Table of contents

The authors have measured time-resolved photoluminescence from the dilute magnetic semiconductor Cd_1-xMn_xTe for x up to 0.3. For x=0.09 they observe exciton localisation both by neutral acceptors and by alloy potential fluctuations; for higher x-values they observe localisation in alloy potential fluctuations to be dominant. In the latter case localisation occurs on a timescale of <20 ps. Subsequent rapid transfer to lower-energy localised states, with a characteristic time of about 150 ps, is interpreted as being due in part to magnetic polaron effects.

Dynamics theory of quantum crystals with delocalised defects (defectons) is developed. An exact, non-linear set, consisting of elasticity theory and kinetic equations is derived. The equations deduced may be applied to every uncharged quasi-particle gas (or gases) with an arbitrary dispersion law in deformable solids. The quasi-wave-vector role is correctly taken into account, and its equation of motion is obtained. The defecton distribution function in a rotating quantum solid is calculated.

Transient volume change induced by irradiation with an electron pulse of alpha -Al₂O₃ and Cr doped Al₂O₃ have been measured above 77K. It is found that the volume change induced in alpha -Al₂O₃ shows a single exponential decay with a lifetime of 12 ms below 140K and that the initial value of the volume change is a linear function of the electron fluence and is not influenced by doping with Cr³⁺. It is suggested that the transient volume change observed in alpha -Al₂O₃ is associated with the relaxed excited state resulting from fundamental electronic excitation, possibly the self-trapped exciton.

The far-infrared reflectivity spectra polarised along the crystallographic orientations of ammonium dihydrogen phosphate, were studied for both the para- and antiferroelectric phase, over a wide temperature range: 7 to 300K. The reflectivity spectra are fitted to the factorised form of the dielectric function. The temperature dependence of the optical parameters are reported for both the phases. The mode parameters of a low-frequency heavily damped mode in each B₂ and E spectrum, assigned to the proton collective mode, are studied in detail.

The density functional theory is applied to study the freezing of the hard sphere liquid into the close packed structures. To describe the short-range correlations in the solid phases a reference liquid is introduced, whose density is fixed by a stationary condition. The thermodynamic properties of the two solid structures are calculated for increasing numbers of order parameters until convergence is achieved. According to the calculation the FCC structure is slightly more stable than the HCP one, but the difference in the grand potentials is comparable with the error of the calculation.

The basal plane resistivity rho of stage 2 SbCl₅ intercalated graphite has been measured as a function of temperature T and pressure p in the range 195-300K and 0-1.75 GPa, using a contactless method. rho is found to increase with increasing p. New evidence is found for the existence of the high-pressure (>1 GPa) phase suggested by Houser and co-workers (1984). The p/T phase boundaries for the low T(<230K) and intermediate p (>0.35 GPa) ordered phases are outlined. A re-analysis of previous results for stage 1 SbF₅ intercalated graphite indicates a similar phase diagram for this material.

Structural phase transitions of tetramethyl ammonium manganese trichloride (TMMC) are investigated by using the Brillouin scattering method. The temperature dependences of the elastic coefficients C₁₁ and C₃₃ extracted from the experimental data are analysed by using the Landau theory of phase transitions. The step anomaly of C₃₃ observed at the order-disorder transition temperature T_c can be attributed to a coupling between the order parameter and totally symmetric strains. An anomalous softening C₃₃ above T_c indicates an interaction also between tetramethyl ammonium (TMA) molecules and the MnCl₃ chains. A new lattice anomaly, which the authors attribute to a change in the rotational degree of freedom of the TMA molecules, is observed at about 395K. Doping of TMMC with 2% of Cu²⁺ creates a sequence of phases with transition temperatures at 92, 124 and 130K. An incommensurate phase stabilised by impurities may be involved in this sequence. Strong elastic scattering of light indicating a formation of needle-like domains along the c axis is observed in the doped crystals between 90 and 130K.

As is well known the compound CsCuCl₃ exhibits a Jahn-Teller phase transition at 420K. In order to investigate the nature of the dynamic phase above this temperature, magnetic susceptibility measurements have been carried out around the phase transition point on large single crystals. The transition from a static to dynamic Jahn-Teller effect in the single CuCl₆ octahedra is reflected in a change of the value of the spectroscopic g-factors which can be explained in terms of a vibronic reduction factor (Ham factor). The measurements show a large anisotropy of the hysteresis. This can be explained by assuming a cooperative dynamic Jahn-Teller effect occurring in domains.

A quantum theory of diffusion is proposed based on the following premises. The impurity state is taken in the form of a wave packet constructed out of its Bloch states in the host lattice, and its time evolution is studied including its interaction with the host-lattice phonons. It is shown that a correspondence can be established between the classical diffusion equation and the time evolution of the probability density arising out of the impurity wave packet. The diffusion and trapping coefficients D_T and gamma _T are related to the imaginary part of the energy shift of the impurity caused by its interaction with phonons. The detailed calculations are based on second-order perturbation theory for the energy shift, the Debye model for the host lattice and the effective-mass approximation for the impurity band. At low temperatures D_T is found to be proportional to T³2/, and at high temperatures the Arrhenius formula of Vineyard (1957) is obtained. The estimated migration energies for mu ⁺ diffusion in BCC metals agree reasonably with the experimental values. The conditions for non-existence of gamma _T are also discussed.

