Table of contents

The predictions of possible 2D patterns of corrugations (the 'islands') of the unstable flat phase boundary separating the pre-stressed solid substance and the melt are carried out on the basis of a quasi-static evolution. The dispersion relations for the increment of small disturbances are announced for the model of an isotropic solid of arbitrary thickness and in-plane pre-stresses. It seems that the predictions can be checked in various experiments mentioned by Nozieres (1990) and in particular, in experiments with ⁴He similar to those of Torii and Balibar (1992); also, they can be compared with numerous experimental data related to solid epitaxial films.

The phase diagram and magnetization curves of a ferrimagnetic mixed spin-¹/₂ and spin-³/₂ Ising system are examined by the use of the effective-field theory with correlations. Some outstanding features are found in the temperature dependences of total and sublattice magnetizations.

The peak drift velocity v_p in the Esaki/Tsu miniband conduction of a superlattice is investigated theoretically as a function of the lattice temperature. In contrast to the existing Boltzmann-type theories, the balance-equation calculation predicts a noticeable saturation of v_p at low lattice temperatures, in agreement with the recent experimental observation by Sibille et al. (1992).

Measurements are reported of the Auger (autoionization) spectrum of Ne produced by bombarding Al and Si surfaces with Ne⁺ ions with energies in the range 400 eV to 5 keV. The shift in the Ne peak energies with incident ion energy is shown to follow a very simple Doppler model. The data are found to contain many small Auger peaks in addition to the two characteristic peaks recorded by previous workers. This new structure is shown to be consistent with gas-phase data and with measurements of the autoionizing states in Ne I.

Long-wavelength optical modes of lattice vibrations are studied and a Frohlich-like Hamiltonian of the interaction between an electron and the interface optical phonons is derived. Interesting properties of the interface optical modes and their coupling with electrons are found. The numerical results for the dispersion relations, eigenvectors and coupling functions of the interface optical phonons in several practical systems are obtained and discussed. It is shown that the interface optical modes should be taken into consideration in theoretical investigations of electro-phonon scattering in heterostructures.

Structural relaxations near surfaces and interfaces are analysed in a simple, generic model with first-, second- and third-layer interactions. The relaxations have exponential envelopes with three types of structural distortion: (i) ferrodistortive, (ii) antiferrodistortive, and (iii) modulated (incommensurable). Their stability conditions in the field of control parameters and their relationship with structural phase transitions is derived. A tricritical point is found for three-layer interactions and a transition to an incommensurate phase. All phase transitions to the ferrodistortive phase are first-order in the model. The theory is applied to the analysis of lattice relations near internal interfaces in ionic polytypic materials (e.g. PbI₂). Diffuse X-ray scattering and the shift of diffraction angles are the typical fingerprints for such relaxations. The relevant structure factors are calculated. There is tentative agreement between the calculated and observed diffraction profiles.

The interfacial reactivity of the rare earth metals Yb and Pr deposited on ultra-thin layers of SiO₂ on Si(111) substrates under UHV conditions is investigated using the Auger spectra of silicon, oxygen and the rare earth metals to monitor chemical changes. At room temperature exposure to approximately 10 AA of rare earth metal leads to reduction of the SiO₂ to form rare earth oxide and rare earth silicide, while at elevated temperature reaction to form a silicate phase is demonstrated.

The methods used for formation and observation of HR (highly excited Rydberg) states of alkali are here adopted for the study of hydrogen HR states. Electronically highly excited states are formed in a hydrogen-filled diffusion source with a graphite foil as emitter at high temperature. The excited states are detected by electrostatic methods, mainly by their interaction with a well shielded collector. Excited negative hydrogen and/or alkali ions are formed at the first grid in the analysing device, and give emission of electrons at the collector. The flux from the source can be cut off instantaneously with a beam flag, which reveals time constants of the order of minutes in the signal. It is concluded that surface layers of excited states are formed on the electrode surfaces. A condensed phase of electronically excited so-called Rydberg matter was predicted by Manykin et al. (1981, 1982) and reported by Aman et al. (1990) and Svensson et al. (1992) Aman et al. (1992) later reported on non-alkali Rydberg matter. A similar type of matter appears to be found in the present study.

