Table of contents

The authors calculate the dynamical critical exponent z for an N-vector model with long-range exchange couplings in random magnetic fields with long-range correlations using a 1/N expansion.

It is pointed out that homogeneously mixed-valent and excitonic insulator phases proposed earlier for the Falicov-Kimball model break a local symmetry of the Hamiltonian. An Elitzur-type proof on the impossibility of spontaneously breaking a local symmetry is given to show that such phases are not possible at nonzero temperature. At zero temperature, the possibility of breaking a local symmetry is demonstrated in a related model, but it is argued that such symmetry breaking does not occur in the Falicov-Kimball model.

An optical model is applied to a quantum well resonant tunnelling diode, in which the effect of scattering can be taken account by introducing an absorption coefficient alpha . In the limit of an opaque barrier the transient time and peak-to-valley current ratio are determined by alpha and the width of the quantum well. The numerical estimates of these quantities are shown to be consistent with the experimental values obtained by Sollner and co-workers.

It is conjectured theoretically that La_2-xM_xCuO₄, with M representing tetravalent elements such as Zr, Ce, Hf, Sn etc. may be high-T_c n-type superconductors in which the majority current carriers are electron-like instead of hole-like as in p-type superconductors obtained from, for example, doping divalent elements Ba, Sr, into La₂CuO₄.

The Heisenberg rhombohedral antiferromagnet (RAF) in the classical approximation is characterised by infinite degeneracy of the ground state corresponding to infinite inequivalent helices whose wavevectors Q belong to lines in the reciprocal space called degeneration lines L_Q. A rich unusual scenario that called a degenerate helix (DH) is set up: this entails the presence of 'soft lines' in the magnon energy spectrum, the absence of LRO in 3D at any finite temperature and possible algebraic decay of the correlation function. Zero-point-motion fluctuations destroy this phenomenology, picking out a certain helix of the manifold L_Q. It may be conjectured that the classical DH scenario could reappear at intermediate temperature when thermal fluctuations overcome quantum fluctuations. Indeed, the authors have found that this is the case-and conclude that the DH is not a mere artefact of the classical approximation but a genuine possibility for helimagnetic systems.

Several physical models involving the interaction of electronic and bosonic degrees of freedom have been proposed and solved exactly in recent years by a new recursion technique. The models belong to various contexts, but most applications have been in the theory of electron and photon spectroscopies of molecules and solids. For a class of Hamiltonians which describe closed-shell systems the recurrence relations provide closed-form analytical solutions. Here the authors present the method and discuss its relationship to alternative approaches that are also based on deriving and solving recurrence relations. They review the main results obtained to date and present solutions relevant to optical spectroscopy for the first time. They demonstrate the usefulness of exact analytical solutions for understanding the nontrivial dynamical behaviour of interacting fermion-boson systems.

At full coverage, adsorption of hydrogen on Rh(110) develops a 1*1-2H structure. A LEED structure analysis-though restricted to integer-order beam spectra only-is able to detect the adsorption position, which is 0.78 AA above the first substrate layer occupying nearly but not exactly threefold-coordinated sites at a lateral distance of 0.98 AA from the Rh rows. So, nearly equidistant hydrogen rows are observed. However, because of missing superstructure spots and a weak influence of hydrogen on integer-order spot intensities, comparatively large error bars of up to +or-0.3 AA result for hydrogen structure parameters. Nevertheless the minimum Pendry R-factor is as low as 0.28. Hydrogen removes the multi-layer relaxation observed for the clean surface towards bulk distances with only a 1.6% contraction between the first and second layer. This is in agreement with previous calculations neglecting hydrogen at the price of a less confident R-factor (0.37).

Previously collected high-resolution neutron-diffraction data from KH₂PO₄ (KDP) at room temperature have been re-analysed, including careful correction for thermal diffuse scattering, absorption and extinction and similar, new data have been obtained from 95%-deuterated KH₂PO₄ (DKDP). This has made possible, for the first time, a detailed study of the effects of deuteration alone on the KDP crystal structure. The main structural change on deuteration is a lengthening of the H(D) bonds, in response to which the PO₄ groups rotate in such a way that the oxygen polyhedron around each K atom remains unaltered in size and shape. The (small) changes in the thermal motion of the K, P and O atoms can be attributed to an expansion of the unit cell on deuteration, and to differences between the ferroelectric-mode amplitudes of KDP and DKDP. A comparison of the H and D thermal motion (i) reveals that it is inadequate to treat each minimum of the H(D) double-well potential as 'riding' on the nearer O atom alone, and (ii) underlines the large and important effects on the H(D) potential of thermal fluctuations in the H(D)-bond length.

