Table of contents

We have performed a detailed transverse magnetization study of the pyrochlore antiferromagnet Gd₂Ti₂O₇. A transverse magnetization of about 10⁻³M_sat is observed in the low-temperature ordered phases. These measurements result in the refinement of the Gd₂Ti₂O₇ phase diagrams. Observation of a transverse magnetization indicates loss of the cubic symmetry in some of the magnetic phases and provides new information for a better understanding of the complicated magnetic ordering of Gd₂Ti₂O₇.

Atomistic simulation has been performed on La_1−x(Ca/Sr)_xMnO₃ to investigate the change of Jahn–Teller energy E_JT and its quantitative dependence of the Curie temperature T_C. When the doping density x increases from 0 to 0.33 (0.25), it is found that E_JT of La_1−xCa_xMnO₃ (La_1−xSr_xMnO₃) decreases from 0.5 eV to 0.17 (0.1) eV. When the pressure changes from 0 to 4.5 GPa, E_JT of La_0.75Ca_0.25MnO₃ (La_0.89Sr_0.11MnO₃) decreases from 0.14 (0.18) eV to 0.07 (0.09) eV. With E_JT and estimated bandwidth W, we calculated T_C, which agrees well with experimental data, especially in the case of being under pressure. It is also found that about 75% enhancement of T_C is contributed by E_JT and the rest by W. Therefore, we propose that E_JT plays a main role, whereas W plays a much less but not negligible role on T_C.

Compositionally homogeneous ZrO₂–CeO₂ nanopowders have been characterized by Raman and extended x-ray absorption fine structure (EXAFS) spectroscopies. These techniques revealed a tetragonal-to-cubic phase transition as a function of CeO₂ content, as observed in a previous synchrotron x-ray diffraction study. The tetragonal–cubic phase boundary was found to be at (85 ± 5) mol% CeO₂. The EXAFS study demonstrated that this transition is related to a tetragonal-to-cubic symmetry change of the Zr–O first neighbour coordination sphere, while the Ce–O coordination sphere preserves its cubic symmetry over the whole composition range.

Site-selective laser excitation and luminescence measurements at 15 K of CaMoO₄ single crystal doped with Nd³⁺ and co-doped with Nb⁵⁺ charge compensator have been investigated in the spectral region of laser interest involving the transition. A larger than expected number of Nd³⁺ sites was found and a model to explain this result is presented. A partial diagram of ⁴F_3/2 and ⁴I_11/2 levels was constructed from the acquired data and their features are discussed.

We have performed a detailed study of the magnetic properties of GdCrO₄ at low temperatures by complementary use of different macroscopic and microscopic physical techniques. A ferromagnetic order is established in this oxide below T_C = 22 K. The ordered magnetic moments of the Cr⁵⁺ ions are located along the crystallographic c-axis, forming an angle of ≈24° with the ordered moments of the Gd³⁺ ions. Surprisingly, only 20% of the Gd³⁺ sublattice orders at the Curie temperature of 22 K, while the remaining 80% stays paramagnetic down to around 10 K. This 80% of the Gd³⁺ ions may be ascribed to a low-temperature orthorhombic phase. Moreover, the Gd³⁺ magnetic order seems to be triggered by a relatively large transferred hyperfine field coming from the Cr⁵⁺ sublattice.

β-PbF₂ single crystals doped with YbF₃ (0.2% and 2%) were studied by x-ray diffraction (XRD), electron paramagnetic resonance (EPR) and optical spectroscopy. EPR revealed the presence of only one kind of paramagnetic ion Yb³⁺, in a cubic symmetry site. The optical absorption, emission and excitation spectra enabled us to identify the transitions attributed to Yb³⁺ in the cubic site and to determine its energy level diagram. Site-selective laser spectroscopy also evidenced the presence of another type of Yb³⁺ ions, undetectable by classical EPR. This second type, which dominates in the 2%-doped crystal and exhibits cooperative luminescence, was attributed to Yb³⁺ ions forming clusters.

