Journal of Physics C: Solid State Physics

A new definition of order called topological order is proposed for two-dimensional systems in which no long-range order of the conventional type exists. The possibility of a phase transition characterized by a change in the response of the system to an external perturbation is discussed in the context of a mean field type of approximation. The critical behaviour found in this model displays very weak singularities. The application of these ideas to the xy model of magnetism, the solid-liquid transition, and the neutral superfluid are discussed. This type of phase transition cannot occur in a superconductor nor in a Heisenberg ferromagnet.

A theory is developed for dispersion characteristics of spin waves in ferromagnetic films taking into account both the dipole-dipole and the exchange interactions. An arbitrary orientation of the internal bias magnetic field is assumed. The general case of mixing exchange boundary conditions (surface spin pinning conditions) is considered. The simple analytical dispersion equations are obtained using the classical perturbation theory. The modification of the spin wave spectrum due to surface anisotropy (or pinning conditions) is discussed.

The explicitly soluble Luttinger model is used as a basis for the description of the general interacting Fermi gas in one dimension, which will be called 'Luttinger liquid theory', by analogy with Fermi liquid theory. The excitation spectrum of the Luttinger model is described by density-wave, charge and current excitations; its spectral properties determine a characteristic parameter that controls the correlation function exponents. These relations are shown to survive in non-soluble generalisations of the model with a non-linear fermion dispersion. It is proposed that this low-energy structure is universal to a wide class of 1D systems with conducting or fluid properties, including spin chains.

The critical properties of the xy model with nearest-neighbour interactions on a two-dimensional square lattice are studied by a renormalization group technique. The mean magnetization is zero for all temperatures, and the transition is from a state of finite to one of infinite susceptibility. The correlation length is found to diverge faster than any power of the deviation from the critical temperature. Analogues of the strong scaling laws are derived and the critical exponents, eta , and delta , are the same as for the two-dimensional Ising model.

Oscillatory effects in a strong magnetic field B and magnetic susceptibility are investigated, as applied to 2D systems, in which the twofold spin degeneracy is lifted by the spin-orbit-interaction Hamiltonian H_SO= alpha ( sigma *k). nu . The term H_SO is shown to change greatly the usual patterns of B^-1-periodic oscillations; some oscillations are strongly suppressed due to the diminishing of the gaps between adjacent levels, and new oscillations appear due to intersections of levels.

In a thermodynamic treatment electromagnetic radiation of any kind is described. The difference between thermal and non-thermal radiation is accounted for by introducing the chemical potential of photons. Instead of an effective temperature all kinds of radiation have the real temperature of the emitting material. As a result Planck's law for thermal radiation is extended to radiation of any kind. The concept of the chemical potential of radiation is discussed in detail in conjunction with light-emitting diodes, two-level systems, and lasers. It allows the calculation of absorption coefficients, of emission spectra of luminescent materials, and of radiative recombination lifetimes of electrons and holes in semiconductors. Theoretical emission spectra are compared with experimental data on GaAs light-emitting diodes and excellent agreement is obtained.

When there is a dipole moment in the repeat unit perpendicular to the surface in an ionic crystal, lattice sums in the electrostatic energy diverge and the calculated surface energy is infinite. The cause of this divergence is demonstrated and the surfaces of any ionic or partly ionic material are classified into three types. Type 1 is neutral with equal numbers of anions and cations on each plane and type 2 is charged but there is no dipole moment perpendicular to the surface because of the symmetrical stacking sequence. Both these surfaces should have modest surface energies and may be stable with only limited relaxations of the ions in the surface region. The type 3 surface is charged and has a dipole moment in the repeat unit perpendicular to the surface. This surface can only be stabilised by substantial reconstruction. These conclusions are important for the analysis of the surface structure of ionic crystals.

Angle-resolved photoemission spectroscopy (ARPES) measurements of the E(k/sub ///) band structure are reported for 1T-TaS₂ and 2H-TaSe₂ at temperatures where commensurate CDWS are well developed. Empirical tight-binding calculations of the band structure in the presence of the experimentally known CDW superlattice are also reported. In the case of the square root 13* square root 13 CDW in 1T-TaS₂, experiment and theory both reveal a gross distortion of the band structure, in which the Ta-derived d band collapses into three sub-band manifolds separated by gaps. The thirteenth electron is predicted to reside in a conduction band only 50 meV wide, where it is susceptible to a Mott-Anderson transition. In the case of 2H-TaSe₂, additional peaks are seen in the ARPES data in the presence of the CDW, but the effects are too weak to distinguish between a band-structure distortion and weak Umklapp associated with the 3*3 superlattice. Calculated densities of states, decomposed according to the inequivalent atomic sites, are presented.

The authors present a systematic approach to the derivation of empirical potential parameters for binary oxides; they also consider their modification for use in mixed oxide systems. Shell-model potentials are used but, unlike the case of the alkali halides within which polarisability and short-range interaction parameters can be transferred, modifications must be introduced when transferring potential parameters between different oxides. The anion polarisability varies with structure and with the nature of the host cation, and changes in cation coordination are reflected in the short-range repulsive cation-anion interaction. Parameters are derived for a range of oxides, and trends in these parameters are discussed. They discuss successful applications of the potentials to the calculation of perfect lattice properties. Equal success is enjoyed when defect and surface properties are considered; in particular the models correctly predict the activation energies for dopant diffusion in NiO, and to a large extent model the surface rumpling of MgO.

