Table of contents

The irreversible enthalpy relaxation processes in liquid dibutylphthalate were tracked under constant-temperature conditions by the temperature jump method with an adiabatic calorimeter, and the relaxation functions were characterized in terms of the stretched exponential function with a nonexponentiality parameter, beta . Remarkable nonlinearity, delta , of the relaxation functions with the magnitude of temperature jump, Delta _jT, was found, and reasonably interpreted to be due to the fragility, m, of the liquid from the comparison with the result of computer simulation based on a thermal activation model for the microscopic rearrangement of molecules. The validity of the Tool-Narayanaswamy model is also discussed on the basis of the simulation result.

In this paper, we reanalyse the incoherent intermediate scattering function, determined by a Monte Carlo simulation of a polymer melt, in the framework of the extended mode-coupling theory (MCT). A previous analysis with the idealized MCT showed systematic deviations between theory and simulation at low temperatures, which could be qualitatively attributed to the neglect of hopping processes. If these hopping processes are taken into account quantitatively by the application of the extended MCT, the discrepancies at low temperatures disappear, and a (quite) accurate estimate of the critical temperature becomes possible.

The dielectric properties of m-fluoroaniline have been investigated over seven decades of frequency (1 kHz-20 GHz) in the temperature range 188-303 K, above the calorimetric glass transition temperature. The Arrhenius plot of the inverse alpha peak frequencies reveals a non-linear character and a non-Debye relaxation function indicates a non-exponential temporal decay of the molecular polarization of the liquid near T_g. The dynamic behaviour turns out to be typical of highly fragile liquids and appears closely similar to that obtained from previous ultrasonic studies on the same system. A crossover between two distinct dynamical regimes takes place in the range of intermediate viscosity at a temperature T_c which falls about 40 K above the glass transition temperature T_g.

Low-frequency measurements for extracting hyperfine structure (HFS) parameters directly from the experimental ESR spectra of glasses are described. Glass of molar composition BaO-P₂O₅ with 0.2 wt% Mn₂O₃ was used. The spectra were recorded at room temperature in the UHF band at various frequencies (of 530 to 950 MHz). The HFS parameter A₀ was determined using the frequency dependence of the spectra.

We examine the phase diagram of systems of hard, spherical particles with a short-range attraction. Recent simulations as well as theoretical investigations predict for such systems an fcc-to-fcc isostructural transition which terminates at a critical point, and the existence of a single fluid phase. We use a nonperturbative density functional approach, by making a mapping of the inhomogeneous system onto a uniform fluid by means of the modified weighted density approximation (MWDA). Previous approaches were instead based on the separation of the potential into a hard-sphere repulsion and a short-range attraction with the latter treated in a mean-field fashion. We obtain improved results for the critical temperature, the middle density at the triple point and the overall shape of the phase diagram, but a worsening of the triple temperature with increasing range of interaction.

We have measured the lattice parameters a(T), b(T) and c(T) as a function of temperature T in the range 15 K<T<300 K for RZn₂ (R=Gd, Tb or Dy) with the CeCu₂-type crystal structure. Anisotropic thermal expansion is observed in the a(T), b(T) and c(T) against T plots for all materials investigated below the magnetic transition temperatures T_tr. The linear expansivity above T_tr along the b direction becomes small on varying the rare earth atom R in RZn₂ from R=Gd to Dy to Tb. The sign of the change in c, Delta c/c, for GdZn₂ below T_tr is positive while those for TbZn₂ and DyZn₂ are negative. The cell volume in all materials expands when magnetic orderings appear. The obtained results are qualitatively understood in terms of the phenomenological model in which the crystal field and magnetic exchange contributions to the thermal expansion are taken into account.

The physical properties of the ionic conductor, obtained by dissolution of lithium trifluoromethanesulphonylimide in polyethylene oxide, (PEO)_nLi⁺((CF₃SO₂)₂N)^- have been investigated for several values of n. The phase diagram has been established from both DSC and NMR techniques. The diffusion coefficients of ⁷Li and ¹⁹F containing Species, determined by the pulsed magnetic field gradient (PMFG) technique, are interpreted as the measures of the cationic and anionic transport numbers, which are concentration dependent, and t₊ reaches a value close to 0.3. This study is complemented by a systematic analysis of the behaviour of the ⁷Li relaxation time T₁ versus temperature and concentration which is correlated to the glass temperature T_g.

