Table of contents

Magnetization isotherms on single crystal La_0.73Ca_0.27MnO₃ exhibit a metamagnetic structure generally linked with a first-order transition, a currently accepted feature of the behaviour of this system near this doping level. By contrast, field and temperature dependent ac susceptibility data display a set of pseudocritical maxima which move upwards in temperature and decrease in amplitude as the field increases, a characteristic of continuous transitions. Within experimental uncertainty these features are coincident in the (H–T) plane.

The experimental and theoretical aspects of obtaining the magnetic information carried by laser beams diffracted from an array of nanosized magnetic objects are reviewed. Experimentally it will be shown that the magneto-optic Kerr effect (MOKE) hysteresis loops recorded for diffracted beams can be quite different from those recorded for the reflected beam. We will show that the diffracted MOKE (D-MOKE) loops are proportional to the magnetic form factor or, equivalently, to the Fourier component of the magnetization corresponding to the reciprocal lattice vector of the diffracted beam. In conjunction with micromagnetic simulations, the D-MOKE provides a powerful and non-destructive technique for investigating the magnetization reversal process in submicron-sized magnetic particles. The advantages and disadvantages of the D-MOKE technique will be compared to those of other techniques that yield related information (e.g. magnetic force microscopy, Lorentz electron microscopy and micromagnetic simulations).

The degenerate state in two-dimensional photonic crystal is studied. The photonic analogue of the static Jahn–Teller effect is utilized for realizing tunable propagation through photonic crystal waveguide bends. It is shown that the resonant coupling of the photon modes at the corner can be tuned by the symmetry and magnitude of the distortion of the lattice.

Formation of thermal vacancies in icosahedral Zn₆₅Mg₂₅Er₁₀ quasicrystals has been specifically studied from room temperature to about 720 K by positron annihilation spectroscopy employing two-detector coincident Doppler broadening techniques. Significant vacancy formation was observed for temperatures higher than 0.6T_m. An apparent vacancy formation enthalpy of 1.2 eV was determined. The results are discussed in comparison with high temperature vacancy processes in quasicrystals as well as in other complex solids.

The effect of the ramping rate on oxygen precipitation and the denuded zone in heavily doped Czochralski (CZ) silicon has been investigated. Wafers with different dopants (boron, arsenic and antimony) were subjected to a pre-annealing at 1150 °C and then ramping processes with different rates. It was found that along with the decrease of the ramping rates, the density of oxygen precipitates increased in all the heavily doped silicon wafers; however, the width of the denuded zone decreased in heavily boron doped silicon. It is considered that compared with case for lightly doped wafers, the oxygen precipitation was enhanced in the heavily boron doped wafers; but was retarded in the heavily arsenic and antimony doped wafers during the ramping process. The mechanism of the ramping on the oxygen precipitation and the denuded zone was also discussed.

We present a detailed study of the crystallization induced in an amorphous alloy by monoatomic projectiles in the electronic energy deposition regime. The crystallization phenomenon was investigated by transmission electron microscopy (TEM) for different lead ion energies (0.7–5 GeV), irradiation temperatures (80–300 K) and ion fluences (from 1 × 10¹¹ up to 2 × 10¹³ ions cm⁻²).

We present a detailed study of the crystallization induced in an amorphous alloy by cluster ions. The formation of nanocrystalline iron boride phases was observed by transmission electron microscopy in an amorphous Fe_73.5Cu₁Nb₃Si_13.5B₉ alloy irradiated up to fluences of 1 × 10⁹–2 × 10¹¹ clusters cm⁻² at room temperature with C_n (n = 5, 10, 60) carbon clusters at 20 MeV, but not after irradiation with C₆₀ fullerenes at 30 MeV. The high level of energy deposited in electronic excitations along the path of carbon clusters induces radial pressure waves in the vicinity of the projectile trajectories. The formation of nanocrystallites was interpreted as being due to a structural relaxation which occurs in the wake of the shock front. The mechanical stresses induced by the pressure wave destabilize the amorphous structure and drive the nucleation of nanocrystallites.