The authors have investigated the electronic structure of the rare-earth metal praseodymium using angle-resolved UV photoemission from Pr(0001) and compared the results with band-structure and first-principles photocurrent calculations. Normal-emission valence band spectra in the photon energy range 20-50 eV are dominated by emission from points in the Brillouin zone with a high density of states. The binding energies of the critical points at Gamma are found to he 2.5+or-0.1 eV and 3.6+or-0.1 eV, in good agreement with bulk band-structure calculations. Off-normal-emission spectra show considerable dispersion and are discussed.

By using the transfer-matrix method, the authors have studied the interface plasmon modes of an infinite-layered two-dimensional electron-gas system, in which all the layers are arranged periodically except for one region of quasi-periodic layers mapping the rule of the Fibonacci sequence. It has been found that with the removal of translational symmetry, the spectrum of localized interface plasmon modes, both acoustic-like and optic-like, becomes very rich. The dispersion relation of the interface plasmons is presented in terms of the transfer-matrix elements, and a couple of critical wavevectors is also found.

The author calculates the scattering rates for intrasubband and intersubband transitions in GaAs-AlAs quantum wells due to interface phonons with an applied longitudinal electric field. The electron-interface-phonon (Frohlich) Hamiltonian used is that obtained from the Fuchs-Kliewer slab model, and the electron envelope wavefunction under the influence of an electric field parallel to the growth direction is obtained by a variational method. The usual selection rules for these transitions break down and the scattering rates are found to increase significantly when an electric field is applied. These scattering rates may even become the dominant scattering mechanism for large quantum wells and sufficiently high fields. He observes also that this change in scattering rates has an important dependence on the interface-phonon dispersion.

Some effects of ion bombardment on the growth of non-hydrogenated amorphous silicon films were studied using samples prepared with low-energy ion-beam-assisted deposition (IBAD). Inert gas ions were used to determine the effects of the ion beam on pure amorphous silicon without the complication of simultaneous chemical effects. The ion bombardment was found to increase the film density, improve film stability, increase the effective conductivity activation energies and to move the position of the infrared absorption peak assigned to silicon-carbon bonds to larger wavenumbers. The largest effective conductivity activation energies and the lowest conductivities were obtained from films made with Ar⁺ ions and bombarding energies around 100 eV. However, these 'best' films still do not have a sufficiently low defect density to be useful semiconductors. The improvements are consistent with the thesis that densification of the growing film is the primary effect of the beam. However, for the conditions used, the densification appears to saturate, leaving a relatively large number of dangling bonds in the films. These may result from the presence of Ar known to be in the films, although the results are consistent with the topological model which suggests that the best conditions for forming covalently bonded solids are obtained when the number of bonding constraints is equal to the number of degrees of freedom.

Exact-diagonalization calculations have been performed to study the electronic structure of d band metal surfaces with specific applications to a Ni(001) surface. Results of local-density approximation band-structure calculations are taken as the starting point. A realistic surface model of d electrons has been constructed in a tight-binding scheme, with the strong short-range d-d correlation fully taken into account in a periodic small- cluster approach. Calculated results show some interesting features induced by many-body effects. The implications and consequences of the results are discussed.

The scattering of hydrogen from metal surfaces is of interest in nuclear fusion research for two reasons: (i) the plasma-wall interaction and (ii) the production of intense neutral hydrogen beams for refuelling and heating. The Ni-H system is also of basic interest in surface chemistry and physics for reasons of hydrogen storage, catalysis and fuel cells. Studies of hydrogen interaction with surfaces are proceeding and a discussion follows as to the mechanism of dissociation of the incident molecules.