In liquid helium II second-sound waves with rectangular profiles degenerate ultimately into triangular form and the speed of the wave front is no longer constant. This effect gives rise to a non-linear dependence of the shock speed on the initial wave amplitude, but its magnitude is insufficient to account for the experimental results above a certain limited range of amplitudes. However, the degeneration into triangular form is the dominant effect in the dependence of the wave speed on decreasing pulse width, at constant amplitude, and here the authors analysis agrees well with the experimental results.

A Green function technique is developed to study the spin-dependent phonon Raman scattering in ferromagnetic semiconductors (FMS). The damping gamma _k^zz and the dynamic structure factor S^zz(k,E) are determined. The author obtains three temperature regions with different contributions of the damping terms gamma _ss^zz, gamma _sd^zz and gamma _sp^zz. The temperature dependence of the integrated intensity of S^zz is in agreement with the experimental data for FMS, such as CdCr₂Se₄.

The authors derive a functional integral representation for the partition function of the transverse Ising model. The spin operators are replaced by fermion operators and special care is taken to achieve complete equivalence of the spin model and the fermion model. They discuss several approximations for the functional integral and show that in a recent paper on spin glasses, where a similar method was used, the original spin model and the fermion model, which was obtained by replacing the spin operators by fermion operators, are not equivalent and have different critical temperatures.

Elastic and inelastic neutron scattering experiments have been carried out to investigate magnetic structure and magnetic excitations in anisotropic antiferromagnet FeTiO₃. A detailed structure analysis shows that the spin direction of each Fe²⁺ ion is oblique away from the hexagonal c axis by (2.2+or-0.2) degrees at 4.3K with keeping all spins collinear. The oblique angle is found to be almost insensitive to the temperature variation up to the Neel temperature T_N=57.35+or-0.05K, which was determined from the power law fitting of sublattice magnetisation data. It is shown that single ion anisotropy due to the crystalline electric field are inadequate to explain the magnetic structure. The excitation spectra, on the other hand, exhibit a complicated behaviour, reflecting the oblique easy-axis ground state, in which five distinct branches of magnetic origin are confirmed to exist at 12K. According to the phenomenological theory, in which the effective anisotropy is assumed to have an orthorhombic symmetry due to the obliqueness, four of the observed branches can be well reproduced by identifying them as non-degenerate acoustic and optical magnon modes including a hybridisation with a transverse acoustic phonon.

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 influence of the linear magnetoelectric effect on bulk and surface (Damon-Eshbach) magnetostatic modes is considered for uniaxial ferromagnetic systems. This effect is shown to give rise to a broadening of the band allowed for propagation of the bulk waves. It is shown that the linear magnetoelectric properties result also in change of the frequency and angle regions which are allowed for surface waves.

The spin-lattice relaxation times of Fe³⁺ in LiAl₅O₈ have been obtained at temperatures as high as 750K, using Mossbauer spectroscopy, for the first time. The Raman relaxation process is found to have an insignificant contribution from the acoustic modes of lattice vibrations. On the other hand, the internal mode of FeO₄ tetrahedra at 192 cm^-1 is found to contribute dominantly to the Raman relaxation of the Fe³⁺ ions. The contribution from the direct mode of spin-lattice relaxation due to the acoustic phonons is found to be relatively significant even at higher temperatures.

Mossbauer spectra of the layer compound Sr₁2/Sm₃2/Li₁2/Fe₁2/O₄ show significant differences from those of similar compounds, studied previously, where the rare-earth ion was La, Pr or Nd. The Sm spectra are more complicated but can be fitted in an internally consistent way, requiring at least four components. This suggests a significantly greater degree of disorder in the Li/Fe site occupation in the Sm case.

The Mossbauer spectra of single crystals of alpha -Fe₂O₃ have been measured (i) close to the Morin transition at T_M approximately=260K, (ii) at 4.2K in magnetic fields parallel to the (111) axis in the region of the spin-flop transition, and (iii) at temperatures below T_M in magnetic fields perpendicular to the (111) axis. The first two transitions show a coexistence of two magnetic phases over a range of temperatures and fields respectively, confirming their first-order character, although no hysteresis was observed. The width of the transitions showed a strong dependence on small levels of impurity and on strains. In the third transition a gradual reorientation of all the spins towards a direction in the (111) plane perpendicular to the applied field was observed. This transition was incomplete at the highest fields and temperatures used (10.0 T at 245K).

Through a critical review of previous work, it is shown that, in the case of sheet silicate crystals or mosaics, errors occur frequently in the information drawn from Mossbauer spectra taken along different directions. A method is proposed, which is based on (i) the acknowledgement of how far the theoretical doublets follow the experimental absorption for all chosen orientations, and (ii) the clear description of the often incomplete information provided on the EFG at each iron site. A study of an iron-poor muscovite crystal is presented. It is based on new fitting programs, simultaneously processing the spectra taken along different directions. As thickness is carefully taken into account, a new method is presented for fast measurements of the resonant percentage of the radiation.