The electronic band structure, metallization, structural phase transition and superconducting transition of silver chloride (AgCl) and silver bromide (AgBr) under pressure are studied using the TB-LMTO method. The ground state properties and bandgap values are compared with the experimental and previous theoretical results. These silver halides become metals and superconductors under high pressure, but before this they undergo structural phase transition from NaCl phase to CsCl phase. It is found that the charge transfers between Ag 5s, 4d and X np, nd (X = Cl, Br; n = 3, 4) states cause metallization. It is found that the metallization pressure increases with decrease of lattice constant. The density of states at the Fermi level (N(E_F)) is enhanced as the pressure is further increased, which leads to the superconductivity in AgCl and AgBr. Like CsI, AgCl and AgBr come under the class of pressure induced superconductors. The pressure effects on λ (electron–phonon mass enhancement factor) and μ^* (electron–electron interaction parameter) clearly suggest that AgCl and AgBr are electron–phonon mediated superconductors. The non-occurrence of metallization, phase transition and onset of superconductivity simultaneously in ionic solids is also confirmed.

The polaron features for long-range electron–phonon interaction are investigated by extending a variational approach previously proposed for the study of systems with local coupling. The ground state spectral weight, the average kinetic energy, the mean number of phonons, and the electron–lattice correlation function are discussed for a wide range of model parameters focusing on the adiabatic regime and comparing the results with the short-range case (Holstein model). A strong mixing of electronic and phononic degrees of freedom for small values of the electron–phonon coupling constant is found in the adiabatic case due to the long-range interaction. Finally a polaron 'phase diagram' is proposed.

The band structure of SrO terminated Co/SrTiO₃ superlattices is investigated within the first principle augmented spherical wave method allowing us to discuss the oxide induced polarization and to examine the decay of the evanescent states into the oxide layer which is essential for the understanding of tunnelling phenomena through the insulator. The calculated d polarization of the Co electrode at the interface confirms qualitatively the interpretation of the measurements and presents a crossover from positive to negative values around E_F−0.9 eV. It is also shown that, due to the high d character of the band structure of Sr and Ti, d states do not decay more rapidly into the oxide layer than s or p states, and they have a preserved polarization.

In this work we study electron transport through quantum wires coupled to a quantum dot Aharonov–Bohm ring by using a noninteracting Anderson tunnelling Hamiltonian. In the case of an asymmetric coupling and three dots within the Aharonov–Bohm ring we have observed the appearance of Fano-like resonances in the conductance. We have investigated the asymmetry and the position of the conductance resonances and found closed expressions. We have also observed Aharonov–Bohm oscillations. Finally the local density of states as well as the probability amplitude phases have been studied.

We report a study of the effect of hydrostatic pressure up to 9 kbar on selected compounds of the Gd₅(Si_xGe_1−x)₄ series (x = 0.8, 0.45, 0.1) by means of ac magnetic susceptibility, compressibility, and linear thermal expansion measurements. The pressure-induced increase of the transition temperatures at the second-order boundaries of the phase diagram is rather moderate: (x = 0.8) and (x = 0.1). This effect is stronger in the range, where (x = 0.45,0.1), indicating that the ferromagnetic ordering can be simultaneously driven through a pressure-induced structural transformation. The values of d lnT_C/d lnV calculated with the use of the measured value of compressibility () are significantly lower than those estimated from the concentration dependence of the lattice cell volume, thus demonstrating that the dependence of the transition temperatures upon changing the Si/Ge ratio across the series cannot be explained by a pure volume effect.

A-site cation disorder effects on the structural, electrical transport and magnetic properties have been studied in the Nd_0.7(Ca,Sr,Ba)_0.3MnO₃ series with a fixed A-site mean radius  Å by resistivity, dc susceptibility, infrared spectra and thermopower measurements. The disparity of transition temperature obtained from resistivity, magnetization and thermopower increases with increasing amounts of A-site cation disorder quantified by the size variance σ². The insulator-like ferromagnetic behaviour and the sign change of thermopower induced by σ² have been observed below the Curie temperature T_C. As σ² increases, the size of the ferromagnetic domains decreases, and the carriers seem to be trapped strongly in the interfaces between ferromagnetic domains; such behaviours may help to understand the insulator-like ferromagnetic state induced by σ² below T_C.

