Table of contents

Experimental results are presented for the ultrasonic velocity and attenuation of shear waves at 2.4 MHz in proustite. The effects of the polarization and wavelength of irradiation in the range 450-900 nm on the photoinduced phase transition at 210 K are reported.

The quasiparticle damping and the phase space are studied through the Fermi liquid analysis for a non-half-filled two-dimensional weakly interacting tight-binding Fermi system. The T² ln T or omega ² ln omega dependence of the damping is obtained, in the same way as for 2D Fermi systems with effective-mass dispersions.

Measurements of the thermal expansion and elastic constants of dilute antiferromagnetic Cr alloys are reviewed and analysed to obtain the strain dependence of characteristic temperatures (energies) for the Neel transition and the phase transition between the incommensurate (I) and commensurate (C) spin-density-wave (SDW) phases. The results are related where possible to the behaviour of the transition temperatures under pressure, which correspond normally to positive and negative volume dependence, respectively, of the Neel temperature T_N and the IC SDW phase transition temperature T_IC, as indicated by the minimum at T_N and maximum at T_IC in the temperature dependence of the thermal expansivity. The unique anomaly in the temperature dependence of the shear strain modulus in CrRu alloys is analysed along the same lines.

The first-order isotopic difference method of neutron diffraction has been used to study the interatomic coordination of the ferric ion in three concentrated ( approximately 2 M) heavy water solutions, each prepared under different conditions with differing degrees of acidity: (i) 1.52 M Fe(NO₃)₃; (ii) 1.72 M Fe(NO₃)₃; and (iii) 2 M Fe(ClO₄)₃. In all solutions, a well defined hydration structure was obtained, with nearest neighbour Fe³⁺. . .O and Fe³⁺. . .D correlations at 2.01(2) and 2.68(3) AA respectively. A variety of coordination numbers were calculated: in the 1.52 M nitrate solution a complete six-fold hydration shell was found, whereas for the 1.72 M nitrate solution, which was heated prior to preparation, significant hydrolysis was observed. By contrast the Fe³⁺ ion in the perchlorate solution showed six-fold coordination but with significant hydrolysis.

Neutron diffraction experiments were carried out on aqueous solutions of 2.2 M chromium (III) perchlorate in water, heavy water and a 'null' mixture of water and heavy water. The isotopic difference methods of neutron diffraction were applied to the scattering data, and the results were used to obtain structural information concerning the Cr³⁺ hydration. The first hydration shell of Cr³⁺ contains six water molecules with oxygen and hydrogen atoms situated at 1.98(2) and 2.60(3) AA, respectively, consistent with a trigonal pyramidal coordination mode for the water molecules. With a coordinated water molecule geometry appropriate to the crystal structures a mean tilt angle of 34+or-6 degrees is calculated. A second-order difference was calculated from the results of the water and heavy water solutions, and g_CrH(r) was determined. The result for the 'null mixture', which is predominantly g_CrO(r) is compared with the appropriate combination of the results for the water and heavy water solutions, and in this way it is demonstrated that the Cr³⁺ ion does not show preferential hydration in perchlorate solutions. There is also clear evidence for orientational correlations in the second hydration sphere with broad peaks centred at 4 and 4.5 AA which are assigned to Cr-O and Cr-D correlations respectively.

The first-order isotopic difference method of neutron diffraction was applied to the iron ions of an acidic 1 molal solution of iron chloride in heavy water. Results were obtained for the Fe²⁺ hydration; these show that this ion is hexahydrated with nearest neighbour Fe. . .O and Fe. . .D distances of 2.12(2)AA and 2.75(5)AA, respectively. There is also evidence of a weak second hydration shell.

The authors show that below the lambda point a microscopic (pseudospin) model of liquid ⁴He, that incorporates hard-core repulsion and nearest-neighbour attraction between atoms, is equivalent to a Gross-Pitaevskii (GP) model of weakly interacting (soft-core) bosons in the presence of a self-generated gauge field, by specializing to cylindrically symmetric vortex solutions. Requiring consistency in the behaviour of the condensate density in the two models, they are able to identify the gauge field in the GP model as that part of the total velocity that has a non-vanishing curl in the pseudospin model. It becomes evident that the gauge field is the depletion velocity.

