Alfred Q. R. Baron

An X-ray scattering beamline for studying dynamics

Abstract We describe the design of an instrument for the investigation of sample dynamics: the SPring-8 High Resolution Beamline (BL35XU). The primary purpose of the beamline is high resolution (∼meV) inelastic X-ray scattering measurements. The secondary purpose is nuclear resonant scattering measurements. Construction of the beamline will begin in the summer of 1999 and commissioning will start in the spring of 2000.

Kohn anomaly in MgB2 by inelastic X-ray scattering.

We study phonons in MgB2 using inelastic x-ray scattering (1.6 and 6 meV resolution). We clearly observe the softening and broadening of the crucial E(2g) mode through the Kohn anomaly along GammaM, in excellent agreement with ab initio calculations. Low temperature measurements (just above and below T(c)) show negligible changes for the momentum transfers investigated and no change in the E(2g) mode at A between room temperature and 16 K. We report the presence of a longitudinal mode along GammaA near in energy to the E(2g) mode that is not predicted by theory.

Softening of Cu-O bond stretching phonons in tetragonal HgBa2CuO4+delta.

Phonons in nearly optimally doped HgBa(2)CuO(4+delta) were studied by inelastic x-ray scattering. The dispersion of the low-energy modes is well described by a shell model, while the Cu-O bond stretching mode at high energy shows strong softening towards the zone boundary, which deviates strongly from the model. This seems to be common in the hole-doped high-T(c) superconducting cuprates, and, based on this work, not related to a lattice distortion specific to each material.

Liquid iron‐sulfur alloys at outer core conditions by first‐principles calculations

We examined the density, bulk sound (compressional) velocity, and Gruneisen parameter of liquid pure Fe, Fe100H28 (0.50 wt % H), Fe88H40 (0.81 wt % H), and Fe76H52 (1.22 wt % H) at Earths outer core pressure and temperature (P-T) conditions (~100 to 350 GPa, 4000 to 7000 K) based on first-principles molecular dynamics calculations. The results demonstrate that the thermodynamic Gruneisen parameter of liquid iron alloy decreases with increasing pressure, temperature, and hydrogen concentration, indicating a relatively small temperature gradient in the outer core when hydrogen is present. Along such temperature profile, both the density and compressional velocity of liquid iron containing ~1 wt % hydrogen match seismological observations. It suggests that hydrogen could be a primary light element in the core, although the shear velocity of the inner core is not reconciled with solid Fe-H alloy and thus requires another impurity element.

Bond stretching phonon softening and kinks in the angle-resolved photoemission spectra of optimally doped Bi2Sr1.6La0.4Cu2O6+delta superconductors.

We report the first measurement of the Cu-O bond stretching phonon dispersion in optimally doped Bi2Sr1.6La0.4Cu2O6+delta using inelastic x-ray scattering. We found a softening of this phonon at q=( approximately 0.25,0,0) from 76 to 60 meV, similar to the one reported in other cuprates. A comparison with angle-resolved photoemission data on the same sample revealed an excellent agreement in terms of energy and momentum between the angle-resolved photoemission nodal kink and the soft part of the bond stretching phonon. Indeed, we find that the momentum space where a 63+/-5 meV kink is observed can be connected with a vector q=(xi,0,0) with xi > or =0.22, corresponding exactly to the soft part of the bond stretching phonon.

Bond stretching phonon softening and angle-resolved photoemission kinks in optimally doped Bi2Sr1:6La0:4Cu2O6+sigma superconductors

We report the first measurement of the Cu-O bond stretching phonon dispersion in optimally doped Bi2Sr1.6La0.4Cu2O6+delta using inelastic x-ray scattering. We found a softening of this phonon at q=( approximately 0.25,0,0) from 76 to 60 meV, similar to the one reported in other cuprates. A comparison with angle-resolved photoemission data on the same sample revealed an excellent agreement in terms of energy and momentum between the angle-resolved photoemission nodal kink and the soft part of the bond stretching phonon. Indeed, we find that the momentum space where a 63+/-5 meV kink is observed can be connected with a vector q=(xi,0,0) with xi > or =0.22, corresponding exactly to the soft part of the bond stretching phonon.

Lattice Dynamics of LaFeAsO1-xFx and PrFeAsO1-y via Inelastic X-Ray Scattering and First-Principles Calculation

The lattice dynamics of LaFeAsO 1- x F x ( x =0,0.1) and PrFeAsO 1- y ( y ∼0.1) are investigated using inelastic x-ray scattering and ab-initio calculation. Measurements of powder samples provide an approximation to the phonon DOS, while dispersion is measured from a single crystal of PrFeAsO 1- y . A model that agrees reasonably well with all of the data at room temperature is built from results of ab-initio calculations by reducing the strength of the Fe–As bond by 30%.The lattice dynamics of LaFeAsO 1- x F x ( x =0,0.1) and PrFeAsO 1- y ( y ∼0.1) are investigated using inelastic x-ray scattering and ab-initio calculation. Measurements of powder samples provide an approximation to the phonon DOS, while dispersion is measured from a single crystal of PrFeAsO 1- y . A model that agrees reasonably well with all of the data at room temperature is built from results of ab-initio calculations by reducing the strength of the Fe–As bond by 30%.

Carbon-depleted outer core revealed by sound velocity measurements of liquid iron–carbon alloy

The relative abundance of light elements in the Earths core has long been controversial. Recently, the presence of carbon in the core has been emphasized, because the density and sound velocities of the inner core may be consistent with solid Fe7C3. Here we report the longitudinal wave velocity of liquid Fe84C16 up to 70 GPa based on inelastic X-ray scattering measurements. We find the velocity to be substantially slower than that of solid iron and Fe3C and to be faster than that of liquid iron. The thermodynamic equation of state for liquid Fe84C16 is also obtained from the velocity data combined with previous density measurements at 1 bar. The longitudinal velocity of the outer core, about 4% faster than that of liquid iron, is consistent with the presence of 4–5 at.% carbon. However, that amount of carbon is too small to account for the outer core density deficit, suggesting that carbon cannot be a predominant light element in the core.

Nearly perfect large-area quartz : 4 meV resolution for 10 keV photons over 10 cm2

The perfection of two commercial top-grade quartz crystal wafers has been investigated using Bragg reflection at theta(B)=89.77 degrees of a 2.0 meV bandwidth beam of 9.979 keV X-rays by the (7\bar4\bar34) lattice planes. Topographic images show small defect concentrations (<or=5 cm(-2)). Energy scan widths are below 3 meV over 8 mm x 8 mm areas and 4 meV over the whole wafer ( approximately 11 cm(2)). This suggests that quartz can be a useful optical material.

A REFRACTIVE COLLIMATOR FOR SYNCHROTRON RADIATION

We demonstrate that a simple and inexpensive plastic refractive lens may be used to collimate synchrotron radiation. The divergence of an 18.5 keV beam of radiation from an undulator at SPring-8 is reduced from 11 μrad, full width at half maximum, to less than 3.5 μrad, while preserving 75% of the incident intensity.