Table of contents

We have measured the electrical resistivity of an antiferromagnetic Kondo compound CeNiGe₃ under pressure. The Néel temperature initially increases with pressure P up to 3 GPa, then decreases rather steeply with further increasing pressure, and becomes zero at a critical pressure  GPa. The A and ρ₀ values of the resistivity ρ = ρ₀+AT² in the Fermi liquid relation become maximum around P_c, the A value attaining an extremely large value which is comparable with that in a heavy fermion superconductor CeCu₂Si₂. Superconductivity is found below 0.48 K in a wide pressure region from 4 to 10 GPa. The upper critical field H_{c 2}(0) is about 2 T, indicating heavy fermion superconductivity.

A consideration of the lack of history dependence in the non-equilibrium steady state of a quantum system leads us to conjecture that in such a system there is a set of quantum mechanical observables whose retarded response functions are insensitive to the arrow of time, and which consequently satisfy a quantum analogue of the Onsager reciprocity relations. Systems which satisfy this conjecture can be described by an effective free energy functional. We demonstrate that the conjecture holds in a resonant level model of a multi-lead quantum dot.

The magnetic anisotropy of nanometer thin films and of nanosize structures is discussed. Experimental methods for the quantitative determination of magnetic anisotropy are described. Magnetocrystalline, shape, and magnetoelastic anisotropy contributions are reviewed, and recent examples for the non-bulk-like magnetic anisotropy and of the temperature dependence of both the magnetization and magnetic anisotropy of nanoscale materials are presented. It is shown that film strain and its relaxation give rise to film thickness dependent anisotropy, which can be misinterpreted as a surface anisotropy. The decisive role of the surface anisotropy for adsorbate-induced spin-reorientation transitions (SRT) is elucidated. The application of x-ray magnetic circular dichroism (XMCD) for the determination of magnetic anisotropy of nanosize islands down to the single atom size is presented.

A birefringence-imaging study of a crystal of SrTiO₃ shows a purely second-order phase transition close to 105 K with no measurable indications of critical fluctuations. The near perfectly linear development of the birefringence near the phase transition is consistent with normal Landau mean-field behaviour with an order-parameter critical exponent close to 0.5.

Large aggregates of self-interstitials are known to be responsible for the rod-like defects which have a {113} habit plane. In regions where there is a supersaturation of vacancies, similar defects are formed by the aggregation of lattice vacancies. We present the results of first principles calculations that show such structures to be particularly stable in comparison to isolated vacancies and stable vacancy clusters, but less stable than the {113} self-interstitial structures.

Raman and IR studies of KAl(MoO₄)₂, RbAl(MoO₄)₂ and CsAl(MoO₄)₂ are reported. The assignments of modes are given on the basis of lattice dynamics calculations. The temperature dependence of the KAl(MoO₄)₂ vibrational modes shows that this compound exhibits a second-order phase transition around 90 K from the to most probably a monoclinic and ferroelastic phase.

The full potential linearized augmented plane wave (FP-LAPW) method within density functional theory is applied to study, for the first time, the structural and electronic properties of CaFI and to compare them with CaFCl and CaFBr, all compounds belonging to the tetragonal PbFCl structure group with space group P4/nmm. We used the generalized gradient approximation (GGA) based on exchange–correlation energy optimization to calculate the total energy and also the Engel–Vosko GGA formalism, which optimizes the corresponding potential for band structure calculations. Ground state properties such as the lattice parameters, c/a ratio, bulk modulus, pressure derivative of the bulk modulus and cohesive energy are calculated as well as the optimized internal parameters, by relaxing the atomic position in the force directions. The variations of the calculated interatomic distances and angles between different atomic bonds are discussed. CaFCl was found to have a direct band gap at Γ whereas CaFBr and BaFI have indirect band gaps. From these computed bands, all three materials are found to be insulators having band gaps of 6.28, 5.46 and 4.50 eV, respectively. We also calculated the valence charge density and the total density of states at equilibrium volume for each compound. The results are in reasonable agreement with the available experimental data.

