Table of contents

The smaller-than-anticipated asymmetry of the electronic Raman scattering in TmPO₄ is related to the effects of virtual excited states of the type 4f^N-1g. These effects almost exactly cancel the contributions coming from states belonging to 4f^N+1d^+or-1. Revised calculations for ErPO₄ are presented.

The successive phase transitions reported in triammonium hydrogen disulphate (TAHS) are investigated by EPR using Cu²⁺ and Mn²⁺ ions as probes. At room temperature, in the spontaneously strained monoclinic phase, Cu²⁺ and Mn²⁺ ions were found to be in two magnetically inequivalent sites. At the phase II (monoclinic) to (+140 degrees C) phase I (trigonal) transition the magnetic inequivalence of Cu²⁺ ions vanished, resulting in a broad single line at g=2.240+or-0.002 due to a possible dynamic Jahn-Teller effect that may be present for the ²E state of the Cu²⁺ ion in trigonal symmetry. This transition exhibited a hysteresis of about 45 degrees with the reverse transition appearing at +95 degrees C on cooling. The II to III transition resulted in the splitting of the Cu²⁺ resonance into two components with a temperature-dependent intensity ratio and separations. These results are consistent with the model that the Cu²⁺ ion occupies the NH₄⁺ (I) position at room temperature, and takes an off-centred position in phase III with an associated electronic dipole moment, responding to the onset of antiferroelectric ordering.

The effect of simple structural randomness, such as stretching branching and surrounding, on the electronic structure of polysilane is investigated using molecular orbital calculation. The frontier orbital (HOMO and LUMO) states consist mainly of skeleton Si. The fluctuation in the frontier orbital states due to such randomness are found to be quite small. Moreover, tailing of the resulting gap states into the energy gap is negligible.

A modulated-impurity (MI) mechanism of pinning is put forward for K₂Pt(CN)₄Br_0.3*3.2H₂O (KCP) based on the Q-quasi-modulated distribution of the bromine anions along the chain axis (Q/2 being the Fermi momentum reduced to the first Brillouin zone). The spectrum of the collective excitations (amplitudons and phasons) as well as the dielectric function are calculated for the charge density wave (CDW) state. Fair agreement is obtained with the optical and neutron scattering data in KCP. A possible explanation is suggested of the difference between the DC gap and the optical one.

For a standard small-polaron Hamiltonian, a closed formula for the electron memory functions governing the nonMarkovian electron transport is derived in a systematic way applicable to arbitrary lattices. The correspondence with some older treatments is discussed in detail.

The NMR linewidths of hydrogen and fluorine in the complex ionic salt NiTiF₆.6H₂O have been examined from 77 to 400K. Separate abrupt changes in the ¹H width (122-126K) and the ¹⁹F width (131-134K) confirm the existence of two transitions in the known structural transformation region, and an anomaly near 205K suggests a possible additional transition. The overall temperature dependence is discussed in terms of reorientations of the octahedral complex ions.

The mean-field approach of Grunewald and co-workers (1982) provides a consistent way of extending the one-electron hopping theory to account for the effect of intersite Coulomb interactions. In this paper the authors show that the density of states of 4096-donor random simulations obtained in the limit T=0 is very similar to that obtained from the established energy-minimising procedure. They provide numerical results that imply that, remarkably, the states obtained by iteration on the mean-field equations, with no regard to energy minimisation, are in fact likely to be pseudo-ground-states for large enough systems.

The electronic structure of the transition-metal impurities Cr, Mn, Fe, Co, Ni and Cu and of a vacancy in InP have been studied theoretically using the multiple-scattering X alpha -SCF method. It is found that the dangling bond hybrid t₂ level moves into the band gap. For Fe this is in the middle of the gap providing a clue to the semi-insulating effect of Fe in InP. The crystal-field resonance level e goes from the gap into the valence band as one goes from Cr to Cu. The corresponding t₂ level goes successively deeper into the valence band from Cr to Cu. The population density study shows that the general trend is an increase of transfer of charge as one scans the latter part of the first transition-metal series.

The authors report the first clearly identified neutral double acceptor in any II-VI semiconductor, observed in Zn-annealed ZnTe. Optical properties of the associated bound exciton (BE) clearly identify the number of holes present in the exciton and the acceptor ground state and establish T_d site symmetry. Zeeman and uniaxial stress perturbation spectroscopies were both required to establish the character of the double acceptor ground state in which the crystal-field ( Gamma ₃- Gamma ₅) splitting of the J=2 hole-hole state ( approximately 4.8 meV) is unexpectedly large compared to the J=0-J=2 splitting, ( approximately 0.9 meV), so that the Gamma ₃(J=2) state of the neutral double acceptor lies lowest and is approximately 2 meV below the Gamma ₁(J=0) state. A fourfold splitting is seen in the (2s¹,1s¹) excited state of the double acceptor when this is observed in the satellites of the main BE line. This is attributed to the presence of states with J=0, 1, 2 and 3 due to the coupling of the spins of the two holes, although residual crystal-field splittings may also be significant.

Within a real-space renormalisation group framework, the authors treat the q-state Potts ferromagnet in a simple cubic lattice consisting of two different semi-infinite bulks (respectively characterised by the coupling constants J₁ and J₂) separated by a (1,0,0) interface (characterised by J_s). The use of a rather sophisticated two-terminal cluster allows a quite reliable discussion of the phase diagram and its universality classes. Four physically different phases are obtained, namely the paramagnetic and the double-bulk, single-bulk and surface ferromagnetic ones. The critical surface contains a multicritical line (associated with J₁ not=J₂) which in turn contains a special point associated with J₁=J₂.

