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We study the origin of the compensation in disordered mixed systems of the Wannier-Stark ladder effects observed previously as strong jumps of the transmission coefficient in ordered and disordered systems with potential wells and barriers subjected to a bias voltage. The one-dimensional Kronig-Penney model is used to investigate this problem by means of the transmission coefficient. We found that the band spectrum of the systems with barriers is shifted in comparison with the corresponding spectra of those with wells. Therefore the delocalization of each system in an electric field corresponds to the localization of the other. We found also that the disorder conserves the bandwidth of the ordered system if the average potential strength corresponds to the potential strength of this system.

The low-temperature specific heat for non-magnetic Mg₄₂Y₈Zn₅₀ quasicrystal has been investigated to get information on the electronic structure. The electronic specific heat coefficient gamma is estimated to be 0.63 mJ mol^-1 K^-2, which is about 70% of the value expected from the free-electron model. This small gamma value strongly suggests the existence of a pseudogap of the electronic density of states in the vicinity of the Fermi level. The low-temperature specific heats of icosahedral Mg₄₂Gd₈Zn₅₀ and Mg₄₂Tb₈Zn₅₀ quasicrystals, which exhibit spin-glass behaviour, have also been investigated. The specific heat of Mg₄₂Gd₈Zn₅₀ exhibits a broad maximum at a temperature that is a few kelvins above the spin freezing temperature T_f determined by AC susceptibility measurements. The magnetic entropy at T_f for the Mg₄₂Gd₈Zn₅₀ quasicrystal reaches about 30% of the theoretical value of R ln 8, being comparable to those of crystalline spin-glass systems such as magnetically dilute AuFe and CuMn alloys. The magnetic specific heat of Mg₄₂Tb₈Zn₅₀ quasicrystal is smaller than that of Mg₄₂Gd₈Zn₅₀, suggesting the splitting of the ground state due to the local electrostatic field.

Spin-glass-like behaviour and low-temperature specific heat have been investigated for bulk amorphous Er_xN_100-x, (x=33, 50 and 80) alloys prepared by high-rate direct-current sputtering. The competition between ferromagnetic and antiferromagnetic interactions arising from the exchange fluctuations is negligibly small in this system, in contrast to that in amorphous Er-Cu alloys. The random magnetic anisotropy (RMA) suppresses the ferromagnetic coupling, resulting in a spin-glass-like state. The magnetization per Er atom becomes smaller with increasing Er content owing to the increase in the RMA. At the spin freezing temperature T_f the low-temperature specific heat exhibits a broad maximum, which becomes narrower with the decrease in Er content. It should be noted that the temperature at which the magnetic specific heat of the amorphous Er₃₃Ni₆₇ alloy reaches a maximum value coincides with the T_f determined by the alternating-current magnetic susceptibility measurement. The splitting of the ground state from J=15/2 into a Kramers doublet is caused by the electrostatic field, being accompanied by a Schottky-type specific heat with a linear temperature dependence. The magnetic entropy at T_f is estimated to be about 45-60% of the theoretical value, being much larger than those for crystalline dilute spin-glass systems. Moreover, the plot of the magnetic specific heat versus T^3/2 is linear at low temperatures. Therefore, it is considered that a ferromagnetic-like spin wave is excited, although its magnetic structure is not ferromagnetic.

In this paper the behaviour of the specific heat on NH₃ deuteration in nickel hexammine nitrate is determined from the analysis of different contributions of the crystal lattice. For a non-deuterated crystal the experimental specific heat outside the phase transition regions was reproduced well by the sum of harmonic oscillator specific-heat functions plus a correction for thermal dilatation. In calculating the specific heat, maximum use has been made of frequencies derived from spectroscopy. NH₃ deuteration provides a good test of the analysis procedure. Since deuteration generally shifts the frequencies of some of the vibrations, a hydrogenous compound and its deuterated analogue have different vibrational heat capacities. One would expect a larger heat capacity for a deuterated crystal; this is indeed observed by calorimetry. Hence, this model has to be verified by calculating and comparing with experiment how NH₃ deuteration affects the specific heat. The difference between the experimental NiDNO₃ and NiHNO₃ specific-heat values, referred to as Delta c_exp^D-H, is expressed as the sum of the contributions from the frequency shifts in the vibrational spectrum of the deuterated crystal compared with that of the non-deuterated crystal. The results of the analysis show that the calculated Delta c^D-H is very close to the experimental values. This result strongly supports the simple Debye plus Einstein model.