The magnetic pyrochlore oxide materials of general chemical formula R₂Ti₂O₇ and R₂Sn₂O₇ (R  rare earth) display a host of interesting physical behaviours depending on the flavour of rare earth ion. These properties depend on the value of the total magnetic moment, the crystal field interactions at each rare earth site and the complex interplay between magnetic exchange and long range dipole–dipole interactions. This work focuses on the low temperature physics of the dipolar isotropic frustrated antiferromagnetic pyrochlore materials. Candidate magnetic ground states are numerically determined at zero temperature and the role of quantum spin fluctuations around these states is studied using a Holstein–Primakoff spin wave expansion to order 1/S. The results indicate the strong stability of the proposed classical ground states against quantum fluctuations. The inclusion of long range dipole interactions causes a restoration of symmetry and a suppression of the observed anisotropy gap leading to an increase in quantum fluctuations in the ground state when compared to a model with truncated dipole interactions. The system retains most of its classical character and there is little deviation from the fully ordered moment at zero temperature.

The charge transfer antiferromagnetic (T_N = 220 K) insulator EuNiO₃ undergoes, at ambient pressure, a temperature-induced metal–insulator (MI) transition at T_MI = 463 K. We have investigated the effect of pressure (up to ) on the electronic, magnetic and structural properties of EuNiO₃ using electrical resistance measurements, ¹⁵¹Eu nuclear resonance scattering of synchrotron radiation and x-ray diffraction, respectively. With increasing pressure we find at p_c = 5.8 GPa a transition from the insulating state to a metallic state, while the orthorhombic structure remains unchanged up to 20 GPa. The results are explained in terms of a gradual increase of the electronic bandwidth with increasing pressure, which results in a closing of the charge transfer gap. It is further shown that the pressure-induced metallic state exhibits magnetic order with a lower value of T_N ( at 9.4 GPa) which disappears between 9.4 and 14.4 GPa.

Muon spin rotation/relaxation experiments on the HoBaCo₂O₅ perovskite are presented. At low temperatures, two spontaneous frequencies are observed, which can be related to the weighted sum and the difference of the magnetic moments of Co²⁺ and Co³⁺ in the charge ordered state of HoBaCo₂O₅. A temperature-dependent depolarization of the muon spin polarization changes its behaviour at the charge order transition. These results are discussed in the context of the charge order transition and the transport properties of HoBaCo₂O₅.

Large unit cell calculations of the properties of charged point defects in insulators largely neglect dielectric polarization of the crystal, because the periodically repeated cells are so small. Embedded quantum cluster calculations with shell-model crystals, representing a single defect in a large crystal, are able to represent the polarization more realistically. For such embedded quantum clusters, we evaluate the optical excitation energy for the nitrogen vacancy in charge state (+3): v_N³⁺ in AlN. This is done with and without dielectric polarization of the embedding crystal. A discrepancy of a few per cent is found, when both ground and excited state orbitals are well-localized within the vacancy. We show that the discrepancy rises rapidly as the excited state becomes more diffuse. We conclude that an embedded cluster approach will be required for transitions that involve even somewhat diffuse states. The investigation is based on a new model for AlN that shows promise for quantitative accuracy.

The quantized electron-diffusion thermoelectric power S is studied for a general one-dimensional band structure in ballistic channels with applications to a single-quantum-well channel and tunnel-coupled double-quantum-well channels in a perpendicular magnetic field. We find a field-induced sign reversal of S and oscillations in double-well channels.