The exchange coupling J(l) between magnetic layers across a non-magnetic spacer is observed to oscillate as a function of the spacer thickness l. In an earlier work a theory of the oscillatory exchange was proposed which shows that the oscillation periods are characteristic of the spacer. The theory relied on the assumption of an infinitely large exchange splitting in the magnetic layers, which leads to complete confinement of magnetic carriers of one spin in the ferromagnetic configuration of the sandwich. While this may be valid for strong ferromagnets such as Co or Ni, the complete confinement model is not a realistic approximation for iron which has holes in its majority-spin band. The theory is now generalized to the case of partial confinement of carriers in the spacer appropriate to a sandwich with weakly ferromagnetic layers. An exactly solvable hole-gas model of the coupling as well as numerical tight-binding results are presented which demonstrate that the oscillation period is unaffected but the amplitude and phase depend critically on the degree of confinement. Asymptotic expansions for J(l) valid at finite temperature and for an arbitrary single tight-binding band are also obtained. They show that the period and temperature dependence of the oscillations are directly related to the properties of the spacer Fermi surface but the amplitude and phase depend also on the exchange splitting in the ferromagnetic layers.

(Fe-Si)/Pd multilayered films with different layer thicknesses have been obtained by RF sputtering. When the thickness of the Pd layers is fixed, the specific saturation magnetization M_s of films increases with decrease in the thickness of Fe-Si layers. This result is caused by the polarization effect of Pd atoms. On varying the thickness d_p of the Pd layers, with a fixed Fe-Si layer thickness, it is found that for Pd layers thinner than 36 AA the polarization effect of Pd is reduced and even disappears at d_p=18 AA; then magnetic coupling between the magnetic layers appears on further decrease in d_p.

The superhyperfine (SHF) interactions of Fe³⁺ impurities with five shells of Li neighbours were measured in a nearly stoichiometric LiNbO₃ crystal by applying electron nuclear double resonance (ENDOR). Similarly to what was found for LiTaO₃, the site of Fe³⁺ in LiNbO₃ has a threefold symmetry. The analysis of the ENDOR spectra clearly shows that Fe³⁺ is on a substitutional Li⁺ site. The variations of shape and intensity of ENDOR lines belonging to specific ligand nuclei when rotating the crystal in the static magnetic field are attributed to local disorder.

Both the temperature and the pressure dependences of the b₄⁰ spin-Hamiltonian parameter for RbCaF₃, CsCaF₃, RbCdF₃ and TlCdF₃ doped with Gd³⁺ have been reported. On the assumption that b₄⁰ depends on the metal-ligand distance according to a law b₄⁰ approximately r^k the following values of k were obtained: -20.9, -27.1, -24.9 and -29.3, respectively. It was found that the temperature dependence of b₄⁰ is predominantly caused by spin-phonon interactions which were estimated to contribute about 68% to the total dependence observed. The effectiveness of some theoretical models predicting the role of vibronic contributions is also discussed.

The integrated intensities of the absorption lines due to local vibrational modes of defects of T_d and C_3v symmetry are calculated as a function of the parameter measuring the strength of the coupling between the mode and the electric field associated with the light beam. In the case of C_3v symmetry defects, the derivation does not assume this parameter to be the same for the longitudinal and transverse modes. The model is applied to the centres resulting from the neutralization by hydrogen of shallow dopants in crystalline semiconductors.

The authors report neutron scattering observations of the stabilization of the locked-in phase with spiral pitch of 1/4c* in holmium by a magnetic field applied along the b-axis. In contrast to a theoretical suggestion that stabilization of this phase by a c-axis field was due to imperfect alignment causing a small component of field in the basal plane, it is shown that the effect of a field of 3 T along the b-axis is to stabilize the phase over a somewhat smaller temperature range than the stabilized range in an identical field along the c-axis. The centre of the locked-in phase moves from 96 K in zero field to 103 K in a b-axis field of 3 T, while in a similar c-axis field the phase remains centred at around 96 K. The authors also observe a locked-in phase at a wave-vector of 5/18, in the temperature range from 125 K to the Neel transition.