Inelastic neutron scattering (INS) has been used to study the magnetic excitations of Ho³⁺ ions in grain-aligned samples of HoBa₂Cu₃O_x in both orthorhombic (x approximately=7) and tetragonal (x approximately=6.2) phases. The magnetic excitations associated with the lowest ground state crystalline electric field transition exhibit dispersion in the (a,b) plane but remain constant in energy along the c axis. The data are analysed in the random-phase approximation in terms of a generalized Heisenberg model including anisotropies in the coupling of the spin operators.

A method is presented to calculate ab initio exchange constants and spin-wave excitations of multi-sublattice magnetic structures on the basis of total-energy calculations of incommensurate magnetic structures. Here the exchange energies, dispersion curves and Curie temperature for magnetite (Fe₃O₄) are obtained and compared with experimental results.

The coupling of spin waves for a Heisenberg system with a finite size is studied. It is found that the final results are boundary condition dependent. For a periodic system, the finite-size effect weakens the coupling of spin waves and an additional term proportional to -T⁷2//N appears with N, the system's size, in the expression for the number density of spin waves at temperature T. The free boundary greatly complicates the coupling of spin waves. There are three new kinds of scattering process: between spin waves parallel to the free surfaces of the system, between spin waves parallel to the surface and other waves, and between spin waves with the same components in the direction perpendicular to the free surface of the system. Unlike the others, the last effective interaction tends to depress the excitation of spin waves at low temperatures.

The crystal structure and magnetic properties of Er₂Fe_17-xSi_x and Er₂Fe_17-xSi_xC compounds were investigated. The results of the powder X-ray diffraction indicate that the Er2Fe_17-xSi_x compounds crystallize in the hexagonal Th₂Ni₁₇-type structure and the Er₂Fe_17-xSi_xC compounds exhibit two phases of the hexagonal Th₂Ni₁₇-type structure and the rhombohedral Th₂Zn₁₇-type structure. The addition of Si results in a decrease in the lattice constants and the unit-cell volumes. The Curie temperature T_C of the Er₂Fe_17-xSi_x, compounds is found to increase from 327 K for x=0 to 485 K for x=2.5 and then to decrease slightly with increasing x, whereas the T_C of their carbides Er₂Fe_17-xSi_xC undergoes a small enhancement with increasing Si concentration. An approximately linear decrease in the saturation magnetization with increasing x is observed when the non-magnetic Si atom is substituted for Fe. Spin reorientation transitions are observed in Er₂Fe_17-xSi_x, and Er₂Fe_17-xSi_xC. Substitution of Si for Fe in Er₂Fe₁₇ leads to an increase in the uniaxial anisotropy of the Er sublattice; therefore, the spin reorientation temperature T_sr, increases with increasing Si concentration. The introduction of carbon has a marked effect on shifting the T_sr, towards higher temperatures.

The ferromagnetic Cu₂MnAl Heusler alloy has been prepared via a combination of mechanical alloying and heat treatment. The metastable Cu₂MnAl phase can be formed directly when Cu, Mn and Al powders are mechanically alloyed. The resulting single-phase nanocrystalline material is highly disordered. When heat treated at 473 K, the Heusler phase can be ordered. The saturation magnetization is then approximately half the value achieved in highly ordered alloys, prepared via conventional melting and casting techniques. When heat treated at 973 K, the magnetic properties of the resulting material are similar to those of previous reports. The Cu₂MnAl phase can be disordered by mechanical milling, resulting in a decrease in saturation magnetization and increase in coercivity. The Heusler phase has been found to be unstable when mechanically milled over extended periods.

EPR and optical (4f-4f photoluminescence and photoluminescence excitation) spectra due to Er centres are studied on bulk ZnSe crystals, which were grown by the high-pressure Bridgman technique and doped with ErF₃ (and partly, in addition, with Li₂CO₃) during crystal growth. Besides the well known, almost isotropic signal with g=5.94, which has been assigned to isolated Er³⁺ on Zn lattice sites, we observe three strongly anisotropic EPR spectra (A, B and B') which are due to transitions in non-cubic Er³⁺ (spectrum A) and Er³⁺ (spectra B and B') centres. The symmetry axes of these centres have directions close to (111) (A) and parallel to (110) (B and B'). The angular dependences of the signals are influenced by twinning effects. In the crystals doped with ErF₃ alone, only a single type of centre, which is obviously identical with the type A EPR centre, manifests itself in photoluminescence. A crystal-field analysis of the corresponding EPR and optical spectra shows that this centre has a Gamma ₆-type ground level and is characterized by a crystal-field parameter ratio As<r⁶>/A₄<r⁴>=-0.22. We think that this centre is a complex consisting of Er³⁺ on a Zn site and F on a nearest-neighbour interstitial site. The g-factors found for the EPR signals B and B' can be explained on the basis of non-Kramers doublet ground levels of Er²⁺ 4f¹² which result from the splitting of cubic Gamma ₅ triplets due to non-cubic crystal-field components. These two signals are ascribed to Er³⁺ on the two zincblende-type interstitial sites, respectively, each forming a complex with some other kind of atom on its next-nearest-neighbour interstitial site. A discussion is given of the result that the EPR spectra (unlike the optical spectra) were detected only in the case of Li₂CO₃ codoping.