The electrical resistivity ρ(T) of Y_1−xU_xPd₃ (x = 0,0.05 and 0.2) and the lattice constants for x = 0 have been measured at high pressure. It is found that the cubic Cu₃Au structure is stable up to 12 GPa at room temperature. The Kondo temperature T_K was extracted from the ρ(T) curve and it was found that it increases with pressure. A logarithmic temperature dependence characteristic of the Kondo effect was found for x = 0.2 in the temperature range above about 0.5T_K. Fermi liquid behaviour in ρ(T) for x = 0.05, i.e., , is observed and its stability at high pressure is discussed. The pressure dependence of the Kondo temperature T_K is discussed using the Grüneisen parameters at T_K, Γ_K. It appears that the values of Γ_K are the same for these two compounds (x = 0.05 and 0.2): Γ_K = 12. The T-linear behaviour in ρ(T) for x = 0.2, which is characteristic behaviour for non-Fermi liquids, is collapsed by an application of pressure and typical Fermi liquid quadratic temperature dependence recovers at high pressure. From the result for x = 0.2, the power n of the temperature in is determined as 1.0 at ambient pressure and 1.9 at 5.8 GPa. It is pointed out that the hybridization effect due to the application of pressure gives rise to a crossover from a non-Fermi liquid state to Fermi liquid state. But the crossover temperature T_cr shows a pressure dependence different from that predicted by the two-channel Kondo model.

A formalism based on non-local dielectric response theory and Green function techniques has been developed to describe the interaction of quantum well excitons with an evanescent optical wave of a planar waveguide. Reflection spectra of a system in which a quantum well placed behind a dielectric interface at which light experiences total internal reflection have been calculated. It is shown that the spectral feature corresponding to the exciton resonance becomes much more pronounced if the angle of incidence is close to the critical angle of total internal reflection. The concept of a generalized Snell law has been applied to provide simplification of the formalism.

A selected intersubband transition in the asymmetric quantum well is theoretically proposed by using the superposition of two identical time delayed and phase shifted broadband pulses. Three conduction subbands in the semiconductor quantum well structure are optically coupled with the ultrafast infrared pulses. By adjusting the delay between these two pulses, the carriers at ground level can be selectively pumped to one of the upper levels, while the other upper level remains unoccupied. Thus selective transitions in the three level model can be manipulated by optical interference. At the same time, terahertz radiation will be emitted by coherent controlled charge oscillations. The phase and amplitude of THz radiation is found to be sensitive to the optical interference of the coupling pulses.

We present the results of high magnetic field (up to 30 T) and temperature (50 mK–80 K) dependent transport measurements on a 2DEG in GaN/AlGaN heterojunctions. A high mobility (above 60 000 cm² V⁻¹ s⁻¹ at 4 K) 2DEG was obtained by MBE growth of dislocation free GaN and AlGaN layers on semi-insulating bulk GaN substrates. A cyclotron gap and spin splitting are observed. Results from two studies are reported: (i) a tilted field experiment determining the 2DEG g^*-factor from the angular modulation of the amplitude of SdH oscillations; (ii) quantum Hall effect measurements determining the activation energies for spin and cyclotron energy gaps at even and odd filling factors. The observed 'cyclotron gap' enhancement is attributed to the effect of electron–electron interaction and it is estimated using the model of a 2D-screened Coulomb potential. The analytic result for the enhancement of the 'cyclotron gap' yields an addition to the activation energy, , , which is proportional to the magnetic field and resembles the mass renormalization.

We have investigated metallic break junctions in the heavy-fermion compound UPd₂Al₃ at low temperatures between 0.1 and 9 K and in magnetic fields up to 8 T. Both the current–voltage I(V) characteristics and the d V/d I(V) spectra clearly showed the superconducting ( K) as well as the antiferromagnetic ( K) transition at low temperatures when the bias voltage was raised. The junctions with lateral size of order 200 nm had huge critical current densities around 5 × 10¹⁰ A m⁻² at the antiferromagnetic transition and hysteretic I(V) characteristics. Degrading the quality of the contacts by in situ increasing the local residual resistivity reduced the hysteresis. We show that those hysteretic I(V) curves can be reproduced theoretically by assuming the constriction to be in the thermal regime. It turns out that these point contacts represent non-linear devices with N-shaped I(V) characteristics that have a negative differential resistance like an Esaki tunnel diode.

