Table of contents

We have discovered that Si is able to stabilize the structure of the 2:17-type carbides. The carbides, R₂Fe₁₅Si₂C_x (R=Sm and Er), were made by heating powders of R₂Fe₁₅Si₂ in CH₄ at 700 degrees C. For Sm₂Fe₁₅Si₂C_2.3, the Curie temperature is 630 K and the anisotropy field is estimated to be over 70 kOe at room temperature; however, the saturation magnetization is relatively low, only about 88 emu g^-1, also at room temperature.

Neutron diffraction experiments over the momentum transfer range 0<Q(AA)^-1<40 have been used to investigate the structural properties of liquid sulphur from 140 degrees C up to 300 degrees C, i.e. from conditions near the melting point of sulphur up to temperatures well above its lambda -transition (T_lambda =159 degrees C), and to study how the structure of sulphur rapidly quenched from different liquid temperatures changes in relation to the corresponding liquid structure. The data have been Fourier transformed to give an accurate pair correlation function g(r) from which absolute numbers and distances of atoms in given molecular configurations may be obtained. The results indicate which molecular bonds are changed in the different solid and liquid states and shed new light on the lambda -transition from a molecular point of view. In particular we show that the number of nearest neighbours probably increases with temperature above the lambda transition. The data on g(r) are also interpreted with recent reverse Monte Carlo calculations.

The local structures of molten ZnCl₂, RbCl and Rb₂ZnCl₄ are measured with the EXAFS technique. Several models are used to analyse the data, and some structural parameters as well as the radial distribution functions of the nearest-neighbour atoms around Zn or Rb atoms are obtained. The tetrahedra consisting of four Cl atoms around Zn are found to be dominant and stable in molten ZnCl₂. The skewed behaviour of the radial distribution function for Rb-Cl in molten RbCl is also found, while the local structures around Rb can be regarded as a persistence of the solid state but with strong short-range disorder. In molten Rb₂ZnCl₄, a network linked by Zn Cl₄ groups is not found and the nearest-neighbour structures around Rb atoms are more disordered than in molten RbCl. This has also confirmed that the EXAFS method can provide reliable information about the nearest-neighbour atom-atom coordination for some molten salts, which is helpful in specifying the painwise interaction potential.

The wetting transition in a closed system is analytically discussed based on the Sullivan model. The wetting transition is first order for the finite system, while it becomes of second order for the infinite one. In addition, the temperature T_W of the wetting transition becomes lower as the separation of the two walls decreases.

We have measured the thermopower, S, of pure Gd from about 300 degrees C to 1375 degrees C, well above the melting temperature of 1312 degrees C, special care has been taken to remove H impurities from these samples. S_solid(T) is similar to S for the light trivalent rare earths; evidence of a solid-solid phase transition is seen at about 1285+or-10 degrees C. S_liquid=-4.3+or-0.5 mu V degrees C^-1, and there is no apparent change in S across the melting temperature. S_liquid is compared with calculations via several theoretical models, including the new Bachelet-Hamann-Schluter pseudopotentials.

The temperature dependences of the elastic constants and thermal expansion of a Cr-Al-alloy single crystal containing 2.2 at.% Al are reported. This is a critical concentration, being simultaneously close to the very steep incommensurate-commensurate spin-density wave transition line on the magnetic phase diagram as well as to the triple point, where the incommensurate and commensurate spin-density wave states coexist with the paramagnetic state. Very large magnetic contributions at 0 K of up to 28% for c_L= 1/2 (c₁₁+c₁₂+₂c₄₄), 24% for c₁₁ and 48% for the adiabatic bulk modulus, B, were observed, while the magnetic contributions to the shear elastic constants, c₄₄ and 1/2 (c₁1-c₁₂), amount to less than 1% at 0 K. The magnetovolume, the longitudinal elastic constants as well as B vary linearly with temperature over a large temperature range, This cannot be explained by existing theories. The absolute value of the zero-temperature magnetic Gruneisen parameter is exceptionally large, reflecting the much larger magnetic contribution to B than to the volume of the crystal at 0 K.

