M. Krisch

Spin crossover in ferropericlase at high pressure: a seismologically transparent transition?

An iron spin transition has no effect on the seismologic properties of lower-mantle minerals. Seismic discontinuities in Earth typically arise from structural, chemical, or temperature variations with increasing depth. The pressure-induced iron spin state transition in the lower mantle may influence seismic wave velocities by changing the elasticity of iron-bearing minerals, but no seismological evidence of an anomaly exists. Inelastic x-ray scattering measurements on (Mg0.83Fe0.17)O-ferropericlase at pressures across the spin transition show effects limited to the only shear moduli of the elastic tensor. This explains the absence of deviation in the aggregate seismic velocities and, thus, the lack of a one-dimensional seismic signature of the spin crossover. The spin state transition does, however, influence shear anisotropy of ferropericlase and should contribute to the seismic shear wave anisotropy of the lower mantle.

Inelastic X-ray scattering in YBa2Cu3O6.6 reveals giant phonon anomalies and elastic central peak due to charge-density-wave formation

1 Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany 2 European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France 3 CNR-SPIN, CNISM and Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy 4 Institut für Festkörperphysik, Karlsruher Institut für Technologie (KIT), P.O.B. 3640, D-76021 Karlsruhe, Germany

A perfect crystal X-ray analyser with 1.5 meV energy resolution

Abstract A new spherical silicon crystal analyser, operated in backscattering geometry at the Si(11 11 11) reflection, allows to perform inelastic X-ray scattering experiments with a total energy resolution of 1.5 ± 0.2 meV full-width-half-maximum (FWHM) at 22 keV incident photon energy. This analyser was constructed by improving a procedure recently developed.

Studies of coherent and diffuse x‐ray scattering by porous silicon

The microstructure of porous silicon layers has been studied by means of x‐ray diffraction. Using a double‐crystal diffractometer, the observed diffraction patterns give directly the mismatch between the lattice parameters of the porous layers and of the substrate, and the curvature of various porous silicon samples obtained in different conditions. From measurements with the same experimental set‐up, but with a larger scan range, broad diffuse bumps produced by the pore structure have been observed. This new feature allows us to obtain structural informations on the porous silicon in a nondestructive way. In particular, we have observed the anisotropic pattern showing a preferential elongation of the pores perpendicular to the (100) surface.

Phonon dispersion and lifetimes in MgB2

We measure phonon dispersion and linewidth in a single crystal of MgB2 along the Gamma-A, Gamma-M, and A-L directions using inelastic x-ray scattering. We use density functional theory to compute the effect of both electron-phonon coupling and anharmonicity on the linewidth, obtaining excellent agreement with experiment. Anomalous broadening of the E(2g) phonon mode is found all along Gamma-A. The dominant contribution to the linewidth is always the electron-phonon coupling.

Suppression of thermal conductivity by rattling modes in thermoelectric sodium cobaltate

The need for both high electrical conductivity and low thermal conductivity creates a design conflict for thermoelectric systems, leading to the consideration of materials with complicated crystal structures. Rattling of ions in cages results in low thermal conductivity, but understanding the mechanism through studies of the phonon dispersion using momentum-resolved spectroscopy is made difficult by the complexity of the unit cells. We have performed inelastic X-ray and neutron scattering experiments that are in remarkable agreement with our first-principles density-functional calculations of the phonon dispersion for thermoelectric Na(0.8)CoO2, which has a large-period superstructure. We have directly observed an Einstein-like rattling mode at low energy, involving large anharmonic displacements of the sodium ions inside multi-vacancy clusters. These rattling modes suppress the thermal conductivity by a factor of six compared with vacancy-free NaCoO2. Our results will guide the design of the next generation of materials for applications in solid-state refrigerators and power recovery.