The behaviour of topological defects of the spiral type is studied numerically within a two-dimensional XY model with an applied in-plane magnetic field. The energy dependence of the 'spiral–antispiral' pair on the field value and direction is investigated. It turns out to be linear when the field is directed along the pair axis and exhibits a combined linear–logarithmic behaviour for the perpendicular orientation.

Using many-body Green function theory for thin ferromagnetic Heisenberg films, we compare the static susceptibilities (transverse and parallel) calculated for uniaxial single-ion anisotropy and exchange anisotropy, respectively. Although there are qualitative differences in the results of these calculations with respect to the temperature dependence of the easy and hard axis magnetizations and susceptibilities, the calculated values of these observables are quantitatively so similar that it is unlikely that experimental measurements could decide on which type of anisotropy is acting in a real ferromagnetic film.

The magnetoresistance of pressed powder compacts of the manganese pyrochlore Tl₂Mn₂O₇, including samples diluted with Al₂O₃ powder and the substituted materials (Tl_1.8Bi_0.2)Mn₂O₇ and Tl₂(Mn_1.8Te_0.2)O₇, is compared with that of dense ceramic samples of the same materials. The resistivity of the pressed powders, which is several orders of magnitude greater than that of the ceramics, usually shows a negative magnetoresistance which varies as H² both above and below the Curie point T_C; the quadratic variation is observed in the ceramics only above T_C. The H² variation below T_C, which is unlike that found in other half-metallic oxides, is associated with the semimetallic character of Tl₂Mn₂O₇ which leads to the presence of misaligned spins at the surfaces of the powder particles. When Tl₂Mn₂O₇ powder is mixed with different volume fractions of Al₂O₃, the magnetoresistance near the percolation threshold at 54% Tl₂Mn₂O₇ is found to be dominated by a few tunnel barriers, and there is evidence for Coulomb blockade in both the I:V and R:T curves.

The microscopic origin of the spin Hamiltonian (SH) parameters for Ni²⁺(3d⁸) ions in a trigonal type I symmetry (C_3v,D_3d,D₃) crystal field (CF) is studied. In addition to the spin–orbit (SO) interaction, we consider also the spin–spin (SS) and spin–other-orbit (SOO) interactions. The relative importance of the four (SO, SS, SOO, and combined SO–SS–SOO) contributions to the SH parameters is investigated using the CFA/MSH package and the complete diagonalization method (CDM). The SO mechanism is dominant for all CF parameter (CFP) ranges studied, except where the contributions D_SO to the zero-field splitting (ZFS) parameter D change sign. For the trigonal CFP, due to the other three mechanisms exceeds D_SO. Although |D_SOO| is quite small, the combined |D_SO−SOO| is appreciable. The SO-based perturbation theory (PT) works generally well for the g-factors: and , while it fails for D in the vicinity of v_c and for large and v>0. The high percentage discrepancy ratio δ_D = 2020% for v_c indicates unreliability of D_SO (in PT). Applications to Ni²⁺ ions at trigonal symmetry sites in LiNbO₃, α-LiIO₃, and Al₂O₃, are provided. The theoretical SH parameters are in good agreement with the experimental data. The low symmetry (C₃) effects induced by the angle φ are tentatively studied, but appear to be quite small.

Classical shell-model potentials for describing the complex ferroelectric behaviour of barium titanate and strontium titanate are developed and used to simulate Ba_xSr_1−xTiO₃ solid solutions. The temperature versus composition phase diagram is very well described and the local behaviour of the structure and polarization is analysed. It is shown that the ferroelectric properties of the solid solution can be understood in terms of the effects of average density and the local chemical environment. The experimentally observed static dielectric peak broadening around T_c at low x is reproduced in the simulation and seems to be related to the average volume rather than to the local chemical environment.