Table of contents

The results of neutron diffraction investigations of TbMn₂D₂ deuteride are presented and analysed. The location of deuterium atoms in the lattice was determined. Magnetic ordering in this compound appears below 300 K. The magnetic structure can be described by the propagation vector [½½½]. The Tb sublattice couples antiferromagnetically, with magnetic moments equal to 4.8 µ_B, and the Mn sublattice is also coupled antiferromagnetically, with different magnetic moments.

The Josephson effect for a heavy-fermion superconductor CeIrIn₅, which has two characteristic temperatures T₀ and T_c, has been investigated for CeIrIn₅-Cu-Nb junctions, where T₀ and T_c are the temperature of the transition to the zero-resistivity state and the bulk, thermodynamic transition temperature, respectively. For all the junctions fabricated on the (110) and (001) planes of CeIrIn₅, the temperature below which the Josephson effect is observed is near T₀ = 0.8 K and much higher than T_c = 0.3 K. This result suggests that at least the surface of CeIrIn₅ is in a superconducting state below T₀, in which phase coherence between CeIrIn₅ and Nb is possible.

Electronic and atomic structures for high-pressure orthorhombic selenium were studied with first-principles calculations. Structural optimizations for four atoms in the orthorhombic unit cell were performed and the space group of the optimized structure was determined as P2₁/m, which is a monoclinic space group. The atomic structure optimized and the space group determined are consistent with the result of the x-ray diffraction measurement. The electronic density of states of the optimized structure is quite similar to that of the structure in the next high-pressure phase. This fact corresponds well to the experimental result that the transition to the next phase is a second-order one.

The high-temperature phenomena exhibited by KH₂PO₄ and RbH₂PO₄ have been investigated by differential thermal analysis, thermogravimetric methods, and thermo-polarizing microscopy. The thermal transformations which appear at T_p = 196 °C in KH₂PO₄ and T_p = 96 °C in RbH₂PO₄ are endothermic in addition to showing weight loss. On heating further to beyond T_p, the thermal transformation shows several endothermic peaks and there is weight loss in KH₂PO₄ and RbH₂PO₄. It has been observed by thermo-polarizing microscopy that until T_p is exceeded, uniaxial interference figures are exhibited by crystals of KH₂PO₄ and RbH₂PO₄, with cracks and chemical change at the surface of KH₂PO₄ near T_p~192 °C and RbH₂PO₄ near T_p~92 °C. The high-temperature phenomena exhibited by KH₂PO₄ and RbH₂PO₄ near T_p could indicate not changes from tetragonal to monoclinic structure but chemical decomposition at the surface of the crystals such as that described by nMH₂PO₄→M_nH₂P_nO_{3n + 1} + (n-1)H₂O (M = K, Rb)

Crystal structure investigations, electrical resistivity and magnetoresistance measurements were carried out for GdMn_xAl_12−x (2.6 ≤ x ≤ 6.1), TbMn_xAl_12−x (2.3 ≤ x ≤ 7.2) (ThMn₁₂ structure-type, space group I4/mmm) and Gd₂Mn_xAl_17−x (6.6 ≤ x ≤ 12.2), Tb₂Mn_xAl_17−x (8.5 ≤ x ≤ 10.2) (Th₂Zn₁₇ structure-type, space group R-3m) ternary compounds. These aluminides are characterized by either disordered or partially ordered distributions of Mn and Al atoms in the crystal structure. However, the RMn_xAl_12−x phases exhibit a fully ordered structure (CeMn₄Al₈ structure-type) at the 1:4:8 stoichiometry. A relation between the crystal structure and conductivity of the R–Mn–Al ternaries was evaluated. Hopping conductivity in terms of variable range-hopping was used to describe electrical transport below 150–200 K. The width of the hopping conductivity regime appears to be related to the atomic disorder of the crystal structure. The maximum range was detected for Gd₂Mn_9.5Al_7.5 (50–200 K). The magnetic state of rare earth atoms does not substantially influence this type of conductivity since the magnetoresistance is below 1% in a large temperature range (T > 0.5 K) and magnetic fields up to 12 T.