A density functional approach is proposed to study the ⁴He liquid-gas interface. The free energy density, which depends on the particle density and temperature, has been adjusted to reproduce the liquid density and the vapour pressure along the liquid-gas coexistence line, as well as the zero-temperature surface tension. After achieving a fairly good description of the phase transition, the calculated surface tension agrees well with the experimental results. The calculated density profile is used to discuss a recent experimental determination of the surface thickness.

The particle-insertion method of Widom (1963) has been widely used in numerical simulations for the purpose of calculating the excess chemical potential, mu _ex. It is known, however, that values of mu _ex obtained by Widom's method are strongly dependent on N, the number of particles in the system. The authors use the formalism of the grand-canonical ensemble to derive an expression for the leading term in the finite-size correction to mu _ex. The new expression reduces to the exact result for hard rods and gives numerical results in good agreement with those of simulations of hard-disk and hard-sphere fluids. It is, in both respects, an improvement over an expression previously derived by Smit and Frenkel via a thermodynamic argument.

A characterization of the dielectric properties in incommensurately modulated phases is presented. First a microscopic description is adopted. It takes into account the structural restrictions imposed on a microscopic scale by the superspace group. The macroscopic optical properties are derived from it. The optical activity as observed in the incommensurate phases of several compounds cannot be explained within a plain microscopic approximation. Therefore, a mesoscopic level is considered, implying averaging distances large with respect to the basic unit cell parameters or even the modulation wavelength, but small with respect to the size of the finite crystal. The phenomenological approach presented is compatible both with the microscopic description based on a superspace group symmetry, as well as with the macroscopic one involving the point group of the average structure. In the case considered here (one-dimensional incommensurate modulation) the optical properties of the dielectric medium are analysed in terms of subperiodic groups, periodic along the direction of the modulation wave and homogeneous along the other two directions. It is shown that the optical properties are affected by the incommensurability and that small changes in symmetry related to boundary conditions can lead to macroscopic effects. To interpret the experimental data more quantitatively a Jones model is applied, which describes the mesoscopic spatial dispersion of the modulated dielectric tensor to vary periodically along the modulation and being constant in the plane perpendicular to it. This model is compared with the experimental results for the ellipticity angle chi of (N(CH₃)₄)₂ZnCl₄ obtained for light propagating along the modulation.

The birefringence and optical activity of (N(CH₃)₄)₂ZnCl₄ in phases I, II, III and IV have been measured with a high-accuracy universal polarimeter for six samples, all with different orientations. The birefringences clearly show the phase transitions. From the data, the diagonal components g_ii of the second-rank gyration pseudotensor could be determined and the off-diagonal components g_ij can be estimated. The results can be interpreted with the mesoscopic description of Dijkstra, Janner and Meekes which is especially applicable to the incommensurately modulated phase (II), but also to the multidomain monoclinic phase (IV).

The temperature dependence of the lattice parameters in the beta -phase (BCC structure) of pure Zr and Zr-2 at.%Co was determined between 900 and 1550 degrees C by neutron diffraction. All diffraction patterns show a high diffuse background, which is identified as thermal diffuse scattering and originates from particular low-energy phonons in BCC Zr and its alloys. The lattice parameter of pure BCC Zr is given by a_Zr=3.14(17)*10^-5T+3.579(5) in AA (T in K). Alloying with Co results in a contraction of BCC Zr, from which it is concluded that Co is dissolved substitutionally. The consequences of these results for the fast diffusion of Co in BCC Zr are discussed.

The authors have prepared the alloys Al₆₅Cu₂₀(Fe_1-xMn_x)₁₅ with x=0.0, 0.2, 0.4 and 0.6 by the liquid-quenching method. The alloys with x=0.0 and 0.6 are examined by the transmission electron microscopy. Their electron diffraction patterns demonstrate fivefold, threefold and twofold symmetries of the typical icosahedral symmetries of quasi-crystals. The X-ray diffraction patterns of the samples with x=0.0, 0.2, 0.4 and 0.6 can be indexed with six independent Miller indices of the icosahedral structure of quasi-crystals. Thus they can determine the quasi-lattice constant, which was found to increase with increase in the manganese concentration. The differential thermal analysis measurements show that the icosahedral quasi-crystals Al₆₅Cu₂₀(Fe_1-xMn_x)₁₅ with x=0.0, 0.2, 0.4 and 0.6 are in thermodynamically good stable phases. The room-temperature resistivity of the samples are in the range 15.47-30.93 mu Omega m, which increases with increase in the manganese concentration. The very high resistivities may be caused by the high density of distorted icosahedral structure defects and the possible s-d resonant scattering. The inversed susceptibility versus temperature for x=0.4 and 0.6 can fit the Curie-Weiss formula. The local magnetic moments are 1.64 mu _B and 1.27 mu _B per Mn atom for samples with x=0.4 and 0.6, respectively. In contrast the samples with x=0.0 and 0.2 do not display a Curie-Weiss-like behaviour.