Different shell models for Nd₂CuO₄ reproduce the phonon dispersion curves comparably well. Even though the phonon frequencies are practically the same in the different models the phonon eigenvectors and thus the phonon spectra depend very sensitively on the shell model parameters. In this paper we compare theoretical neutron and synchrotron one-phonon scattering cross-sections.

Structural phase transitions are considered in which the effective ordering interaction J(R_ij) arises from local stresses induced by the ordering in cell i and propagated elastically to a distant cell j. The generalized Landau free energy functional is set up and four applications made. Firstly the origin of metastable tweed microstructures is shown to lie in the dense medium of strong embryonic tweed-like microdomains existing as fluctuations at temperatures as high as 2T_c and above. Secondly the tweed-like pattern originates from the very anisotropic interaction associated with domain walls, and four cases are distinguished. Thirdly it is shown that the Landau-Ginzburg theory of the width and shape of a domain wall can be carried over in some cases but not others. Fourthly the magnitude of critical fluctuations is considered together with corresponding corrections to the Landau theory of the phase transition in the four cases.

A complete set of helically symmetric wave functions is constructed and is used to set up the Bloch-like states for describing the electronic band structure of nanoscale systems with helical symmetry. Various helical arrangements leading to different graphitic tubules may be obtained by rolling the two-dimensional honeycomb lattice into a cylinder subjected to appropriate periodic boundary conditions. A discussion of the symmetries of these helices is given. This method is applied to study the electronic structure of graphitic tubules and is compared with other methods found in the literature.

The Fermi surfaces of bismuth single-crystal samples doped with tin and lead have been investigated by means of the techniques of ultrasonic quantum oscillations (UQO) and Shubnikov-de Haas (SdH) oscillations. Experiments were carried out in the temperature range 1.2-4.2 K using longitudinal ultrasonic waves of frequencies up to 70 MHz and in magnetic fields up to 2.3 T. The excess hole densities of single-crystal samples obtained from the analysis of UQO and SdH oscillations data, residual resistivity ratio (RRR) and Hall-effect measurements are in the range of 3*10²³ to 1.4*10²⁵ m^-3. The Fermi-surface geometry of carriers was determined from the angular dependence of the periods of UQO and SdH oscillations. The major effect of doping of bismuth with Sn (or Pb) is to decrease the Fermi energy. On increasing the amount of doping, the pockets in the L-point conduction band became progressively smaller. Further increase of the dopant concentration leads to the disappearance of the L-electron pockets and the appearance of L holes. The experimental results have shown that the Fermi surfaces of acceptor-doped bismuth crystals are similar to those of pure bismuth and that the L_v valence band is the mirror image of the L_c conduction band.

We consider an integrable model consisting of two one-dimensional parabolic bands of opposite mass (m=+or- 1/2 , respectively) separated by a gap, 2 Delta . The bands contain spinless fermions, one band corresponds to the conduction band and the other one to the valence band of a semiconductor or semimetal. The holes in the valence band and the spinless electrons in the conduction band are locally attracted via a delta -function potential. This model can be mapped onto the two-component Fermi gas with delta -function interaction, so that the two components label the bands: the chemical potential corresponds to a magnetic field, and Delta to the chemical potential. We use Gaudin and Yang's exact solution of the many-body problem to study the formation of exciton bands. The properties of the ground and metastable states, the excitation spectrum and the thermodynamics of the model are obtained. In the ground or metastable states the particles (electrons in the conduction band and holes in the valence band) are either paired in exciton bound states or unpaired. Their spectrum of elemental excitations is approximately parabolic. At finite T many-particle bound states (string solutions of the Bethe ansatz equations) can be populated; at low T these states are strongly delocalized and can be interpreted as electron-hole droplets. The low-T properties of the model are discussed.

