Pressure-dependent electronic structures in multiferroic DyMnO(3): A combined lifetime-broadening-suppressed x-ray absorption spectroscopy and ab initio electronic structure study

Variations in the electronic structure and structural distortion in multiferroic DyMnO(3) were probed by synchrotron x-ray diffraction, lifetime-broadening-suppressed x-ray absorption spectroscopy (XAS), and ab initio electronic structure calculations. The refined x-ray diffraction data enabled an observation of a diminished local Jahn-Teller distortion of Mn sites within MnO(6) octahedra in DyMnO(3) on applying the hydrostatic pressure. The intensity of the white line in Mn K-edge x-ray absorption spectra of DyMnO(3) progressively increased with the increasing pressure. With the increasing hydrostatic pressure, the absorption threshold of an Mn K-edge spectra of DyMnO(3) shifted toward a greater energy, whereas the pre-edge line slightly shifted to a smaller energy. We provide the spectral evidence for the pressure-induced bandwidth broadening for manganites. The intensity enhancement of the white line in Mn K-edge spectra is attributed to a diminished Jahn-Teller distortion of MnO(6) octahedra in compressed DyMnO(3). A comparison of the pressure-dependent XAS spectra with the ab initio electronic structure calculations and full calculations of multiple scattering using the code FDMNES shows the satisfactory agreement between experimental and calculated Mn K-edge spectra.     

Strong coupling between 4f valence instability and 3d ferromagnetism in Yb(x)Fe4Sb12 studied by resonant x-ray emission spectroscopy

We have investigated the temperature and pressure dependency of the electronic structure of Yb-filled skutterudites, YbFe(4)Sb(12) and Yb(0.88)Fe(4)Sb(12), using x-ray absorption and emission spectroscopies. An anomalous increase of the Yb valence, which is beyond the conventional Anderson model picture, is found to coincide with the onset of the ferromagnetic order in the x=0.88 sample below 20 K. In contrast, the nearly stoichiometric YbFe(4)Sb(12) is paramagnetic down to 2 K and the Yb valence is independent of temperature. This evidences a close interplay between the magnetic instability of the Fe 3d electrons and valence instability of the Yb 4f electrons. Under pressure, a sudden increase in the valence is found to occur around 13 GPa for YbFe(4)Sb(12) and 17 GPa for Yb(0.88)YbFe(4)Sb(12).     

Treatment of interfaces in particle image velocimetry

A first-order accurate method of extending the capability of image velocimetry to interfaces is presented. In this method, the image fields are locally extended across interfaces using fields from the other image of an image pair. During this image parity exchange, the extension of the image fields amounts to locally reversing and reflecting the relative velocity field across the interface. Numerous experimental examples are given to demonstrate and validate the accuracy of the method. These are the plane Couette flow and the laminar pipe flow demonstrating straight rigid boundaries; uniform flow past a sphere and a sphere moving in a stagnant fluid demonstrating curved rigid surfaces; and a free-surface flow and a liquid–liquid interface flow demonstrating compliant interfaces. Received: 3 November 1998/Accepted: 18 August 1999     

Charge confinement effect in cuprate superconductors: an explanation for the normal-state resistivity and pseudogap

The fact that the stripe phase and pseudogap in the cuprate superconductors occur in the same doping regime is emphasized. A model based on charge confinement in self-organized nanometer-scale stripe fragments is proposed to understand various generic features of the normal-state energy gap including the magnitude of the gap, its anti-correlation with the superconducting gap, and the d-wave symmetry in its -dependence. This model also provides a basis for understanding other anomalous normal-state properties such as the linear temperature dependence of electrical resistivity. Received 7 December 1998     

The critical specific heat of a glassy ferromagnet

Using standard ac calorimetry, we have measured the specific heat of the amorphous ferromagnet Fe₇₅P₁₅C₁₀ in the neighborhood of its Curie temperature T_C ? 600 K. Even though this material is structurally disordered, a sharp lambda-like cusp was observed, typical of a pure crystalline substance. We report the first determination of the critical specific heat exponent α = δ′ = -.18 ± .04 and the amplitude ratio A⁺/A^- = 1.2 ± .3 for an amorphous ferromagnet and find that these values are typical of a Heisenberg system. Our results are in agreement with recent renormalization group calculations which indicate that the fixed point which characterizes the critical behavior will be stable against the addition of disorder to the ordered system if α < 0.     

The ratio of energy gap to transition temperature in amorphous superconductors

Using results of tunneling measurements, values of 2Δ(o)/kT_c for a number of amorphous transition-metal based superconductors are found to be 3.5±0.1 in agreement with the BCS theory in the weak-coupled limit unlike previously reported results suggesting strong electron-phonon coupling in amorphous non-transition metals such as Ga and Pb-Bi alloys. A comparison of our results with existing data for amorphous soft-metal alloys and A-15 compounds suggests that non-crystallinity alone does not give rise to a significantly enhanced electron-phonon interaction.     

Charge transfer and dd excitations in transition metal oxides

Electronic excitations in transition-metal oxides MnO, FeO, CoO and NiO are investigated by inelastic X-ray scattering and optical reflectivity measurements. The dielectric functions are derived from the experimental data as a function of the momentum transfer, q. Based on the derived q-dependent dielectric functions, two types of the charge transfer excitations, i.e., dipolar and non-dipolar charge transfer, are clearly identified. We show that the Mott gaps around 5 eV are defined by the former whereas the latter occurs at higher energies of 8–12 eV. Based on a molecular orbital analysis, we associate the dipolar and the non-dipolar excitations with non-local charge transfer and conventional charge transfer, respectively. These types of excitations are shown to be common for the 3d metal monoxides. On the other hand, the dd excitations observed in NiO and CoO at energy <4 eV do not appear in FeO and MnO. The reasons are addressed in this report.     

Layer dependence of the superconducting transition temperature of HgBa2Can−1CunO2n+2+δ

High-pressure methods have been used to synthesize multiphase compositions in the Hg---12{n−1}n homologous series. The phase assemblages were examined by optical, electron diffraction and X-ray diffraction techniques, and their stoichiometries verified by electron microprobe. Transport and magnetic susceptibility measurements were combined with the results of the phase analysis to establish superconducting transition temperatures for both as-prepared and O₂- or Ar-annealed materials. It was found that the transition temperature peaks at T_c = 134 K for n = 3 and then decreases abruptly for n>4, reaching T_c<90 K for n7.     

Electronic versus lattice match for metal-semiconductor epitaxial growth: Pb on Ge(111)

Lattice match is important for epitaxial growth. We show that a competing mechanism, electronic match, can dominate at small film thicknesses for metal-semiconductor systems, where quantum confinement and symmetry requirements may favor a different growth pattern. For Pb(111) on Ge(111), an accidental lattice match leads to a √3 × √3 configuration involving a 30° in-plane rotation at large film thicknesses, but it gives way to an incommensurate (1 × 1) configuration at small film thickness. The transformation follows an approximately inverse-film-thickness dependence with superimposed bilayer oscillations.