Table of contents

Crystallization, melting and structural evolution upon crystallization in Nd₆₀Al₁₀Fe₂₀Co₁₀ bulk metallic glass (BMG) are in situ investigated by x-ray diffraction with synchrotron radiation under high pressure. It is found that the crystallization is pressure promoted, while the melting is inhibited. The crystallization and melting process are also changed under high pressure. The features of the crystallization and melting under high pressure are discussed.

A procedure for finding the equilibrium states of a hcp structure under pressure from minima of the Gibbs free energy using first-principles electronic theory has confirmed the existence of structural anomalies in hcp Zn and hcp Fe. Calculations of elastic constants as functions of pressure then show that the anomalies are large effects which cover a substantial range of pressure. These calculations permit reinterpretation of the experimental data to show that the structural anomaly, although small, can be seen in the data. These calculations contradict a recent first-principles calculation that found that the structural anomaly in hcp Zn disappeared when a large number of k-points were used in the Brillouin zone integrations.

We study the oxygen isotope (¹⁶O,¹⁸O) and finite size effects in Y_1−xPr_xBa₂Cu₃O_7−δ by in-plane penetration depth (λ_ab) measurements. A significant change of the length L_c of the superconducting domains along the c-axis and λ_ab² is deduced, yielding the relative isotope shift for x = 0, 0.2 and 0.3. This uncovers the existence and relevance of the coupling between the superfluid, lattice distortions and anharmonic phonons which involve the oxygen lattice degrees of freedom.

Positronium formation has been studied at a graphite surface in the presence of a physisorbed monolayer of argon. In addition to the expected formation when a partial fluid monolayer is present, it is observed that formation is increased when the monolayer becomes an incommensurate solid.

We review and summarize recent theoretical and experimental work on electron spin dynamics in quantum dots and related nanostructures due to hyperfine interaction with surrounding nuclear spins. This topic is of particular interest with respect to several proposals for quantum information processing in solid state systems. Specifically, we investigate the hyperfine interaction of an electron spin confined in a quantum dot in an s-type conduction band with the nuclear spins in the dot. This interaction is proportional to the square modulus of the electron wavefunction at the location of each nucleus leading to an inhomogeneous coupling, i.e. nuclei in different locations are coupled with different strengths. In the case of an initially fully polarized nuclear spin system an exact analytical solution for the spin dynamics can be found. For not completely polarized nuclei, approximation-free results can only be obtained numerically in sufficiently small systems. We compare these exact results with findings from several approximation strategies.

In this review, results of imaging coherent electron flow through a two-dimensional electron gas (2DEG) are presented. The images show the modal pattern of electron wavefunctions passing through a quantum point contact. At longer distances, unexpected branching of the electron flow is observed due to the small bumps and dips in the potential caused by the donor atoms. Images of the electron flow being bent by an electrostatic prism are also presented. All of the images are decorated by interference fringes spaced by half the Fermi wavelength, which are used to spatially profile the electron density. A technique for imaging the inelastic scattering length in the 2DEG is also demonstrated.

In recent years, disorder has been shown to be crucial for the understanding of diluted magnetic semiconductors. Effects of disorder in these materials are reviewed with the emphasis on theoretical works. The types and spatial distribution of defects are discussed. The effect of disorder on the intimately related transport and magnetic properties are considered from the viewpoint of both the band picture and the isolated-impurity approach. Finally, the derivation and properties of spin-only models are reviewed.

We examine the effect of spatial confinement on the hyperfine exchange interaction between two donor nuclei in a semiconductor quantum dot. On the basis of an exactly solvable model we look at the strength of the interaction in the ionized and neutral cases as one of the nuclei approaches the dot surface.

We report on a molecular dynamics study of the cross-plane lattice thermal conductivity in GaAs/AlAs superlattices. The layers of the superlattice are modelled by a three-dimensional face centred cubic lattice with cubic anharmonicity, and with atomic scale roughness at the interfaces. We perform the simulation of heat flow for a section of a superlattice with high- and low-temperature thermal reservoirs attached to opposite ends. The calculation reproduces qualitatively the features observed experimentally, i.e., the dramatic reduction of the conductivity relative to the conductivity of the bulk constituent materials, and the variation of the thermal conductivity with the superlattice repeat distance. The results are also in agreement with those obtained previously by Daly et al (2002 Phys. Rev. B 66 024301) who determined the thermal conductivity from the time taken for an initially inhomogeneous temperature distribution to relax.