Elasticity of hexagonal boron nitride: Inelastic x-ray scattering measurements

We report a high-pressure investigation of the relaxor ferroelectric lead zinc niobate PbZn{sub 1/3}Nb{sub 2/3}O{sub 3} (PZN) up to 46 GPa, which is the highest pressure yet attained in the study of relaxors. The evolution of both Raman and x-ray scattering with pressure gives evidence for important pressure instabilities, which find its expression in three successive phase transitions. The observed pressure-induced suppression of diffuse scattering above 5 GPa is similar to recent reports and supports the hypothesis that this is a general feature in relaxors at high pressures.Stable pairing states of superfluid {sup 3}He in aerogel are examined in the case with a global uniaxial anisotropy which may be created by applying a uniaxial stress to the aerogel. Due to such a global anisotropy, the stability region of an Anderson-Brinkman-Morel (ABM) pairing state becomes wider. In a uniaxially stretched aerogel, the pure polar pairing state with a horizontal line node is predicted to occur, as a three-dimensional superfluid phase, over a measurable width just below the superfluid transition at T{sub c}(P). A possible relevance of the present results to the case with no global anisotropy is also discussed.A series of Ca{sub x}CoO{sub 2} (0.15{<=}x{<=}0.40) materials have been prepared by means of an ion exchange reaction from Na{sub x}CoO{sub 2}. Transmission electron microscopy (TEM) measurements revealed a rich variety of structural phenomena resulting from cation ordering, structural distortion, and twinning. Systematic structural analysis, in combination with the experimental data of Na{sub x}CoO{sub 2} (0.15{<=}x{<=}0.8) and Sr{sub x}CoO{sub 2} (1.5{<=}x{<=}0.4) systems, suggests that there are two common well-defined cation ordered states corresponding, respectively, to the orthorhombic superstructure at around x=1/2 and the 3{sup 1/2}ax3{sup 1/2}a superstructure at around x=1/3 in this kind of system. Multiple ordered states, phase separation, and incommensurate structural modulations commonly appear in the materials with 0.33<x<0.5. The TEM observations also reveal an additional periodic structural distortion with q{sub 2}=a{sup *}/2 in materials for x{<=}0.35. This structural modulation also appears in the remarkable superconducting phase Na{sub 0.33}CoO{sub 2}{center_dot}1.3H{sub 2}O.Electrical resistance, thermoelectric power, dc magnetization, ac susceptibility, and electron spin resonance (ESR) are investigated for the polycrystalline Nd{sub 1-x}Sr{sub 1+x}CoO{sub 4} (x=0.25, 0.33, and 0.60). Powder x-ray diffraction (XRD) confirms that these compounds crystallize in K{sub 2}NiF{sub 4}-type structure with space group I4/mmm. The specimens exhibit ferromagnetic and semiconducting behaviors. With Sr doping, the lattice parameter c increases, the cusp intensity related to spin-glass states weakens, and the ferromagnetic property intensifies. The transport mechanisms in high temperature range obey Arrhenius law and might be understood by small polaron models. The magnetic properties present spin-glass states at {approx}18 K and Griffiths singularity at {approx}210 K.In this work we report on a low-energy electron diffraction (LEED) study of MnO(100) thick films grown on Ag(100) in order to determine their surface geometry. The LEED results indicate a topmost layer rumple of (4.8{+-}2.0)% with the oxygen ions moving towards the vacuum side. These results are in line with other surface structure determinations carried out on the (100) surface of different oxides having rock-salt structure but are in disagreement with MEIS results reported in the literature for the MnO(100) using a MnO single crystal.We report the observation of Co{sup 3+}/Co{sup 4+} short-range charge ordering in 10% Ho-doped SrCoO{sub 3-x} by means of high resolution neutron powder diffraction. The associated one-dimensional commensurate modulation, which can be described with the propagation vector q{sub CO}=(0 0 1/2) with respect to the cubic perovskite cell Pm3m, occurs for compositions close to x=0.20, corresponding to a 1:1 Co{sup 3+}/Co{sup 4+} ratio and extends over clusters of finite size (D{approx}250 A). The bond valence sums for the Co{sup 3+} and Co{sup 4+} sites are +3.07(7) and +3.95(11) (x=0.19), very close to their nominal values +3 and +4. We attribute this astonishing observation to the one-dimensional (1D) character of the associated distortion pattern, whose elastic stabilization is eased with respect to the 3D arrays reported for other charge-ordered perovskite oxides.The compounds RNi{sub 2}Mn (R=Tb, Dy, Ho, and Er) with a MgCu{sub 2}-type structure have been synthesized. The R to transition metal atom ratio is confirmed to be 1:3 using the energy dispersive spectroscopy. The structural and magnetic properties have been investigated by various experimental methods. The x-ray diffraction patterns (XRD) can be well indexed with a cubic Laves cell and space group Fd3m. The refinement results of the XRD patterns show the presence of vacancies in the crystallographic structure. The