The hydrodynamic equations for a narrow slab when the state of ³He-A is deformed slightly by a magnetic field are solved. Then the torque exerted on the torsional oscillator in which the slab is contained is calculated, and the changes in the resonant frequency as a function of the magnetic field are determined.

In-quadrature frequency-resolved photocurrent (FRPC) measurements were performed on amorphous SeTe films between 20 K and 290 K, and as a function of excitation intensity. These measurements yield lifetime distributions directly. The results show that the recombination takes place between distant pairs and that there is a continuous distribution of states within the mobility gap.

We study the magnetic response properties of both site and spatially disordered Anderson-Hubbard models via a random-phase-type approximation for collective excitations about stable, inhomogeneous mean-field ground states. For the site-disordered model, zero-temperature transitions between paramagnetic, disordered antiferromagnetic and spin-glass-like ground states are examined. Within broken symmetry phases, a microscopic picture of the response of the inhomogeneous distribution of local magnetic moments to an external field is obtained, and the role of disorder in leading to a strong site differential enhancement in local susceptibilities is highlighted.

The magnetic ordering of the S=2/3 Heisenberg antiferromagnet on a rhombohedrally stacked triangular lattice LiCrO₂ is studied by susceptibility, neutron diffraction and polarization analysis measurements using a single crystal. The temperature dependence of the susceptibility strongly suggests that a 120 degrees structure is established in each quasi-two-dimensional layer, which is parallel to the c plane. Assuming the 120 degrees structure in which magnetic moments are confined in a plane including the c axis because of an Ising-type anisotropy, neutron scattering shows that the magnetic ordering is a double-Q structure with non-equivalent wave numbers q=(1/3 1/3 0) and (-2/3 1/3 1/2). It is characterized by an alternating sequence of rotational direction of the 120 degrees structure along the c axis.

In order to investigate the magnetic interaction in the paramagnetic and the antiferromagnetic state of NdCu₂, we measured the Cu nuclear magnetic resonance (NMR). In NdCu₂ an antiferromagnetic long-range order exists below T_N=6.5 K and a change of the spin structure around 4.1 K has been reported. For the paramagnetic state, we have deduced the hyperfine coupling constant and the electrostatic parameters at the Cu sites, such as the principal axis of the electric field gradient, the nuclear quadrupole frequency and the anisotropy parameter. The nuclear quadrupole resonance line splits into several lines in the ordered state due to the onset of the hyperfine field, which can be calculated assuming two different magnetic Cu sites. We discuss the consistency between these NMR results and the magnetic structure proposed by recently performed neutron diffraction experiments.

We have measured the electrical resistivity, thermoelectric power, magnetoresistance, magnetic susceptibility and magnetization of the antiferromagnetic Kondo lattice compound CePd₂Ga single crystal. A clear anisotropy and an antiferromagnetic transition at 2.9 K have been observed for both the electrical resistivity and the magnetic properties. The crystal-field level scheme has been investigated on the basis of the anisotropy and the magnetic specific-heat data; the first and the second excited states have been estimated to lie at 66 K and about 280 K, respectively.

The magnetic anisotropy coefficients and constants of single-ion origin as functions of magnetization have been computed and classified. They are valid for a whole class of untrivial theories and are thus universal or canonic within this class. The role of the anisotropy coefficients as a finite basis of functions spanning the variation of the constants with magnetization and, hence, with temperature and applied field is clarified. The ratio of the zero-temperature anisotropy constants is of crucial importance for the observed type of magnetization dependence of the first anisotropy constant when two basis functions are considered. The method is directly applicable to the analysis of magnetostriction of single-ion origin and serves to identify and quantify three generic types of variation of anisotropy and magnetostriction. It is demonstrated that a compensation point lambda _S=0 for the macroscopic magnetostriction constant of amorphous ferromagnets may result from the competition between the lowest and the next-lowest single-ion contributions even if no two-ion contributions are considered. Prospective extensions of the method are given.

