Table of contents

It is shown that the Auger transition probability for excitons bound to deep neutral impurities cannot be derived by extrapolating the results obtained for the series of shallow donors and acceptors. Although the probability increases with localisation over the range of binding energies characteristic of hydrogenic impurities, the breakdown of the effective mass approximation in the case of deep levels greatly reduces the Auger probability. Exceptional situations where this may not be so are also predicted.

For original paper see ibid., vol.16, p.L109 (1983). A recent claim that the one-parameter scaling theory of localisation contains an inherent inconsistency is refuted.

For comment see ibid., vol.16, p.L741 (1983). It is maintained that the one-parameter scaling theory is inconsistent with the physics of Anderson localisation.

Starting from the formalism of force-correlation functions the electrical resistivity R, the thermoelectric power S and the Hall coefficient R_H are calculated for a system containing electron-libron scattering. In the framework of a two-band model these transport quantities can be calculated in the case of weak scattering and high temperatures. The results obtained for R, S and R_H are compared qualitatively with measurements on TTF-TCNQ.

The concept of a scaling stiffness for frustrated systems is introduced. Physical arguments supported by numerical calculations on a 3d Heisenberg spin glass suggest that its apparent lower critical dimensionality is time dependent, being equal to three on short time scales and bigger than three at sufficiently long times.

Dielectric permittivity and losses measurements were carried out on crystalline ammonium hydrogen bis-chloroacetate in the frequency range 100-2000 MHz near the transition temperature. The observed dielectric relaxation process was nearly monodispersive. From the analysis of the various experimental results it is suggested that the most probable model of the transition observed in this crystal is the so-called pseudo-spin lattice coupled model given by Nath and co-workers (1979).

The authors have observed quite strongly-phonon-coupled photoluminescence (PL) in V-doped InP with a sharp no-phonon line near 705 meV and an excited state splitting of 1.55 meV. Detailed Zeeman data on the cold line are reported. The ground state is shown to be a pure spin triplet, g approximately=2 and must arise from the V_In³⁺(d²) charge state. The PL is ascribed to transitions between the ³T₂(³F)-³A₂(³F) levels of V_In³⁺. A marked tetragonal anisotropy is found in the excited states of the transitions. This is tentatively attributed to an almost static Jahn-Teller effect in the ³T₂(³F) state. A similar attribution is proposed for qualitatively similar PL spectra observed by U. Kaufmann et al. (1982) in GaAs and GaP.

The author has calculated the absorption energy for the F⁺ centre in lithium oxide by using the point-ion model as well as the approximate pseudopotential method formulated by Bartram and co-workers (1968). The result obtained using the point-ion model agrees reasonably well with that from experimental work. However, the result obtained using the approximate pseudopotential calculation is higher than the experimental result by 1 eV.

The authors show that charge-transfer excitations (like 2M²⁺ to M⁺+M³⁺) can be lowered greatly in energy near grain boundaries, where sites are no longer equivalent. In special cases the excitations may be exothermic ('negative-U' behaviour); likely cases include (320) and (122) grain boundaries in FeO. Consequences include effects on conductivity, segregation of impurities with different valence, and on other charge-state-dependent properties.

The phonon dispersion curves of two rocksalt-structure uranium pnictides, USb and UAs, measured by neutron inelastic scattering, are presented. Both materials display marked anomalies in the disposition of certain branches, similar to those observed in some intermediate-valence compounds. The data are compared with the prediction of simple lattice dynamical models.

It is shown that in the classical limit the entropy of disorder in the harmonic chain of particles with masses M_i and nearest-neighbour force constants K_i((K/M)=1) is given by S_dis=-Nk_B(log(K/M)¹2/) (average over chain) compared with the standard expression S_dis=-Nk_B(log omega ) (average over normal modes).

The critical dynamics of a class of displacive second-order phase transitions is studied in the limit T_c to 0 (T_c being the critical temperature). Both the renormalisation-group recursion relations method and direct calculation are used for the derivation of the critical exponents up to order epsilon ² ( epsilon =3-d). The static exponents remain, in some sense, universal, whereas the dynamic exponent has no epsilon corrections.

The effect of random-field-induced domain wall wandering on the nature of the uniaxial incommensurate-commensurate (IC) transition is studied in the framework of a d-dimensional 'random-rod' model. In this model, the random field h(r) couples linearly to the wall elongations. Both short- and long-range field correlations (h(r)h(0)) approximately mod r mod ^-D+1 are considered. It is found that the domain wall density l^-1 vanishes with the driving parameter delta (e.g. temperature, pressure) as ( delta - delta _c)^beta with beta =(5-D)/2(D-3) for 3<D<5. For D>d, D has to be replaced by d. If D(d) is linear in d then beta -(d_R-d)/2(d-d_c^-) (*) for d_c^-<or=d<d_R-where d_R and d_c^- denote the upper critical dimensionality for the roughening transition and the lower critical dimensionality for the occurrence of a conventionally ordered phase, respectively-D(d_R)=5, D(d_c^-)=3. For D>or=5 or d>or=d_R the classical result l^-1 approximately ln^-1( delta - delta _c)^-1 applies; for D<or=3 or d<or=d_c^- there is no IC transition. The results for a corresponding bulk random-field model follow from the 'random-rod' model by means of the Villain relation D(d)=(5+2d)/3 for all d, i.e. from (*) with d_R=5, d_c^-=2. It is argued that the formula (*) applies for a variety of IC transitions with d_R and d_c^- specifying different universality classes.