The site occupancy in the Re–W and Re–Ta sigma phases is studied using a first-principles statistical thermodynamics approach. The factors which govern the stability of the sigma phase were analysed using selected supercells for total energy calculations. Cluster variation calculations in the tetrahedron approximation were performed to study the effect of composition and of finite temperature on the ordering phenomena in the sigma phase. The difference between the site occupancy of both Re–W and Re–Ta sigma phases is discussed.

Transport of Na⁺ and Li⁺ under the influence of an electric field in twinned quartz is simulated using molecular dynamics techniques. Comparison between bulk transport and transport along twin boundaries shows that the cations are trapped inside twin walls for weak fields along the crystallographic c-axis. Stronger fields lead to transport along twin walls with significantly lower mobility than in the bulk. With E along [110], transport in the wall is faster than in the bulk. We observe cation trapping preferentially in the twin walls when E is applied out of the plane of the wall.

The unphysical solutions of the periodic Anderson model obtained by Keiter and Leuders (Keiter H and Leuders T 2000 Europhys. Lett. 49 801) in dynamical mean-field theory (DMFT) are shown to result from the authors' restricted choice of the functional form of the solution, leading to a violation of the analytic properties of the exact solution. By contrast, iterative solutions of the self-consistency condition within the DMFT obtained by techniques which preserve the correct analytic properties of the exact solution (e.g., quantum Monte Carlo simulations and the numerical renormalization group technique) always lead to physical solutions.

The Perdew, Burke and Ernzerhof (PBE) generalized gradient approximation (GGA) is the most popular exchange-correlation energy used in today's ab initio studies. The GGA is tested here in relation to the intrinsic uncertainty in choosing the degree of localization of the exchange-correlation hole (the κ-coefficient in the spin-polarized enhancement factor). The proposed and most commonly used value of κ = 0.804 (best suited for atoms and molecules) works well for some solids but should be modified in many cases in order to predict lattice parameters in good agreement with experiments. The effect on the structural and magnetic properties of 3d, 4d and 5d metals including the structural phase order of Fe is examined using two different state-of-the-art ab initio implementations of density functional theory: the full-potential linearized muffin-tin orbital and full-potential linearized augmented-plane-wave methods. This study gives examples for the case of elemental d metals of the errors associated with these properties when using the PBE-GGA in state-of-the-art ab initio electronic structure studies.

The variational and fractional-dimensional space approaches are used in a thorough study of the virial theorem value and scaling of the shallow-donor binding energies versus donor Bohr radius in GaAs/(Ga,Al)As semiconductor quantum wells (QWs) and quantum-well wires (QWWs). In the case of the fractional-dimensional space approach, in which the three-dimensional actual anisotropic semiconductor heterostructure is modelled by a fractional-dimensional isotropic effective medium, we have shown that if the ground-state wave function may be approximated by a D-dimensional hydrogenic wave function, the virial theorem value equals 2 and the scaling rule for the donor binding energy versus Bohr radius is hyperbolic, both for GaAs/(Ga,Al)As wells and wires. In contrast, calculations within the variational scheme show that the scaling of the donor binding energies with quantum-sized Bohr radius is in general nonhyperbolic and that the virial theorem value is nonconstant. Moreover, calculations for the donor binding energies versus well widths or wire radii, within both the fractional-dimensional and the variational approaches, indicate that any general conclusion based on a given virial theorem value or donor energy versus Bohr radius scaling rule should be examined with caution.

We have studied the substitution of a 3d element, namely Mn, at the Ru site of the metallic ferromagnet SrRuO₃. We find that the substitution gives rise to short-range cluster formation which coexists with long-range ferromagnetism. The clusters show a characteristic blocking at a temperature lower than T_c (the ferromagnetic transition temperature). We find that the magnetoresistance of the sample with substitution is higher than that of the parent SrRuO₃ and that the magnetoresistance peaks at the cluster freezing temperature.