Infrared absorption studies are reported for CdS containing lithium (⁶Li and ⁷Li isotopes) introduced by diffusion. The absorption bands, that are observed in the spectral region 350 to 550 cm^-1, are explained in terms of localised vibrational modes for three kinds of defects: (i) complexes comprising weakly coupled interstitial Li impurities in irregular positions, (ii) complexes comprising weakly coupled Li as an interstitial species in regular sites, and (iii) well oriented complexes comprising at least three strongly coupled Li impurities in substitutional sites.

Anomalies in the elastic constants of terbium iron garnet have been observed below room temperature. The longitudinal wave velocity for (111) propagation behaves anomalously at the compensation temperature (244 K) but that for (001) and (110) propagations does not. For shear wave propagation the temperature dependence of the ultrasonic attenuation is rather complex with new shear modes appearing below 80 K. The intricate behaviour of the elasticity is ascribed to the changes in the crystal and magnetic structures that occur below the compensation temperature.

The tetragonal-orthorhombic phase transition in arsenic pentoxide has been studied by X-ray powder diffraction and is found to be a proper ferroelastic transition. The spontaneous strain behaves as the order parameter following Landau theory with epsilon _s varies as mod T_c-T mod ^beta , beta =¹/₂, T_c=578 K. The order parameter susceptibility has been determined from line broadening and follows mod T_c-T mod ^-y, gamma =1. No deviation from Landau behavior has been observed experimentally.

For pt.I see ibid., vol.18, p.2647 (1987). The authors present here an experimental study of the variation in the shape of Ta(100) 4f core-level lines with temperature. These spectra exhibit a broad structure in the higher-binding energy side of the ⁵/₂ levels which can be assigned to the satellite predicted in the preceding paper. Moreover, a theoretical simulation of the total spectrum including the surface, first-underlayer and bulk peaks compares very satisfactorily with the experimental data. In particular, the overall shape of the spectrum and the variation with temperature of the linewidths are well reproduced. The presence of the satellite due to electron-hole pair formation thus explains the apparent anomalies of the Doniach-Sunjic parameters of the Ta surface peak put forward in earlier experiments. Finally, they note that it explains also the apparent different broadenings of the two spin-orbit lines.

The effect of a transverse field on the spin-glass freezing transition in the Sherrington-Kirkpatrick version of the quantum Ising model is studied using the thermofield dynamics (the real-time, finite-temperature quantum field theory) due to Takahashi and Umezawa (1975) as a substitute for the 'n-replica trick'. The field-dependent transition line to the spin-glass phase is found and the averaged local static metastable susceptibility in the transverse direction is evaluated in the simple mean-field approximation for the effective dynamic Lagrangian.

Recently a weak, orthorhombic, displacive superlattice was observed in both mineral and synthetic samples of NiAs. The symmetry-adapted displacement eigenvectors and corresponding atomic displacement patterns of the normal modes of the unmodulated sub-cell associated with the observed modulation wavevector (q=¹/₃ (1120)) are derived. Comparison with an experimental convergent-beam electron diffraction study strongly suggests that the deviation of the modulated structure from the unmodulated parent structure can largely be ascribed to the condensation, or freezing-in, of a q=¹/₃ (1120), Sigma ₆ phonon mode.

For pt.XI see J. Less-Common Met., vol.129, p.335 (1987). The theory of nuclear spin relaxation due to dipolar coupling between diffusing spins in crystals requires the calculation of spectral density functions for particular jump models of the diffusion. The results of such calculations have always been presented in a tabular form that is inconvenient to use compared with the simpler, but much less accurate, Bloembergen-Purcell-Pound theory. An approximate analytic function is proposed which, when fitted to the results of detailed calculations, has comparable accuracy to these calculations but has the simplicity of use of the BPP theory.

Measurements are reported on two-dimensional magneto-plasma resonances in pools of positive and negative ions trapped below the surface of superfluid ⁴He over the temperature range from 300 to about 20 mK. Values for the cyclotron masses of both types of ion have been deduced. The masses so obtained are independent of both the density of ions in the pool and the distance at which they are trapped below the surface. The cyclotron mass of the negative ion is independent of temperature, within experimental error, but that of the positive ion exhibits the same strong temperature dependence that has been deduced in earlier experiments from the observed temperature dependence of plasma resonant frequencies in the absence of a magnetic field.

