Table of contents

The authors report on the electrical resistivity of a quasi-one-dimensional organic system having asymmetric donor molecules like quinolinium under high pressure up to 8 GPa at room temperature. A low-temperature resistivity study at normal pressure has also been made. The single crystals were grown by an electrochemical process. The pressure studies reveal that there is a strong negative pressure derivative of resistivity in the low-pressure region, but at high pressures there is a marked tendency towards saturation.

Transverse Ising models with arbitrary spin S are studied by use of the pair approximation with the discretized path-integral representation. The equation for the phase diagram is derived for an arbitrary number of nearest-neighbour spins z and critical lines are obtained numerically for a few values of the spin. It is easy to show that in the limit z to infinity the results for the infinite-range interaction model can be derived within the present formulation.

The concept of 'speromagnetism' in insulators describes a state of matter in which the antiferromagnetic exchange interaction between neighbouring magnetic ions is frustrated, leading to a random, isotropic spin structure. The detection of speromagnetism has relied heavily on applied-field Mossbauer spectroscopy. The authors consider the Mossbauer data on three ferric insulators said to be speromagnetic, and find that they may be most simply modelled in terms of antiferromagnetism.

Monte Carlo computer simulation of the passage of antiprotons and protons through a silicon crystal under axial channelling conditions is performed. The energy spectra and angular distribution of channelled protons and antiprotons moving along the (110) direction are calculated and compared. The projectile energy range is from 300 keV to several megaelectronvolts. It is shown that the energy loss corresponding to the maximum of the antiproton spectrum is close to the energy loss of particles moving in a random direction. Moreover, the most probable energy loss of the channelled part of the antiproton beam is slightly less than the random energy loss. The Barkas correction to the energy loss has little effect on the energy spectra of channelled particles and can hardly be studied with this type of experiment. The angular distributions of protons and antiprotons differ considerably. The characteristic features of the angular distribution may serve as evidence of channelling of low-energy antiprotons.

The Phi⁴ potential is widely employed for modelling systems that undergo displacive phase transition of the soft mode or order/disorder. A molecular dynamic simulation is used to establish where the crossover lies between the two regimes, considering both limits of long range and short range intersite interactions. It appears that with both ranges of interaction the crossover occurs when the on-site potential changes from a single to a double well. The behaviour of the soft mode frequency omega is also investigated, and in particular the ratio of the slopes d omega ²/dT between T>T_c and T<T_c. The authors find this ratio attains the value -2 given by standard renormalized phonon theory only the double limit of long range coupling and extreme soft mode behaviour. The order parameter varies as (T_c-T)^1/2 over a wide temperature range, as observed in several materials, in this double limit only.

The authors study the dynamical behaviour of discommensuration patterns that have been observed in commensurate-incommensurate phase transitions. Using a method based on a phenomenological dynamics model proposed by Enomoto and Kato, two-dimensional simulations are performed to discuss the time evolution of the model system towards equilibrium states from the initial disordered state.

The symmetry of the different phases of the 'textbook' pure ferroelastic compound lead phosphate (Pb₃(PO₄)₂) was reinvestigated. Neutron experiments were performed with a four-circle goniometer and a triple-axis spectrometer. A non-centrosymmetry test was carried out by the optical second-harmonic generation method. It is shown that the ferroelastic phase of this compound is non-centrosymmetric up to T_c=180 degrees C (453 K) with C2 space group; above this temperature, lead phosphate transforms into an R3m paraelastic phase. Structural refinements in the C2 space group only show slight distortions of the PO₄ tetrahedra compared with the results obtained with the centrosymmetric C2/c space group. The non-centrosymmetry of the monoclinic phase leads to the possibility of spontaneous polarization. This result shows that lead phosphate is not a pure ferroelastic compound as previously reported; the ferroelasticity of this material could be coupled to other physical properties.

