Table of contents

Point defects in fumed ∼7 nm sized silica nanoparticles have been studied by means of K- and Q-band electron spin resonance following photodissociation of H from passivated defects. Generally, the spectra are dominated by the presence of intrinsic oxygen-associated hole centres (OHCs), probably of two types, one of which could be identified as the peroxy radical . Determination of the upper limit of the density of all defects observed suggests that, in conflict with previous inference, OHCs cannot account for the observed optical band gap narrowing. Probably that effect is related instead to nm size induced structural alterations in the SiO₂ network.

We present a comprehensive investigation of the structural, electronic, and magnetic properties of Pd_N and Rh_N clusters with up to N = 13 atoms, based on ab initio density functional calculations. The novel aspects of our investigation are the following. (i) The structural optimization of the cluster by a symmetry-unconstrained static total-energy minimization has been supplemented for larger clusters (N≥7) with a search for the ground state structure by dynamical simulated annealing. The dynamical structural optimization has led to the discovery of highly unexpected ground state configurations. (ii) The spin-polarized calculations were performed in a fixed-moment mode. This allowed us to study coexisting magnetic isomers and led to a deeper insight into the importance of magnetostructural effects. For both Pd and Rh the larger clusters adopt ground state structures that can be considered as fragments of the face-centred cubic crystal structure of the bulk phase. For Pd clusters, the magnetic ground state is a spin triplet (S = 1) for N≤9, a spin quintuplet (S = 2) for N = 10, and a spin septet (S = 3) for the largest clusters. Large magnetic moments with up to S = 8 have been found for Rh clusters. Magnetic energy differences have been found to be small, such that there is an appreciable probability of finding excited magnetic isomers even at ambient temperatures. In several cases, the structural energy differences are also sufficiently small to permit the coexistence of polytypes.

The first-principles density-functional method is used to study the recently high-pressure synthesized compound PtC. It is confirmed by our calculations that platinum carbide has a zinc-blende ground-state phase at zero pressure and that the rock-salt structure is a high-pressure phase. The theoretical transition pressure from zinc-blende to rock-salt structure is determined to be 52 GPa. Furthermore, our calculation shows the possibility that the PtC experimentally synthesized under high pressure conditions might undergo a transition from rock-salt to zinc-blende structure after a pressure quench to ambient conditions.

The heat capacity (C_p) and dilatation (α) of YB₁₂ and LuB₁₂ are studied. C_p of the zone-melted YB₁₂ tricrystal is measured in the range 2.5–70 K, of the zone-melted LuB₁₂ single crystal in the range 0.6–70 K, and of the LuB₁₂ powder sample in the range 4.3–300 K; α of the zone-melted YB₁₂ tricrystal and LuB₁₂ single crystals is measured in the range 5–200 K. At low temperatures a negative thermal expansion (NTE) is revealed for both compounds: for YB₁₂ at 50–70 K, for LuB₁₂ at 10–20 K and 60–130 K. Their high-temperature NTE is a consequence of nearly non-interacting freely oscillating metal ions (Einstein oscillators) in cavities of a simple cubic rigid Debye lattice formed by B₁₂ cage units. The Einstein temperatures are ∼254 and ∼164 K, and the Debye temperatures are ∼1040 K and ∼1190 K for YB₁₂ and LuB₁₂ respectively. The LuB₁₂ low-temperature NTE is connected with an induced low-energy defect mode. The YB₁₂ superconducting transition has not been detected up to 2.5 K.

We investigate the influence of an external voltage V₀ on conductance G through a quantum dot (QD), which is side-coupled to a quantum wire of length L_W, whose two ends are weakly connected to leads. In our calculation, the poor man's scaling law and slave-boson mean-field method are employed. With V₀ increased, a series of resonant regions is formed and G exhibits different properties in and out of these regions, which is the universal result of the finite-size effect on the Kondo correlation. In symmetric structures, the would-be resonant regions corresponding to odd wavefunctions are removed. If the symmetry is broken by changing the QD position, those regions will be recovered. In two asymmetric structures with their wire lengths being L_W and L_W+1, respectively, the two sets of resonant regions intersect with each other. These symmetry-related phenomena characterize side-coupled QD structures. With the barrier width increased, the number of resonant regions is increased, too.