Above the semiconductor-to-metallic transition (SMT) temperature (T_p), transport properties of the La_1−xPb_xMnO_3+δ (0 ≤ x ≤ 0.5)-type mixed valence oxides with T_p between 230 and 275 K (depending on x) have been thoroughly examined for a small-polaron hopping conduction mechanism of the carriers. Although the variable range hopping (VRH) model was used earlier to fit the entire conductivity data above SMT, we noticed two distinct regions (above and below θ_D/2; θ_D is the Debye temperature) where different types of conduction mechanisms are followed. The high temperature (T > θ_D/2) conductivity data of all the Pb-doped samples follow the adiabatic hopping conduction mechanism, while those of LaMnO₃ (x = 0) showing no SMT follow the non-adiabatic hopping conduction mechanism of Mott or Emin with reasonable values of polaron radius, hopping distance, polaron binding energy, activation energy, etc being different for different systems. The VRH model, however, fits the corresponding low temperature (T < θ_D/2) data of all the samples. Both resistivity ρ(T) and thermoelectric power S(T) follow a similar microscopic theory above T_p supporting the small-polaron hopping mechanism. Thermoelectric power also showed appreciable magnetic field dependence around SMT.

We report a theoretical study of the spectral statistics of a quasi-one-dimensional surface superlattice in perpendicularly applied magnetic fields. The energy-level-spacing distribution and the Dyson–Mehta Δ₃ statistic of the magnetic band structure of the system are calculated. The calculations show that for the system with inversion symmetry, the magnetic band structure at the wave vector k = 0 is well described by the statistic derived by a superposition of two independent Gaussian orthogonal ensemble (GOE) statistics. This result is consistent with the fact that the system shows a false time-reversal violation and a real-space symmetry. The calculations show also that when the wave vector k is moved away from the k = 0 point, the statistical properties of the magnetic band structure are excellently described by the GOE statistics. The GOE statistics are also found in the magnetic band structure when the inversion symmetry is removed from the system.

In this work we clarify the nature of frequent oscillations of the conductance of open quantum dots that were reported by Liang et al (Liang C-T et al 1998 Phys. Rev. Lett. 81 3507). Continuous and almost periodic oscillations superimposed upon ballistic conductance features are observed when the conductance G of the dot changes within a wide range 0<G<6e²/h. We confirm the single-electron origin of the conductance oscillations by means of measurements in a perpendicular magnetic field and calculation of capacitances of the quantum dot with respect to two-dimensional (2D) electron gas reservoirs and gates. The calculations of the three-dimensional electrostatics of the device and 2D transport through the dot show that the progression of the Coulomb oscillations into the region G>2e²/h is the consequence of suppression of inter-one-dimensional-subband scattering. The theory of Coulomb blockade and the Landauer formula are modified for the case of the quasi-one-dimensional system to describe combined charging and ballistic transport through the dot. Measured dependences of the conductance on the gate voltages and its temperature behaviour are correctly reproduced by the calculations.

The hysteretic voltage gap ΔV(m) - the difference between the threshold voltages for single-charge-soliton tunnelling into and escape from a single-electron dual-junction-array trap (which consists of 2N gated small junctions with equal junction capacitances C, equal stray capacitances C₀, equal input gate capacitances C₁, and coupling capacitance C_C) through an m-junction cotunnelling process - is investigated for various charge solitons including a single electron, an exciton, and a combined soliton. Our results show that ΔV(m) has a strong dependence on m, C₀/C, C₁/C, C_C/C, and N, and that no hysteresis loop exists beyond a critical value η_c of C₀/C. For finite stray capacitance and strong coupling capacitance, the exciton can be a candidate for use in constructing more stable single-electron circuits, as in the case of no stray capacitance and weak coupling capacitance previously discussed in the literature.

The crystal structure, magnetic and electrical transport properties of the sodium-doped lanthanum manganites La_1-xNa_xMnO₃ (0.07⩽x⩽0.40) have been studied in detail using x-ray powder diffraction, atomic absorption spectroscopy, a SQUID (superconducting quantum interference device) magnetometer and the four-probe resistivity measurement technique. A rhombohedrally distorted perovskite structure has been observed in the range 0.07⩽x⩽0.20. Both the lattice parameter and unit-cell volume decrease with increase in the Na content. A ferromagnetic-to-paramagnetic phase transition associated with a metal-insulator transition is observed for all the La_1-xNa_xMnO₃ compounds. There is a systematic change in both the Mn-O-Mn bond angle and the tolerance factor with Na content. The compositional variation of the magnetic and metal-insulator transition temperatures is explained as due to the distortion of the MnO₆ octahedron and increase in the tolerance factor that controls the hopping interaction. In the metallic region a ρ~AT² behaviour is observed due to the magnon excitation effect. The resistivity shows a field-dependent minimum at low temperature that has been explained as due to the intergrain transport phenomenon.

