Table of contents

The Mattis-Gallinar effect predicts that the scalar mass of an exciton depends upon its internal kinetic energy, and that the exciton mass may be larger than the sum of the masses of the electron and the hole. The author generalizes this effect to the tensorial case of a crystal structure which is not necessarily cubic. By assuming that the electron and hole share the same band structure, it is found that M_ij^-1=-(1/4) Sigma _RR_iR_jK_R where K_R represents an internal excitonic kinetic energy associated to the vector R of the crystal lattice, and M_ij^-1 is the ijth component of the inverse mass tensor of the exciton. Thus, if the exciton becomes localized in the sense that K_R to 0, the mass tensor M may become arbitrarily large.

Sub-micron split-gate structures have been fabricated on three pseudomorphic GaAs-In_xGa_1-xAs-AlGaAs quantum well structures, with indium fraction, x of 0.04, 0.10, and 0.14. The conductance characteristic of the device on the x=0.04 material showed clear plateaux due to the quantization of ballistic conduction in one dimension. Devices fabricated from the x=0.10 and x=0.14 material did not show plateaux, but exhibited a complex structure that varied quantitatively between nominally identical devices. This structure is thought to be due to both the random nature of the In_xGa_1-xAs alloy, and the roughness of the In_xGa_1-xAs interfaces. The spatial variation of the potential between the arms of the split gate due to these effects was probed by applying a differential voltage between the two gates.

The author explores the effect of carrier interaction with tail states on the mobility in amorphous silicon hydride for various model density of states profiles. Upon the assumption that multiple trapping of electrons dominates the transient response at short times, it is found that a shift in the demarcation energy, E_d*, between multiple trapping and tunnelling, to shallower energies can lead to an enhancement in the mobility for temperatures lower than 100 K.

The analogy between electromagnetic wave propagation in multidimensionally periodic structures and electron wave propagation in real crystals has proven to be a very fruitful one. Initial efforts were motivated by the prospect of a photonic band gap, a frequency band in three-dimensional dielectric structures in which electromagnetic waves are forbidden, irrespective of propagation direction in space. Today many new ideas and applications are being pursued in two and three dimensions, and in metallic, dielectric and acoustic structures, etc. The author reviews the early motivations for this work, which were derived from the need for a photonic band gap in quantum optics. This led to a series of experimental and theoretical searches for the elusive photonic band-gap structures, those three-dimensionally periodic dielectric structures which are to photon waves what semiconductor crystals are to electron waves. Then he describes how the photonic semiconductor can be 'doped', producing tiny electromagnetic cavities. Finally he summarizes some of the anticipated implications of photonic band structure for quantum electronics and the prospects for the creation of photonic crystals in the optical domain.

Raman spectra were measured for ethanol solid solutions of LiCl of various concentrations between 5 and 77 K. With increasing LiCl concentration, the O-H stretching and the intermolecular vibrational bands changed abruptly from broad bands to sharp peaks at a mole ratio n approximately=8 of EtOH to LiCl. This spectral change is explained by a phase transition from glass to a crystalline hydrate-like cluster. For the solutions at n<or=8, a marked spectral change was observed, at temperatures around 68 K, in the O-H stretching bands, and was ascribed to a lowering of the symmetry of the crystal structure due to thermal contraction on decreasing temperature. Other solid solutions of alkali halides also showed similar spectral changes with alkali halide concentration, except for methanol solutions, but none of them showed any spectral changes on decreasing temperature. These results imply that crystallization commonly takes place in rapidly cooled concentrated solutions of alkali halides, and that the ethanol solid solution of LiCl has a unique crystal structure which causes the spectral change with temperature.

The large deviation from the ideal mixture behaviour and the concentration-dependent asymmetry in the thermodynamic properties of Mg-Sn liquid alloys is investigated within a simple theoretical model of heterocoordination (i.e. a model in which there is a preference for unlike atoms to be paired as nearest neighbours). This has been utilized to extract additional microscopic information such as concentration fluctuations and the chemical short-range order parameter. The analysis suggests that heterocoordination leading to the formation of chemical complexes Mg₂Sn is likely to exist in the melt but is only of a weakly interacting nature. The interaction energies between the species of the melt are found to depend considerably on temperature and the alloy is chemically more ordered towards the Mg-rich end of the phase diagram.

The crystal structure of intermediate-valence (IV) samarium hexaboride has been studied on single-crystal double-isotope samples ¹⁵⁴Sm¹¹B₆ by X-ray diffractometry at room temperature and by high-resolution powder neutron diffraction in the temperature range 23 K<or=T<or=300 K. The X-ray experiment revealed the occurrence of vacancies at the boron site, larger thermal vibrations of the Sm atom than of La in the isostructural non-IV material LaB₆ and an aspherical charge distribution around the Sm nucleus. The neutron diffraction experiment confirmed the anomalous temperature dependence of the lattice parameter and revealed both a peculiar temperature dependence of the anisotropic thermal vibrations of the boron atom and a temperature-dependent change in the ratio of the isotropic thermal parameters of the Sm and B atoms. Thermal vibrations of the Sm ion can be satisfactorily described by the Einstein model with characteristic temperature Theta _E approximately=120 K within the whole temperature range. The data obtained are discussed in terms of the influence of the fluctuating valence of the Sm ion on the structural parameters of atoms.