ordering temperatures T{sub C} have been derived to be 131, 94, 75, and 50 K for R=Tb, Dy, Ho and Er, respectively, which are much higher than those of their corresponding RNi{sub 2} and RMn{sub 2} compounds. A large difference of M-T curves between zero-field-cooling and field-cooling magnetization for all samples at a certain temperature range is observed in a low field, which can be understood in the terms of narrow-domain-wall pinning and a sensitive temperature dependence of coercivity.The structure of liquid CdTe was investigated at pressures up to 23.5 GPa using synchrotron x-ray diffraction. The structure factor, S(Q), and the pair distribution function, g(r), drastically change in two pressure regions, 1.8-3.0 and 7.0-9.0 GPa, accompanied with marked increase in the average coordination number. These findings suggest that there exists at least three stable liquid forms below 23.5 GPa. The pressure interval of the structural change is much smaller compared to other liquids of tetrahedrally bonded materials. Comparing the shapes of S(Q) and g(r) and other structural parameters with the respective data for the reference materials reveals that the lowest- and intermediate-pressure forms have the same local structures as the crystalline counterpart (zinc-blende-like local structure and a NaCl-like local structure), while the highest-pressure form has a different local structure from that in the crystalline form.The charge distributions of slow atomic particles that are singly scattered, multiply scattered, recoiled, and sputtered from metal surfaces are analyzed in terms of both nonadiabatic particle-substrate electron transfer and electron transfer from electronically excited substrates. The results are compared to experimental data for 50 eV Na{sup +} ions scattered from Cu(001), and Al atoms sputtered and recoiled from Al(100). The comparison allows for a quantitative determination of the role of substrate excitations in surface charge exchange. In addition, an analysis of kinetic electron emission (KEE) is carried out using similar low-energy metal projectile-metal substrate systems. Contributions to KEE from various nonadiabatic processes are quantitatively evaluated, including the same process that is responsible for charge formation in single-scattering experiments. The results are compared to experimental KEE data induced by Na{sup +} impinging on Ru(0001). The contributions of nonadiabatic one-electron processes are shown to be small when realistic particle-substrate parameters are used. Many-electron interactions are assumed to play an important role in explaining KEE and, as an illustration, a simplified hot-spot model is outlined.Neutron powder diffraction and transport measurements have been used to investigate the PrBaCo{sub 2}O{sub 5.48} compound between room temperature and 820 K. A structural phase transition, involving a rearrangement of oxygen vacancies, was found at T{sub OD}=776 K. Across the transition the perovskite structure loses its vacancy ordering, and the crystal symmetry changes from orthorhombic Pmmm to tetragonal P4/mmm. The resistivity measurements for temperatures above {approx}350 K yield high values of {rho}, indicating that the compound is rather semiconducting than metallic as usually accepted. A model in terms of thermally activated hole (polaronic) hopping is proposed.Granular films composed of well defined nanometric Co particles embedded in an insulating ZrO{sub 2} matrix were prepared by pulsed laser deposition in a wide range of Co volume concentrations (0.15<x<0.43). High-resolution transmission electron microscopy (TEM) showed very sharp interfaces between the crystalline particles and the amorphous matrix. Narrow particle size distributions were determined from TEM and by fitting the low-field magnetic susceptibility and isothermal magnetization in the paramagnetic regime to a distribution of Langevin functions. The magnetic particle size varies little for Co volume concentrations x<0.32 and increases as the percolation limit is approached. The tunneling magnetoresistance (TMR) was successfully reproduced using the Inoue-Maekawa model. The maximum value of TMR was temperature-independent within 50-300 K, and largely increased at low T, suggesting the occurrence of higher-order tunneling processes. Consequently, the tunneling conductance and TMR in clean granular metals are dominated by the Coulomb gap and the inherent particle size distribution.The five independent elastic moduli of single-crystalline hexagonal boron nitride (h-BN) are determined using inelastic x-ray scattering. At room temperature the elastic moduli are in units of GPa C{sub 11}=811, C{sub 12}=169, C{sub 13}=0, C{sub 33}=27.0, and C{sub 44}=7.7. Our experimental results are compared with predictions of ab initio calculations and previously reported incomplete datasets. These results provide solid background for further theoretical advances and quantitative input to model elasticity in boron nitride (BN) nanotubes.I argue that certain bosonic insulator-superfluid phase transitions as an interaction constant varies