The electric field strengths in a long rectangular device carrying a quantised Hall current are calculated. In the ideal system an analytic approximation for the field near the edge is obtained which is in agreement with the numerical results of MacDonald et al. (1983). It is pointed out that even a small departure of the Hall angle from pi /2 leads to a uniform field, and therefore current, across the system. Diamagnetic corrections are shown to be negligible. The fields in the source and drain are calculated, both for a two-dimensional electrode, for which an analytic solution is available, and for a three-dimensional electrode. In the three-dimensional case, most of the energy dissipation occurs within a distance l of two of the corners of the rectangle, where an expression for l is derived.

Damping effects of boson elementary excitations in the Dyson-Maleev representation of one-dimensional spin S chain using the spectral density method (SDM) are studied. For the spectral density the authors use the 'gaussian ansatz', and the reasonable results, obtained in the low-temperature limit, confirm that it is able to describe damping effects in the SDM.

In the Sherrington-Kirkpatrick (SK) model the effect of a transverse field Delta on the spin glass freezing is studied on the basis of the theory of Thouless, Anderson and Palmer (1977). It is shown from the logarithmic singular point of the free energy in the high-temperature region that T_c decreases with increasing Delta . To be consistent with this result the mean-field equation is set up for T<T_c. Due to the correlation effect spin flips associated with the transverse field are unlikely to occur at low temperatures and the system eventually undergoes the spin glass transition irrespective of the magnitude of Delta , which is characteristic of the SK model.

The Ising model with frustration covering either only the x-y planes or all three planes of a simple cubic lattice is studied using Monte Carlo methods. In agreement with mean-field and renormalisation group analysis using epsilon =4-d expansions, the authors find that the transition in the partially frustrated model is continuous with alpha <0, where alpha is the specific heat exponent. For the fully frustrated model, finite-size scaling analysis suggests that the transition is most likely to be of first order, although a continuous transition with alpha >0 cannot be ruled out. In both cases, a second ordered phase is found for very low temperatures.

The evaluation at T=0 of the kinematical effects in the spin wave theory for S=1/2 Heisenberg ferro- and antiferromagnets has been achieved, projecting out the nonphysical magnonic contribution following Herbert's method. Moreover, the projected average value has been generalised to any operator and extended to any number of projection operators by means of a suitable diagrammatic technique. As an example, the method here developed has been applied to the S=1/2 Heisenberg ferromagnet.

For pt.I see ibid., vol.18, p.6247 (1985). The ground state properties of the S=1/2 antiferromagnetic Heisenberg Hamiltonian are analysed in the light of the projected spin wave theory previously reported, which takes into account the kinematical interaction (KI). It is shown that the inconsistency found for the zero point energy as obtained from different boson mappings is due to the fact that in the usual spin wave theory the KI is either neglected or not considered. Moreover, values of zero point spin reduction and zero energy shift are calculated with increasing accuracy and compared with the ground state estimates reported in the literature. The validity of the assumptions for the antiferromagnetic ground state is discussed and found to become worse as the lattice dimensionality becomes lower.

The EPR spectra of the paramagnetic complexes M(CN)₄Cl₂:NaCl obtained by X-ray irradiation on trivalent Co, Rh, and Ir hexacyanides inserted in NaCl, as well as RhCl₆^4- and IrCl₆^4- complexes in NaCl, present an additional hyperfine structure besides the well known two-chlorine first-ligands one. This structure is proved to arise from the interaction between the unpaired electron and the nearest-neighbour sodium atoms. The observation of this structure in sodium chloride and not in the analogous potassium chloride host lattice is explained by overlap and covalent effects in a molecular orbital approach.

Electron spin resonance and photoacoustic absorption spectra of vanadium-doped LaPO₄ have been studied at room temperature. Two nonequivalent centres are observed. Both can be described by a spin Hamiltonian of axial symmetry. The Hamiltonian parameters are: site 1: g/sub ///=1.9280, g_{perpendicular to} =1.9780, A/sub ///=-181*10^-4 cm^-1A_{perpendicular to} =-68*10^-4 cm^-1; site 2: g/sub ///=1.9070, g_{perpendicular to} =1.9840, A/sub ///=-195*10^-4 cm^-1, A_{perpendicular to} =-81*10^-4 cm^-1. Analysis of ESR and PAS data suggests that both centres can be attributed to VO²⁺ ions and are analogous to similar centres found in phosphate glasses.

Raman and infrared reflectivity spectra of Zn_1-xMn_xTe mixed crystals were measured in the composition range 0<x<0.15. The experimental data obtained are analysed using the modified random-element isodisplacement model. It is found that the x dependence of the optical-phonon frequencies displays an intermediate behaviour between one- and two-mode type. A satisfactory agreement of the model with the experimental results is obtained only after the inclusion of force constants due to next-nearest neighbours.

By tuning the excitation energy through the exciton bound to a NN₃ pair ground-state energy, in GaP:N at 5K, the resonant Raman behaviour for the bulk LO mode of the crystal is obtained. Because the electronic levels associated with nitrogen in GaP are highly localised, a simple expression for the Raman cross section can be derived from the general Loudon expression. A calculation of the cross section is made, which involves three sublevels of the NN₃ electronic states and then compared with the experimental results. This experiment shows interference between the three Raman contributions.