The behaviour of the sound velocity and damping in the phase transition vicinity in the weak ferroelectric Li₂Ge₇O₁₅ has been investigated by the ultrasonic technique (30-150 MHz) and by Brillouin scattering (about 40 GHz). A large critical contribution to the acoustic anomalies has been found in the paraelectric phase. Analysis of the sound velocity and damping dispersion revealed the relaxation parts of anomalies under conditions when the critical parts are large. The Landau theory is used to interpret the results obtained. Some coefficients in the thermodynamic potential expansion are determined. Increasing suppression of the relaxation anomalies by the depolarizing field as T to T_c is considered for both the sound velocity and the damping of longitudinal phonons with the wavevector along the polar axis. This is a specific feature of weak ferroelectrics.

Reflectivity spectra of Te have been measured in the energy range from 0.5 to 3.8 eV at pressures up to 96 kbar using a diamond anvil cell. Optical conductivity spectra sigma ( omega ) have been derived from the reflectivity by using an oscillator fitting analysis. The sigma ( omega ) spectrum for trigonal Te at 1 kbar has a main peak at 2.2 eV and a shoulder around 1.2 eV. These peaks shift to lower energy and grow with increasing pressure. When Te is transformed into a monoclinic phase around 40 kbar, a Drude term appears in the low-energy region of the sigma ( omega ) spectrum, which suggests that monoclinic Te has a metallic nature. In the orthorhombic phase at 73 kbar, a main peak at 0.5 eV and a shoulder around 1.2 eV are observed for the sigma ( omega ) spectrum. These changes in the sigma ( omega ) spectra of Te with pressure are discussed in comparison with the results of band calculations.

Far-infrared transmittance and reflectance of PbTiO₃, PbZr_0.53Ti_0.47O₃ and PbZr_0.75Ti_0.25O₃ thin films deposited by a sol-gel technique on sapphire substrates were measured between room temperature and 650 degrees C. The spectra were fitted with a classical oscillator model to calculate the dielectric response. Polar-mode parameters are more accurate than data on bulk ceramic materials and are in good agreement with them. An analysis of the soft-mode behaviour and comparison with the low-frequency permittivity data clearly indicate the existence of additional relaxation that is always present in the several cm^-1 range, even at room temperature.

Up-conversion processes in PbF₂+TeO₂+WO₃ glass-ceramics doped with Tm³⁺ and codoped with Tm³⁺ and Yb³⁺ ions were investigated under 680 nm excitation. Emission, excitation and time-resolved spectra and decay profiles were measured at temperatures between 12 and 300 K. A strong blue emission centred at 478 nm originating from the ¹G₄ level of Tm³⁺ was observed in a doubly doped sample via a two-step energy transfer mechanism, (Tm) ³F₄ to (Yb) ²E_5/2 energy transfer and a positive feedback (Yb) ²F_5/2 to (Tm) ¹G₄ process, raising the excited Tm³⁺ ion from the ³F₄ to the ¹G₄ level following the absorption of 680 nm excitation light. The time-resolved emission spectra prove the validity of these energy transfer processes. The optical properties of the excited levels and the energy transfer processes involved in the up-conversion were interpreted using a kinetic model describing the interactions between rare-earth ions in terms of energy migration and luminescence quenching. The microscopic interaction parameter for Tm-Tm intersection in the absence of Yb³⁺ ions was determined to be 6.1*10^-40 cm⁶ s^-1 in the sample having 1.0 mol% TmF₃. The same parameter for the energy transfer from Tm to Yb was calculated to be 1.2*10^-38 cm⁶ s^-1 in the sample having 0.15 mol% TmF₃ and 15 mol% YbF₃. No effect of temperature on the emission intensity of the (Tm) ³F₄ level was observed although the emission intensities of the (Tm) ¹G₄ and (Yb) ²F_5/2 levels were decreased with increasing temperature. The former decrease accounts for the migration of energy between Yb³⁺ ions and controls the temperature dependence of the blue-up conversion emission.