloy. A formal solution from the self-consistent theory of Moleko et al (Moleko L K, Allnatt A R and Allnatt E L 1989 Phil. Mag. A 59 141) is derived for collective diffusion and compared with the corresponding solution for the binary random alloy. Tracer diffusion in the ternary alloy is treated from the perspective of a special case of the quaternary random alloy. Results from Monte Carlo calculations for tracer and collective correlation factors (for the bcc ternary random alloy) are found to be in excellent agreement with this self-consistent theory but in only semi-quantitative agreement with the earlier theory of Manning (Manning J R 1971 Phys. Rev. B 4 1111).

We probe near-surface regions in hydrogenated amorphous and microcrystalline silicon by recording the transient photocurrent after application of a green laser pulse with short absorption depth through the glass-substrate/silicon interface. Depending on the spatial defect inhomogeneity close to the illuminated surface the transient photocurrent shows a different decay behaviour under strongly absorbed green light as compared with more uniformly absorbed red illumination. We apply a Fourier transform technique to the photocurrent decay, which reveals spatial inhomogeneities in the deep-defect density in amorphous silicon. For a highly crystalline sample of microcrystalline silicon we find depth homogeneity in the electronic properties, in agreement with information from structural investigations.

Dielectric and conductivity spectra of the compound Li_1−xNi_{1 +x}O₂ (x = 0.03) are reported and analysed. The spectra were recorded for compacted powders within the broad frequency range 10–10¹⁰ Hz at temperatures varying between 210 and 300 K. The evolution of the spectra of the sample with respect to the damage under air allows us to define two regimes: the first one is an interfacial regime and the second a bulk regime. The frequency of crossover (ν_co) between the two regimes lies in the interval 1 × 10⁹ and 2 × 10⁹ Hz at room temperature. At frequencies below ν_co, the spectral analysis shows dielectric relaxations due to interfacial polarization phenomena in the sample. The complex resistivity plots (for ν≤ν_co) allowed the determination of the dc conductivity of Li_1−xNi_1+xO₂ versus temperature. At frequencies greater than ν_co, one dielectric relaxation corresponding to small-polaron hopping has been observed. The corresponding relaxation frequency is thermally activated with an activation energy of 0.15–0.16 eV and a pre-exponential factor ν₀ ≈ 3 × 10¹²–4 × 10¹² Hz, which is of the order of the longitudinal-optical-phonon frequency ν_LO. The conduction behaviour is interpreted in terms of hopping of small polarons (holes) on an oxygen network.

We propose a model for the nature of the low-temperature phase of a geometrically frustrated antiferromagnet with a Kagomé lattice, SrCr_8−xGa_4+xO₁₉. We propose that the long-range dipolar interaction between the magnetic Cr³⁺ ions introduces correlations in their zero-point dynamics. The dipolar ground state has the spins performing correlated oscillations in a coherent state with a well defined global phase. We calculate the magnon excitations of such a dipolar array and find good agreement with the spin-wave velocities inferred from measurements of the specific heat. We can further explain the unusual muon spin-relaxation signal of this phase.

Two Bloembergen–Purcell–Pound (BPP) models for analysing nuclear spin relaxation data for translational diffusion in disordered systems are compared with Monte Carlo simulations. One model (the a-BPP model, 'a' standing for average) is commonly used for disordered systems and the other (the Cameron–Sholl BPP model) is more rigorously based and can distinguish between site-and barrier-energy disorder. Simulated relaxation data produced using Gaussian distributions of energy disorder are analysed using the models, and the parameters obtained from the fits are compared with the values used for the simulations. It is found that both models can give reasonable fits to the data. Both models also give reasonable agreement with the simulation parameters provided that the standard deviation of the energy distribution for the a-BPP model is interpreted as the average of the site-and barrier-energy standard deviations. Quantitative estimates are given of the accuracy of the parameters from the fits.

The structure of two oxygen-related luminescence centres in oxygen-doped BaF₂ was investigated by means of photoluminescence (PL) and photoluminescence-detected electron paramagnetic resonance (PL-EPR). One of the oxygen-related luminescences peaking at 2.83 eV is associated with an excited triplet state (S = 1) of an oxygen–vacancy complex with the z-axis of the fine-structure tensor parallel to the ⟨110⟩ direction. This complex can be described as an oxygen on a fluorine lattice site with a next-nearest fluorine vacancy along the ⟨110⟩ direction. The luminescence at 2.25 eV is also associated with a triplet state. Its PL-EPR spectrum is probably due to oxygen–vacancy complexes with a nearest fluorine vacancy along the ⟨100⟩ direction.

In the framework of the concept of charge-transfer transitions, we suggest a semi-quantitative model which explains the strong increase of the circular magneto-optical effects in the iron garnets (IG) R₃Fe₅O₁₂ upon doping with Bi³⁺ or Pb²⁺ ions. The covalent admixture of Bi³⁺, Pb²⁺ 6p orbitals (with giant one-electron spin–orbit coupling constant) with oxygen 2p states causes growth of the oxygen contribution to the spin–orbit coupling constant of (FeO₆)⁹⁻, (FeO₄)⁵⁻ complexes (the main magneto-optically active centres of IG). The enhanced spin–orbit interaction of the oxygen is not the only effect of substitution; it also gives rise to the effective anisotropic tensor addend to the spin–orbit interaction and the circular magneto-optical effects in IG as well. A computer simulation of the magneto-optical spectra of Y_3−xBi_xFe₅O₁₂ for the case of an inhomogeneous bismuth distribution is carried out. Estimates of various contributions to the IG magneto-optical effects are given. The anisotropic tensor mechanism yields a contribution to the Faraday effect which is comparable in magnitude with the 'ordinary' ferromagnetic contribution.

Analysis of experimental data on the magneto-optical effects in garnets supports our theoretical model.

Polarization-dependent x-ray absorption measurements were performed on crystalline ZnO three-dimensional arrays consisting of highly oriented microrods as well as on particulate thin films consisting of monodisperse spherical nanoparticles. Strong anisotropic effects have been observed for the highly oriented ZnO rods, but not for the isotropic spherical nanoparticles. Full-potential calculations of the orbital-resolved x-ray absorption of a ZnO wurtzite periodic crystal, including Zn 3d among the valence states, show very good agreement with the experimental findings. Comprehensive fundamental knowledge of the electronic structure of ZnO is obtained by probing and demonstrating the orbital symmetry of oxygen and its contribution to the conduction band of this important II–VI semiconductor.

We discuss the origin of the maximum in the electrical resistivity ρ(T) of quasicrystals (QCs) and show that it can be consistently explained as a magnetic effect due to the presence of localized magnetic scattering centres within the quasiperiodic lattice. On the experimental side we present a comparative electrical resistivity–magnetic susceptibility study of icosahedral AlPdMn and CdCa QCs and show correlation between the temperature of the ρ(T) maximum and the electronic paramagnetic magnetization of localized magnetic centres. We propose a theoretical explanation in terms of the Korringa–Gerritsen (KG) model, originally developed for noble metals with diluted transition metal impurities. Unlike the other existing models of electrical conduction in QCs, the KG model describes the ρ(T) maximum as a magnetic effect and predicts its shift to higher temperatures for an increased concentration of magnetic moments.