Status report on the advanced photon source: Spring 1994

Facility construction is presented for the conventional facilities, central laboratory/Office building, and the user residence facility. Accelerator systems are described in terms of commissioning and control system hardware. Insertion devices, front-end components, and monochromators are depicted in the experimental facilities. The operating energy and beam stability are discussed in terms of operations objectives. Collaborative access teams efforts are presented for the memoranda of understanding, CAT funding, and foreign participation in research at APS

S=1/2 chains and spin-Peierls transition in TiOCl

We study TiOCl as an example of an S=1/2 layered Mott insulator. From our analysis of new susceptibility data, combined with LDA and LDA+U band structure calculations, we conclude that orbital ordering produces quasi-one-dimensional spin chains and that TiOCl is a new example of Heisenberg-chains which undergo a spin-Peierls transition. The energy scale is an order of magnitude larger than that of previously known examples. The effects of non-magnetic Sc impurities are explained using a model of broken finite chains.Comment: 5 pages, 5 figures (color); details on crystal growth added; to be published in Phys. Rev.

Dilatometry study of the ferromagnetic order in single-crystalline URhGe

Thermal expansion measurements have been carried out on single-crystalline URhGe in the temperature range from 2 to 200 K. At the ferromagnetic transition (Curie temperature T_C = 9.7 K), the coefficients of linear thermal expansion along the three principal orthorhombic axes all exhibit pronounced positive peaks. This implies that the uniaxial pressure dependencies of the Curie temperature, determined by the Ehrenfest relation, are all positive. Consequently, the calculated hydrostatic pressure dependence dT_C/dp is positive and amounts to 0.12 K/kbar. In addition, the effective Gruneisen parameter was determined. The low-temperature electronic Gruneisen parameter \Gamma_{sf} = 14 indicates an enhanced volume dependence of the ferromagnetic spin fluctuations at low temperatures. Moreover, the volume dependencies of the energy scales for ferromagnetic order and ferromagnetic spin fluctuations were found to be identical.Comment: 5 page

Electromagnetic Response of Layered Superconductors with Broken Lattice Inversion Symmetry

We investigate the macroscopic effects of charge density waves (CDW) and superconductivity in layered superconducting systems with broken lattice inversion symmetry (allowing for piezoelectricity) such as two dimensional (2D) transition metal dichalcogenides (TMD). We work with the low temperature time dependent Ginzburg-Landau theory and study the coupling of lattice distortions and low energy CDW collective modes to the superconducting order parameter in the presence of electromagnetic fields. We show that superconductivity and piezoelectricity can coexist in these singular metals. Furthermore, our study indicates the nature of the quantum phase transition between a commensurate CDW phase and the stripe phase that has been observed as a function of applied pressure.Comment: 9 pages, 1 figure. Final version. Accepted in Phys.Rev.

Status report on the Advanced Photon Source -- Fall 1993

The Advanced Photon Source Project is well along on installation of injector components and is beginning installation of the storage ring. All major buildings have been occupied with the exception of the experiment hall, which will be completed by January 1994. Research and development on all major technical components is nearing completion. Construction of user laboratory/office modules has begun. Plans for a central laboratory/office complex are well advanced. Fifteen Collaborative Access Teams have been approved for research on the initial set of 40. This talk will review the progress of construction and the highlights of the currently planned research program

Neutron scattering observations on the magnetic phases of rareearth ternary superconductors

A number of ternary compounds become superconducting even though they contain a chemically ordered sublattice of magnetic rare-earth ions. Studies of the physical properties of these materials have revealed anomalies below T/sub c/ which have been attributed to magentic ordering transitions. Usng neutron spectrometers at the Brookhaven National Laboratory, a group of us has demonstrated that simple magnetic structures with long-range order do occur, and we have solved some of the magnetic structures of these superconductors. Specifically, we have found that in DyMo/sub 6/S/sub 8/ and TbMo/sub 6/S/sub 8/ an antiferromagnetic structure coexists with superconductivity. In two other compounds, ErRh/sub 4/B/sub 4/ and HoMo/sub 6/S/sub 8/ we have found that the development of ferromagnetism is responsible for the quenching of superconductivity. A study of the critical magnetic neutron scattering near the superconducting ..-->.. ferromagnetic transitions shows the presence of fluctuations into a state with an oscillatory magnetization of wave length lambda = 100A

The Advanced Photon Source: Performance and results from early operation

The Advanced Photon Source at Argonne National Laboratory is now providing researchers with extreme-brilliance undulator radiation in the hard x-ray region of the spectrum. All technical facilities and components are operational and have met design specifications. Fourteen research teams, occupying 20 sectors on the APS experiment hall floor, are currently installing beamline instrumentation or actively taking data. An overview is presented for the first operational years of the Advanced Photon Source. Emphasis is on the performance of accelerators and insertion devices, as well as early scientific results and future plans

Physics research opportunities with synchrotron x-radiation

New x-ray sources of substantially increased brilliance would be available from undulator magnets operating on a new-generation 6 GeV storage ring. To understand what research opportunities would be provided by such improved sources, a number of existing x-ray scattering techniques are briefly described with a qualitative analysis of their requirements for source brilliance. In addition to improvements of existing techniques which will permit application to a generally broader range of problems, new opportunities for magnetic and inelastic x-ray scattering are discussed. 25 references, 15 figures

High-resolution x-ray scattering

Beamline and spectrometer instrumentation for high-resolution x-ray scattering at the Stanford Synchrotron Radiation Laboratory is described. The combination of photon intensity at the sample of approx. 10/sup 13/ photons/sec/10 mm/sup 2/ and the momentum-transfer resolution of ..delta..Q approx. 10/sup -4/ A/sup -1/ make this a unique facility. Examples are given of data obtained on two-dimensional phase transitions both in studies of thin (2 molecular layers) liquid-crystal films and rare-gas monolayers on pyrolytic-graphite substrates