A DTA test, performed within the 300–700 K range, showed that the series of NaNbO₃:yMn single crystals exhibit first-order phase transition known from pure sodium niobate. The introduced Mn dopant lowers the temperature of this phase transition at a rate ΔT_P–R/Δ%(Mn) ≈ −15 K wt%⁻¹. The electric permittivity measured at ambient pressure showed a clear Curie–Weiss maximum at this antiferroelectric phase transition. The applied axial compression caused gradual suppression of this anomaly towards step-like characteristics. Flat ε(T, X, y) curves appeared when the axial pressure X exceeded a few hundreds bars. The stress field was calculated around the Mn ions built into the host. It was pointed out that due to this the stress field and external compression are lower than misfit stress estimated for the P–R structural phase transition and any new phase was not induced with dopant manganese ions at the obtained real concentration y level. The recorded ε(T, X, y) characteristics were fitted with a generalized Curie–Weiss formula. It was shown that the molecular field approximation remains valid. Both linear and nonlinear elastic effects were necessary to describe the experimental ε(T, X, y) characteristics.

The phase transition of ferroelectrics is one of their most important bulk-nature attributes. In well-epitaxial ferroelectric thin films, the crystal structures are highly strained and the ferroelectric phase transitions tend to dissappear because of an epitaxial effect. We investigated the epitaxial effect by structural analysis and clarified the growth conditions of BaTiO₃ films that show their bulk nature. As a result, epitaxial crystals of BaTiO₃ with a thickness of 67 Å were grown on a MgO substrate with a Pt electrode by the activated reactive evaporation method. A clear thermal anomaly was observed at 140 °C that suggests the phase transition of BaTiO₃.

We present and analyse high-resolution absorption, luminescence and upconversion-luminescence spectra of Cs₂GeF₆: 2% Re⁴⁺. The crystals of Cs₂GeF₆: 2% Re⁴⁺ were grown from solution. Upon excitation around 11 000 cm⁻¹ in the near infrared, visible upconversion luminescence around 17 000 cm⁻¹ is observed. On increasing the temperature from 15 K to room temperature the integrated upconversion luminescence intensity decreases to 2%. This is attributed to the decreasing absorption cross-section at the monochromatic laser-excitation energy. The upconversion excitation spectra and the results obtained from time-dependent measurements indicate a dominant energy-transfer upconversion mechanism.

In this paper we report measurements of thermoluminescence in the temperature range of 20–370 K, isothermal decays, pulsed vacuum ultraviolet and γ-excited luminescence time profiles at various temperatures on cerium-activated orthoaluminate (LuAlO₃:Ce, LuAP), a new and promising scintillator material. We demonstrate that results of all these experiments can be consistently explained by assuming a recombination mechanism of scintillation light production in the LuAP scintillator. Using a simple first-order kinetic model that includes Ce³⁺ ions as recombination centres and a number of electron traps, we extract from experimental data the basic trap parameters (energy depths and frequency factors). Consequently we identify nine traps that are responsible for undesired features of the LuAP scintillator, such as a reduced scintillation light output, a relatively long scintillation rise time and slow scintillation components (afterglow) at room temperature. We demonstrate that some of these traps are responsible for large variations of the scintillation light yield with temperature as reported earlier. Although the deepest traps do not alter scintillation time profiles, they are responsible for a significant scintillation light loss and are, therefore, detrimental to scintillation performance of the material. We observe that there is an apparent correlation between trap depths and frequency factors for at least five of the traps that may fit some more general pattern involving various groupings of all the traps. This, in turn, would indicate that traps in LuAP are not unrelated and are due, most likely, to a series of native defects in the LuAP crystal structure. Although the specific identity of traps remains unknown, the performance of the LuAP scintillator is now, in practical terms, fully understood and can be described numerically at any temperature using a model and a set of parameters given in this paper. It is clear that any major improvement of the material would require that traps are eliminated or that their influence on the scintillation process is minimized.

