Table of contents

The Jahn-Teller instability for oxygen ions in Cu-O planes and Ba-O planes of YBa₂Cu₃O_{approximately 7} systems is investigated. These ions have two Cu-O bonds in a straight line and the degenerate O 2p levels are partly filled. The Jahn-Teller effect tends to move the oxygen ions into the nearby vacancy site in the Cu-O planes so that the Cu-O bonds are bent and the degeneracy is removed. As a result restoring forces, which avoid the oxygen hoppings to the vacancy sites, are softened. On the other hand the pairing mechanism of conduction carriers via exchange of oxygen hoppings may play an important role for superconductivity if the softening takes place. The Jahn-Teller phase transition is estimated by quite a simple model, and its relation to the superconducting transition is discussed. Some anomalous properties of YBa₂Cu₃O_{approximately 7} systems can be expected.

The electrical resistivity of the superconducting ceramics M_xY_1-xBa₂Cu₃O_{7- delta} , where M=Zr or Hf, has been studied for a series of Zr-substituted (x<0.40) and Hf-substituted (x<0.50) systems. All these ceramics exhibit superconductivity above liquid nitrogen temperature. With increasing x the superconducting transition temperature T_c decreases and the room-temperature resistivity increases. On the other hand, the mechanical strength and the resistance to aging and humidity increases significantly. Some metallographic and local chemical analysis results are also presented and discussed.

The effect of fluctuation of the crystal-field interaction D on the transition temperature in an amorphous (or a crystalline) spin-one Ising ferromagnet with a coordination number z=3 is investigated by the use of the effective-field theory with correlations. The author finds that even for D>or=0 the amorphous system with the structural fluctuation delta ( delta = Delta J/J) in the range of 0.9< delta <1.0 shows an increase in Curie temperature T_c with the increase in fluctuation of D, although in the other region of delta the system shows a decrease in T_c.

A multiple-shell, curved-wave EXAFS analysis of the spectra on the absorption edges of all three cation species in the high-temperature superconductor Ba₂YCu₃O₇ is reported. No change in structure is observed between liquid nitrogen and room temperatures. However, there are considerable differences in Debye-Waller factors which demonstrate that the Cu-O bonds are extremely rigid in this material.

X-ray diffraction from the antiferroelectric superlattice of SrTiO₃ has been observed in a grazing geometry for several values of the X-ray penetration depth. The samples were a polished single crystal grown by the Verneuil technique and a polished and etched single crystal grown from a top-seeded solution. The variation with temperature of the order parameter was found to depend strongly on the penetration depth for the Verneuil-grown sample. This dependence was much less for the sample grown from a top-seeded solution. The observed X-ray intensities for both samples have been fitted by a mean-field theory that takes into account a surface free-energy density that is different from that in the bulk, such as might arise from a strain localised in the surface.

The deviation Delta Gamma in the linewidth Gamma due to finite extension of an experimental sample is studied in the context of the mode-coupling theory. Near the 3D-2D crossover onset, this deviation is obtained for a spherical ball and a planar film by introducing a cut-off wavenumber in the mode-coupling equation. The ratio of the leading-order deviation Delta Gamma / Gamma ^crit is of the order of 1/qL, where q is the wavevector of the corresponding mode and L is the film thickness.

The general formula for Lyapunov exponents for any one-dimensional Anderson model with both diagonal and off-diagonal disorder is given using the formalism of the so-called supersymmetric integrals. A weak disorder expansion of Lyapunov exponents and of the density of states is presented. The nonanalyticity of both quantities for states with wavevector pi r/s, with r and s integers, is proven.

A model is introduced for Anderson localisation in the integer quantum Hall regime. The model represents a system with a disordered potential that varies slowly on the scale of the magnetic length, but includes quantum tunnelling and interference effects. Numerical calculations indicate that the localisation length diverges only at the centre of each Landau band. The scaling behaviour near the mobility edge is analysed: results suggest that quantum tunnelling induces crossover at the classical percolation threshold to critical behaviour similar to that found previously for a rapidly varying potential.

The authors report the results of two types of experiments on the thermo-electric coefficients of GaAs-Ga_1-xAl_xAs heterojunctions. In the first set of experiments, the density and mobility of a two-dimensional electron gas has been varied over a wide range by illumination. The zero-field thermopower is reduced by the increased carrier density but the oscillation amplitude at any particular field is insensitive to it. In the second set of experiments they have increased the phonon mean free path in the GaAs substrate by polishing the rear surface. This results in an increase in magnitude of the thermo-electric coefficients which is consistent with phonon drag effects.