The crystal and magnetic structure of strontium ferrite Sr₄Fe₄O₁₁ (SrFeO_2.75) has been investigated using neutron powder diffraction and magnetic susceptibility measurements over the temperature range 1.5–293 K and by Mössbauer spectroscopy at room temperature. Sr₄Fe₄O₁₁ is orthorhombic (symmetry group Cmmm) over the temperature range 1.5–293 K. Its crystal structure contains two iron sites occupied by trivalent and tetravalent iron. The trivalent magnetic moments order antiferromagnetically below T_N = 232(4) K and the magnetic unit cell coincides with the chemical cell. The ordered moments are parallel to the b axis and the tetravalent iron moments form a magnetically frustrated sublattice. The extrapolated value of the Fe³⁺ moment is 3.55(5) μ_B at 0 K. The antiferromagnetic coupling occurs by the superexchange mechanism and is mediated by oxygen ions. There are no signs of long-range magnetic order in the tetravalent sublattice down to 1.5 K.

The kinetics of an amorphous-to-Cu₅₁Zr₁₄ phase transformation in an as-cast Cu₆₀Ti₂₀Zr₂₀ rod have been investigated by differential scanning calorimetry. The relative volume fractions of the transferred crystalline phase as a function of annealing time, obtained at 713, 716, 723, 728, and 733 K, have been analysed in detail using 14 nucleation and growth models together with the JMA model. A time-dependent nucleation process is revealed. A steady-state nucleation rate of the order of 10²²–10²³ nuclei m⁻³ s⁻¹ in the temperature range 713–733 K and an activation energy of the order of 550 kJ mol⁻¹ for the phase transformation in the as-cast Cu₆₀Ti₂₀Zr₂₀ rod were detected, for which some possible reasons are suggested.

Low-temperature (300–4.2 K) electrical resistivity behaviours of as-cast glassy, relaxed, and crystallized Pd₄₀Ni₁₀Cu₃₀P₂₀ alloys have been investigated. It is found that the resistivity of the as-cast glassy alloy is about 257 µΩ cm at ambient temperature and decreases with a temperature coefficient of resistivity (TCR) of 2.1 × 10⁻⁴ K⁻¹ in the temperature range of 280–50 K and 89 × 10⁻⁴ K⁻¹ in the temperature range of 12–4.2 K. The relaxation effect slightly increases the resistivity by a factor of about 10% while crystallization decreases the resistivity of the alloy. Surprisingly, we found that all samples, as-cast glassy, relaxed, crystallized, and well-crystallized alloys, exhibit similar temperature dependences of the electrical resistivity in the temperature range investigated. TCR changes at around 10–20 K for all samples are linked with rapid increase of the magnetization in the alloys.

We show that the Vickers microhardness, measured on flux grown single crystals of the Yb_xY_1−xInCu₄ alloy system, although sample dependent, exhibits clear concentration dependence; it increases with decreasing x. Such a dependence is not expected because the cubic lattice parameter increases with decreasing x and one expects then a decrease of hardness with decreasing x. Also, such a concentration dependence is in accordance with neither the Mott–Nabarro theory nor other known experimental results. We ascribe the observed dependence to the change of the electronic structure of the Yb_xY_1−xInCu₄ alloy system with x.

The phase transition in K₃Na(CrO₄)₂ single crystals has been studied by electron paramagnetic resonance (EPR) of Mn²⁺ ions. The values of the spin-Hamiltonian parameters have been evaluated. Below the phase transition temperature, the magnetic z-axis is deflected in ac and bc crystallographic planes by the same angle. Anomalous line shapes recorded below the phase transition temperature suggest the existence of an incommensurate phase.

The problem of quantum-confined heterostructures with elliptic shape is studied with elliptic cylinder coordinates and a basis of symmetry-adapted Mathieu functions. The conventional effective mass Hamiltonian is written in two-dimensional and three-dimensional geometries. The problem is not separable in radial-like and angular-like coordinates. A variational method using a basis of symmetry-adapted Mathieu functions is proposed. Energy splitting, optical intersubband and interband transition spectra, and polarization effects are analysed. A calculation for self-organized InAs and InAs/InAlAs quantum dots on InP is performed. Various island shapes and morphologies are observed for such quantum dots, which may be important for potential applications in fibre optic telecommunication systems.

We present conductivity measurements of quasicrystalline AlPdRe ribbons in a wide range of temperature (20 mK–300 K), for icosahedral samples spanning a broad resistivity range (R = ρ_4 K/ρ_300 K from 2 to 209). We focus here on a detailed analysis of the temperature dependence of the conductivity σ(T) for the insulating samples (R>16). Three successive regimes are revealed as the temperature is increased to 300 K: a low temperature variable range hopping-like behaviour, followed by a Thouless regime and a high temperature critical regime. The temperature dependence of the inelastic scattering time found elsewhere by magnetoresistance data analysis accounts well for the change of curvature in σ(T) and the trends observed as R varies. The present results point to a similar behaviour between quasicrystalline AlPdRe samples and disordered systems close to the metal–insulator transition, in accordance with our previous analysis of the very low temperature insulating regime and of the finding of conductivity scaling laws.