Previous results on fluctuation-induced domain wall wandering in continuous systems and its influence on the nature of the uniaxial incommensurate-commensurate transition are unified in a phenomenological interface theory. A relation between the exponents beta and psi describing the vanishing of the domain wall density with the driving parameter delta , and the divergence of the effective wall width for vanishing pinning potential, respectively, is established. If psi is linear in the dimensionality, d, we get for d_R>or=d>or=d_c, beta =(d_R-d)/2(d-d_c). The critical dimensionalities d_R and d_c depend on the model under consideration. The changeover from the weak to the strong fluctuation regime is characterised by the different behaviour of the wall correlation length xi _{perpendicular to} . If walls separate domains of opposite polarisation, dipolar interaction increases the stiffness of the walls and decreases d_R and d_c. Their values for thermal, quantum and random field fluctuations are calculated. In the dipolar case, frozen impurities lead to a lower critical dimensionality d_c=2 and beta =¹/₂ for d=3, in agreement with experiments.

In an earlier paper (see ibid., vol.15, p.L381, 1982) the author gave a simple proof that the static, adiabatic and intermediate coupling schemes give the same nonradiative multiphonon transition rates to leading order in the off-diagonal part of the electron-phonon interaction. In that proof, the time dependence in the interaction picture of the matrices describing the transformation from one representation to another was neglected using a heuristic argument. In this paper it is shown explicitly that this time dependence can be neglected, hence firmly establishing the earlier result. The equivalence between the various coupling schemes is also established for the time rate of change of a general observable; for example the charge density. The present work is discussed in relation to the recent papers by Peuker et al. (1982) and by Wagner (1982). In particular it is shown that there is no contradiction between the present work and that due to Wagner. It is also shown that Wagner's alternative adiabatic base is none other than the base defined by Condon electronic wavefunctions and displaced lattice oscillators and that this result holds independently of the strength of the off-diagonal part of the electron-phonon interaction.

The condition for the Anderson transition in systems where the atoms are completely randomly distributed in space with concentration c is obtained by assuming the coincidence of the onset of extended states and of the infinite cluster in a special percolation model. This model consists of the random system of sites with the probability of connection between sites depending on the distance between them and on their local surroundings. The thresholds of percolation in this model, determined in the three-dimensional case by considering the dependence of the average numbers of sites connected to a given site on the configuration of unbroken connections and by direct computer simulation are in good agreement. They yield for a parameter M=c^-1/3 alpha (where alpha ^-1 is a characteristic length of the exponentially decreasing transfer integral) the critical value M_c=5.8+or-0.4. In two dimensions the model predicts a transition for M_c=3.9+or-0.2.

Auger recombination in direct-gap semiconductors is reinvestigated taking into account a realistic band structure instead of using the usual parabolic approximation. It is found that the direct conduction band process is negligible in large-gap semiconductors (E_g>or=0.6 eV), contrary to what has been found using the parabolic approximation. The new values for the probabilities of the valence band process and of the phonon-assisted Auger processes are also found to be smaller, but not by so much. Altogether, smaller values for the total Auger recombination are obtained and these are in agreement with experimental results. In addition, the effect of carrier degeneracy is shown to be weak, apart from where carrier concentrations are extremely high (n>or=10²⁰ cm^-3). Finally, the uncertainties involved in the quantitative values of the Auger coefficients are discussed.

DLTS measurements have been made on a large number of InP samples. Eight different electron traps are observed in this material, and their emission characteristics are reported. For three of these levels, detailed capture cross section measurements were possible, leading to an accurate description of their thermodynamic properties.

The phenomenological model is proposed for amorphous magnets with ferromagnetic exchange and small random anisotropy. A new type of spin-glass state is described. Its behaviour in an external magnetic field is shown to deviate strongly from mean-field theory predictions. The effect of coherent anisotropy is discussed.

Randomly diluted one-dimensional Ising chains of spin S are studied by breaking the systems down to noninteracting finite segments. Exact closed-form expressions are obtained for the specific heat and perpendicular susceptibility of S=¹/₂ chains and numerically exact results are given for spins up to ⁵/₂.

The spin-1 Heisenberg model with bilinear and biquadratic exchange is studied by means of exact calculations on finite cells of up to ten sites, in one and two dimensions. The authors investigate the nature of the ground state for different values and signs of the exchange constants and obtain estimates of the ground-state energy. Anomalous ground states are found for some, but not all, of the cases studied. The specific heat is also calculated.