Thomson scattering by rare-earth multipoles is a valuable technique for determining orbital structures in rare-earth systems. In the present paper, the formalism describing the relation between the magnetic and multipolar structures in cubic systems is extended to the fourth- and sixth-order multipoles. The associated x-ray scattering amplitudes are written using the cubic irreducible representations and calling in play appropriate Stevens equivalent operators. This paper is intended to provide a rigorous framework for the interpretation of multipolar scattering experiments on magnetically or orbitally ordered systems.

The segregation effects of Nb and V on bcc FeΣ3[10](111) grain boundary cohesion are studied by the first-principles DMol method within the framework of density functional theory. The calculated segregation energy difference between the grain boundary and the corresponding free surface is -0.51 eV (-0.58 eV) for solute Nb (V), which indicates that both Nb and V could enhance the grain boundary cohesion in body-centred-cubic Fe. We found that the chemical effect and the geometry effect of Nb (V) play crucial but opposite roles in determining whether a material is brittle or ductile. The chemical effect is dominant and advantageous for grain boundary cohesion. Also, Nb and V show very different behaviours: in chemical effect, Nb is more conducive to ductility than V; while in the geometry effect, Nb is less conducive to ductility and more conducive to embrittlement than V.

A theoretical model is suggested which describes the formation of nano-wires associated with compositional inhomogeneities in multilayered films. General formulae are found relating characteristic length scales of compositional inhomogeneities in film layers and geometric parameters (layer thickness values, misfit parameters) of the multilayered film. The exact relationship between such length scales and parameters is revealed and analysed in detail in the exemplary case of three-layer films.

The temperature dependencies of the resistivity and magnetization of a series of Ni_2+xMn_1-xGa (x = 0-0.09) alloys were investigated. Along with the anomalies associated with ferromagnetic and martensitic transitions, well-defined anomalies were observed at the temperature of premartensitic transformation. The premartensitic phase existing over the temperature range 200-260 K in the stoichiometric Ni₂MnGa is progressively suppressed by the martensitic phase with increasing Ni content and has vanished in the Ni_2.09Mn_0.91Ga composition.

The crystal structure of borophosphates ABPO₅ (A = alkaline earth or Pb) was resolved on a polycrystalline sample using the Rietveld method. The x-ray diffraction patterns data show that ABPO₅ crystallize in a centrosymmetric space group P3₁21 and their structure is related to the borogermanates REBGeO₅ with a stillwellite-type structure. Pr³⁺ ion was used as a local structural probe to corroborate the structural resolution results. Absorption and fluorescence spectra of A_1−xPr_xBP_1−xGe_xO₅ (A = alkaline earth or Pb; x = 0.05) have been investigated at different temperatures. At 9 K the ³H₄ → ³P₀ transition of trivalent praseodymium ion (4f² configuration) is observed as a single line. This indicates a unique crystallographic site for the rare earth ion in these compounds replacing the divalent cation. Energy level schemes were deduced from the low-temperature spectroscopic measurements. Comparing the electronic level splittings of studied compounds with the already reported data on REBGeO₅ doped with Pr³⁺ ion, it is possible to dispel the doubt existing about structural determination. Moreover, under 460 nm excitation, intense red emission of trivalent praseodymium is observed corresponding to ¹D₂ → ³H₄ transition. The lifetime measurements of ¹D₂ level have been performed for all the title compounds.

In a recent comment (Prellier W and Raveau B 2001 J. Phys.: Condens. Matter13 2749) on our published letter (Lee Y P et al 2000 J. Phys.: Condens. Matter12 L133), have proposed that our Pr_0.65Ca_0.35MnO₃ films may have (hk0)-oriented epitaxy rather than the (00l)-oriented epitaxy. We point out herein that always, owing to the substrate-film lattice mismatch, lattice strains are accumulated during the heteroepitaxial growth of perovskite-like manganite films, which allows us to explain the physical results obtained.