The DC magnetoresistivity measurements of a compensated and barely insulating indium phosphide sample yield results showing variable-range hopping (VRH) in the temperature range 0.09 K<T<4.2 K for magnetic fields up to 11 T. Analysis of the conductivity data in terms of the relation sigma = sigma ₀ exp(-(T₀/T)³) indicates that s=0.25 for B<6 T, i.e. near the metal-insulator transition, while for larger magnetic fields (B>6T) s=0.5. This suggests that, in the vicinity of the metal-insulator transition, the VRH is in agreement with Mott's predictions (1968) assuming a constant density of states N(E_F) at the Fermi level whereas, far from the metal-insulator transition, a Coulomb gap is opening, in agreement with the predictions of Shklovskii and Efros (1984).

The authors describe a model of halogen-bridged mixed-valence linear chain complexes (transition-metal ions bridged by halogen ions) incorporating electrons coupled to on-site and inter-site phonons. They show that the charge-transfer situation appropriate to many members of the class corresponds to a charge-density wave with strong electron-phonon coupling. In this regime, they calculate intrinsic defect states, including kinks, polarons, bipolarons and excitons. These are strongly localised and are important in view of the growing attention given to local modes in solids. Available structural and optical data can be explained if an on-site Coulomb interaction is also included. The resulting parameter values suggest that, despite the large distortions observed, the mixed-valence state in many of these compounds is only marginally stable.

After a brief introduction to the semiconductor heterojunction devices in which a two-dimensional electron gas can be realised and can be subjected to a periodic potential through a grating, the linear response function for this periodically modulated system is calculated in the random phase approximation. Two solvable models have been considered, based on the anisotropic electron kinematics arising from the periodic potential. For the first model, approximate and exact results are obtained when (the Fermi level) E_F>2t (half the band width) while, for the second model, exact results are obtained when E_F>2t as well as E_F<2t. Results for the static polarisability and the plasmon dispersion are explicitly displayed.

⁵⁷Fe Mossbauer spectra of CsCo_0.997⁵⁷Fe_0.003Br₃ and RbCo_0.99⁵⁷Fe_0.001Cl₃ between 5 K and room temperature are reported. In both compounds, the quadrupole slitting is found to be negative, and it is deduced that the Fe²⁺ ion has a singlet ground state with nearly zero orbital and spin angular momentum for temperatures above the three-dimensional magnetic ordering temperature T_N1. The Mossbauer spectra of CsCo_0.997⁵⁷Fe_0.003Br₃ and RbCo_0.99⁵⁷Fe_0.01Cl₃ are consistent with a transition to a partially disordered antiferromagnetic phase at T_Nl, and a transition to a fully ordered ferrimagnetic phase at T_N2. The transition temperatures T_N1 are not significantly lower than those in the pure compounds, i.e. 28.3 K in CsCoBr₃ and 28 K in RbCoCl₃. T_N2 is found to be 12 K in CsCo_0.997⁵⁷Fe_0.003Br₃, and 4-14 K in RbCo_0.99⁵⁷Fe_0.01Cl₃. Two types of relaxation process are found in the Mossbauer spectra of these two compounds at temperatures below T_Nl. One is attributed to electronic relaxation between the lowest states of the Fe²⁺ ion; the other, which is present only at T_N2 and above, to solitons in the chains of magnetic ions that are excluded from the three-dimensional order between T_N1 and T_N2. The temperature variation of the linewidths in the Mossbauer spectra of RbCo_0.99⁵⁷Fe_0.01Cl₃ above T_N1 is consistent with solitons being present in this material.

Further details of a new method for studying strongly coupled orbital triplet Jahn-Teller systems (JT) are presented. The procedure, which involves unitary transformation and energy minimisation, allows coupling to both e- and t₂-type cluster vibrations. Detailed results are presented for systems in which trigonal wells in the potential energy surface are energy minima. An estimate is made of the range of e- and t₂-type couplings for which the method is valid. Results are also presented for systems with tetragonal energy minima. These are compared with well known results of T(X) e JT problem for the case when the t₂-type coupling can be neglected.

The size scaling exponents alpha of the DC conductivity of Sierpinski carpets made of conductive paper are measured, and the possible relations between the exponent alpha and the fractal dimension d_f, the symmetry and the topology of carpets are studied. For some sequences of the carpets, the authors find that alpha can be expressed as the convenient power law alpha ^{varies as} (2-d_f)^eta and the exponent eta is not sensitive to the geometry of the carpets and is almost constant (about 0.96) in some region beta (from 0.6 to 1.6), where beta is a control parameter used to describe other properties then fractal dimension, such as the symmetry of the sequences of carpets and is defined by the ratio of the void sections along the x direction to those along the y direction in the carpet. The relation between the spectral dimension d_s and the geometry of the carpet is also discussed.

