R. Colella

Proposal for a free electron laser in the x-ray region

Abstract It is proposed that a free electron laser can be operated in the X-ray region, in the range 2–3 A. An analysis is presented of the machine parameters and the characteristics of the mirrors that are required for operation in the Angstrom region.

Transport properties and the semiconducting nature of TiS2

Abstract For a range of temperatures between 4.2 K and 700 K, we have measured the resistivity, Hall coefficient, thermoelectric power and infrared reflectivity of TiS2 single crystals with varying degrees of stoichiometry, and a corresponding variation in the carrier concentration by a factor of 15. The strong correlations established among different properties over this large range of doping provides convincing evidence that TiS2 is a semiconductor rather than a semimetal. However, our most stoichiometric samples continued to be metallic with carrier concentrations of n ⋍ 2 × 10 20 cm -3 . The origin of these electrons is unknown. The temperature dependent resistivity follows approximately a power law θ(T) - θ(4.2 K) ∝ Tm over the entire temperature range of the measurements (77 K -700 K). The exponent m is not strictly 2 as would be expected for carrier-carrier scattering but varies from 2.2 for our most stoichiometric sample to 1.85 for the least stoichiometric. We also find that θ(300) - θ(4.2 K) varies with doping as n -2 3 rather than the n -5 3 dependence reported previously and presented as evidence for carrier-carrier scattering. We have considered optical phonon scattering, but find that this mechanism alone cannot fit our data. We regard the scattering mechanism as unknown.

Phase determination of X-ray reflections in a quasicrystal

Multiple Bragg diffraction effects have been observed in an Al–Cu–Fe quasicrystal. The experimental data are analyzed by means of a multibeam perturbation theory. Good fits are obtained between experimental and calculated profiles. The feasibility for phase determination of structure factors is demonstrated. It is found that there is no inversion symmetry in Al–Cu–Fe.

Phase determination of the forbidden reflection 442 in silicon and germanium using multiple Bragg scattering

The sign inversion of the forbidden reflection 442 in silicon between room temperature and 700 K has been verified using multiple beam diffraction and the concept of virtual Bragg scattering. A similar determination in the case of germanium confirms that the 442 is mostly due to anharmonic effects at room temperature.

Resonant scattering and multiple Bragg X-ray diffraction in LaMnO3; a classical view

The mechanism for resonant scattering in LaMnO(3) has been the subject of some controversy between two models. Model (A) is based on orbital ordering, whereas model (B) invokes the Jahn-Teller distortion. From the structural point of view, the two models lead to different d-electron densities. The forbidden reflection 300 is turned on by resonance, leading to a well defined scheme for the lobes of the Mn d electrons [model (A)]. It turns out that the same experimental results could have been interpreted according to model (B). The only difference is that this alternative interpretation would have required a structure factor F(300) of opposite sign. This is a typical example of a phase problem. An experimental technique has been applied for phase determination based on multiple Bragg scattering. This shows that the multiple-beam results are in agreement with model (B), not with model (A), laying to rest a long overdue controversy.

Temperature dependence of intensities from silicon with glancing incidence HEED

Peak reflectivity for the 2nd to 7th order Bragg reflections of the hhh systematic set have been obtained at different temperatures. The results have been interpreted using an n-beam dynamical theory adapted to the Bragg case at glancing incidence. On the basis of this analysis it is concluded that the component of the mean-square vibrational amplitude of the surface atoms normal to the surface is about 40% greater than that of the bulk.

Bulk and surface evidence for the long-range spatial modulation of X-ray absorption in the Al–Pd–Mn quasicrystal at Bragg incidence

Abstract An X-ray standing waves experiment was performed at the ID32 beam line of the ESRF on an Al–Pd–Mn quasicrystal with the X-ray beam at normal incidence. The X-ray photoemission core levels for each element were recorded to probe the surface. The drain current, i.e. the electron current from ground needed to neutralize the irradiated sample and to compensate the photoemitted electrons was also recorded as a bulk probe that integrates over all atoms of the sample. Within the two-beam approximation applied to quasicrystals, a simple analysis of this effect is proposed. This provides a direct-space illustration of the diffraction process in a quasiperiodic structure.

Plane-wave theory of three-crystal Laue interferometer

The plane-wave theory of a three-crystal Laue interferometer is presented in terms of the amplitudes diffracted by a single slab in the Laue case, using Zachariasens formalism. Successive applications of the single-slab expressions for the amplitudes lead to the final intensities of the interfering beams present on the back side of the third crystal slab. Numerical examples for X-ray and neutron diffraction show clearly the different contrast relationships in the two cases.

Technical Report on DOE project: X-ray physics of materials (proposal No.Z817)

The SRI-CAT was able to order the construction of the First Optics Enclosure, the second enclosure housing the monochromator and the first user station, and various motors, controllers, and electronics for the control of the hard x-ray beamline components.

X-Ray and Neutron Interferometry: Basic Principles and Applications

Interferometry is based on the idea of creating spatially separated coherent beams, which can be brought together and produce interference effects. Optical interferometry is a well established field. Coherent beams can be realized by using half-reflecting glass slides, and easily manipulated and directed along assigned directions by mirrors. The same optical elements cannot be used in the X-ray region, and this is the reason why we had to wait until 1965 before we could see an X-ray interferometer in operation (Bonse and Hart, 1965a; 1965b.