BaFBr is usually contaminated with oxygen which influences the performance of this well known X-ray storage phosphor material. Oxygen is incorporated as a diamagnetic O^2- ion on F- sites with a nearby charge-compensating vacancy in the Br^- sublattice, as is known from the paramagnetic O^-_F and F(Br^-) centres created by room-temperature X-irradiation and studied in detail with electron paramagnetic resonance (EPR) techniques. It is shown that BaEBr:O^2-_F has a luminescence band excited at 4.95 eV, peaking at 2.43 eV with two radiative lifetimes of 0.2 ms and 1 ms, respectively. With luminescence-detected EPR (i.e. optically detected magnetic resonance (ODMR)), it is shown that the luminescence is due to an excited triplet state of an O^2-_F-Br^- vacancy pair. Time-resolved ODMR measurements reveal the same radiative lifetimes as seen in luminescence. No singlet emission was observed. The zero-field splitting can be explained assuming a F(Br^-)-O^-_F pair defect as the excited triplet state.

Phase transitions in a highly polarizable crystal of cubic symmetry induced by two types of random site electric dipole orientated correspondingly along the (100)- and (111)-type directions are considered. The distribution function of random electric fields produced by these dipoles was calculated. This function made it possible to obtain the order parameters, the critical concentrations of dipoles and the transition temperatures for ferroelectric phase transitions with tetragonal and rhombohedral symmetries. All these quantities were calculated as functions of the ratio of dipole moments and the concentrations of the two types of dipole considered. It was shown that, at certain concentrations and dipole moment ratio, the order parameters of both aforementioned ferroelectric phases had to coexist. The mean values of random elastic fields produced by the random electric fields considered through the electrostriction effect are calculated. It was found that in crystals such as KTaO₃ the elastic fields of E_g symmetry may be larger than those of T_2g symmetry. The eight-well adiabatic potential of the cluster, which includes the impurity dipole and its nearest neighbours, was shown to be transformed into a six-well potential under the action of E_g symmetry deformations. This transformation was supposed to be the main reason for the tetragonal symmetry phase transitions observed earlier in K_{1- eta} Li_eta Ta_{1- xi} Nb_xi O₃ ( eta <or=0.06; xi <or=0.028) even at very low lithium ion concentrations. The calculation of the transition temperature confirmed this supposition and made it possible to explain qualitatively the transition temperature concentrational dependence observed in the aforementioned mixed disordered system.

We report on the optical detection of electron paramagnetic resonance at 36 GHz, 67 GHz and 70 GHz for all the absorption bands of Bi₁₂GeO₂₀ (BGO) in the visible spectral range. This completes our previous studies and demonstrates clearly that the three major magnetic circular dichroism features do belong to the same isotropic centre, i.e., a Bi³⁺_Ge+h defect with the hole being delocalized onto the four surrounding oxygen atoms. We provide a simple explanation for the unusual sign of ODMR and the transient behaviour of the magnetic circular dichroism at resonance.

The electron density distribution map of alpha -tin has been drawn by the maximum-entropy method (MEM) with 11 independent and two combined structure factors, which were determined by the powder pattern decomposition from X-ray data. The final R and R_w factors were 0.80% and 0.89%, respectively, though a small amount of beta -tin was contained in the specimen. The obtained map indicates that the bonding electrons are clearly seen between the two adjacent tin atoms in the (110) plane with the 0.6 e AA^-3 level. This value was about twice that of germanium. The structure factor F(222)e^-M for the 222 forbidden reflections at 293 K calculated from the MEM density is +0.4350. The results are discussed by comparing with the electron density maps of C (diamond), Si and Ge.

The 'new' irreducible Green function theory suggested by Mitra and Chakraborty (J. Phys. Condens. Matter 7 (1995) 379) is shown to be inconsistent and incorrect.

For original paper see ibid., vol. 7, p. 379 (1995). The consistency of the new irreducibility condition and that of the related irreducible Green function theory is discussed in the light of Brown's arguments (see ibid., vol. 7 (1995)).