Low-frequency light scattering spectra of the incommensurate phase of quartz show the presence of weak additional peaks in the 6-12 cm^-1 range. These peaks are shown to correspond to 'folded' acoustic phonons with a wavevector close to the incommensurate modulation wavevector. A transverse acoustic mode and a longitudinal mode can be identified in the (ZZ) and (X+Y, Z) polarization configurations, respectively. Selection rules for Raman activation of acoustic modes are derived in the special case of the 'triple-q' incommensurate structure, and the results are compared with experimental observations.

The authors have studied a random dimer model using the transfer matrix method and used the Landauer formula to compute conductances of large dimer chains as a function of the Fermi energy of the incident electron. They find very interesting resonance structures in this system around two special energies, namely the site energies (taken from a binary distribution). The localization length around these energies is found to diverge quadratically as a function of the energy difference as one approaches one of these energies from either side. It is also found that the conductance vanishes for this random dimer chain at almost all the allowable energies, just as it does in the case of a random 'monomer' chain. Thus a localization-delocalization transition (and more certainly a mobility edge) is absent in this system as opposed to what is claimed in some recent articles.

The Hubbard model in the limit of strong Coulomb interaction (the (t-J) model) is studied by the diagram technique for Hubbard operators. The generalized random-phase approximation (GRPA) is formulated as an approximation taking into account electron loop-type diagrams. Within the framework of this approximation, both the dynamic magnetic and dielectric susceptibilities are calculated for a wide interval of electron concentrations, 0<n<1. The magnetic susceptibility shows a crossover in the magnetic behaviour of the system from pure itinerant magnetism to magnetism with localized magnetic moments. The crossover occurs at a critical point, n_c, when the chemical potential for electrons of the lower Hubbard band changes sign. Magnetic phase diagrams are constructed on the (t/U, n) plane at T=0 for different types of crystal lattices. The behaviour of magnetic phases with temperature is also studied. The GRPA leads to a too drastic crossover, especially at low temperatures, because of insufficient allowance for the charge and spin fluctuations on a site. Summation of special diagram series shows that the Gaussian fluctuations of the effective electric and magnetic fields lead to a smoother picture of the crossover.

A semiempirical local approach to the problem of electron correlation at localized defect states in solids is presented. The approach modifies any quantum-chemistry scheme by adding a certain correction to the Fock matrix, thus permitting one to obtain self-consistent correlation corrections to all ground-state parameters of the defect. The theory has been applied to the large-unit-cell models of the dangling-bond defects Si:V⁰ and Si:(VH₃)^-. The results of the calculations are useful for investigation of negative-U properties.

The energy relaxation of a photogenerated electron-hole plasma is investigated in highly excited CdS at room temperature using time-resolved luminescence in the picosecond regime. It is established that the non-equilibrium carrier excess energy dissipation process exhibits a two-stage pattern with increasing excitation density. These stages reflect the sequential process of anharmonic decay of the non-equilibrium phonon system generated during plasma cooling. Experimental data are explained in terms of the kinetic model, taking into account two generations of non-equilibrium phonons. A high excitation level is shown to be a crucial condition for the manifestation of a second-generation phonon bottleneck in hot-carrier energy relaxation.

The resistivity and Hall coefficient of single-crystal UNi₂Si₂ have been studied in detail for the temperature range 4.2-300 K. The resistivity of UNi₂Si₂ is largely due to magnetic scattering and the phonon scattering contribution is estimated to be about 14% at room temperature. At low temperatures, the resistivity can be described by a gapped spin-wave model plus a T² term. The temperature dependence of the Hall coefficient is accounted for by a theoretical model invoking skew scattering of conduction electrons by localized magnetic moments. Among the three magnetic phase transition temperatures, the two lower ones are found to be magnetic field dependent and shift with the field applied along the tetragonal c axis. Using the resistivity measurement in an applied magnetic field, a field-temperature phase diagram of UNi₂Si₂ is presented.