are driven by emergent geometric properties of insulating states. I examine the renormalized chemical potential and population of disordered bosons at different energy levels. These quantities define the geometric aspect of an effective low energy Hamiltonian which I employ to investigate various resonating states and quantum phase transitions. In a mean field approximation, I also demonstrate that the quantum phase transitions are in the universality class of a percolation problem.The electronic structure and physical properties of {gamma}-Sn{sub 3}N{sub 4} in the spinel structure are investigated by first-principles calculations. The calculated band structure, electronic bonding, and optical properties are compared with two well-studied spinel nitrides {gamma}-Si{sub 3}N{sub 4} and {gamma}-Ge{sub 3}N{sub 4}. {gamma}-Sn{sub 3}N{sub 4} is a semiconductor with a direct band gap of 1.40 eV and an attractive small electron effective mass of 0.17. Its optical properties are different from that of {gamma}-Si{sub 3}N{sub 4} and {gamma}-Ge{sub 3}N{sub 4} because of the difference in the conduction band minimum. The Sn K, Sn L{sub 3}, Sn M{sub 5}, and N K edges of the x-ray-absorption near-edge structure spectra in {gamma}-Sn{sub 3}N{sub 4} are calculated using a supercell approach and are found to be rich in structures. These spectra are discussed in the context of the electronic structure of the unoccupied conduction band in the presence of the electron core-hole interaction. These calculated spectra can be used for the characterization of this novel compound.The structure of the incommensurate phase of Rb{sub 2}ZnCl{sub 4} has been determined at 194 K (2 K above the lock-in transition) within the soliton regime using satellites up to fifth order. The rather anharmonic modulation functions agree with the expected steplike functions supported by theoretical arguments. In addition, the constancy of the ratio between the amplitudes of the fifth-order and first-order harmonics, a relation predicted by theory, indicate the correctness of the model and imply a value of 0.4 for the soliton density n{sub s}. A symmetry mode analysis shows that the incommensurate structure is consistent with the one of the lock-in phase in the sense that the displacement pattern of every symmetry mode remains unaltered in the transition except for a global change in the amplitudes.X-ray diffraction of SnO{sub 2} (cassiterite) at high pressures and temperatures demonstrates the existence of four phase transitions to 117 GPa. The observed sequence of phases for SnO{sub 2} is rutile-type (P4{sub 2}/mnm){yields}CaCl{sub 2}-type(Pnnm){yields}pyrite-type(Pa3){yields}ZrO{sub 2} orthorhombic phase I (Pbca){yields}cotunnite-type (Pnam). Our observations of the first three phases are generally in agreement with earlier studies. The orthorhombic phase I and cotunnite-type structure (orthorhombic phase II) were observed in SnO{sub 2} for the first time. The Pbca phase is found at 50-74 GPa during room-temperature compression. The cotunnite-type structure was synthesized when SnO{sub 2} was compressed to 74 GPa and heated at 1200 K. The cotunnite-type form was observed during compression between 54-117 GPa with additional laser heating carried out at 91 and 111 GPa. Fitting the pressure-volume data for the high-pressure phases to the second-order Birch-Murnaghan equation of state yields a bulk modulus of 259(26) GPa for the Pbca phase and 417(7) GPa for the cotunnite-type phase.We report x-ray photoelectron spectroscopy (XPS) study of Na and K adlayers on icosahedral Al{sub 70.5}Pd{sub 21}Mn{sub 8.5} (i-Al-Pd-Mn) quasicrystal. The Na 1s core-level exhibits a continuous linear shift of 0.8 eV towards lower binding energies (BE) with increasing coverage up to one monolayer (ML) saturation coverage. In the case of K/i-Al-Pd-Mn, a similar linear shift in the K 2p spectra towards lower BE is observed. In both cases, the plasmon related loss features are observed only above 1 ML. The substrate core-level peaks, such as Al 2p, do not exhibit any shift with the adlayer deposition up to the highest coverage. Based on these experimental observations and previous studies of alkali metal growth on metals, we conclude that below 1 ML, both Na and K form a dispersed phase on i-Al-Pd-Mn and there is hardly any charge transfer to the substrate. The variation of the adlayer and substrate core-level intensities with coverage indicates layer by layer growth.We report the magnetic properties of the ZnL{sub 2}S{sub 4} (L=Er,Tm,Yb) olivines, in which the magnetic lanthanide ions are in a potentially frustrated geometry consisting of sawtooth chains of corner-sharing triangles. Fits to the high-temperature magnetic susceptibility yielded Curie-Weiss temperatures of {theta}{sub W}{approx_equal}-4, -13, and -75 K for the Er, Tm, and Yb compounds, respectively. None of the compounds displayed magnetic long-range order above T=1.8 K. The lack of ordering at temperatures near {theta}{sub W} may be attributed to either the low dimensionality of the structure or the frustrating effect of the triangular geometry.

