Table of contents

The reverse Monte Carlo modelling technique is commonly applied for the analysis of the atomic structure of liquid and amorphous substances. In particular, partial structure factors of multi-component alloys can be determined using this method. In the present study we use the example of the liquid Ni₃₃Ge₆₇ alloy to investigate the impact of different input data on the result of RMC modelling. It was found that even two experimental structure factors might be sufficient to obtain reliable partial structure factors if the contrast between them is high enough.

We study reflection and transmission of waves in a random tight-binding system with absorption or gain for weak disorder, using a scattering matrix formalism. Our aim is to discuss analytically the effects of absorption or gain on the statistics of wave transport. Treating the effects of absorption or gain exactly in the limit of no disorder allows us to identify short- and long-length regimes relative to absorption or gain lengths, where the effects of absorption/gain on statistical properties are essentially different. In the long-length regime, we find that a weak absorption or a weak gain induce identical statistical corrections in the inverse localization length, but lead to different corrections in the mean reflection coefficient. In contrast, a strong absorption or a strong gain strongly suppress the effect of disorder in identical ways (to leading order), both in the localization length and in the mean reflection coefficient.

The perturbed Lennard-Jones chain (PLJC) equation of state is formulated based on first-order variational perturbation theory. The model uses two parameters for a monatomic system, segment size, σ, and segment energy, ε/k. In this work, we employed the PLJC equation to calculate the liquid density of 26 metals, including alkali and alkali earth metals, iron, cobalt, nickel, copper, silver, gold, zinc, cadmium, mercury, aluminium, gallium, indium, thallium, tin, lead, antimony, and bismuth, for which accurate experimental data exist in the literature. The calculations cover a broad range of temperatures ranging from the melting point to close to the critical point and pressures ranging from the vapour-pressure curve up to pressures as high as 2000 bar. The average absolute deviation in the liquid density predicted by the PLJC equation of state in the saturation line compared with experimental data is 1.26%. Also, using the normal melting temperature and liquid density at melting point (T_m, ρ_m) as input data for the estimation of the equation of state parameters provides a good correlation of liquid density at saturated and compressed pressures.

Good-quality and fine-grain Bi₆Fe₂Ti₃O₁₈ magnetic ferroelectric films with single-phase layered perovskite structure have been prepared successfully via the metal organic decomposition (MOD) method. The results of low-temperature magnetocapacitance measurements reveal that an ultra-low magnetic field of 10 Oe can produce a non-trivial magnetodielectric response in zero-field cooling conditions, and the relative variation of dielectric constants in a magnetic field is positive, i.e. [ε_r(H)− ε_r(0)]/ε_r(0) = 0.05, when T<55 K, but negative with a maximum of [ε_r(H)− ε_r(0)]/ε_r(0) = −0.14 when 55 K<T<190 K. The magnetodielectric effect shows a sign change at 55 K, which is due to a transition from an antiferromagnetic to a weak ferromagnetic, and vanishes abruptly at around 190 K, which is thought to be associated with an order–disorder transition of iron ions at the B site of perovskite structures. Our results allow the expectation of low-cost applications of detectors and switches for extremely weak magnetic fields over a wide temperature range of 55–190 K.

While La_0.7Ba_0.3MnO₃ is a ferromagnetic metal (T_C = 340 K)with long-range ordering, Nd_0.7Ba_0.3MnO₃ shows a transition around 150 K with a small increase in magnetization, but remains an insulator at all temperatures. Gd_0.7Ba_0.3MnO₃ is non-magnetic and insulating at all temperatures. Low field dc magnetization and ac susceptibility measurements reveal the presence of a transition at around 150 K in Nd_0.7Ba_0.3MnO₃, and a complex behaviour with different ordering/freezing transitions at 62, 46 and 36 K in the case of Gd_0.7Ba_0.3MnO₃, the last one being more prominent. The nature of the field dependence of the magnetization, combined with the slow magnetic relaxation, ageing and memory effects, suggests that Nd_0.7Ba_0.3MnO₃ is a cluster glass below 150 K, a situation similar to that found for La_1−xSr_xCoO₃. Gd_0.7Ba_0.3MnO₃, however, shows non-equilibrium dynamics characteristic of spin glasses, below 36 K. The difference in nature of the glassy behaviour between Gd_0.7Ba_0.3MnO₃ and Nd_0.7Ba_0.3MnO₃ probably arises because of the larger disorder arising from the mismatch between the sizes of the A-site cations in the former. Our results on Nd_0.7Ba_0.3MnO₃ and Gd_0.7Ba_0.3MnO₃ suggest that the magnetic insulating states often reported for rare earth manganates of the type Ln_1−xA_xMnO₃ (Ln = rare earth, A = alkaline earth) are likely to be associated with glassy magnetic behaviour.