Detailed measurements of the spontaneous magnetization of single-crystal La_0.73Ba_0.27MnO₃ are presented; these show a marked reduction in this property below 60 K which is consistent with a clearly discernible moment reduction, one of the few definitive conclusions about such effects provided by bulk measurements.

A new series of 3:29 compounds in the Fe–Nd–Ti–Zr quaternary system were synthesized successfully. The formation and structural properties were studied by means of powder x-ray and powder neutron diffraction. The results reveal that only a small amount of Zr content can exist in the 3:29-type compounds in the Fe–Nd–Ti–Zr quaternary system. Higher Zr content induces the formation of rhombohedral Th₂Zn₁₇-type phase. In the structures of 3:29-type compounds, the Zr content partially substitutes for Nd and enters the 2a and 4i crystallographic sites exclusively. Upon Zr substitution, the lattice parameters a, b, c and the unit cell volumes V of (Nd,Zr)₃(Fe,Ti)₂₉ compounds decrease monotonically and their intrinsic magnetic properties, including the Curie temperature and saturation magnetization, lessen as well.

The small rhombohedral distortion (at  K), observed for the V₄ tetrahedral cluster involved in the cubic compound GaV₄S₈, has been investigated by means of nuclear magnetic resonance (NMR). ⁵¹V NMR experiments successfully monitored the considerable change in the electronic state inside the cluster. The isotropic term of ⁵¹V hyperfine coupling is reduced markedly, and instead large anisotropy appears below T_S. The ⁵¹V nuclear spin–lattice relaxation rate is reduced markedly below T_S, suggesting a reduction of the density of states in the spin excitation spectrum. The structural transition can be interpreted as the Jahn–Teller instability resulting from the orbital degeneracy of the highest occupied 3d cluster orbitals in the V₄ tetrahedral cluster. The drastic change in the NMR quantities at T_S is explained as a transition from the electron occupation in nearly spherical degenerate orbitals to that in a strongly asymmetric non-degenerate orbital.

The temperature variation of the electrical resistivity ρ and the Seebeck coefficient S of Heusler-type Fe₂VAl_1−xSi_x (0≤x≤1) alloys has been investigated. We have shown that the transport parameters are very sensitive to doping. For the x = 0 sample, high values of ρ and negative temperature coefficient of resistivity (TCR) have been observed. As the Si concentration increases, ρ decreases and the TCR changes its sign, while S shows significant changes in magnitude as well as sign when Al is replaced with Si. These changes appear to be reminiscent of a metal to semiconductor transition. It has been shown that the conventional transport theories proposed for intermetallic alloys or semiconductors cannot explain the transport behaviour in the whole temperature range of the present study. Low-temperature resistivity data of x = 0–0.02 samples could be described with a gapless semiconductor model. The strong composition dependence of S and ρ is attributed to the sharp variations in electronic density of states at the Fermi energy. It is also shown that by optimum doping one can achieve very large values of power factor (P). The estimated power factor at room temperature is observed to be highest (2.23 × 10⁻³ W mK⁻²) for x = 0.06 and comparable to that of conventional thermoelectric material. At lower temperatures P is found to be even higher than that of conventional thermoelectric material.

The crystal lattice of bulk grains and state-of-the-art films of indium nitride was investigated at the atomic scale with perturbed angular correlation spectroscopy using the ¹¹¹In/Cd radioisotope probe. The probe was introduced during sample synthesis, by diffusion and by ion implantation. The mean quadrupole interaction frequency ν_Q = 28 MHz was observed at the indium probe site in all types of indium nitride samples with broad frequency distributions. The observed small, but non-zero, asymmetry parameter indicates broken symmetry around the probe atoms. Results have been compared with theoretical calculations based on the point charge model. The consistency of the experimental results and their independence of the preparation technique suggest that the origin of the broad frequency distribution is inherent to indium nitride, indicating a high degree of disorder at the atomic scale. Due to the low dissociation temperature of indium nitride, furnace and rapid thermal annealing at atmospheric pressure reduce the lattice disorder only marginally.

