E. D. Isaacs

Compound refractive optics for the imaging and focusing of low-energy neutrons

Low-energy neutrons are essential for the analysis and characterization of materials and magnetic structures. However, both continuous (reactor-based) and pulsed (spallation-based) sources of such neutrons suffer from low fluence. Steering and lensing devices could improve this situation dramatically, so increasing spatial resolution, detectable sample volume limits and even perhaps opening the way for the construction of a neutron microscope. Neutron optics have to date exploited either Bragg diffraction,, such as bent crystals, or reflection, as in mirror guides or a Kumakhov lens,. Refractive optics remain an attractive alternative as they would permit full use of the beam cross-section, allow a compact and linear installation and, because of similarity to conventional optics, enable the use of commercial design and simulation tools. These advantages notwithstanding, single-element refractive optics have previously been considered impractical as they are too weakly focusing, too absorptive and too dispersive. Inspired by the recent demonstration of a compound refractive lens (CRL) for high-energy X-rays, we have designed, built and tested a prototype CRL for 9–20-Å neutrons by using readily available optical components: our CRL has gains greater than 15 and focal lengths of 1–6 m, well matched to small-angle neutron scattering.

Direct measurement of antiferromagnetic domain fluctuations

Measurements of magnetic noise emanating from ferromagnets owing to domain motion were first carried out nearly 100 years ago, and have underpinned much science and technology. Antiferromagnets, which carry no net external magnetic dipole moment, yet have a periodic arrangement of the electron spins extending over macroscopic distances, should also display magnetic noise. However, this must be sampled at spatial wavelengths of the order of several interatomic spacings, rather than the macroscopic scales characteristic of ferromagnets. Here we present a direct measurement of the fluctuations in the nanometre-scale superstructure of spin- and charge-density waves associated with antiferromagnetism in elemental chromium. The technique used is X-ray photon correlation spectroscopy, where coherent X-ray diffraction produces a speckle pattern that serves as a ‘fingerprint’ of a particular magnetic domain configuration. The temporal evolution of the patterns corresponds to domain walls advancing and retreating over micrometre distances. This work demonstrates a useful measurement tool for antiferromagnetic domain wall engineering, but also reveals a fundamental finding about spin dynamics in the simplest antiferromagnet: although the domain wall motion is thermally activated at temperatures above 100 K, it is not so at lower temperatures, and indeed has a rate that saturates at a finite value—consistent with quantum fluctuations—on cooling below 40 K.

Resonant inelastic x-ray scattering from valence excitations in insulating copper oxides

We report resonant inelastic x-ray measurements of insulating La{sub 2}CuO {sub 4} and Sr{sub 2}CuO {sub 2}Cl{sub 2} taken with the incident energy tuned near the Cu K absorption edge. We show that the spectra are well described in a shakeup picture in third-order perturbation theory which exhibits both incoming and outgoing resonances and demonstrate how to extract a spectral function from the raw data. We conclude by showing {bold q} -dependent measurements of the charge transfer gap. {copyright} {ital 1999} {ital The American Physical Society }

Crystallization of charge holes in the spin ladder of Sr14Cu24O41

Determining the nature of the electronic phases that compete with superconductivity in high-transition-temperature (high-Tc) superconductors is one of the deepest problems in condensed matter physics. One candidate is the ‘stripe’ phase, in which the charge carriers (holes) condense into rivers of charge that separate regions of antiferromagnetism. A related but lesser known system is the ‘spin ladder’, which consists of two coupled chains of magnetic ions forming an array of rungs. A doped ladder can be thought of as a high-Tc material with lower dimensionality, and has been predicted to exhibit both superconductivity and an insulating ‘hole crystal’ phase in which the carriers are localized through many-body interactions. The competition between the two resembles that believed to operate between stripes and superconductivity in high-Tc materials. Here we report the existence of a hole crystal in the doped spin ladder of Sr14Cu24O41 using a resonant X-ray scattering technique. This phase exists without a detectable distortion in the structural lattice, indicating that it arises from many-body electronic effects. Our measurements confirm theoretical predictions, and support the picture that proximity to charge ordered states is a general property of superconductivity in copper oxides.

Momentum-resolved charge excitations in a prototype one-dimensional Mott insulator.