The authors present detailed investigations of vibrational modes in a hierarchy of rational (or commensurate) approximants to icosahedral quasicrystals based on exact diagonalization of the dynamical matrix and recursion calculations of the vibrational spectrum. The results demonstrate the existence of well defined longitudinal and transverse acoustic modes with isotropic dispersion relations in the vicinity of quasiperiodically distributed special points in wavenumber space, the ' Gamma points' of the reciprocal quasilattice. Stationary eigenmodes are found around other high-symmetry points in reciprocal space corresponding to quasi-Brillouin zone boundaries. The authors show that strictly localized ('confined') modes exist and that their origin is a local topological frustration, i.e. in a local deviation from ideal icosahedral packing.

Using a method derived in a previous paper (see ibid., vol.4, p.1947 (1992)), information at g=0 about the pseudoatoms in the polar semiconductor GaAs is obtained from total-energy calculations. The latter were carried out using the Kohn-Sham equations within the local density approximation. The authors briefly review the method and the difficulties, particularly over the charges, of applying it to polar semiconductors. The linear dependence of the equations for the parts of the pseudoatoms at any given reciprocal lattice vector g is shown to arise because first-order terms can only have three independent equations whereas point group symmetry in general gives rise to four equations. The results suggest that the rigid ions are well-localized and well-behaved functions of position. Contour plots of the charge densities in the (110)-(001) plane are drawn. They show that most of the changes take place around the As ion and at the bond charges. The deformation takes place mainly at the bond charges and shows a charge transfer between the bond charges themselves; it also smooths out, very slightly, effects caused by the movement of the rigid ions.

Tomonaga's idea of describing the one-dimensional jellium model in terms of bosons is adopted for the three-dimensional case, but worked out in a completely different way. An algorithm is given for the construction of a boson Hamiltonian that takes into account the full dynamics of the jellium model at all densities. The algorithm is applied to a reduced form of the jellium model, which has the same ground state energy as the jellium model in the high-density limit. The resulting boson Hamiltonian is compared with Sawada's Hamiltonian, which also has this ground state energy in the limit of high density. Finally, the present boson formulation is discussed briefly.

Previous theories of inhomogeneous broadening by point defects of impurity transitions in crystals have invoked a continuum approximation for the defect positions. The authors generalize these theories by treating the crystal as a discrete lattice. At low defect densities they recover the results of the continuum theory, finding a Lorentzian lineshape. At high defect densities the lineshape is approximately Gaussian. At intermediate densities, the lineshape displays satellite structure due to different configurations of nearby defects. For all densities they derive approximate analytic expressions for the lineshape that are in good agreement with exact numerical results.

Low-temperature measurements of the conductivity, the Hall effect and the magnetoresistance have been performed on an n-GaAs sample of impurity concentration close to its critical value for the metal-insulator transition. The magnetoconductivity is analyzed in the context of the weak-localization theory, and the inelastic scattering time tau _epsilon is deduced from the same model in two different ways for each temperature. It is found that tau _epsilon varies as T^-1 in the whole temperature range. This behaviour is compared with the theoretical predictions. Discrepancies are found between the weak-localization theory and the experimental data for magnetic fields larger than 0.4 T.

The problem of the ground state of a V²⁺ ion in GaAs is discussed once more. A V²⁺ centre, labelled as V²⁺(II), with a ⁴T₁ ground state, has been observed previously by thermally detected electron paramagnetic resonance (TD-EPR) and identified as probably part of a complex. Further TD-EPR studies on GaAs:V are described here. They show a new line which is attributed to a V²⁺ centre having a ²E ground state in agreement with the theoretical prediction for the isolated substitutional V²⁺ ion in this material. All the TD-EPR results and previous phonon scattering investigations on GaAs:V and GaP:V are compared. They appear to confirm this hypothesis.

An analytic expression for the dynamical conductivity sigma ( omega ) of spinless fermions with local disorder and nearest-neighbour repulsion is derived on lattices with infinite coordination number Z. The model is exactly solvable in the whole parameter range assuming two possible phases: a homogeneous phase and a checkerboard charge-density wave (CDW). Away from half filling the system displays anomalous behaviour: weak particle-density fluctuations favour spontaneous symmetry breaking. First, the authors investigate the effects of this anomaly on the conductivity in the AC and DC regimes. Second, they focus on the Mott transition occurring at zero temperature at half filling. The critical exponents for the conductivity are computed for the dependence on the interaction U, the disorder gamma , the filling n, the temperature T and the frequency omega .