The low-temperature lattice specific heat of very dilute FCC Al-Li alloys has been studied by applying the Green function method to an isolated substitutional Li impurity. The specific heat is very sensitive to force constant changes induced by the defect. It is observed that the contribution of the T_1u mode is not preponderant. The authors include the effect of the second neighbours of the Li atom.

An optimization procedure for determining the force constant parameters of a simple 'eleven-parameter' rigid-ion model (RIM) has been developed that yields good predictive power for the eigendisplacements (EDs). The disregard that the RIM has fallen into in the last few decades is undeserved, and here the authors show that given an appropriately optimized set of parameters the RIM can predict good results. The optimization procedure is presented and applied here to SiC and GaAs. For SiC the optimized parameters not only give calculated phonon frequencies which agree with the measurements to better than 6% but also predict the EDs which agree with the ab initio (LDF) method to better than 1%. In the case of GaAs the RIM, using the optimized parameters, predicts the observed EDs to well within the quoted experimental uncertainties. Where the measurements do not exist, the predicted EDs in GaAs agree with those calculated by the LDF method to better than 1%. As is the case of the LDF method calculations on GaAs, the present RIM calculations for the transverse acoustic (TA) frequencies show large variations from the measured values towards the zone boundaries. This implies that short-range forces from further neighbours need to be taken into account if the calculated frequencies are also to agree with the measurements. The RIM calculations presented here of EDs in SiC and GaAs using the set of force constant parameters optimized for that purpose, suggest that these calculations are essentially model independent.

Electronic structures of Fe₁₆X₂ (X=B, C and N) were calculated by the LMTO-ASA method within the framework of the local-spin-density approximation, and the effect of X atoms on the magnetic properties were investigated. The magnetic moments of the Fe(4e) atoms which are nearest to the X atoms decrease. This is due to the hybridization between Fe(4e) d states and X valence states. On the other hand, the Fe(4d) atoms farthest from X atoms carry a very large magnetic moment. The Fe(4d) moment is indirectly influenced by X atoms through the neighbouring Fe and the enhancement increases for X=B, C and N, in that order. The hyperfine fields at Fe(4e) and Fe(8h) in Fe₁₆N₂ are nearly equal, though their magnetic moments are different. This fact can be explained on the basis of the difference of the contributions from the valence s electrons.

Recent optical measurements of a 1.4 eV doublet in diamond have revealed distinct optical polarization features which are associated with a nickel defect. A model for the defect is investigated taking into account the large electronic hybridization that will take place about the metal ion. Relative spectroscopic intensities and estimates of the spin-orbit interaction are considered within the LCAO approximation. Finally, a cluster model calculation is made specifically to investigate the electronic structure of the nickel defect and estimate the LCAO coefficients. From the results, suggestions are made about the possible charge states and lattice locations of nickel in diamond.

The mode-coupling equations for a random glass (structural or spin-glass systems) are derived by using the Keldysh technique. For this derivation the assumption about the initial equilibrium state of the system is crucial. The glass transition corresponds to the appearance of a time-symmetry-breaking solution. Therefore the order parameter occurring below the critical temperature has a purely dynamic origin. The final dynamic equations are reduced to a model introduced by Gotze and Sjogren (1984). The other characteristic feature of the glassy state is the conservation of a slightly non-equilibrium distribution. In this quasi-equilibrium case the dynamics is described by Parisi-Sompolinsky-Zippelius equations.

The local electronic density of states (LDOS) has been calculated for Fe-Ge(110), Fe-Ge(111) and Fe-Ge(100) interfaces and neighbouring atomic planes using the recursion method. Interface states are found to exist within the mutual gaps of the constituent atoms and strongly depending on the local atomic environments. The most excess LDOSs are found for Fe-Ge(111) interface and the least for Fe-Ge(110). The magnetic moments for Fe atoms are found to decrease when the Fe layer approaches the interface boundary, which is in accord with the experiments. The electron spin polarization parameters evaluated from the LDOS are qualitatively consistent with experimental measurements.