The Coulomb interaction between localized electrons is shown to create a 'soft' gap in the density of states near the Fermi level. The new temperature dependence of the hopping DC conductivity is the most important manifestation of the gap. The form of the density of states within the gap is discussed.

The effective-mass Hamiltonian H=¹/₂m^alpha pm^beta pm^alpha +V_c+ upsilon for non-homogeneous semiconductors is studied. Here m is the position-dependent effective mass, 2 alpha + beta =-1. V_c is the position-dependent conduction band edge and upsilon is a localised potential. Through an exact model calculation the authors show that when effective-mass theory is applicable, alpha =0 and beta =-1,.

A novel experiment is described in which frequency-dependent loss due to carriers that are optically excited and then trapped in the band tails of hydrogenated amorphous silicon is observed. The decay of the signal over long periods gives information about recombination in this material at low temperatures.

The title compound undergoes a metal-insulator transition at 210 K under normal pressure: the transition temperature increases with increasing pressure. The phase diagram has been studied up to 7 kbar and it corresponds approximately to that obtained for (DMDCNQI)₂Cu above 5 kbar.

A method is presented for diagonalising a non-Hermitian Hamiltonian that can be used to plot the giant polariton dispersion relation.

A perturbed angular correlation study on Au₈₅Fe₁₅ in different metallurgical states and over a wide temperature range is reported. The hyperfine field and its temperature variation reflect a cluster glass behaviour due to chemical clusters of different sizes rather than a magnetic double transition.

Theory and experiments on steady-state and femtosecond time-resolved optical nonlinearities in semiconductors are reviewed. A simple description of the physical processes underlying the nonlinearities is given. The discussion is focused on the spectral region around the fundamental absorption edge, and it covers coherent oscillations, the optical Stark effect as well as the bleaching of the exciton resonance with increasing excitation intensity, plasma screening and band-filling phenomena.

A local-pair mechanism for high-temperature superconductivity in the quaternary copper oxide families based on (La/Sr)₂CuO₄ and YBa₂Cu₃O₇ is developed. The available experimental data (as of 29 June 1987) are assessed particularly from a materials point of view, to make clear what the combination of features is in these systems that is so unusual as effectively to restrict high-temperature superconductivity to the mixed-valent copper oxides-and then only to certain of these. All the principal enabling factors ensue from the position of copper in the periodic table, and the potential core-like stability of the d¹⁰ configuration. The latter permits monovalence to be added to di- and trivalence for this element, as for silver and gold, though it is in copper that all three valencies are most readily stabilised. The proposed mechanism does not involve disproportionation, but fluctuations on pre-existing 3+d⁸ sites to 1+d¹⁰, using electrons drawn from the d⁹ sites over which the superconductivity is dominantly mediated. The late position of copper in the transition-metal series secures heavy p/d hybridisation which leads to the absence of pair-breaking local moments being established both at the di- and trivalent sites. Support for the present negative-U model is extracted from the author's earlier work on the metal-insulator transition. A more magnetically oriented model like the resonant valence bond model which has recently associated with observations in d¹ TiOCl, etc., does not really seem suited to the geometry and electronic conditions in the present copper oxides. However it is suggested that TiB₂, La₃S₄ and the Chevrel phases might be open to treatment by such a model. Comment is also made about how the superconductivity of PdH_x and Ba(Pb/Bi)O₃ might be seen in the light of the present model, and attention is directed once again to what might be happening in pressure-quenched CuCl and CdS.

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.

The paper reviews, at an introductory level, the theory of photon scattering from condensed matter. Magnetic scattering, which arises from first-order relativistic corrections to the Thomson scattering amplitude, is the main topic. A general theoretical framework is discussed for the cross section and secondary polarisation. Specific results of immediate interest are presented, including the case of circular polarisation of the incident and secondary beams. Use of the magnetic neutron and photon scattering techniques is also contrasted.

The positive muon is a unique microscopic probe. When it is implanted in condensed matter, the evolution of its spin polarisation may be readily monitored to give information on the crystallographic or molecular sites it occupies, the local fields it experiences and the time dependence of these fields. This article describes the experimental techniques in current use, and the manner in which the muon behaviour is characterised. The implications for solid state and chemical physics are discussed. The comparison between muon and proton behaviour in similar circumstances is emphasised. For chemical aspects, the equivalent comparison is between muonium and hydrogen, with muonium considered as a radioactive light isotope. Examples of studies are reviewed for magnetic materials, for metals and metal hydrides, for semiconductors and ionic crystals, and for certain organic materials and liquids. Novel information is obtained on topics including spin dynamics, dynamic correlations and critical phenomena, the localisation and quantum mobility of light interstitial particles, charge screening, the local electronic structure of hydrogen-like defect centres, hyperfine interactions, various kinetic and dynamic isotope effects and the role of vibrational effects, including zero-point motion, in determining molecular structure. Comparisons are drawn with studies by other techniques, notably magnetic resonance. Techniques and applications appropriate to pulsed muon sources are considered. The major achievements to date and the potential for future developments and new science are identified.