A method for calculating the thermodynamic properties of both classical perfect crystals and defects by performing a single zero-temperature energy minimization is described. This method is based upon the calculation of a local harmonic free energy as performed by LeSar et al. and by Sutton, and involves determining the dynamical matrix, Gruneisen parameters, and the elastic properties of the system. The dependence of the free energy and the lattice parameter of a perfect Au crystal on temperature are accurately determined with this method. The validity of this method is demonstrated by accurately determining the temperature dependence of the vacancy formation energy, the excess free energy of a (100) surface, and the excess free energy of a Sigma 13 (001) (22.62 degrees ) twist grain boundary.

The effect of strong electron-electron interactions (described by the single-band Hubbard Hamiltonian) on the electronic spectrum and cohesive energies of substitutionally disordered alloys has been studied. The authors use the Hubbard III solution to treat the correlations and coherent-potential approximation for configurational averaging. The use of the Lorentzian form of the host metal density of states enables them to obtain analytical expressions for various quantities of interest. Questions related to the problem of the calculations of cohesive energies in disordered systems with correlations are discussed and partially resolved.

Second-order vibronic reduction factors are derived for strongly coupled orbital triplet systems coupled to t₂ modes of vibration using the recently derived symmetry-related method. This involves the calculation of oscillator overlaps which are projected out of cubic vibronic states. Details are presented for examples where the perturbations are both of E-type symmetry where they are both of T₂-type symmetry and where they are of mixed E- and T₂-type symmetries. The symmetry properties of the results are discussed and compared with those obtained previously for perturbations of T₁ symmetry, such as those for spin-orbit coupling. The new calculations are appropriate when a description of second-order uniaxial stresses for strongly coupled systems is required-such as in the study of deep-level impurities in semiconductors.

The present study deals with electron intersubband scattering in real quantum wire structures. Both the multi-subband structure and confined phonon modes are considered together. The rates of scattering by confined longitudinal-optical (LO) phonons and by surface-optical (SO) phonons are calculated taking into account all possible LO phonon modes as well as all possible electron intersubband transitions. The estimations of transition rates for GaAs/AlAs QWIS have shown that intrasubband electron scattering and most intersubband transitions are due primarily to scattering by confined LO phonons, but in resonant intersubband transitions the contribution of SO phonons may be dominant when the phonon energy is close to the intersubband energy separation. Moreover, electron-SO-phonon scattering might play an important part in low-temperature electron transport because the GaAs-like SO mode is shifted towards lower frequencies compared with that of LO phonons. The energy dependence of the total scattering rate in an ideal quantum wire exhibits multiple sharp peaks related to each intersubband transition. These peaks originate from the resonant nature of the density of states in ideal one-dimensional systems. It is demonstrated that in real quantum wires with variable thickness the resonant peaks broaden or even disappear due to variation of subband energies.

Electrical transport properties of La_2-xSr_xCuO₄ up to x=0.4 have been systematically investigated. A gradual change taking place in the sequences semiconducting-non-superconducting, semiconducting-superconducting, metallic-superconducting and metallic-non-superconducting has been observed. The results provide the experimental evidence for a transition from metallic behaviour at high temperatures to a variable-range hopping (VRH) regime at low temperatures. A crossover from 3D VRH to 2D VRH has been observed when the system changes from a semiconducting-non-superconducting to a semiconducting-superconducting type. The samples with different Sr concentrations were further doped with zinc at the Cu site, and the resulting effect on superconductivity and normal-state resistivity has been examined. The two competing processes, namely the increase in the hole concentration with Sr addition and the localization of charge carriers due to Zn substitution have been discussed.

The superconductivity of YBa₂Cu₃O_y ceramics doped with SnO₂ was studied. The superconducting transition temperature T_c was increased slightly as the SnO₂ content changed from 0 to 5 wt.%. The results of the Mossbauer effect and the positron annihilation spectra showed that the Sn ions did substitute for Cu(1) sites. The increase in T_c may be attributed to the optimal carrier concentration as well as to the fine and dense microstructure obtained by doping with SnO₂.