Based on the method previously proposed by the authors, the crystal-field analysis and investigation of the polarization and relative intensities of transitions between the sublevels ⁴F_3/2 to ⁴I_13/2, ⁴I_11/2 and ⁴I_9/2 in Nd³⁺:YVO₄ crystals have been performed, in which the Nd³⁺ ions occupy positions with point symmetry D_2d. The predicted polarization characteristics of all the emission lines under discussion are in complete agreement with those observed in experiments by other authors, and the emission lines with highest or comparatively high calculated intensities correspond to the laser emission lines that have been realized. In these calculations, the effect of seventh-order odd crystal field on the emission intensities was first omitted and then estimated by the method suggested by Judd in 1962. It turned out that this effect should be taken into account explicitly and cannot be completely made up only by multiplying a factor suggested by Judd. However, the simple method introduced can be seen as useful in the study of new laser crystals.

The transient drift-velocity response of miniband transport to suddenly turned-on step electric fields is analysed for semiconductor superlattices with and without carrier transverse confinement. Our study is based on the recently developed balance-equation theory, which takes full account of the carrier occupation of the transverse subbands and the realistic electron-impurity and electron-phonon scatterings in the system. Up to 21 transverse subbands are included for two-dimensionally confined superlattices, and four lateral subbands for one-dimensionally confined ones. We find that thermal excitations of the carrier transverse movement suppress the Bloch oscillation. In a high-carrier-density system where carriers are thermalized rapidly in both longitudinal and transverse directions, the Bloch-type oscillatory velocity response, anticipated from purely one-dimensional Esaki-Tsu and Boltzmann theories, can appear only in extreme two-dimensionally confined system.

Some aspects of quantum tunnelling of a composite particle are clarified by a simple model. We calculate the probability that a couple of point particles bound to each other by a square well potential with a hard core tunnel through a rectangular potential barrier. It is shown that resonance structures appear in the transmission spectrum in some cases, which reflects the existence of quasi-bound-states of the centre of mass motion around the potential barrier. It is also shown that an inelastic tunnelling occurs in which the transitions in the relative motion are induced by the tunnelling of the centre of mass.

A possible effect of spin fluctuations on the electric field gradient in Cu-O layers in HTCS is studied by the phenomenological antiferromagnetic Fermi-liquid theory of Millis et al. Our theoretical analysis suggests that the coupling between the quadrupole charge density and the spin fluctuations in the two-dimensional 'metallic' Cu-O layers is the origin of the temperature dependence of ⁶³Cu NQR frequencies at the Cu(2) sites in YBa₂Cu₃O_{7- delta} .

The two-dimensional t-J model at low electron densities is unstable against various forms of electron pairing at low enough temperatures. At parameter values J/t<<1, the leading instability, although only at very low temperature, is to p-wave pairing similar to the Hubbard model. At values of J/t>1, an instability against d-wave pairing sets in at a higher temperature as found numerically by Dagotto et al (1993). In addition, at values of J/t>2, which are beyond the threshold for a bound state in the low-electron-density limit, a region of predominantly s-wave pairing is found.

It is shown analytically that the critical exponents for the 3D Ising model from a renormalized Hamiltonian treatment, given by Girvin, of the thermal averages appearing in an approximate difference equation formulation, are exactly those of the spherical model ( gamma =2, alpha =-1). These are compared to the values computed by Girvin: gamma approximately=1.78, alpha approximately=0.11, which do not satisfy scaling. It is also noted that the behaviour w( lambda ) approximately=W(1)+O(1- lambda )^1/2 for lambda approximately=1^- is reproduced by the Chadi-Cohen method of computation of reciprocal lattice sums only if the criterion q_i²<<1- lambda is met, where q_i are the smallest special points brought in at the order of approximation considered.