The authors study the effects of discreteness on the motion of coupled solitons of two weakly coupled discrete sine-Gordon systems. A collective coordinate method associated with Dirac's formalism of constrained Hamiltonian dynamics is used to derive the equations of motion for the centres of the solitons and for the dressing or discreteness corrections of the continuum solitons. The authors show that the dynamics of the coupled solitons can be described by a set of two non-linear differential equations. It is also shown that the coupling reduces (increases) the trapping processes in the case where the two solitons have the same polarity (different polarities). A numerical analysis of the static dressing equations is performed. They find that the dressing lowers the potential energy of solitons and increases the Peierls-Nabarro barrier.

Thermal treatment of Bi, Pb, Ga, Sr, Ca and Cu oxides mixed to the nominal composition Bi_1.8Pb_0.1Ga_0.1Sr₂Ca₂Cu₃O_y produces a superconducting material with a transition at 72-74 K. X-ray data indicate only the (2212) orthorhombic phase with lattice constants a=b=5.4 AA and c=31 AA. Resistance data of polycrystalline samples of this material show quasi-two-dimensional paraconductivity effects. The effective thickness of the superconducting electronic sheet, d=53 AA, is comparable to the value of 30 AA found for the coherence length from the magnetoresistance data. As in other Bi-based high-T_c materials, this compound shows a substantial dissipation in the presence of an external magnetic field.

URhAl is one of a large class of uranium compounds with the hexagonal ZrNiAl structure. It becomes ferromagnetic at 27 K and the ordered moment is found to be 0.94 mu _B per formula unit. The authors have performed experiments with polarized neutrons in order to measure the magnetization density in the unit cell. A crystallographic study gives atomic positions in agreement with previous work, but they find that the crystals, which were grown by a Czochralski method, are highly perfect and exhibit a large amount of extinction. Corrections for these effects are discussed. The magnetization density shows that the total moment as measured by magnetization is made up of four contributions. The moments (in mu _B) on uranium, Rh_I, Rh_II, and from the conduction electrons are, respectively 0.94(3), 0.28(2), 0.03(2), and -0.11(3). The observation of a large moment at the Rh_I site, which lies in the (001) plane with the U atoms shows that strong anisotropic hybridization occurs between the U and Rh_I. This is the cause of the very large bulk anisotropy. At the uranium site they have analysed the contributions from the spin and orbital moments and find - mu _L/ mu _S=1.81(7). The free ion 5f³ value for this ratio if 2.6. The reduction gives one a quantitative measure of hybridization in the system. The results are compared with those calculated from density functional theory.

The electron paramagnetic resonance spectra of Cr³⁺ in LiNbO₃ and heavily Mg-doped LiNbO₃, have been measured and analysed according to the superposition model. The axial spectrum of Cr³⁺ in LiNbO₃ is consistent with Nb substitution. On the other hand, the dominant isotropic spectrum found in Mg-doped LiNbO₃ can be accounted for if an appropriate ( approximately=0.1 AA) relaxation of the Cr³⁺ along the c-axis is allowed. Moreover, a point-ion calculation shows that the axial shift of the impurity ion is also qualitatively consistent with recent observations of an enhanced pi -polarization character of the optical transitions in LiNbO₃:Mg, Cr.

Memory effects have been studied in the vicinity of the incommensurate-commensurate phase transition in Rb₂ZnCl₄ single crystals. Properties in the incommensurate phase have been observed to depend critically on whether a crystal had been cooled or heated before the temperature is kept constant (thermal memory) or whether it had been subjected to stresses (mechanical memory) or to a static electric field (electric memory) previously. The crystal exhibits pronounced after-effects specific for incommensurate phases which can be understood in terms of soliton pinning on lattice inhomogeneities.

The thermal quenching of (A⁰X) and (D⁰X) emission lines due to processes of the type (A⁰X) to A⁰+X and (D⁰X) to D⁰+X, respectively, is discussed in terms of nonradiative phonon-induced transitions. The onset and the strength of the quenching are determined by the exciton-impurity binding energy as well as the amount of lattice relaxation accompanying these transitions. The latter effect, which has obviously been overlooked in earlier discussions, depends upon the degree of localization of the 'persistent' particle (hole or electron, respectively) and is relatively strong in the case of the 'deep' acceptors (such as Cu_Cd and Ag_Cd in CdTe). The quantitative correlation of the quenching parameters (and also of the capture coefficients related to the inverse processes) with the ionization energies of A⁰ and D⁰, which is expected on this basis, is confirmed by experimental results for CdTe and InP.