Transparent oxyfluoride glass-ceramics, containing β-Pb_1−yYb_yF_2+y nanocrystallites, were also synthesized and studied by XRD, EPR and optical spectroscopy. Two types of Yb³⁺ ions were found, as in β-PbF₂ single crystals. The optical properties of the oxyfluoride glass-ceramics turn out to be similar to those of ytterbium activated β-PbF₂ single crystals. Moreover, the Yb environments found in PbF₂ single crystals seem to already occur in the parent glass. Therefore, these materials are expected to be good laser media, like the rare-earth doped fluorite crystals, either in bulk or fibre form.

We present an ab initio study of the electron–phonon (e–ph) coupling and its contribution to the phonon linewidths and to the lifetime broadening of excited electron and hole states in bulk Pd. The calculations, based on density-functional theory, were carried out using a linear-response approach in the plane-wave pseudopotential representation. The obtained results for the Eliashberg spectral function α²F(ω), e–ph coupling constant λ, and the contribution to the lifetime broadening, Γ_e−ph, show strong dependence on both the energy and momentum of an electron (hole) state. The calculation of phonon linewidths gives, in agreement with experimental observations, an anomalously large broadening for the transverse phonon mode T₁ in the Σ direction. In addition, this mode is found to contribute most strongly to the electron–phonon scattering processes on the Fermi surface.

The surface and grain-boundary effects on the in-plane thermal conductivity of polycrystalline platinum nanofilms have been investigated. The thicknesses of the nanofilms range from 15.0 to 63.0 nm and the mean grain sizes measured by x-ray diffraction vary from 9.5 to 26.4 nm. The thermal conductivities of the nanofilms measured by a direct electrical heating method are greatly reduced from the bulk values. The measured results are compared with the values predicted by the Qiu and Tien model and the Kumar and Vradis theory. It is found that the reduction in the thermal conductivity is mainly caused by grain-boundary scattering and the reflection coefficient of electrons striking the grain boundaries is around 0.35. The relaxation time model is also applied to study the size effects to check whether the Matthiessen rule is still valid in predicting the in-plane thermal conductivity of polycrystalline metallic nanofilms. The results indicate that by considering only grain-boundary scattering and background scattering the Matthiessen rule is still valid. If surface scattering, however, is included, deviations of the Matthiessen rule from other theories mentioned above have been found.

Nd³⁺ dissolved in solid poly(acrylic acid) was synthesized by polymerization of the monomer partially neutralized with neodymium hydroxide in aqueous solution. The monomer modification and the coordination of ligands to Nd³⁺ were confirmed by ¹H NMR spectroscopy. The measured oscillator strengths for transitions from the ground state to the main excited state manifolds compared favourably with calculated electric dipole oscillator strengths. The spontaneous emission rates, the fluorescence branching ratios and the stimulated emission cross sections of the , and transitions, as well as the radiative lifetime and the quantum efficiency of the ⁴F_3/2 emitting level, were determined.

Transient carrier transport phenomena in an In_0.53Ga_0.47As-based p–i–n semiconductor nanostructure have been studied by using subpicosecond transient/time-resolved Raman spectroscopy. We observe an instability of the GaAs-like optical phonon population in this nanostructure semiconductor that occurs when electrons are accelerated to very high velocities by the application of intense electric fields. The results open up a new channel for creating coherent THz frequency that can be used in THz electronic devices. We suggest that the observed phenomena will have enormous impact on the carrier dynamics and carrier transport in nanoscale semiconductor electronic devices.

By means of magnetic susceptibility and specific heat measurements, x-ray and unpolarized neutron diffraction investigations on powder and single-crystal samples, simultaneous long-range antiferromagnetic Fe and Nd ordering in NdFe₃(¹¹BO₃)₄ with R32 chemical structure has been found at temperatures below T_N = 30.5(5) K down to 1.6 K. At temperatures down to 19 K the propagation vector is k_hex = [0,0,3/2] and becomes slightly incommensurate at lower temperatures. Combined with symmetry analysis, best powder neutron profile fits are obtained with magnetic spiral configurations with the magnetic moments oriented parallel to the hexagonal basal plane according to the irreducible representations τ₃ in the commensurate case. This is in agreement with the easy directions of magnetization perpendicular to the c-axis as determined by magnetic susceptibility measurements. At 1.6 K the magnetic Fe moment amounts to 4.9 μ_B close to the free ion moment of Fe³⁺. The magnetic Nd³⁺ moment saturates presumably due to crystal-field effects at 2.7 μ_B.