We study an incommensurate long-range order induced by an external magnetic field in a quasi-one-dimensional bond-alternating spin system, F₅PNN, focusing on the role of the frustrating interaction which can be enhanced by a high-pressure effect. On the basis of the density matrix renormalization group analysis of a microscopic model for F₅PNN, we present several H–T phase diagrams for typical parameters of the frustrating next-nearest-neighbour coupling and the interchain interaction, and then discuss how the field-induced incommensurate order develops by the frustration effect in such phase diagrams. A magnetization plateau at half the saturation moment is also mentioned.

A new zinc brome boracite Zn₃B₇O₁₃Br has been grown by a chemical transport reaction in closed quartz ampoules at 920 K. The crystal structure was characterized by Rietveld refinement. Ferroelectric nano and micro reorientable domains were found in this material using polarizing optical microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Chemical analysis was performed with x-ray energy dispersive spectroscopy (EDX). In the crystal, a new structure transition at 586 K from orthorhombic (Pca 2₁) to cubic cell () has been found. This transition was corroborated by differential scanning calorimetry (DSC).

The longitudinal strains ξ₃ of initially unpoled polycrystalline (ceramic) ferroelectrics having different composition were measured as a function of the electric field strength E. The electric field dependences of the longitudinal piezoelectric coefficients d₃₃(E) and longitudinal electrostriction coefficients M₃₃(E) were calculated from the virgin ξ₃(E) curves and analysed. It was shown that taking into account the polarization nonlinearity (that is, the dependence of dielectric susceptibility on E) leads to nonmonotonic field dependences d₃₃(E) and M₃₃(E). In a nonlinear system, the electrostrictive effect is due not only to polarization but also to the dependence of dielectric susceptibility on the electric field strength. The large magnitude of the dielectric susceptibility of soft and relaxor ferroelectric ceramics is responsible for the giant electrostriction being positive in low electric fields and negative in strong ones. The possibility of giant negative electrostriction existing has been found for the first time. In strong electric fields, the strain gain has a limitation because of the competition between the positive contribution of the piezoelectric effect and the negative contribution of electrostriction to the strain.

The electronic and atomic structure of SrTiO₃ crystals below the antiferrodistortive phase transition observed at 105 K is calculated using the hybrid B3PW functional as implemented in the ab initio CRYSTAL-2003 computer code. Such a combination of non-local exchange and correlation permits the calculation for the first time of the TiO₆ octahedron rotational angle and the ratio c/a of tetragonal lattice constants in excellent agreement with experimental data. The level splitting of the bottom of the conduction band is found to be very small, <1 meV. The predicted phase-transition induced change of the optical gap from indirect to direct is confirmed by experimental photoconductivity data.

We demonstrate that the experimental findings of the magnetic properties of the weakly coupled trilayer system Ni₄/Cu_N/Co₂ are reproduced by a theory that combines first principles calculations of the exchange interactions in a classical Heisenberg model with Monte Carlo simulations. Through an analysis of the spin–spin correlation function we show that two distinct temperatures can be identified; a higher temperature where long range magnetic order disappears and a lower temperature where the spin–spin correlation of the Ni atoms undergoes a drastic change. We argue that our findings hold in general for 'weak exchange link' systems.

Magnetic measurements were performed on Gd_xLa_1−xNi₅ compounds in the temperature range 1.7–300 K and fields up to 90 kOe. There is a transition from a spin fluctuation behaviour, characteristic for LaNi₅,to a ferrimagnetic type ordering for x≥0.2. The critical field for appearance of induced Ni moment was estimated. Then, the nickel moments, at 1.7 K, increase linearly with the exchange field. Band structure calculations were performed on RNi₅ and Gd_xLa_1−xNi₅ systems. The Gd 5d band polarization is due to both local 4f–5d exchange and 5d–3d short range interactions with neighbouring atoms. There are also 5d–5d interactions through orbitals having lobes oriented along the c-axis. The differences between magnetic moments at 2c and 3g sites, obtained from band structure calculations, were correlated with their local environments. The mean effective nickel moments decrease when increasing gadolinium content, an opposite behaviour to that evidenced for ordered Ni moments at 1.7 K. This behaviour was attributed to gradual quenching of spin fluctuations by internal field.