X-ray photo-electron spectroscopy of Tl core lines show that Tl impurities in PbTe exist in only one charge state. This contradicts theoretical ideas that link the superconductivity of Tl-doped PbTe with the existence of Tl⁺ and Tl³⁺ sites.

The low-temperature spin waves in the competing-anisotropy antiferromagnet K₂Co_xFe_1-xF₄ have been calculated for various concentrations in the range 0<x<1 by the method of computer simulation. These calculations show that there is not a soft mode at the phase boundary between the uniaxial and mixed phases and predict spin-wave dispersion relations and ground-state properties that are in better agreement with the available experimental data on K₂Co_xFe_1-xF₄ than calculations using an average-crystal approximation.

In the orthorhombic compound holmium trifluoride, HoF₃, the trivalent holmium ion has a singlet ground state, with another singlet at 5.90 cm^-1. Optical spectroscopy, magnetic and thermal measurements are used to investigate the behaviour of single crystals at 4.2 K and below. The results largely confirm the crystal-field analysis of Ram and Sharma (1985), but give a value of 6.59 cm^-1 for the separation of the two singlets. It is found also that the system orders antiferromagnetically at T_N=0.53 K, with the holmium moments aligned parallel to the a axis. The enhancement of the ¹⁶⁵Ho NMR frequency is determined by a resonance experiment at 3.5 GHz. A simple theory of two electronic levels including hyperfine structure is used to interpret the results. Despite exceptionally large corrections required for the internal field, the magnetic and thermal measurements can be fitted using molecular-field theory.

The linear thermal expansion, AC susceptibility and magnetostriction have been measured on a single crystal of HoAl₂. The authors' results clearly show two phase transitions associated, respectively, with establishment of magnetic order and with spontaneous spin reorientation. The critical exponent gamma _SR of the susceptibility for T>or approximately=T_SR has been determined. From magnetostriction measurements along the major symmetry directions of the crystal, they have obtained the possible deformation modes compatible with the point symmetry of the cubic crystal structure. A complete study of the magnetoelastic behaviour has allowed them to determine the reduced magnetoelastic coupling coefficients M₂^gamma /C^gamma , M₂^epsilon /C^epsilon , M_l^alpha /C^alpha (l=4,6) and D^alpha /C^alpha . The model used is based on a single-ion origin for the crystal electric field interaction of the rare-earth ion, and they have taken into account the spontaneous reorientation of the magnetic moment in the molecular field, which represents the exchange interaction.

In this work the authors review the different models and methods used to analyse the IR reflection spectra of iron, zinc and magnesium fluorides, between 4.2 and 300 K. They especially show that the four-parameter oscillator model, presented by Gervais (1974), gives results similar to those obtained by other methods, but also a more accurate determination of the longitudinal-mode frequencies. The authors have shown clearly, for the mode A_2u, the occurrence of abnormal behaviour of the longitudinal frequencies with temperature. This result allowed them to consider, for zinc and magnesium fluorides, the presence of an incipient ferroelectric order at low temperature. They show also how they have been forced to consider an additional oscillator in the symmetry spectrum E_u.

Copper has been incorporated into high-purity ZnSe single crystals in the concentration range from 0.005 to 0.5 p.p.m. using the distribution effect during dipping into molten Zn-Cu alloys. The change in the I₁^d emission line reveals that the occupation state of the incorporated Cu atoms can be converted from interstitial to substitutional sites through annealing at 573 K after heat treatment under a saturated Se pressure. It becomes clear that there are two origins, V_Zn and Cu_Zn, for the I₁^d emission as suggested by Huang and co-workers (1983).

In a recent Letter to the Editor, Thalmeier (1987) calculates the amplitude of the de Haas-van Alphen effect to be expected in heavy-electron compounds. The authors argue that the result is in error and arises from a confusion over the relationship between the resistivity and the self-energy appropriate for a description of de Haas-van Alphen amplitudes.

This Comment points out that a recent Letter to the Editor by Thalmeier is in total disagreement with present experimental results. This disagreement arises because the theory he uses is invalid; the correct theory is given in a previous publication by Stamp (1987) under the assumption that one is dealing with a Fermi liquid.

In a recently published conductivity study of an InP sample very close to the metal-insulator transition, Finlayson and co-workers (1987) interpreted their data in terms of the relation σ(T)= σ_c exp(-(T₀/T)^1/4). They regarded additional magnetoresistance measurements as confirmation of a phase diagram proposed by Shapiro (1984). Both these conclusions are criticised. For T>or approximately=80 mK the temperature dependence of the conductivity can be understood in terms of a phenomenological model developed previously for amorphous Si_1-xCr_x.