Anomalies of magnetic properties caused by the interaction of energy levels of the rare-earth ion in HoPO₄ in a high magnetic field along the [100] and [110] axes are investigated experimentally and theoretically. Jumps in the magnetization curves M(H) and maxima in their derivatives d M(H)/d H are observed at critical fields H_c = 200 and 320 kOe where one of excited singlets approaches or crosses the lowest energy level of the Ho³⁺ ion. The experimental data are found to be described rather accurately by the calculated magnetic moments M(H) along the [100] and [110] axes when taking into account the quadrupolar interactions and magnetocaloric effect at the adiabatic magnetization in pulsed fields. It is shown that a jump-like change of the quadrupolar interactions of α and γ (or δ) symmetry in HoPO₄ caused by a change in the corresponding quadrupolar moments upon level crossing leads, according to the experiment, to a decrease of the critical field H_c and a sharper variation of the M(H) and d M(H)/d H curves near the crossover.

Two extreme pictures of electron–phonon interactions in nanoscale conductors are compared: one in which the vibrations are treated as independent Einstein atomic oscillators, and one in which electrons are allowed to couple to the full, extended phonon modes of the conductor. It is shown that, under a broad range of conditions, the full-mode picture and the Einstein picture produce essentially the same net power at any given atom in the nanojunction. The two pictures begin to differ significantly in the limit of low lattice temperature and low applied voltages, where electron–phonon scattering is controlled by the detailed phonon energy spectrum. As an illustration of the behaviour in this limit, we study the competition between trapped vibrational modes and extended modes in shaping the inelastic current–voltage characteristics of one-dimensional atomic wires.

In considering a novel function in ferromagnetic tunnel junctions consisting of ferromagnet (FM)/barrier/FM junctions, we have theoretically investigated the multiple-valued (or multi-level) cell property, which is in principle realized by sensing the conductances of four states recorded with magnetization configurations of two FMs; that is, (up, up), (up, down), (down, up), (down, down). To obtain such 4-valued conductances, we propose FM1/spin-polarized barrier/FM2 junctions, where FM1 and FM2 are different ferromagnets, and the barrier has spin dependence. The proposed idea is applied to the case of the barrier having localized spins. Assuming that all the localized spins are pinned parallel to the magnetization axes of FM1 and FM2, 4-valued conductances are explicitly obtained for the case of many localized spins. Furthermore, objectives for an ideal spin-polarized barrier are discussed.

Keldysh formalism is used to find the current–voltage characteristics of a small system of interacting electrons described by a Hubbard model coupled to metallic wires. The numerical procedure is checked and the well known results for an Anderson impurity are found. When larger interacting regions are considered, quite different results are obtained depending on whether the Hubbard part is half-filled or not. At half-filling, the existence of an energy gap for charge excitations manifests itself by making the current exponentially small as a function both of the number of interacting sites and the value of U. The behaviour changes at large voltages above the gap energy when activated charge transport takes place. In contrast, for filling factors other than half, the current goes through the interacting system and suffers just a small amount of scattering at both connections. Conductance depends slightly on U and much more on the filling factor but not on the length of the interacting region.

We investigate the low energy acoustical and optical modes in HgBa₂CuO_4+δ using inelastic x-ray scattering (IXS). The experimental phonon dispersion and the dynamical structure factor are compared with an atomic shell model, and the set of atomic potentials obtained is discussed. Our results are also compared with those obtained by Raman spectroscopy and with density-of-state data measured by inelastic neutron scattering.

Ce₃NiGe₂ was studied by means of DC and AC magnetic susceptibility, magnetization, electrical resistivity, magnetoresistivity and specific heat measurements. The compound undergoes two subsequent magnetic phase transitions at T_N = 6.2 K and T₁ = 5.2 K, from paramagnetic to antiferromagnetic and to ferromagnetic-like ground state, respectively. The electrical and thermodynamic behaviour is governed by interplay of RKKY, Kondo and crystal field interactions, with the Kondo temperature of the order of 10 K and the total crystal–field splitting of about 690 K.