The leading non-static contribution to the effective potential in a RVB (resonating valence bond) phase for the t-J model is characterized in terms of a canonical integral. It is shown that a Chern-Simons term does not arise from such an integral at low doping; however, at higher doping this integral suggests that such a term could appear.

The magnetic structure of TiFe₂ which crystallizes with the C15 Laves phase structure has been determined from neutron diffraction measurements made on both single-crystal and polycrystalline samples. In the antiferromagnetic structure the Fe2 atoms in the (6h) sites with the same z coordinate are coupled ferromagnetically with the moments in successive layers alternately parallel and antiparallel to the c-axis. The Fe1 atoms which are in the (2a) sites are at an anti-centre of symmetry and carry no ordered moment. Magnetization measurements support this structure, giving a positive paramagnetic Curie temperature and showing the metamagnetic transition characteristic of ferromagnetically coupled sheets. Polarized neutron measurements indicate that in the antiferromagnetic phase the two types of Fe atom contribute almost equally to the susceptibility and suggest that the magnetic moment on the Fe1 atoms is only induced by the molecular field due to the aligned moment on the surrounding Fe2 atoms.

The magnetic structures previously proposed for the low-temperature antiferromagnetic phase (AF1) of the intermetallic compound Mn₅Si₃ have been revised in the light of the results of neutron polarimetry in conjunction with unpolarized-neutron single-crystal integrated-intensity measurements. At 4.2 K the commensurate magnetic unit cell is orthorhombic with a=6.889, b=11.901 and c=4.805 AA and it is related to the hexagonal cell of the paramagnetic phase by b approximately= square root 3a. The hexagonal cell contains four Mn1, six Mn2 and six Si atoms. The non-collinear magnetic structure of the AF1 phase is stable below 66 K and has monoclinic symmetry. At 4.2 K the Mn1 atoms have moments of 1.20(5) mu _B inclined parallel and antiparallel to the polar direction theta =116(1) degrees , phi =105(1) degrees , where theta is measured from (001) and phi from (010). One third of the Mn2 atoms carry no moment, one third have moments of 2.30(9) mu _B at +or-( theta =70(1) degrees , phi =93(1) degrees ) and the remainder have 1.85(9) mu _B at +or-( theta =21(1) degrees , phi =11(7) degrees ). The non-collinearity is attributed to topological frustration and the wide variation in the magnetic moments to the combined effects of Mn moment instability, frustration and single-ion anisotropy.

The authors have measured the ultrasonic elastic constants C₃₃, C₄₄, C₁₁ and C₆₆ and their associated attenuation coefficients ( alpha _ij) for a single crystal of erbium as a function of temperature. They have observed anomalous behaviour in both C_ij and alpha _ij which they attribute to commensurate spin-slip structures as observed by Gibbs et al. and Lin et al. using synchrotron X-ray scattering and neutron diffraction techniques.

The authors have carried out a neutron diffraction study on a single crystal of Gd₆₆Y₃₄ to examine the behaviour of the helical q-vector close to the helix-ferromagnet II transition. A novel approach to the peak fitting of the diffraction peaks has enabled inter-layer turn angles of less than 1 degrees to be measured, establishing strong evidence to support the proposition that this is a continuous transition. The authors have not observed a modulated c-axis component of magnetization in the helical phase and therefore have no evidence of threefold symmetry in this system.

Magnetization of a single-grained quasi-crystal of nominal composition 66.32 at.% Al-20.41 at.% Cu-13.27 at.% Fe was investigated in the temperature range from 2 to 250 K and for magnetic fields below 5 T along fivefold, threefold and twofold axes. No anisotropy was found, which is consistent with the symmetry consideration based on the symmetry of the diffraction spots. Electrical conductivity along the twofold axis showed a minimum at around 20 K and a positive longitudinal magnetoresistance at low temperatures. The conductivity is discussed on the basis of the weak-localization theory.