Single-phase AlN nanopowders, with average diameter of about 70 nm, were synthesized through an AlCl₃ aided CVD route; the formation process is also discussed in this paper. XRD and HRTEM revealed that the AlN nanopowders obtained are wurtzite structured with lattice constants of a = 3.1114 Å and c = 4.9792 Å. The photoluminescence spectrum showed that there is a deep-level blue luminescence band centred at 2.54 eV in AlN nanocrystals. This method is a simple and efficient way to synthesize AlN nanopowders.

Neutron and x-ray diffraction experiments of high resolving power with neutrons from a spallation source and high-energy photons from a synchrotron have been performed on compositional series of binary Zn, Nb and on mixed Zn/Nb tellurite glasses. The Te–O, Zn–O and Nb–O coordination numbers are determined by Gaussian fitting of the first-neighbour peaks in the neutron and x-ray data simultaneously. The transition of TeO₄ to TeO₃ units with increasing fraction of a second component is indicated by decreasing total Te–O coordination numbers. This transition appears different for glasses with ZnO or Nb₂O₅ additions. Details of the Te–O peaks suggest there are two species of Te–O bonds with lengths of and  nm. The change of their fractions shows excellent agreement with the existence of TeO₄ trigonal bipyramids and TeO₃ trigonal pyramids. All oxygen atoms from ZnO and Nb₂O₅ are used for rupture of Te–O–Te bridges, which is accompanied with a change of nearly all participating TeO₄ to TeO₃ groups. The tendency for a change decreases for glasses of higher second component content which is accompanied by the occurrence of TeO₄ groups with non-bridging oxygens. The Nb tellurite glasses show transition to network-forming behaviour with the formation of Nb–O–Nb bridges. The fractions of TeO₃ units of ternary Zn/Nb tellurite glasses agree with an additivity behaviour of the modifying effects of ZnO and Nb₂O₅ additions.

Mössbauer spectroscopy of La_0.67Ca_0.33Mn_0.9Fe_0.1O₃ shows phase separation above and below the magnetic transition temperature T_C. These phase separations are found to be independent and related to different phenomena. The relative volumes of the two phases above and below T_C are also very different. Mössbauer spectra above T_C show that the phase splitting is due to different lattice distortions around Fe in the two phases. The paramagnetic component showing large distortion disappears when the magnetic hyperfine splitting appears on lowering the temperature. The phase splitting below T_C is due to a difference in the nature of magnetic ordering and the degree of covalence of the magnetic ions in the two phases. DC magnetization shows that the spin ordering in the two phases is collinear and non-collinear, respectively. At low temperatures, the non-collinear phase converts into the collinear phase when the applied magnetic field exceeds a certain value. When the field is withdrawn, the non-collinear phase is not fully regained.

In a recent publication, Kuzovkov et al (2002 J. Phys.: Condens. Matter.14 13777) announced an analytical solution of the two-dimensional Anderson localization problem via the calculation of a generalized Lyapunov exponent using signal theory. Surprisingly, for certain energies and small disorder strength they observed delocalized states. We study the transmission properties of the same model using well-known transfer matrix methods. Our results disagree with the findings obtained using signal theory. We point to the possible origin of this discrepancy and comment on the general strategy of using a generalized Lyapunov exponent for studying Anderson localization.

We reply to comments by Markoš et al (2004 J. Phys.: Condens. Matter 16) on our recent paper (Kuzovkov et al 2002 J. Phys.: Condens. Matter 63 13777). We demonstrate that our quite different viewpoints stem for the different physical assumptions made prior to the choice of the mathematical formalism. The authors of the comment expect a priori to see a single thermodynamic phase while our approach is capable of detecting the co-existence of distinct pure phases. The limitations of the transfer matrix techniques for the multi-dimensional Anderson localization problem are discussed.