The effect of partially substituting praseodymium for lanthanum in the superconducting materials La_2-xSr_xCuO₄ has been investigated. Powder neutron diffraction work demonstrated that only minor structural changes occur; however, marked differences in electrical properties were found. Compounds of the general formula, La_1.62-xPr_xSr_0.18CuO₄, synthesised using a precipitation method from Pr³⁺ compounds, were found to be semiconductors in the range 0<x<0.27. In the La_1.62-yPr_0.18Sr_yCuO₄-phase region a change from semiconducting to superconducting behaviour was found on increasing the level of strontium doping.

The functional form of the volume pinning force F_p for high-temperature type II superconductors is derived as the degree of distortion in the flux-line lattice changes. A single mechanism operates in both NbTi and Nb₃Sn, namely the activation of Frank-Read sources where F_p=(1/100)( mu ₀H²_c2/DK²)b(1-b/0.95) for 0.3<or=b<or=0.9 and F_p=(1/21)( mu ₀H²_c2/DK²)b¹2/(1-b)² for 0.1<or=b<or=0.9 respectively. Above the critical state, flux flow is first characterised by the motion of core dislocations.

The authors compute the first-order quantum corrections to the spin-wave spectrum and spin-spin correlation functions of an easy-plane antiferromagnet. Results show that the classical approximation often adopted is totally inadequate at low temperatures, and in particular a quantum treatment of the out-of-plane fluctuations is crucial. These fluctuations are very strong, demonstrating, for instance, the inadequacy of a mapping to the quantum sine-Gordon model. They apply our calculation to (CH₃)₄NMnCl₃ (TMMC), and S=⁵/₂ spin system.

Projection techniques have been used to analyse the S=¹/₂ heisenberg anisotropic model. The zero-point spin reduction (ZPSR) and the zero-energy-shift, (ZES) values, to first-order approximation in the projectors, and for any anisotropy value are reported. Moreover, an analytical expression for the ZPSR is derived. The analysis of the results as a function of the anisotropy proves that, for one dimension, the ZPSR correction of kinematical interaction yields physically meaningful results. Moreover, there are ZPSR discontinuities at the Heisenberg isotropic ferromagnetic and antiferromagnetic limits, which are just the signals of the transitions to the isotropic fixed-point Hamiltonian. For two and three dimensions, both ZPSR and ZES smoothly interpolate for any anisotropy value.

A study of the EPR spectra of K₂MgF₄:Ni after X-irradiation is presented. Room-temperature X-irradiation produces three types of Ni⁺ defects having tetragonal symmetry and with the unpaired electron in a mod x²-y²) orbital. The values of the spin-Hamiltonian parameters of these centres are given and tentative models for them are presented. In one of these centres the Ni⁺ ion is at the centre of an elongated octahedron of fluorines (Ni⁺-I) while in the other two the monovalent nickel has one (Ni⁺-II) and two (Ni⁺-III) fluorine vacancies placed along the tetragonal crystal axis. Only Ni⁺-I is produced by 77 K X-irradiation. These results are compared with those recently reported for KMgF₃ and an analysis of the spin-Hamiltonian parameters of the different Ni⁺ centres in KMgF₃ and K₂MgF₄ is also presented.

The influence of high-frequency ultrasonic waves on the dynamic diffraction of Mossbauer gamma quanta in a perfect Si crystal is studied. The scattering intensities as well as the diffractional Mossbauer spectra for different reflections and ultrasonic frequencies nu _s are measured. Above the threshold frequency ( nu _s> nu _th), enhancement of elastic scattering is observed. At nu _s< nu _th, 'pumping' of the elastic component into the inelastic component occurs. The experimental data are in good agreement with the theoretical calculations.

The ultraviolet-induced reduction of Eu³⁺ to Eu²⁺ in CaSO₄:Eu has been found to proceed via a charge transfer (CT) transition of the Eu³⁺-O₈ complex followed by the liberation, migration and trapping of the hole generated by the CT. A kinetic model of this process based on the thermally stimulated transfer of the electronic population between Eu³⁺ upper 4f⁶ states and the Eu³⁺-O charge transfer state is proposed and some of its implications for the production of thermoluminescence (TL) in CaSO₄:Eu discussed. The UV-induced TL response of CaSO₄:Eu was found to be correlated with the production rate of Eu²⁺ for wavelengths greater than 230 nm, however the same correlation was not apparent for shorter wavelengths.