Emission spectra of diamond films grown by microwave-plasma-assisted CVD have been carried out at 77 K. The Raman spectra are not the same for different wavelengths of excitation. Deconvolution of these spectra shows two major components (A and B) in addition to the first-order diamond Raman line. The scattering cross sections of these components are large by comparison with the scattering cross section of diamond. The B component decreases in intensity with respect to the diamond Raman line as the wavelength of excitation is decreased. Component A is very similar to the spectrum from amorphous Si_0.3C_0.7:H by Ramsteiner et al. (1988). It is suggested that the B component may be associated with a silicon-rich form of amorphous silicon carbide. There is a broad-band luminescence at 1.95 eV whose excitation behaviour and temperature dependence is similar to that associated with donor-acceptor recombination radiation in amorphous Si_0.3C_0.7:H as reported by Sussmann et al. (1981).

The use of resonant Raman scattering at X-ray energies for studies of the near-edge structure of the absorption edge is presented. The potential of the method is shown in the case of the Ho L_III-edge, which is measured using Cu K alpha 1 radiation and a focusing crystal spectrometer. The results are interpreted within a model where the density of final electron states is constant plus a narrow band where the density is high (white line). A comparison with the other methods used in X-ray absorption near-edge structure spectroscopy is also given.

Photoemission is performed on TlSbS₂, a layered compound with a large unit cell (16 atoms) and a calculated structure of 28 valence bands. The angle-integrated emission data confirm the existence of three groups of bands as obtained in the tight-binding calculation. In the angle-resolved modem, the observed emission presents a large energy dispersion with an apparent period larger than the reciprocal lattice. This effect, also present in other materials having a large number of atoms by unit cell, is attributed to strong variations versus k/sub /// of the transition matrix elements. In this case, photoemission data are not directly comparable with the band structure and a complete calculation of emission probability is needed to assign the observed bands.

The authors study the chemical shift of the Cu 2p core level for Cu₂O, CuO and NaCuO₂, where Cu is formally monovalent, divalent and trivalent, respectively. They find that the binding energy increases with the valence, as expected, although the variation in the number of 3d electrons is small and the trivalent Cu atom is found to have a smaller net positive charge than the monovalent or divalent Cu. The origin of the calculated chemical shifts is analysed and the relation with the chemical valence is discussed.

The authors re-examine the electronic structure of 3d impurities in niobium and molybdenum using local density functional theory and an improved version of the KKR-Green function method. The calculations reveal an extraordinary sensitivity of the local 3d moments with respect to the angular momentum cutoff l_max of the wavefunctions. Whereas previous calculations with l_max=2 yielded very large moments for Cr, Mn and Fe impurities in Nb, these moments are completely suppressed, if higher angular momenta (l_max=3 or 4) are added. A qualitatively similar trend is found for the Mo host; however the reduction of the local moments is more moderate. The present results are in good agreement with the experimental data.

The thermoelectric effect in a semiconductor superlattice in a nonquantized electric field is investigated for electrons of the lowest miniband in the linear approximation of Del T. Analytical expressions for the thermopower and the heat conductivity coefficient are obtained as functions of the superlattice parameters Delta and d, temperature, concentration and electric field E. The results confirm the fact that depending on the relation between Delta and other characteristic energies of the carrier charge (kT, xi and h(cross)/ tau ) the carrier charges can behave either as a quasi-two-dimensional or as a three-dimensional electron gas. The prospect of using a superlattice as a good-quality and highly efficient thermoelement is also proposed.

Spin fluctuations in the paramagnetic phase of the spin-density-wave antiferromagnet Cr are greatly reduced by doping with V, as observed directly by inelastic neutron scattering, and indirectly by the effect on the nuclear magnetic relaxation time, the nature of the Neel transition, the electrical resistivity, the thermal expansion and bulk modulus, and the magnetic susceptibility. The collation here of these previously reported experimental results shows their significance, while the effect of V doping on the magnetoelastic properties of Cr is represented directly, rather than through the change in the magnetic Gruneisen parameter, as previously.