Magnetization studies have been carried out on a single crystal of TbBaCo₂O_5.5 in the temperature range 2–350 K and magnetic fields up to 50 kOe. Several spontaneous magnetic phase transitions have been observed and analysed in both Co (T_C = 277 K, T_N1 = 210 K) and Tb (T_N2 = 3.44 K) subsystems. It is shown that the spontaneous first-order phase transition at T_N1 from an antiferromagnetic to a weak ferromagnetic state is accompanied by an intermediate state. The properties of this state are strongly determined by the thermo-magnetic history of the sample. Metamagnetic type transitions have been observed at 140 K< T<190 K for the Co subsystem and at T<T_N2 for the Tb subsystem. The analysis of the magnetic properties of TbBaCo₂O_5.5 points to the strongly anisotropic character of the Tb and Co subsystems. The Tb subsystem was shown to be of the Ising type. For this subsystem, the exchange interactions between nearest neighbours and next-nearest neighbours were determined. The observed properties of the Co subsystem also point to the Ising character of the Co ions. The effect of the twinning structure on magnetization processes of the weak ferromagnetic phase, especially hysteresis curves, is discussed.

A study of crystal structure, elastic, and magnetic properties of low-doped Nd_1−xCa_xMnO₃ () perovskites has been carried out. The ferromagnetic component is shown to increase under hole doping and, simultaneously, the temperature of the orbital order–disorder phase transition decreases. The mechanism of the concentrational transition from a weak ferromagnetic state (x = 0) to a ferromagnetic one (x>0.15) is discussed using a two-phase model, according to which the samples consist of weak ferromagnetic and ferromagnetic phases exchange coupled at their boundary. It is found that interaction between different magnetic phases leads to spin reorientation which takes place for compounds around  K. In the temperature range from 5 to 20 K, metamagnetic behaviour is revealed for the Nd_0.92Ca_0.08MnO_2.98 sample. H versus T as well as T versus x magnetic phase diagrams, which are characterized by the missing of a canted phase, are proposed. The appearance of orientational transitions is explained on the basis of a magnetic analogue of the Jahn–Teller effect taking into account that the magnetic moments of Nd ions are ordered parallel to the moments of Mn ions in the ferromagnetic phase, and opposite to the direction of the weak ferromagnetic vector at T>T_eff in the weak ferromagnetic phase.

The ac susceptibility as a function of temperature, dc magnetic field and frequency as well as the magnetization in quasistatic and pulsed magnetic fields have been measured for new molecule-based heterospin complexes [Mn(hfac)₂BNO_R] (R = H, Cl). These compounds exhibit a quasi one-dimensional chain structure, for which the interchain exchange interaction is small in comparison with the intrachain one . A strong frequency dependence of both the real and imaginary parts of the ac susceptibility is found at low frequencies (below 100 Hz) in both antiferromagnetic (AF) [Mn(hfac)₂BNO_H] and ferrimagnetic (FI) [Mn(hfac)₂BNO_Cl] compounds within the field interval, where the magnetization process is controlled by motion of the domain walls. A number of peculiarities regarding the reversal of magnetization are observed in pulsed fields. In particular, the remanent magnetization appears in the AF compound [Mn(hfac)₂BNO_H] after application of a unidirectional pulse of field. The remanence relaxes within about 1000 ms through a two-stage thermally activated process associated with the nucleation of the AF phase within the field-induced FI phase and displacement of the domain walls separating AF and FI phases. The activation energies for these two stages are estimated to be about 1.66 and 1.80 meV respectively. The activation energy for domain wall motion in the ferrimagnetic [Mn(hfac)₂BNO_Cl] compound is found to be . The slow dynamics of the magnetization in these materials is ascribed to the complicated domain wall displacement that presumably includes the lateral motion of the intrachain wall along the separate chains because of the strong 1D character of these compounds.

The results of polycrystalline electron paramagnetic resonance (EPR) on tetra- and penta-ammino copper(II) fluoroborate are reported. The EPR spectra of penta-ammino Cu(II) complexes in perchlorate and fluoroborate [Cu(NH₃)₅]X₂· xNH₃ depend on the presence of 'lattice ammonia', x, the number of ammonia molecules per Cu(II) ion above n = 5 that are not coordinated to Cu(II). In the high temperature cubic phase the spectrum is always an isotropic line. Furthermore, there is a substantial dependence of the averaged g-factor on the amount of 'lattice ammonia'. In the tetragonal phase of [Cu(NH₃)₅](BF₄)₂·0.1NH₃, below T_c, there are two components to the EPR spectrum: the anisotropic one due to the square pyramid coordination and the isotropic one due to the centres where the averaging mechanism is still efficient. The contribution of the averaged signal to the total EPR spectrum in fluoroborate is significantly different than in isomorphous perchlorate salt. The higher content of isotropic signal in the total EPR spectrum suggests that fluoroborate exhibits a stronger tendency to retain the cubic structure in the low temperature phase than perchlorate.