We have performed theoretical studies of the solubility within Nb_2−xW_xAlC by means of ab initio total energy calculations. If x is increased from 0 to 2 the bulk modulus can be increased by as much as 31%. The bulk modulus deviates from Vegard's rule, which may be understood based on substitution-induced changes in the CNb_2−xW_x bond angle resulting in flattening of the Nb_2−x W_xC layers upon solid solution formation. This rather extensive increase in the bulk modulus can be understood by considering the changes caused by the substitution of Nb through W for the equilibrium volume and chemical bonding. Based on the energy of formation analysis we suggest that the investigated system shows complete solubility. The bond length calculations suggest that both the Nb–C bond length as well as the W–C bond length are not significantly affected by variations in x. Based on a comparison to other solid solutions we suggest that this anomaly may be specific to the family of nanolaminates investigated here.

Landau–Lifshitz equations and spin wave damping are derived from first principles by the spin operator diagram technique for the Heisenberg model with magnetic dipole and exchange interactions. It is found that spin excitations, which are determined by poles of effective Green functions, are given by solutions of the linearized pseudodifferential Landau–Lifshitz equations and the equation for the magnetostatic potential. For a normal magnetized ferromagnetic film the spin wave damping has been calculated in the one-loop approximation for a diagram expansion of the Green functions at low temperature. In the framework of the Heisenberg model the magnetic dipole interaction makes a major contribution to the long-wavelength spin wave relaxation in comparison with the exchange interaction. It is found that the damping decreases with increasing film thickness and applied magnetic field and increases directly proportionally to the temperature. For modes of high orders the damping is higher than for the first spin wave mode.

By means of the density matrix renormalization group (DMRG) method, the magnetic properties of the J–J–J ' quantum Heisenberg chains with spin S = 1/2, 1, 3/2 and 2 in the ground states are investigated in the presence of a magnetic field. Two different cases are considered. (a) When J is antiferromagnetic and J ' is ferromagnetic (i.e. the AF–AF–F chain), the system is a ferrimagnet. The plateaus of the magnetization are observed. It is found that the width of the plateaus decreases with increasing ferromagnetic coupling, and disappears when J '/J passes a critical value. The saturated field is observed to be independent of the ferromagnetic coupling. (b) When J is ferromagnetic and J ' is antiferromagnetic (i.e. the F–F–AF chain), the system becomes an antiferromagnet. The plateaus of the magnetization are also seen. The width of the plateaus decreases with decreasing antiferromagnetic coupling, and disappears when J '/J passes a critical value. Though the ground state properties are quite different, the magnetization plateaus in both cases tend to disappear when the ferromagnetic coupling becomes more dominant. Besides, no fundamental difference between the systems with spin half-integer and integer has been found.

The structural phase transitions occurring at the first heating of as-grown Na₂SO₄(V) are so peculiar that there have been many controversial reports about them, especially with regard to the possible existence of several intermediate phases (II–IV) between the room-temperature (V) and high-temperature (I) phases. From this first detailed study of polarized Raman spectra based on oriented single crystals, we have obtained new results regarding the existence and nature of intermediate phases between phases V and I. We normally found that single-crystal Na₂SO₄(V) transforms directly to phase I at 242 °C without any occurrence of an intermediate phase (type-1 transition), but occasionally it transforms to a mixed phase of II and III near 234 °C, and then the phase II regions transform to phase I near 240 °C, and finally the phase III regions transform to phase I near 250 °C (type-2 transition). These results were confirmed from single-crystal ionic conductivity measurements and thermal calorimetry studies. For polycrystalline powder, however, both of these two types of transition sequences occur simultaneously, resulting in a complex and sluggish behaviour with temperature change. Hence it is now evident for powdered samples why many authors have hitherto reported so many diverse and contradictory results regarding the structural transitions of phase V.

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