Table of contents

We carried out electron tunnelling spectroscopy for single crystals of Bi₂Sr₂CaCu₂O₈ in the superconducting phase with use of STM. The tunnelling differential conductance curve with low noise was obtained for the cleaved surface and compared with that for the lateral surface. The two spectra were found to be essentially identical. It is concluded that tunnelling spectra obtained by our STM experiment contain contributions from every wave number and represent the total electronic density of states. From the functional form of the tunnelling conductance, the simple d wave was excluded for the symmetry of the superconducting pair wave function. The conductance curve obtained was well explained by the model, in which the gap is finite on the whole Fermi surface while it is highly anisotropic in k-space. It was suggested that the symmetry of the superconducting pair wave function is of the mixed symmetry of the s and d waves.

We present in this paper a thermostatistical model of formation of vitreous B₂O₃ which is based on a description of agglomeration of BO₃ triangles resulting in medium-range clusters and on the construction of the partition function related to the space of all possible pathways leading to this set of clusters. With one fitted parameter which is the boroxol formation energy of 5.3+or-0.7 kcal mol^-1, the model predicts the usual shape of the internal energy function and the heat capacity curves, as well as the fraction of boroxol rings of 83%. In the last section, we investigate the structural evolution of molten B₂O₃ during the glass transition process.

New hybrid weighted-density approximations (HWDA) based on both local and global average densities are proposed. In one of the approximations the weighting function is constructed to satisfy the same homogeneous properties as the local weighted-density approximation (LWDA) proposed by Tarazona (1985); in the other, the weighting function is constructed to agree with that of Leidl and Wagner (1993) for the homogeneous fluid. Free energy functionals are derived from these proposed weighted-density approximations by using expansions in terms of the density. They are applied to predict the density profiles of hard-sphere fluids restricted by hard and permeable walls and in spherical cages. For the density profiles of fluids confined between hard flat walls, the results are in good agreement with the computer simulations and comparable with those of the free energy functional approximation of Tarazona even at the high density rho sigma ³=0.9135. For the density profiles of hard-sphere fluids restricted by permeable walls with a finite height and in spherical cages, the results are also in good agreement with the computer simulations. These results show that although the free energy functionals differ, the new approximations and LWDA produce equally accurate results for the inhomogeneous hard-sphere fluids investigated. The new approximations have the advantage of being simpler to apply.

The calculation of the torque T on a permanent ellipsoidal magnet immersed in a magnetic liquid is more delicate than one might be inclined to expect. This problem, for instance, has a bearing on fundamental discussions carried out in the past about the choice between different formulations of electromagnetic theory. The present paper analyses the calculation of T anew, emphasizing the relation to, and the explanation of, the important experiment carried out by Whitworth and Stopes-Roe in 1971. The outcome of the experiment is explained in terms of standard magnetostatics, based upon use of the scalar potential phi . The relation to the so-called Kennelly-Sommerfeld controversy is commented upon. The physical interpretation of the situation is elucidated by carrying out in detail an examination of the total energy balance in the special case of spherical magnet geometry.

Quasielastic light scattering (QLS) in the glassy polymer poly(methylmethacrylate) is investigated in the temperature range 12-470 K. Raman spectra were measured in the frequency interval 3-600 cm^-1. The detailed temperature dependence of the Raman intensity at some fixed frequencies, omega =5, 8, 20 cm^-1 is obtained. The influence of the quenching on the QLS is investigated. It is shown that there are two characteristic temperature regions with different dependences of the QLS intensity on temperature. The first one is for T<50-80 K, where the reduced Raman intensity increases with a characteristic activation energy of the order of 30 K. The second region is the high-temperature one: T>80-100 K. Here the relaxation time is nearly constant while the reduced Raman intensity increases with an activation energy of the order of 700 K up to the glass transition temperature T_g. The spectral dependence of the relaxation contribution, found by a subtraction of the lowest-temperature (12 K) spectrum from the current one, I(T, omega )/(n( omega )+1), demonstrates a relaxational beta peak which weakly depends on temperature. Its characteristic frequency is of the order of its width and is equal to 8-10 cm^-1. The peak can be well fitted by a Lorentzian. The data obtained are considered within the frames of the existing models of QLS.

Formulae for X-ray dynamical Bragg diffraction are derived for a plane-parallel crystal with arbitrary thickness. These formulae are applicable even when the contribution of the real part of the atomic scattering factors to the structure factor vanishes. Based on these formulae, dynamical Bragg reflection and transmission induced only by the imaginary part of the atomic scattering factor are studied in detail.

We present results from neutron diffraction studies of single crystalline HoFeO₃ in a pulsed magnetic field by means of the spectrometer SNIM-2 at JINR-Dubna. The weak antiferromagnetism induced by the applied field in the Fe³⁺ and Ho³⁺ ion sublattices is measured in the temperature range 79-154 K. The temperature dependence of the antiferromagnetic susceptibility of the holmium subsystem is derived and found to obey the Curie-Weiss law.

Lead magnesium tungstate Pb₂MgWO₆ undergoes a structural phase transition at about 310 K. This phase transition has been characterized by Raman spectroscopy. The main features of the Raman spectra, collected between 173 and 523 K, are consistent with group theoretical predictions though unexpected weak and broad signals are observed in the high-temperature phase. From the comparison between these results and the structures previously determined, it is concluded that the phase transition has a behaviour intermediate between pure displacive and order-disorder.