Effective dielectric responses of graded cylindrical composites are investigated when an external uniform field is applied to the composites. Considering linear random composites of cylindrical particles with a specific dielectric function, which varies along the radial direction of the particles, we have studied three cases of dielectric profiles: exponential function, linear and power-law profiles. For each case, the effective dielectric response of graded composites is given on the basis of exact solutions of the local potentials of composites in the dilute limit. For a larger volume fraction, we have proposed an effective medium approximation to estimate the effective dielectric response.

A new scheme of the modified embedded atom method (MEAM) is developed by modifying the analytic form of the embedding function. The new MEAM parameters for Mo, W, V, Nb, Ta and Fe have been determined by relating them to not only bulk properties but also some non-bulk properties. The new scheme was applied to calculate the elastic stiffness of the crystal, the vacancy formation energy, the lattice stability, the surface energies for low-index crystal faces and the bond length and the binding energy for the dimer. The results give a fairly good agreement with the experimental data.

The electric and optical properties within 90° ferroelectric domain walls of tetragonal barium titanate (BaTiO₃) are theoretically examined at room temperature using a microscopic model which is based on the orbital approximation in correlation with the dipole–dipole interaction. We find that when perpendicularly crossing the domain wall, the variation of the spontaneous polarization and refractive indices does not depend on the thickness of the domain wall which varies between 1.5 and 2.5 nm. Moreover, within the domain wall, the refractive indices n₂ and n₃, corresponding to light polarized parallel to the principal axes y_p and z_p, respectively, exhibit an important transition in which n₂ transforms from the ordinary to the extraordinary state, while n₃ behaves oppositely to n₂.

Controlled doping of quaternary alloys of In_xGa_1−xAs_ySb_1−y with tellurium is fundamental to obtain the n-type layers needed for the development of optoelectronic devices based on p–n heterojunctions. InGaAsSb epitaxial layers were grown by liquid phase epitaxy and Te doping was obtained by incorporating small Sb₃Te₂ pellets in the growth melt. The tellurium doping levels were in the range 10¹⁶–10¹⁷ cm⁻³. We have used low-temperature photoluminescence (PL) spectroscopy to study the influence of the Te donor levels on the radiative transitions shown in the PL spectra. The PL measurements were done by exciting the samples with the 448 nm line of an Ar ion laser with varying excitation powers in the range from 10 to 200 mW. For the low-doped sample the PL spectrum showed a narrow exciton-related peak centred at around 610 meV with a full width at half maximum (FWHM) of about 7 meV which is evidence of the good crystalline quality of the layers. For higher Te doping, the PL spectra show the presence of band-to-band and donor-to-acceptor transitions which overlap as the Te concentration increases. The peak of the PL band shifts to higher energies as Te doping increases due to a band-filling effect as the Fermi level enters into the conduction band. From the peak energy of the PL spectra, and using a model that includes the band-filling and band-shrinkage effects due to the carriers, we have estimated the effective carrier concentration due to doping with Te in the epilayers.

The theory of free-carrier absorption is given for a quasi-one-dimensional semiconducting structure in a quantizing magnetic field for the case where the carriers are scattered by polar optical phonons and acoustic phonons and the radiation field is polarized perpendicular to the magnetic field direction. The usual resonance condition Pω_c = Ω+ω₀, where P is an integer and ω₀ and ω_c are the optical-phonon frequency and cyclotron frequency, respectively, becomes , with equal to . The magnetic field dependence of the absorption for the transverse configuration can be explained in terms of a phonon-assisted transition between the various Landau levels of the carriers.

The slow acceptance by the scientific community of only ferromagnetic ordering below T_C in UCu₂Ge₂, and the general acceptance of the same magnetic situation in UCu₂Si₂, both with the BCT ThCr₂Si₂-type crystallographic structure, are reviewed chronologically. Observations by neutron diffraction on annealed polycrystalline samples of UCu₂Ge₂ have finally overcome many conclusions of antiferromagnetic (AF) ordering at low temperatures (LT), deduced from the magnetization overlooking the ferromagnetic domain structure. Observations by magnetization measurements on Cu-flux-grown UCu₂Si₂ single crystals, claiming 'a 50 K AF transition below the 100 K ferromagnetic transition', published recently in this journal (2003 J. Phys.: Condens. Matter15 S1917), are shown to have been misinterpreted by omitting any reference to ferromagnetic domain structure. Comments are made on other features disputing the LT ferromagnetism of UCu₂Si₂.

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