We reconsider the results concerning the extreme-quantum S=1/2 square-lattice Heisenberg antiferromagnet with frustrating diagonal couplings (the J₁-J₂ model) drawn from a comparison with exact-diagonalization data. A combined approach, also using some intrinsic features of the self-consistent spin-wave theory, leads to the conclusion that the theory strongly overestimates the stabilizing role of quantum fluctuations with respect to the Neel phase in the extreme-quantum case S=1/2. On the other hand, the analysis implies that the Neel phase remains stable at least up to the limit J₂/J₁=0.49, which is larger than some previous estimates. In addition, it is argued that the spin-wave ansatz predicts the existence of a finite range (J₂/J₁<0.323 in linear spin-wave theory) where the Marshan-Peierls sign rule survives the frustrations,.

Reorientations of Fe³⁺ spins have been studied in TbFeO₃ single-crystal absorbers by Mossbauer spectroscopy. At low temperature we observed two closely spaced changes of direction of the Fe³⁺ spins. Below 6.5 K the spins moved from the a towards the c orthorhombic axis with decreasing temperature. This movement was reversed at approximately 4.2 K and the movement back to the a axis was complete at 3.5 K. The maximum deviation from the a axis was about 55 degrees and somewhat dependent on crystal stress. These rotations are attributed to the temperature dependence of the Fe-Tb and Tb-Tb interactions.

The dielectric response (DR) of centrosymmetric (space group, Ibam, at 293 K) Na_4.6FeP₂O_8.6F_0.4 single crystals has been studied in a wide temperature range 290-700 K and frequency interval 5-5*10⁵ Hz. The DR reveals dipolar features. The corresponding dielectric relaxation has a non-Debye character. As a consequence. The frequency dependences of both in ' and in " can be fitted with a stretched-exponent approach. The value of the stretched exponent p significantly depends on temperature, exhibiting two peaks: the first peak is at the glass-crystal transition in the sodium subsystem of the crystal and the second peak is above the phase transition point to a modulated phase.

The effect of structural disorder on the first-order Raman scattering in glasses is studied by perturbation series approximations to the averaged Green function of an appropriately chosen model system. The proposed theoretical approach is applied to calculating the Raman spectra of Na₂O-SiO₂ glass systems on the basis of the disorder-modified spectrum of a Si₂O₇ dimer.

Brillouin scattering measurements have been performed on isostructural ferroelectric crystals KTiOPO₄ (KTP), RbTiOPO₄ (RTP) and TITiOPO₄ (ITP) as functions of the temperature. At room temperature, all the elastic constants have been determined. The temperature variations in the longitudinal elastic constants (for TTP and RTP) reveal similar behaviours; they agree with a Landau model for a second-order phase transition where the order parameter is quadratically coupled to the strains. An intense quasi-elastic light scattering signal, related to the ionic conductivity, is observed in the spectra. In addition, a central peak apparently connected with the locking of the soft mode is observed for RTP near its phase transition.

The thermoluminescence (TL) of Dy³⁺-doped BaSO₄ gamma irradiated at room temperature was studied in the temperature range 300-573 K. The TL glow curve of this phosphor material consists of four peaks at 407, 420, 442 and 487 K. The spectral character of the emission recorded during TL was found to be the same for all glow peaks and consists of several bands associated with the characteristic emission of Dy³⁺ ions due to transitions from the lowest excited state ⁴F_9/2 to the ground-stare muitiplets ⁶H_j. Optical absorption measurements before and after irradiation, as well as after a TL run were also performed, in order to obtain better insight into the mechanism of TL production in this phosphor material. These measurements revealed that optical absorption in gamma -irradiated BaSO₄:Dy³⁺ arises predominantly from two sources: firstly defect centres intrinsic to the barium sulphate structure, and secondly oxidation of the Dy³⁺ impurity ion to the 2⁺ valence state. A mechanism for TL based on a charge-transfer process between dysprosium ions and host-related hole centres is proposed. Some of the main dosimetric characteristics of this phosphor material are also reported.