Electron doping in A_2−xL_xFeMoO₆ (where L is a trivalent lanthanide and A is a divalent cation) double perovskites has been established as a suitable technique for increasing their Curie temperature. However, it was found that the magnetoresistance gradually decreases with increasing lanthanide substitution. Here we analyse in detail the magnetoresistance as a function of the magnetic field for several series of ceramic A_2−xL_xFeMoO₆ oxides, showing that the data can be well described by assuming a gradual loss of spin polarization of the conduction band upon electron doping. This observation introduces some constraints to models of ferromagnetic coupling in double perovskites.

We propose a computational method that drastically simplifies the inclusion of the spin–orbit interaction in density functional theory when implemented over localized basis sets. Our method is based on a well-known procedure for obtaining pseudopotentials from atomic relativistic ab initio calculations and on an on-site approximation for the spin–orbit matrix elements. We have implemented the technique in the SIESTA (Soler J M et al 2002 J. Phys.: Condens. Matter 14 2745–79) code, and show that it provides accurate results for the overall band-structure and splittings of group IV and III–IV semiconductors as well as for 5d metals.

We have performed ab initio electronic structure calculations of C and S adsorption on two vicinal surfaces of Pd with different terrace geometries and widths. We find that both adsorbates induce a significant perturbation of the surface electronic and geometric structure of Pd(533) and Pd(320). In particular, C adsorbed at the bridge site at the edge of a Pd chain in Pd(320) is found to penetrate the surface to form a sub-surface structure. The adsorption energies show an almost linear dependence on the number of adsorbate–metal bonds, and lie in the ranges 5.31–8.58 eV for C and 2.89–5.40 eV for S. A strong hybridization between adsorbate and surface electronic states causes a large splitting of the bands, leading to a drastic decrease in the local densities of electronic states at the Fermi level for Pd surface atoms neighbouring the adsorbate, which may poison catalytic activity of the surface. Comparison of the results for Pd(533) with those obtained earlier for Pd(211) suggests a local character of the impact of the adsorbate on the geometric and electronic structures of Pd surfaces.

We analyse the manner in which local atomic structure affects the identification of Bi valence states in the x-ray absorption near-edge structure (XANES) of the Bi L₃ edge, by comparing simulations of the Bi L₃ edges in various polymorphs of Bi₂O₃, NaBiO₃ and Ag₂₅Bi₃O₁₈. We find that while the XANES is certainly sensitive to the ionicity of Bi, it is better described in terms of its sensitivity to the local bond lengths and coordination associated with the valence states of Bi.

Ternary intermetallics of La₃Ni₄Si₄, La₃Ni₄Ge₄, and La₃Pd₄Si₄ have been prepared by arc melting. These compounds take a body-centred lattice with an orthorhombic unit cell. The crystal structure of these compounds is U₃Ni₄Si₄ type with the space group Immm, consisting of the combination of structural units of AlB₂-type and BaAl₄-type layers, as observed for La₃Pd₄Ge₄. DC magnetization and electrical resistivity measurements indicate that La₃Pd₄Si₄ is a type II superconductor with a critical temperature (T_c) of 2.15 K. The lower critical field H_c1(0) is estimated to be 28 Oe. On the other hand, the upper critical field H_c2(0) estimated by the WHH theory is 2.2 kOe. The coherence length ξ(0) of 38 nm and the penetration depth λ(0) of 376 nm are derived. The other compounds La₃Ni₄Si₄ and La₃Ni₄Ge₄ do not show superconductivity above 1.8 K.

In this paper we present calculations of the zero-pressure melting temperature of a series of face-centred cubic (fcc) metals, including Ag, Rh, Cu, Ir, Au, Pt, Pd, Ni, Al and Pd. Our calculations employed the many-body potential due to Cleri and Rosato (1993 Phys. Rev. B 48 22) to model these systems; in the particular case of Pb, we also employed the 'glue' model of Lim et al (1992 Surf. Sci. 270 1109). Melting temperatures were obtained by calculating the Gibbs free energy of the solid and liquid phases, and finding the temperature at which they match. A wealth of other data of interest, ranging from enthalpies of fusion to transport properties, is also reported. Our findings indicate that the models considered in this study account reasonably well for the melting temperature of fcc metals, although there is a tendency to underestimate the experimental values. For the cases of Al and Cu we have also calculated the melting line, up to a pressure of 20 GPa in the case of Al and 100 GPa in the case of Cu.