X-ray Monochromator with 2 × 108 Energy Resolution

An X-ray beam of 3 x 10(7) photons s(-1) with 2 x 10(8) relative energy resolution has been obtained at a third-generation synchrotron undulator X-ray source using the (13 13 13) Bragg reflection from a silicon perfect crystal. The production of these 25.70 keV X-rays with 450 +/- 50 mueV bandpass opens up new possibilities in X-ray optics and spectroscopies.

Liquidlike Behavior of Supercritical Fluids

The high frequency dynamics of fluid oxygen has been investigated by inelastic x-ray scattering, at high pressures and room temperature. In spite of the markedly supercritical conditions (T approximately 2Tc, P>10(2)Pc), the sound velocity exceeds the hydrodynamic value of about 20%, a feature which is the fingerprint of liquidlike dynamics. The comparison of the present results with literature data obtained in several fluids allow us to identify the extrapolation of the liquid-vapor-coexistence line in the (P/Pc, T/Tc) plane as the relevant edge between liquidlike and gaslike dynamics. More interestingly, this extrapolation is very close to the non-metal-metal transition in hot dense fluids, at pressure and temperature values as obtained by shock wave experiments. This result points to the existence of a connection between structural modifications and transport properties in dense fluids.

Diffuse scattering in relaxor ferroelectrics: true three-dimensional mapping, experimental artefacts and modelling

The available body of experimental data in terms of the relaxor-specific component of diffuse scattering is critically analysed and a collection of related models is reviewed; the sources of experimental artefacts and consequent failures of modelling efforts are enumerated. Furthermore, it is shown that the widely used concept of polar nanoregions as individual static entities is incompatible with the experimental diffuse scattering results. Based on the synchrotron diffuse scattering three-dimensional data set taken for the prototypical ferroelectric relaxor lead magnesium niobate-lead titanate (PMN-PT), a new parameterization of diffuse scattering in relaxors is presented and a simple phenomenological picture is proposed to explain the unusual properties of the relaxor behaviour. The model assumes a specific slowly changing displacement pattern, which is indirectly controlled by the low-energy acoustic phonons of the system. The model provides a qualitative but rather detailed explanation of temperature, pressure and electric-field dependence of diffuse neutron and X-ray scattering, as well as of the existence of a hierarchy in the relaxation times of these materials.