Inelastic X-ray scattering investigations of lattice dynamics in SmFeAsO1−xFysuperconductors

We report measurements of the phonon density of states as probed with inelastic X-ray scattering in SmFeAsO_1−xF_y powders. An unexpected strong renormalization of phonon branches around 23 meV is observed as fluorine is substituted for oxygen. Phonon dispersion measurements on SmFeAsO_1−xF_y single crystals allow us to identify the 21 meV A_1g in-phase (Sm,As) and the 26 meV B_1g (Fe,O) modes to be responsible for this renormalization, and may reveal unusual electron-phonon coupling through the spin channel in iron-based superconductors.     

Nature of the magnetic order in the charge-ordered cuprate La1.48Nd0.4Sr0.12CuO4

Using polarized neutron scattering we establish that the magnetic order in La(1.48)Nd(0.4)Sr(0.12)CuO(4) is either (i) one dimensionally modulated and collinear, consistent with the stripe model or (ii) two dimensionally modulated with a novel noncollinear structure. The measurements rule out a number of alternative models characterized by 2D electronic order or 1D helical spin order. The low-energy spin excitations are found to be primarily transversely polarized relative to the stripe ordered state, consistent with conventional spin waves.     

Magnetic-field-induced spin excitations and renormalized spin gap of the underdoped La1.895Sr0.105CuO4 superconductor

High-resolution neutron inelastic scattering experiments in applied magnetic fields have been performed on La1.895Sr0.105CuO4 (LSCO). In zero field, the temperature dependence of the low-energy peak intensity at the incommensurate momentum transfer QIC=(0.5,0.5+/-delta,0),(0.5+/-delta,0.5,0) exhibits an anomaly at the superconducting Tc which broadens and shifts to lower temperature upon the application of a magnetic field along the c axis. A field-induced enhancement of the spectral weight is observed, but only at finite energy transfers and in an intermediate temperature range. These observations establish the opening of a strongly downward renormalized spin gap in the underdoped regime of LSCO. This behavior contrasts with the observed doping dependence of most electronic energy features.     

Spring school on magnetic properties of condensed matter

The sixth joint Stanford-Berkeley summer school on synchrotron radiation and its applications in physical science was held on August 17?22, 2008, at the Stanford Linear Accelerator Center (SLAC). The Stanford-Berkeley summer school is jointly organized by the Stanford University, University of California Berkeley, Lawrence Berkeley National Laboratory (LBNL), and the Stanford Synchrotron Radiation Lightsource (SSRL). Anders Nilsson (Stanford) and Dave Attwood (Berkeley) have been the organizers of this one-week summer school since 2001. It alternates between Stanford and Berkeley. The summer school provides lecture programs on synchrotron radiation and its broad range of scientific applications in the physical science, visits to the Stanford Synchrotron Radiation Lightsource and the Advanced Light Source, where the students also have the opportunity to join a beamline. The program is designed to introduce students and postdocs to the fundamental properties of synchrotron radiation and how to understand and use spectroscopic, scattering and microscopy techniques in various scientific applications. Particular emphasis is given to examples from physics, chemistry, and material science.     

Optical and photophysical properties of light-harvesting phenylacetylene monodendrons based on unsymmetrical branching

The optical and photophysical properties of phenylacetylene dendritic macromolecules based on unsymmetrical branching are investigated using steady-state and time-dependent spectroscopy. Monodendrons, up to the fourth generation, are characterized with and without a fluorescent perylene trap at the core. The higher generation monodendrons without the perylene trap exhibit high molar extinction coefficients (>10(5) M(-1) cm(-1)) and high fluorescence quantum yields (65-81%). When a perylene trap is placed at the core, then the monodendrons typically exhibit high energy transfer quantum yields (approximately 90%), as well as subpicosecond time scale excited-state dynamics, as evidenced by ultrafast pump-probe measurements. The photophysical properties of the unsymmetrical monodendrons are compared to those of phenylacetylene monodendrons with symmetrical branching, which have been described recently. The high fluorescence quantum yields and large energy transfer quantum efficiencies exhibited by the unsymmetrical monodendrons suggest they have potential for applications in molecular-based photonics devices.     

Excited-state dynamics of the intramolecular proton transfer of 3-hydroxyflavone in the absence of external hydrogen-bonding interactions

A single exponential risetime is observed for the lautomer fluorescence of 3-hydroxyflavone in the absence of hydrogen-bonding impurities in the temperature range of 298—77 K. The observed risetime in hydrocarbon solvents is less than 8 ps at 298 K, while at 77 K it is 37 ± 6 ps. The discrepancy with other reports is attributed to hydrogen-bonding solvent impurities which inhibit the intramolecular proton transfer process.     

Determination of the antiferroquadrupolar order parameters in UPd(3)

By combining accurate heat capacity and x-ray resonant scattering results we have resolved the long standing question regarding the nature of the quadrupolar ordered phases in UPd(3). The order parameter of the highest temperature quadrupolar phase has been uniquely determined to be antiphase Q{zx} in contrast to the previous conjecture of Q{x{2}-y{2}}. The azimuthal dependence of the x-ray scattering intensity from the quadrupolar superlattice reflections indicates that the lower temperature phases are described by a superposition of order parameters. The heat capacity features associated with each of the phase transitions characterize their order, which imposes restrictions on the matrix elements of the quadrupolar operators.