The structural, electrical and dielectric properties of the spinel ferrite system Zn_xCo_1−xFeAlO₄ (x = 0.2–0.5) have been studied by means of various experimental techniques. Polycrystalline samples of this series have been prepared by the double sintering ceramic method. The structural parameters such as the lattice constant, x-ray density, pore fraction, cation distribution, interionic distances and polaron radius have been determined for better understanding of the electrical and dielectric behaviour. The compositional and thermal variations of the electrical properties have been studied by means of dc resistivity, thermoelectric power, I–V and C–V measurements. The electrical parameters such as the activation energy, Fermi energy, charge carrier concentration and mobility have been determined and a probable conduction mechanism in the system has been suggested. It is found that the ferrites are electronic conductors, and various results confirm the formation of small polarons. Current controlled negative resistance (CCNR)-type switching is observed in samples with x = 0.3–0.5. The dielectric behaviour of the system has been studied by measuring the dielectric constant, complex dielectric constant and conductivity in the frequency range 100 Hz–1 MHz at selected temperatures. The compositions exhibit normal dielectric behaviour, attributed to Maxwell–Wagner type interfacial polarization. Broadband dielectric spectroscopy in the form of an electric modulus has been applied to investigate the electrical spectroscopy in a wide temperature range. The results clearly indicate the presence of the non-Debye type of dielectric relaxation in these materials.

By means of the analytical expression of the decay of the number of rigid discs as a function of the fraction of covered surface, we are able to evaluate averages of stochastic variables related to the distribution of impenetrable discs. In particular, we consider the edge–edge mean distance between discs, λ_p, and the distance from the edge disc of a point chosen at random, δ. The second moment of δ and its connection to the adatom life time in thin film growth is highlighted and discussed on the basis of the comparison with Monte Carlo results. We also demonstrate numerically that the adatom life time in mean field coalescence is the same as that calculated in genuine coalescence.

The interaction energies of hcp and fcc helium are calculated by using a many-body expansion and cluster approach. The two- to six-body contributions are evaluated based on numerical solution of the Schrödinger equation for N-atomic clusters in the frame of the Born–Oppenheimer approximation. The convergence of the many-body expansion and its truncation position are discussed for the molar volume from 8.4 to 1.744 cm³ mol⁻¹. It indicates that the five-body interactions become manifest at volume smaller than 3.0 cm³ mol⁻¹, providing negative correction to the potential energy; while the six-body interactions emerge at volume smaller than 2.3 cm³ mol⁻¹, with positive correction. With the use of the Einstein approximation for the zero-point contribution, the calculated 0 K equation of state is given up to 180 GPa and compared with measurements.

The Hall effect, electrical conductivity and electron mobility are investigated at temperatures between 55 and 500 K in n-ZnSe crystals doped with Cu, Ag or Au. The presence of a small amount of Cu atoms leads to an inversion of the sign of the Hall coefficient at temperatures above 300 K. Anomalous temperature dependence of the electron mobility is observed in the samples with low Cu concentration (<0.3 at.% in the melt). Different characters of the temperature dependences of kinetic coefficients are found for n-ZnSe doped with Ag and Au. These curves are typical for crystals having several donor levels at different energetic depths. Immediately after doping, silver behaves like a usual compensating acceptor impurity while gold shows amphoteric properties. We propose a model that explains the anomalies of the temperature dependences of the kinetic coefficients in Cu-doped crystals and the lack of the anomalies in Ag-doped and Au-doped crystals. In accordance with this model and our experimental data, copper in n-ZnSe has two charge states, Cu_Zn⁺ (d¹⁰) and Cu_Zn²⁺ (d⁹), and two acceptor levels near the valence band. Silver and gold exist in single-charged states Ag_Zn⁺ and Au_Zn⁺ with d¹⁰ electron configuration forming single energy levels near the valence band. Au atoms form mainly interstitial Au_i donors at low doping concentrations and substitutional Au_Zn and Au_Zn-based acceptors at high doping concentrations. Time stimulation of the amphoteric properties of Ag is discussed.