We report momentum-resolved charge excitations in a one-dimensional (1D) Mott insulator studied using high resolution inelastic x-ray scattering over the entire Brillouin zone for the first time. Excitations at the insulating gap edge are found to be highly dispersive (momentum dependent) compared to excitations observed in two-dimensional Mott insulators. The observed dispersion in 1D cuprates ( SrCuO2 and Sr2CuO3) is consistent with charge excitations involving holons which is unique to spin-1/2 quantum chain systems. These results point to the potential utility of momentum-resolved inelastic x-ray scattering in providing valuable information about electronic structure of strongly correlated insulators.

Diffuse x-ray scattering from La2-xSrxNiO4 and La2-ySryCuO4.

We use synchrotron x-ray scattering from La[sub 1.8]Sr[sub 0.2]NiO[sub 4] and La[sub 1.925]Sr[sub 0.075]CuO[sub 4] to establish a direct relationship between carriers due to Sr doping and strong diffuse scattering. In the nickelate the scattering is peaked at the fourfold symmetric satellite positions ([plus minus][delta],[plus minus][delta],[ital l]), where the basal-plane coordinate [delta] varies with the out-of-plane coordinate [ital l] of the momentum transfer. A similar scattering pattern is observed in the cuprate but with smaller values for [delta].

Diffuse x-ray scattering from La[sub 2[minus][ital x]]Sr[sub [ital x]]NiO[sub 4] and La[sub 2[minus][ital y]]Sr[sub [ital y]]CuO[sub 4]

We use synchrotron x-ray scattering from La[sub 1.8]Sr[sub 0.2]NiO[sub 4] and La[sub 1.925]Sr[sub 0.075]CuO[sub 4] to establish a direct relationship between carriers due to Sr doping and strong diffuse scattering. In the nickelate the scattering is peaked at the fourfold symmetric satellite positions ([plus minus][delta],[plus minus][delta],[ital l]), where the basal-plane coordinate [delta] varies with the out-of-plane coordinate [ital l] of the momentum transfer. A similar scattering pattern is observed in the cuprate but with smaller values for [delta].

Pressure-tuned spin and charge ordering in an itinerant antiferromagnet.

Elemental chromium orders antiferromagnetically near room temperature, but the ordering temperature can be driven to zero by applying large pressures. We combine diamond anvil cell and synchrotron x-ray diffraction techniques to measure directly the spin and charge order in the pure metal at the approach to its quantum critical point. Both spin and charge order are suppressed exponentially with pressure, well beyond the region where disorder cuts off such a simple evolution, and they maintain a harmonic scaling relationship over decades in scattering intensity. By comparing the development of the order parameter with that of the magnetic wave vector, it is possible to ascribe the destruction of antiferromagnetism to the growth in electron kinetic energy relative to the underlying magnetic exchange interaction.

Multiwavelength DFB laser array with integrated spot size converters

We describe the design, fabrication, and performance of a five-element quarterwave-shifted distributed feedback laser array with monolithically integrated spot size converters intended for use as a multiple-wavelength source in dense wavelength-division telecommunications systems. Facet power in excess of 10 mW with less than 150 mA bias and longitudinal side mode suppression greater than 40 dB were routinely achieved. Narrow far-field full-width at half-maximum angles of 6.9/spl deg//spl times/16.3/spl deg/ provided 3.5-dB coupling loss into single-mode fiber with 1.0-dB misalignment tolerances of /spl plusmn/2.0 /spl mu/m. With /spl plusmn/10/spl deg/C thermal tuning, 22 1555-nm channels spaced by 50 GHz were accessed with this device. Thorough field evaluation indicates that such a device is consistent with manufacturing requirements.

ENERGY DISPERSIVE X-RAY DIFFRACTION OF CHARGE DENSITY WAVES VIA CHEMICAL FILTERING

Pressure tuning of phase transitions is a powerful tool in condensed matter physics, permitting high-resolution studies while preserving fundamental symmetries. At the highest pressures, energy dispersive x-ray diffraction (EDXD) has been a critical method for geometrically confined diamond anvil cell experiments. We develop a chemical filter technique complementary to EDXD that permits the study of satellite peaks as weak as 10^(-4) of the crystal Bragg diffraction. In particular, we map out the temperature dependence of the incommensurate charge density wave diffraction from single-crystal, elemental chromium. This technique provides the potential for future GPa pressure studies of many-body effects in a broad range of solid state systems.