We performed first principles calculations to study the As–vacancy interaction and the ring mechanism of diffusion in the presence of Ge in Si in neutral and positively charged states. We discovered that the vacancy barrier decreases substantially when the vacancy hops around the ring. We believe that this indicates that the existence of As atoms can lower the vacancy migration barrier. The vacancy migration barrier is also decreased when Ge atoms are present in the vicinity. We made calculations on the As–vacancy ring mechanism in crystalline Ge and found that the formation energy and migration barriers of the vacancy are smaller than those in crystalline Si. These results support our conclusion that As diffusion can be enhanced by the presence of Ge, and provide physical insight into As diffusion in Si_1−xGe_x.

We study the behaviour of GaAs under a uniaxial compression using an ab initio constant-pressure technique and find that GaAs undergoes a first-order phase transition to a side-disordered orthorhombic Imm 2 structure via an intermediate state having the space group of . The transition pathway and mechanism under uniaxial stress are found to be considerably different from those under the hydrostatic compression.

We have extended Monte Carlo simulations of hopping transport in completely disordered two-dimensional (2D) conductors to the process of external charge relaxation. In this situation, a conductor of area L × W shunts an external capacitor C with initial charge Q_i. At low temperatures, the charge relaxation process stops at some 'residual' charge value corresponding to the effective threshold of the Coulomb blockade of hopping. We have calculated the root mean square (rms) value Q_R of the residual charge for a statistical ensemble of capacitor-shunting conductors with random distribution of localized sites in space and energy and random Q_i, as a function of macroscopic parameters of the system. Rather unexpectedly, Q_R has turned out to depend only on some parameter combination: for negligible Coulomb interaction and for substantial interaction. (Here ν₀ is the seed density of localized states, while κ is the dielectric constant.) For sufficiently large conductors, both functions Q_R/e = F(X) follow the power law F(X) = DX^−β, but with different exponents: β = 0.41 ± 0.01 for negligible and β = 0.28 ± 0.01 for significant Coulomb interaction. We have been able to derive this law analytically for the former (most practical) case, and also explain the scaling (but not the exact value of the exponent) for the latter case. In conclusion, we discuss possible applications of the sub-electron charge transfer for 'grounding' random background charge in single-electron devices.

Magnetostrictive Fe_100−xGa_x alloys over the composition range 0<x<36 have been prepared by combinatorial sputtering methods. These films were investigated by x-ray diffraction and ⁵⁷Fe Mössbauer effect spectroscopy techniques that have been adapted for the efficient study of combinatorial samples. X-ray diffraction measurements show the presence of a disordered bcc phase with a lattice parameter that increases with increasing Ga content. Mössbauer effect measurements show that even for low Ga content the disordered alloys are not truly random but show some degree of short range Ga clustering. For x>20, the Mössbauer effect results suggest the presence of short range D0₃ type order. This persists up to the maximum Ga content studied here (x = 36), although there is no evidence to support the formation of long range D0₃ structural order in any of the films prepared in this study.

In this contribution we focus on a novel way to perform and evaluate magnetization experiments on frozen ferrofluids or magnetic nanoparticles fixed in space. The basic idea is to project the behaviour of a real particle system onto a Stoner–Wohlfarth (SW) particle ensemble behaviour. Therefore first an ideal SW particle system acting as a reference system with unique particle sizes and orientation of easy axis was studied numerically by introducing a particle–particle interaction via a demagnetizing field. We have shown that for non-interacting SW particles a universal mean value rule holds that the magnetization measured after ZFC (zero field cooling) can be expressed as a mean value of the magnetization measured after PHFC and NHFC (positive and negative high field cooling) processes. Any deviation from this mean value rule is therefore due to non-SW behaviour (multidirectional anisotropy and interaction) only and can be derived from the magnetization measurements and be compared with simulations.

By this new method it is possible to determine the mean value of the particle's anisotropy and the mean interaction which is expressed here as a magnetic field.

We will report on experiments performed on frozen ferrofluids because these systems offer the possibility of complete thermal demagnetization after melting. In this contribution we describe experiments performed on two different frozen Co-based ferrofluids by SQUID magnetometry.