The high-field magnetization and temperature dependence of the critical field have been investigated for Laves-phase Lu(Co_1-xGa_x)₂ compounds. A clear itinerant-electron metamagnetic transition is observed and its critical field increases with increasing temperature for x=0.09. The results are discussed on the basis of the Clausius-Clapeyron equation for the magnetic phase transition. This reveals that the magnetic entropy of these compounds decreases above the critical field, suggesting the suppression of the spin fluctuations due to the metamagnetic transition. This is consistent with the recent results on the field dependence of the electronic specific-heat coefficient of these compounds. The Arrott plots for the specimen with x=0.12, which has a very low critical field, have also been investigated. The squared hypothetical spontaneous magnetization M_h² decreases linearly with increasing T² and the reduced M_h versus T plot for Lu(Co_0.88Ga_0.12)₂ is very similar to the reduced magnetization for an Invar-type ferromagnetic Lu(Cu_0.83Al_0.17)₂ compound. This result suggests that Lu(Co_0.88Ga_0.12)₂ in the ferromagnetic state is expected to exhibit marked magnetovolume effects.

Magnetic form factor calculations for the rare-earth ions, made by Balcar and Lovesey, have been tested experimentally, for the case of Sm³⁺ ions in Sm metal and SmPd₃, by using the neutron scattering technique. In the various experiments performed the ⁶H_5/2 to ⁶H_7/2 spin-orbit transition and the intermultiplet transitions ⁶H_5/2 to ⁶F_1/2, ⁶H_5/2 to ⁶F_3/2, ⁶H_5/2 to ⁶F_5/2 and ⁶H_5/2 to ⁶F_7/2 have been observed. The variation of the intensities of these peaks with the neutron momentum transfer is discussed with respect to the predictions of Balcar and Lovesey.

The conduction electron spin resonance (CESR) in two-dimensional (2D) electron systems (inversion layers of semiconductor heterostructures and quantum wells) without 'up-down' symmetry is considered. The pyroelectric-like symmetry of such a layer makes a difference between two normals to the layer and thus leads to the 2D-electron Hamiltonian which includes an additional spin-orbit term H_so=( alpha /h(cross))(p*c). sigma where c is the vector of one of the non-equivalent normals. Accurate quantum kinetic theory with regard for the spin-orbit energy is proposed for the first time, and the paramagnetic linear response is evaluated. It is shown that, if the CESR is excited by a wave of wavevector q, then the decay rate should include a term which reverses its sign with an applied magnetic field B reversal and, for B parallel to the plane of electron motion, is given by Gamma =(h(cross)/ tau )(3( alpha p_F tau /h(cross)²)²+¹/₂(v_F tau q)²+2 alpha mv_F² tau ²h(cross)^-2q.(c*B)) where tau is the mean collision time and v_F is the Fermi velocity. An estimate for the effect in some semiconductor heterostructures is presented, and possibilities of experimental observation are briefly discussed.

The size effect on the ferroelectric phase transition in PbTiO₃ ultra-fine particles is reported. Samples with particle sizes from 20 to 200 nm were prepared by a sol-gel process followed by calcining at different temperatures. The particle size was determined by X-ray diffraction from the integrated width of diffractions. The soft-mode frequency at room temperature was measured by Raman scattering. It decreases with decreasing particle size. The ferroelectric phase transition was traced by specific-heat measurement. The transition temperature decreases and the transition becomes diffused as the particle size decreases. The size dependence of T_C can be described by T_C(D)=766-256/(D-8.8) (K), where 766 K is the T_C for bulk PbTiO₃ and D (nm) is the particle size. This equation gives a critical size of 9.1 nm below which ferroelectricity disappears.

Investigates the influence of Cu²⁺ impurities on the luminescence properties of TMA₂MnBr₄:Cu²⁺ as well as the local structure and orientation of the CuBr₄^2- complexes formed, by means of the excitation and luminescence spectra, lifetime measurements and polarized optical absorption spectroscopy in the 10-300 K temperature range. It is demonstrated that the presence of an intense Br^- to Cu²⁺ charge transfer (CT) band at 555 nm strongly favours a direct energy transfer from Mn²⁺ to the non-luminescent Cu²⁺ impurities. The influence of this energy transfer on the Mn²⁺ luminescence intensity, lifetime and bandshape is analysed as a function of the Cu²⁺ concentration. The results are compared with previous ones obtained in one-dimensional Cu²⁺-doped TMAMnCl₃ and TMAMnBr₃ crystals. Two x, y-polarized bands at 18000 and 28400 cm^-1, and two z-polarized bands at 23800 and 36100 cm^-1 are observed in the CT spectra of TMA₂MnBr₄:Cu²⁺. Their transition energies as well as their polarization are explained in terms of D_2d symmetry distortions of the CuBr₄^2- tetrahedra. The authors also analyse the triplet structure observed in the first CT band which is associated with the tetrahedral ²T₁ CT state, which is split by the effect of both the static D_2d distortion and the large spin-orbit coupling of the Br^- ligands. The absence of discontinuities in the evolution of the CuBr₄^2- CT bands with temperature supports the finding that no structural phase transition occurs below 270 K in these crystals.

The valence state of Bi in pure and doped (Pb, K) BaBiO₃ has been determined by X-ray absorption near-edge structure (XANES) spectroscopy. In all materials the Bi valence is found to be close to IV. A disproportionation of Bi(IV) into Bi(III) and Bi(V) is not indicated by analysis of the XANES data. The valence state of Pb is also found to be IV in BaBi_1-xPb_xO₃.