Superparamagnetic relaxation Mössbauer spectra by phonon–magnon interaction using the multilevel method

The formation of local moments and the effect of charge carrriers in dilute magnetic semiconductors can be well understood using local probe techniques like Mossbauer Spectroscopy. We report here on Mossbauer studies in the systems Fe_0.008Ge_{1 ? x} D _x (D = As, Bi), Fe_0.008Ge_{1 ? x} In _x , and Fe_0.008Ge_{1 ? x} Sn _x . At room temperature magnetic interactions were observed for donor (D) impurities at the Fe site in the Fe_0.008Ge system. No such magnetic ordering was observed for acceptor (In) or neutral (Sn) impurities.     

Dielectric spectroscopy studies of ZnO single crystal

Dielectric relaxation and charge transport induced by electron hopping in ZnO single crystal are measured by using a novocontrol broadband dielectric spectrometer. Typical Debye-like dielectric relaxation originating from electronic hopping between electronic traps and conductive band in surface Schottky barrier region is observed for ZnO single crystal-Au electrode system. However, after insulation of ZnO single crystal by heat treatment in rich oxygen atmosphere, dielectric relaxation and alternating current conductance are observed simultaneously in the dielectric spectra, implying that dielectric relaxation and charge transport can be induced simultaneously by electronic hopping at high temperature in an ordered system. The intrinsic correlation between local dielectric relaxation and long range charge transport offers us a new method to explore complicated dielectrics.     

Order and chaos in soft condensed matter

Soft matter, like colloidal suspensions and surfactant gels, exhibit strong response to modest external perturbations. This paper reviews our recent experiments on the nonlinear flow behaviour of surfactant worm-like micellar gels. A rich dynamic behaviour exhibiting regular, quasi-periodic, intermittency and chaos is observed. In particular, we have shown experimentally that the route to chaos is via Type-II intermittency in shear thinning worm-like micellar solution of cetyltrimethylammonium tosylate where the strength of flow-concentration coupling is tuned by the addition of sodium chloride. A Poincaré first return map of the time series and the probability distribution of laminar length between burst events show that our data are consistent with Type-II intermittency. The existence of a ‘Butterfly’ intensity pattern in small angle light scattering (SALS) measurements performed simultaneously with the rheological measurements confirms the coupling of flow to concentration fluctuations in the system under study. The scattered depolarised intensity in SALS, sensitive to orientational order fluctuations, shows the same time-dependence (like intermittency) as that of shear stress.     

Electron-confined LO-phonon scattering in GaAs-Al0.45Ga0.55As superlattice

We develop a theoretical model to the scattering time due to the electron-confined LO-phonon in GaAs-Al_xGa_1-xAs superlattice taking into account the sub-band parabolicity. Using the new analytic wave function of electron miniband conduction of superlattice and a reformulation slab model for the confined LO-phonon modes, an expression for the electron-confined LO-phonon scattering time is obtained. In solving numerically a partial differential equation for the phonon generation rate, our results show that forx = 0.45, the LO-phonon in superlattice changes from a bulk-like propagating mode to a confined mode. The dispersion of the relaxation time due to the emission of confined LO-phonons depends strongly on the total energy.     

Converged properties of clean metal surfaces by all-electron first-principles calculations

All-electron full-potential linearized augmented plane-wave calculations of the surface energy, work function, and interlayer spacings of close-packed metal surfaces are presented, in particular, for the free-electron-like metal surfaces, Mg(0 0 0 1) and Al(1 1 1), and for the transition metal surfaces, Ti(0 0 0 1), Cu(1 1 1), Pd(1 1 1), and Pt(1 1 1). We investigate the convergence of the surface energy as a function of the number of layers in the slab, using the Cu(1 1 1) surface as an example. The results show that the surface energy, as obtained using total energies of the slab and bulk from separate calculations, converges well with respect to the number of layers in the slab. Obviously, it is necessary that bulk and surface calculations are performed with the same high accuracy. Furthermore, we discuss the performance of the local-density and generalized gradient approximations for the exchange-correlation functional in describing the various surface properties.     

Dissociative adsorption and adsorbate-induced reconstruction on Fe{2 1 1}

The products of CO, NO, O₂ and N₂ dissociation on Fe{2 1 1} have been studied by means of first-principles density functional theory. Preferred adsorption sites for adatoms C, N and O are identified, and trends in charge transfer and surface magnetism described. An experimentally observed (2 × 1) reconstruction induced by O is confirmed to be energetically stable, and a similar reconstruction induced by N is tentatively predicted. It is argued that these reconstructions may be important not only in the context of the catalytic reactivity of the Fe{2 1 1} surface, but also for the initial stages of surface nitridation and oxidation.     

Oxidation process of SiGe on SOI substrates

The oxidation of SiGe film epitaxial grown on top of SOI wafers has been studied. These SiGe/SOI samples were oxidized at 700, 900, 1100 °C. Germanium atoms were rejected from SiGe film to SOI layer. A new Si_1−xGe_x (x is minimal) layer formed at SiGe/Si interface. As the germanium atoms diffused, the new Si_1−xGe_x (x is minimal) layer moved to Si/SiO₂ interface. Propagation of threading dislocation in SiGe film to SOI substrate was hindered by the new SiGe/Si interface. Strain in SOI substrate transferred from SiGe film was released through dislocation nucleation and propagation inner. The relaxation of SiGe film could be described as: strain relaxed through strain equalization and transfer process between SiGe film and SOI substrates. Raman spectroscopy was used to characterize the strain of SiGe film. Microstructure of SiGe/SOI was observed by transmission electron microscope (TEM).     

Dynamical precursor of nematic order in a dense fluid of hard ellipsoids of revolution

We investigate hard ellipsoids of revolution in a parameter regime where no long range nematic order is present but already finite-size domains are formed which show orientational order. Domain formation leads to a substantial slowing down of a collective rotational mode which separates well from the usual microscopic frequency regime. A dynamic coupling of this particular mode into all other modes provides a general mechanism which explains an excess peak in spectra of molecular fluids. Using molecular dynamics simulation on up to 4096 particles and on solving the molecular mode coupling equation we investigate dynamic properties of the peak and prove its orientational origin. Received 19 September 2001 and Received in final form 19 March 2002     

Competing polarization mechanisms in the heavy-fermion paramagnet CeCu<Subscript><Emphasis Type="Bold">6</Emphasis></Subscript>

Nuclear magnetic resonance (NMR) and relaxation of ⁶³Cu and ⁶⁵Cu in a powder sample of the heavy-fermion paramagnet CeCu₆ is measured and analysed quantitatively. Five different Cu sites are accessible to a detailed analysis. We derive quadrupolar splitting frequencies, Ce to Cu transferred hyperfine field coupling constants, and transversal as well as longitudinal relaxation behaviour. Only small relaxation anomalies are observed at the orthorhombic to monoclinic structural phase transition of CeCu₆. We point to the different importance of transferred hyperfine interaction and local conduction electron density for static or dynamic part, respectively, of Cu hyperfine interaction. The different sign of the transferred hyperfine interaction from Ce³⁺ to different Cu neighbours reveals the different competing interaction mechanisms, giving rise to the heavy-fermion paramagnetic behavior of CeCu₆. Received 20 November 2001