The emergence of phase separation is investigated in the framework of a 2D  t–J model by means of a variational product ansatz, which covers the infinite lattice by two types of L × L clusters. Clusters of the first type are completely occupied with electrons, i.e. they carry maximal charge Q_e = L² and total spin 0, and thereby form the antiferromagnetic background. Holes occur in the second type of clusters—called 'hole clusters'. They carry a charge Q_h<L². The charge Q_h and the number N(Q_h) of hole clusters is fixed by minimizing the total energy at given hole density and spin exchange coupling α = J/t. For α not too small (α>0.5) it turns out that hole clusters are occupied with an even number Q_h<L² of electrons and carry a total spin 0. For increasing α the charge Q_h(α) of the hole clusters decreases. Some points on the boundary curve can be extracted from Q_h(α).

The electronic structure, electron g factors and optical properties of InAs quantum ellipsoids are investigated, in the framework of the eight-band effective-mass approximation. It is found that the light-hole states come down in comparison with the heavy-hole states when the spheres are elongated, and become the lowest states of the valence band. Circularly polarized emissions under circularly polarized excitations may have opposite polarization factors to the exciting light. For InAs ellipsoids the length, which is smaller than 35 nm, is still in a strongly quantum-confined regime. The electron g factors of InAs spheres decrease with increasing radius, and are nearly 2 when the radius is very small. The quantization of the electron states quenches the orbital angular momentum of the states. Actually, as some of the three dimensions increase, the electron g factors decrease. As more dimensions increase, the g factors decrease more. The dimensions perpendicular to the direction of the magnetic field affect the g factors more than the other dimension. The magnetic field along the z axis of the crystal structure causes linearly polarized emissions in the spheres, which emit unpolarized light in the absence of magnetic field.

Powder x-ray diffraction and magnetization measurements were employed to investigate the effect of Al on the structural and magnetic properties of Er₂Co_17−xAl_x compounds with 0≤x≤4. Al substitution leads to a monotonic increase in the lattice parameters and an approximately linear decrease in the saturation magnetization. The Curie temperature is also found to decrease with Al concentration. Large thermomagnetic irreversibility seen in these compounds has been attributed to the domain wall pinning effect. The temperature variation of magnetization shows compensation points in the compounds with x = 3 and 4. The field-cooled magnetizations of these two compounds in low fields are found to be negative at temperature below the compensation temperatures, which has been explained on the basis of the temperature variations of the exchange coupling strengths of Er and Co sublattices. The easy magnetization direction at room temperature is found to be along the c-axis in all the compounds. The first order anisotropy constant at room temperature is found to increase with Al concentration, which has been attributed to the increase in the axial anisotropy of both the Er and Co sublattices as a result of Al substitution.

Ground-state and finite-temperature properties of the mixed spin- and spin-S Ising–Heisenberg diamond chains are examined within an exact analytical approach based on the generalized decoration–iteration map. A particular emphasis is laid on the investigation of the effect of geometric frustration, which is generated by the competition between Heisenberg- and Ising-type exchange interactions. It is found that an interplay between the geometric frustration and quantum effects gives rise to several quantum ground states with entangled spin states in addition to some semi-classically ordered ones. Among the most interesting results to emerge from our study one could mention rigorous evidence for quantized plateaux in magnetization curves, an appearance of the round minimum in the thermal dependence of susceptibility times temperature data, double-peak zero-field specific heat curves, or an enhanced magnetocaloric effect when the frustration comes into play. The triple-peak specific heat curve is also detected when applying small external field to the system driven by the frustration into the disordered state.

The absorption and luminescence spectra of a LiBaF₃:Pb²⁺ crystal are studied in the temperature range 10–300 K. At 300 K a structureless absorption band (A band) is found in the energy range 1.4–6.6 eV. The luminescence spectra at 300 and 10 K are composed of two A^' and A₁ bands with maxima at ∼5.0 and ∼4.0 eV, respectively. The A^' band originates from the 'regular' Pb²⁺ centres. The A^' band luminescence decay is single-exponential in the whole temperature range 10–300 K. The properties of the observed absorption and A^' luminescence bands are described well within the semiclassical theory based on the Franck–Condon principle and the Jahn–Teller effect in the excited sp configuration. The possible nature of the A₁ band luminescence is discussed.