The Cousins model (1978) of inner elasticity is applied to the Ni³⁺:Al₂O₃ system. The EPR results under uniaxial stress obtained earlier by Shen and Estle (1979) are reanalysed. It is found that the Al-Al tensors have virtually no effect on the calculation of the isofrequency curves. However, the Cousins model gives isofrequency curves that are in better agreement with the experimental results than those obtained earlier using an 'averaged octahedron' model. From a new analysis of published Raman data under uniaxial stress it is found that, for Ni³⁺:Al₂O₃, V_Eb=43000+or-13000 cm^-1 and r=0.40+or-0.2. The Cousins model is also used to evaluate values of V_Eb for other impurity ions in Al₂O₃ and the results are compared with experimental data.

The cross section sigma _N of multiphonon resonance Raman scattering in polar semiconductors that arises from a process involving free (unbonded by Coulomb interaction) electron-hole pairs is calculated for the case where the number of emitted longitudinal optical phonons N<or=4. It is shown that sigma _N approximately alpha ³, where alpha is the Frohlich electron-phonon coupling constant. The relation between sigma _N and an electron-hole distance function is found. The dependence sigma _N approximately alpha ³ arises from the fact that, after having emitted an arbitrary number of longitudinal optical phonons, the electron-hole pair occupies a finite volume that is proportional to alpha ^-3.

The authors report the carbon K emission bands of carbon fibres, 'single-crystal' graphite highly oriented powder graphite, and evaporated carbon measured at different take-off angles delta . The angular-dependent spectra permit a decomposition into their sigma and pi sub-bands. The intensity ratios of the sub-bands I_pi /I_sigma calculated as a function of delta and of the orientation of the crystallites are in good agreement with the values obtained from the experimental data for sigma >30 degrees . The sigma and pi -sub-bands of carbon fibres and graphite agree in their main features, indicating the close similarity of the electronic structure of both materials.

Experiments have used inelastic electron tunnelling spectroscopy as a method of measuring the phonon density of states of the tunnelling barrier. The magnitude of the measured d²I/dV² characteristic is proportional to the density of phonon states multiplied by a weighting factor, which depends on the strength of the interaction between the electron and the phonon. Without a knowledge of this weighting factor the experiments can only yield qualitative results. The authors present a study of the tunnelling process and derive a weighting factor which allows the phonon density of states to be deduced from the experimental data.

A formula for the frequency-dependent damping rate of long-wavelength surface polaritons in a crystal slab with small intrinsic damping is derived and analysed. Special attention is paid to the dependence of the damping upon the thickness of the slab. It is shown that surface polaritons in thin dielectric slabs at frequencies slightly higher than the TO phonon frequency have lifetimes which are several orders of magnitude longer than the lifetimes of surface polaritons in a semi-infinite dielectric. Their propagation length along the surfaces of an ideally flat slab is a few tens of metres. Similar conclusions also apply to low-frequency (infrared) surface polaritons in a thin metallic slab.

In bulk InSe crystals grown by the Bridgman method, electron accumulation layers are formed in the vicinity of stacking faults with charged impurities adsorbed to the defect plane. Shubnikov-de Haas experiments show that the 2D electron gas in InSe observed at low temperatures has a density of 10¹² cm^-2 and that several dielectric sub-bands are occupied. The g-factor is enhanced by electron-electron interactions. The optical absorption was studied in the frequency range 3-20 meV in perpendicular and tilted magnetic fields up to 8T. The cyclotron resonance at E_res=h(cross) omega _c, the intersubband resonance at E_res=E₀₁ and the combined resonance at E_res=E₀₁+h(cross) omega _c, are all observed for both parallel and tilted polarisation, where E₀₁ is equal to the sub-band separation as deducted from the Shubnikov-de Haas periodicities. With increasing magnetic fields the exciton-and depolarisation-shifted resonance E₀₁ approaches the true sub-band splitting E₀₁, and is the same time reduced in amplitude. Absorption due to bulk 1s-2p transitions in isolated shallow donors is also observed. The 2p level shows a three-dimensional Zeeman splitting (h(cross) omega _c)_3D with a weak anomalous mass anisotropy as observed in cyclotron resonance measurements of bulk carriers at temperatures of 30-100K. The effective electronic Rydberg is in agreement with results of exciton spectra.

The influence of disorder on the Hall resistance of a two-dimensional, degenerate electron gas is examined by studying the scattering of a single particle from an arbitrary potential in crossed electric and magnetic fields. The Hall current in a sufficiently large system is shown to be unaffected by 'islands' of disorder, and corrections in the case of small systems are discussed.

The authors compute exactly the eigenvalues of atomic clusters of arbitrary size with a simple cubic structure in the tight-binding approach (LCAO), including surface effects. To represent the electronic structure of the hypothetical metallic clusters they consider well separated and p states. They find strong deviations from the corresponding states of the bulk cubium where the number of particles is of the order of 10⁴ or less (R<or approximately=50 AA): new states appear near the top of the bands and states disappear near the bottom of the bands. This produces an upward shift of the Fermi level and of the mean energy with decreasing size and an asymmetric and deformed shape of the density of states. Effects of the geometrical shape of the cluster and of the environment are also considered. They discuss the relevance of their results on the interpretation of recent photoemission experiments on small metallic clusters.