We introduce a method for using Fizeau interferometry to measure the intrinsic resolving power of a diffraction grating. This method is more accurate than traditional techniques based on a long-trace profiler (LTP), since it is sensitive to long-distance phase errors not revealed by a d-spacing map. We demonstrate 50,400 resolving power for a mechanically ruled XUV grating from Inprentus, Inc.
High-temperature superconducting (HTS) cuprates are highly anisotropic materials which exhibit metallic-like behavior in the CuO2 planes while retaining dielectric properties in the perpendicular, c-axis, direction. Experimental data show however that in HTS systems the in-plane electronic excitations are strongly coupled to c-axis polarized vibrations. This interaction is manifest in various settings, for example in the resonant Raman profile of phononic excitations, inelastic quasi-particle tunneling, as observation of notch-like features and forbidden scattering for in-plane optical conductivity, colossal c-axis photo-expansion upon in-plane illumination as well as in high-resolution electron energy-loss spectra. We propose that this anisotropic coupling is driven by strong unscreened Coulomb interactions and the preponderance of the Madelung component to the cohesion energy, in particular by the large atomic displacements in the spacer layers induced by charge redistribution within the CuO2 planes.
We discuss recent soft x-ray resonant diffraction studies of magnetic and structural correlations in manganites and cobaltates. In half-doped manganites, the resonant enhancement of super-lattice diffraction peaks resulting from orbital and magnetic order is utilized to make a direct comparison of orbital and spin correlations. The main finding is that the correlation length associated with magnetic order exceeds that of the orbital order by of order a factor of two--a result which appears at odds with orbitally driven magnetic exchange pathways. Similar resonant diffraction measurements at the Co L-edge were performed to study the oxygen-doped cobaltate Bi2Sr2CoO6+δ, in which we find a surprising incommensurate antiferromagnetic order.
Following successful experience using photolithography and high aspect ratio reactive ion etching (RIE) to produce dynamically bent x-ray sagittal focusing crystals, we report on incorporating this optic in a novel high flux, narrow bandwidth, energy scanning monochromator for bend magnet synchrotron radiation. We describe the mono, several modes of operation, and our experience using it. Deep RIE has great utility for the manufacture, in silicon, of mechanical devices with feature as small as a few microns, however aberration free Bragg diffraction focusing requires uniformity in etch depth over large areas. To improve optical performance in terms of minimum focus spot size and maximum x-ray throughput, we are developing "second generation" focusing crystals based on a composite structure concept. We describe some of this new work and suggest areas of application.
It has recently been shown that x-ray diffraction from the doped holes in cuprates can be enhanced by 3-4 orders of magnitude by exploiting resonance effects in the oxygen K shell. This new type of anomalous scattering is direct way of probing ground state inhomogeneity in the mobile carrier liquid of high temperature superconductors. Here we describe a model which quantifies the relationship between experimental count rates and the structure factor for doped holes in this technique. We describe first efforts to detect inhomogeneity in thin films of La2CuO4+δ and report some peculiar observations. We attempt to offer some explanation.
A new technique for making a high resolution X-ray analyzer is presented. The analyzer consists of a silicon wafer with <111> orientation, a Pyrex glass wafer and a concave polished Pyrex glass substrate. The energy resolution of the analyzer was studied on the inelastic scattering beamline of the Synchrotron Radiation Instrumentation Collaborative Access Team on sector 3 of the APS using the Si(777) back reflection at 13.84 keV. Details are presented and compared with other techniques, and we discuss contributions of the measured energy resolution.
A monochromator for use at 13.84 keV with a calculated bandpass of 5.2 meV was designed, built, and tested. Tuning was performed by rotating the inner crystal of a pair of nested silicon channel cut crystals. The inner crystal employs the (884) reflection, and the outer crystal employs a collimating asymmetric (422) reflection (dynamical asymmetry factor, b, equal to -17.5). Tests were done with a double crystal Si(111) pre-monochromator situated upstream of the high resolution monochromator and a Si(777) backscattering crystal situated downstream. For this optical arrangement an ideal value of 6.3 meV as calculated by x-ray dynamical diffraction theory applies for the FWHM of the convolution of the net monochromator reflectivity function with that of the Si(777) reflection. This calculated value is to be compared to the value of 7.1 meV measured by tuning the high resolution monochromator. Measured efficiencies were less than ideal by a factor of 3.2 to 4.9, where the larger flux reduction factors were found with higher positron storage ring currents.
We report on a design and on some experimental results for the performance of a new high energy resolution monochromator. It is a large channel-cut Si crystal with a 197 mm separation between the two faces designed to operate in a near-backscattering regime. The device was tested as a second monochromator on Sector 3 of the Synchrotron Radiation Instrumentation Collaborative Access Team at the Advanced Photon Source using the Si(777) reflection at a photon energy of 13.84 keV. The same monochromator can be used for other energies with reflections of the type (hhh). Special care has been taken to equalize the temperature of the two faces by employing a Peltier heat pump. A Si(111) double-crystal pre-monochromator designed to withstand the high heat load of the undulator radiation was used upstream on the beamline. The measured throughput efficiency of the Si(777) channel-cut monochromator was less than ideal by a factor of 1.9. Dynamical diffraction theory was used to calculate the throughput of an ideally perfect crystal.
We present results of low-temperature two-magnon resonance Raman excitation profile measurements for single layer Sr2CuO2Cl2 and bilayer YBa2Cu3O6.1 antiferromagnets over the excitation region from 1.65 to 3.05 eV. These data reveal composite structure of the two-magnon line shape with peaks at approximately 2.8 and approximately 4 J and strong nonmonotic dependence of the scattering intensity on excitation energy. We analyze these data using the triple resonance theory of Chubukov and Frenkel [Phys. Rev. Lett., 74, 3057 (1995)] and deduce information about magnetic interaction and electronic band parameters in these antiferromagnetic insulators. We study the evolution of the magnetic excitation with hole doping in CuO2 planes of YBa2Cu3O6+(delta ) and YBa2Cu408 single crystals. We find that the spin excitations at energy approximately equal to 3 J, similar to the two-magnon excitations in the insulators, persist with doping and are evidence that antiferromagnetic fluctuations with spatial extent of at least three lattice constants are not overdamped in the underdoped superconductors. The two-magnon resonance study shows the existence of the charge-transfer gap in the underdoped cuprates and provides information about evolution of electronic band parameters with doping. We report a magnetic Raman scattering study of YBa2Cu408 superconductor with substitution of Zn on the Cu(2) site. Two and five-tenths percent of substitution softens the two-magnon excitation frequency from 2900 to 2300 cm-1. This softening is attributed to the absence of a defined spin projection on at least one of eight Cu(2)/Zn sites involved in the superexchange, and it signifies that the Zn- introduced magnetic distortion extends to at lest four Cu sites neighboring a Zn site. The electronic Raman continuum intensity weakens with increasing Zn doping, manifesting a relationship between the Raman continuum and the spin fluctuations.