The authors report the experimental results of electron spin resonance (ESR) studies made on K₂Cu_1-xCo_xF₄, a randomly mixed system of the two-dimensional Heisenberg ferromagnet K₂CuF₄ and the two-dimensional Ising antiferromagnet K₂CoF₄ which have orthogonal anisotropies to each other. From both the temperature and the angular dependence of the resonance field at low temperatures, three different kinds of ferromagnetic phase are confirmed, i.e. XY, oblique and Ising types, which coincides well with the results obtained by neutron scattering and susceptibility measurements. Critical behaviors of both linewidth and resonance field are also reported and discussed. In addition, it is reported that the temperature dependence of the linewidth and the resonance field of the sample on the Co concentration of the spin-glass region show a common behaviour generally observed in many other spin glasses.

EPR measurements have been made at room temperatures on a trigonal V²⁺ centre in CsMgCl₃ that was analysed as a substitutional centre by McPherson et al. to second-order in perturbation theory. The spectrum has been described by direct diagonalization of a 32*32-dimensional spin-Hamiltonian matrix using the computer software developed originally by the authors. The anisotropic parameters g/sub ///-g_{perpendicular to} and b₂⁰ have opposite signs to each other contrary to the expectation from simple electrostatic theory. By rotating the sample while keeping the c-axis normal to the external magnetic field, signal intensities of low- and high-field fine-structure lines interchanged characteristically. From theoretical calculation the intensity variation has been ascribed to the variation of the transition probability. The intensity variation is considered to be the elementary process for the resonant field shift observed in magnetically concentrated materials. The uniaxial direction of a trigonal or tetragonal centre in a microwave cavity can be known from this type of intensity-variation measurement irrespective of the sign of the fine-structure parameter b₂⁰.

Mossbauer absorption spectra have been recorded for single-crystal samples of the antiferromagnetic compound (NH₄)₂FeCl₅.H₂O in applied magnetic fields up to 10 T. A reorientation of the Fe ion spins from the crystal a axis to the c axis was observed. This transition was found to be a continuous and planar rotation at fields around 2 T. Detailed fitting of the spectra in the spin-reorientation region indicated some inequality between the magnetic hyperfine fields associated with oppositely directed ionic spins.

The authors present the polarized far-infrared and Raman spectra of Bi₂CuO₄ single crystals at 300 and 10 K in the spectral range from 30 to 650 cm^-1. All infrared-active modes (10E_u+5A_2u) as well as 21 Raman-active modes were observed. At low temperatures, for x'y' and x'z polarizations, new structure appears which the authors assigned as a two-magnon mode. The assignment of the observed vibrational modes is given according to Cartesian symmetry coordinates and a preliminary force constant calculation on the basis of a rigid-ion model.

The X-ray diffuse scattering intensity was measured at room temperature from disordered Cu-Pd alloys containing 8.0, 9.5, 13.0, 21.8, 28.5 and 42.0 at.% Pd. Twofold and fourfold splittings of diffuse scattering due to short-range order (SRO) were observed at 100, 110 and their equivalent positions respectively from alloys with more than 13.0 at.% Pd, where the separation of the split diffuse maxima increases monotonically with increasing Pd content. No splitting of diffuse scattering was observed for alloys containing 13.0 at.% Pd or less. The diffuse intensities measured in Laue units increase with increasing Pd content except for 42.0 at.% Pd alloy, which shows a much lower intensity compared with that for 28.5 at.% Pd alloy. The SRO parameters were determined from all the six alloys. One representative set of the local atomic arrangements of Pd atoms in the FCC lattice which has been derived from computer simulation of the observed SRO parameters is shown. The authors have calculated the Pd-Pd atom pairs of the first- and second-nearest neighbours for all the six alloys, in both the random and the SRO case. Although the ordered structure for Cu-42.0 at.% Pd alloy is of the CsCl type, no different feature in the SRO structure was observed for this alloy in comparison with the other alloys.