A strong temperature variation in the gyromagnetic factor of the electron paramagnetic resonance line of bis(L-leucinato) copper (II) has been observed in single-crystal samples between 2 and 300 K. In order to characterize the magnetic properties and to evaluate the importance of short-range order effects the author has also measured the magnetic susceptibility on powder samples. The data on the g-shift are interpreted as the combined effect of the dipole-dipole interaction between copper ions, and the spin polarization induced by the applied magnetic field when the temperature is lowered. The layered distribution of the Cu ions in the crystal lattice makes this effect non-negligible and determines the quasi-two-dimensional behaviour of the shift.

An integral equation of the Curie temperature for the Blume-Emery-Griffiths model is derived. Numerical integration is carried out and the results for SC, BCC and FCC lattices are presented.

In this paper the authors have carried out a quantum study of the features of the spin wave spectrum of a superlattice composed of alternating layers of two different ferromagnetic materials or a ferromagnetic material and a non-magnetic material, with a ferromagnetic coupling at the interfaces. Depending on the intensity of the interactions in the interfaces and on the layer thickness very different types of thermal behaviour of the magnetization can be obtained. In the case of the ferromagnetic-non-magnetic system a clear tendency to a bidimensional type of behaviour is shown when the exchange coupling at the interfaces is very small, and a clear dependence of the thermal demagnetization on the layer thickness is observed. In the case of the system composed of two different ferromagnetic materials, in the weak-coupling limit at the interfaces, the authors observe a more rapid decrease of the magnetization with temperature for thinner unit cells. The opposite type of behaviour is observed in the strong-coupling limit.

Measurements of resistivity, susceptibility and magnetization for polycrystalline samples of the alloys CeSi_x (1.6<or=x<or=2.00) in the temperature range 4-300 K show that samples with x<or=1.85 order ferromagnetically while those with x>or=1.90 do not. This correlates well with previous studies of these samples. However, samples with x=1.70 and 1.75 show the presence of an anomalous second peak at a slightly higher temperature than the ferromagnetic ordering temperature. CeSi_x with x=1.90 shows interesting resistive behaviour with characteristics of both magnetic and non-magnetic samples. It exhibits a broad low temperature peak, a Kondo minimum at higher temperatures and the onset of Fermi liquid behaviour below the maximum. The magnitude of the room temperature resistivity is substantially larger than that of the other members of the series, indicating that this sample is very close to the critical concentration for the magnetic-non-magnetic transition in this series. High temperature anomalies are seen in the resistivities of samples with x<or=1.90; nonetheless, no magnetic correlation is observed. Negative magnetoresistance is observed for samples CeSi_1.70 and CeSi_1.90 over all fields and temperatures investigated.

Muon spin relaxation ( mu SR) experiments were carried out in zero field on polycrystalline samples of Hf₂Co and Hf₂CoH₃. In the unhydrided alloy the muon preferentially occupies octahedral interstices and diffuses through a tunnelling mechanism within the network made of adjacent octahedra above 220 K. In contrast, the muon occupies tetrahedral interstices in the hydrided alloy. Muon diffusion takes place through two distinct phonon-assisted tunnelling mechanisms with a possible contribution of overbarrier hopping above 250 K and appears to be strongly enhanced hy the presence of hydrogen. This effect is attributed to the different pathways taken by the muon in hydrided and unhydrided Hf₂Co. A comparison with the hydrogen jump rate measured by perturbed angular correlation in the same hydride provides confirmation that the hydrogen mobility is indeed faster than that of the muon, which appears to be a quite general result in concentrated metallic hydrides.

Gallium is one of the very few elements in which positron trapping at vacancies under equilibrium conditions has not been experimentally observed. The authors present clear evidence of positron trapping in electron-irradiated Ga at 77 K. The recovery of the vacancies produced by irradiation as shown by the recovery of both the lifetime and the Doppler parameter is observed at around 125 K.