This paper reports on the measurement of the optical properties of incommensurately modulated ((CH₃)₄N)₂CuCl₄ by using the High Accuracy Universal Polarimeter. With this technique it is possible to measure, simultaneously, linear birefringence, linear dichroism, circular birefringence, circular dichroism and the rotation of the optical indicatrix. Two different samples are used. The orientation of the first sample allows for the measurement of the gyration tensor element g₃₃. In the other sample g₁₃ can be measured. The optical effects are studied as a function of temperature, both in a region of zero linear dichroism and in a region of finite linear dichroism. The measurements reveal that the crystals have a low defect concentration. The presence of the incommensurate modulation is clearly revealed by the linear birefringence. The linear dichroism, on the other hand, appears to be unaffected by the modulation. It is interesting that if one averages the effect of the incommensurate modulation a crystal structure is obtained that is believed to be orthorhombic and centrosymmetric. Two effects are observed that are forbidden by the symmetry of this average structure. One of them is the rotation of the optical indicatrix, which contradicts an orthorhombic symmetry. The other is the onset of non-zero optical activity, approximately halfway through the incommensurate phase. In centrosymmetric media this effect is not allowed to occur. The observed temperature dependence of the optical activity g₁₃ differs from the behaviour measured by other authors. It is discussed whether the observed effects can be attributed to a symmetry breaking by the incommensurate modulation.

The solubility and thermodynamics of hydrogen in quasi-two-dimensional vanadium lattices have been investigated. The quasi-two-dimensional character of the vanadium lattices were obtained by growing alternating layers of molybdenum and vanadium in Mo/V superlattice structures. The hydrogen solubility was found to be higher in the thin V layers than in bulk vanadium. The enthalpy change, Delta H_H, reached a value of -0.42 eV per H atom after a steep increase from the infinite-dilution limit (-0.28 eV per H atom). The entropy change, Delta S_H, is found to decrease for the thin V layers compared to bulk vanadium for hydrogen concentrations up to H/V=0.2. Above 0.2, Delta S_H reached the bulk values.

The phase diagram of superconducting UPt₃ in the pressure-temperature plane, together with the neutron scattering data is studied within a two-component superconducting order parameter scenario. In order to give a qualitative explanation to the experimental data a set of two linearly independent antiferromagnetic moments which emerge appropriately at the temperatures T_N approximately 10 T_c and T_m approximately T_c and couple to superconductivity is proposed. Several constraints on the fourth-order coefficients in the Ginzburg-Landau free energy are obtained.

Self-sustained oscillations and fluctuations have been observed in undoped hydrogenated amorphous silicon (a-Si:H) thin films when small portions of the sample were biased using coplanar electrodes. After subjecting the samples to very high voltages several times, the current flow becomes unstable. At constant voltage the current continuously increases ending in the irreversible formation of visible channels between the electrodes which exhibit an almost periodic structure. During this process of pattern formation random fluctuation as well as periodic single-frequency and mode-locked multiple-frequency current oscillations and random telegraph noise have been detected. These phenomena are attributed to microcracks in the sample which change their resistivity by the hopping of hydrogen between different binding configurations.

Detailed measurements of the voltage-current (V-I) characteristics for c-axis-oriented Bi_1.8Pb_0.4Sr₂Ca₂Cu₃O_y (Bi(2:2:2:3))/Ag tapes have been performed in a low-magnetic field range. The results show that the V-I curves are determined by a thermally activated flux creep process and possess a power-law behaviour. The activation energy U_J derived from the V-I curves was found to follow a (1-T/T_c)ⁿ/H^0.15 functional relationship, n being 0.37 for H//ab and 0.57 for H//c. In contrast with the (1-T/T_c)³2//H theoretical prediction, this result shows very weak field and temperature dependences of U_J in the low-field region. A small negative curvature of log V versus log I plots near the temperature T_m at which the V-I curves change from being strictly linear to being nonlinear can also be found. This observation was tested using vortex glass (VG) or collective pinning (CP) models. The results showed that the VG or CP descriptions fail probably because of the high current density used in the measurement. We think that the negative curvature can be interpreted from the possible existence of the crossover from flux creep to flux flow.

The AC susceptibility, zero-field-cooled and field-cooled magnetization, hysteresis and Mossbauer studies in the temperature range of 4.2 to 300 K of Zn_0.5Co_0.9Fe_1.2Ti_0.4O₄ are reported. The observed peak in AC susceptibility, the temperature irreversibility in magnetization and the change in coercivity with temperature are shown to be due to the presence of single-domain magnetic clusters. The temperature dependence of the magnetic hyperfine field in Mossbauer spectra indicates the absence of noncollinearity of spins even at low temperatures. The anomalous shapes of the Mossbauer spectra at higher temperatures have been found to result from the presence of ionic spin relaxation. The superparamagnetic fluctuations effect affects the Mossbauer spectrum above 260 K only.

We present an X-ray absorption spectroscopy (XAS) investigation performed at the L edges of the rare earth and at the K edge of iron in the R-Fe intermetallic compounds (La,Ce)₂Fe₁₄BH_x and Ce₂Fe₁₇(H,N)_x, to elucidate the role of the interstitial doping on the electronic and magnetic properties of these systems. Comparison with X-ray circular magnetic dichroism (XCMD) experiments has been carried out to clarify the localization of 4f magnetic moment at the Ce sites upon hydriding. Both XAS and XCMD results evidence the interplay between the structural and magnetic changes that are associated with the modification of the hybridization between the Fe(3d) and Ce(5d) bands.

In a recent paper Likos et al. apply the mean-field approximation to study the phase diagram of simple classical systems with extremely short-ranged attractive interactions. We show in this comment that the mean-field approximation gives rise to important quantitative and qualitative errors in this problem.

We reply to the preceding comment by Rascon and co-workers (J. Phys. Condens. Matter, vol.7, p.8211, 1995), which discusses isostructural FCC-FCC transitions in systems with short-range attractions.