B.A. deHarak

Hydrostatic pressure effects on the magnetic susceptibility of ruthenium oxide Sr3Ru2O7: evidence for pressure-enhanced antiferromagnetic instability

Abstract Hydrostatic pressure effects on the temperature- and magnetic field dependencies of the in-plane and out-of-plane magnetization of the bi-layered perovskite Sr 3 Ru 2 O 7 have been studied by SQUID magnetometer measurements under a hydrostatic helium-gas pressure. The anomalously enhanced low-temperature value of the paramagnetic susceptibility has been found to systematically decrease with increasing pressure. The effect is accompanied by an increase of the temperature T max of a pronounced peak of susceptibility. Thus, magnetization measurements under hydrostatic pressure reveal that the lattice contraction in the structure of Sr 3 Ru 2 O 7 promotes antiferromagnetism and not ferromagnetism. The effects can be explained by the enhancement of the inter-bi-layer antiferromagnetic spin coupling, driven by the shortening of the superexchange path, and suppression, due to the band-broadening effect, of competing itinerant ferromagnetic correlations.

An out-of-plane (e, 2e) spectrometer using a movable electron gun

A coplanar (e, 2e) spectrometer has been modified to make out-of-scattering plane measurements of ejected electron angular distributions. During an experiment, the electron detectors are fixed and the gun moves on the surface of a cone. This special geometry yields the full 360° (e, 2e) angular distribution of electrons ejected into a plane normal to the scattered electron direction. The modifications are compact and fit inside the existing vacuum chamber.

Experimental and theoretical momentum transfer dependence of the He (e, 2e) cross section

The relative triple differential cross section for the electron impact ionization of helium has been measured and calculated for an incident energy of 488 eV. This is sufficiently highly asymmetric geometry that exchange processes may be neglected and the slow and fast outgoing electrons identified as ejected and scattered electrons, respectively. For two fixed ejected electron directions ±90°, and ejected electron energy 34.5 eV, coplanar (e, 2e) measurements have been made for a range of scattering angles between ±30°, corresponding to a momentum transfer range 0.38 → 3 au. The data are presented directly as scattered electron angular distributions and in the manipulated form of their sum, difference and the ratio difference / sum. These are compared to four types of theoretical calculations. Excellent agreement is found between experiment and a calculation that includes significant post collision interaction effects. All four calculations predict an almost identical oscillatory feature in the difference and ratio of the ±90° cross sections, which is consistent with the experimental data and is due to the fact that the cross section passes through zero close to, but not exactly at, 0° scattering.

Experimental and theoretical momentum transfer dependence of the He (e, 2e) cross section for incident electron energies 150 eV and 488 eV

The relative triple differential cross section for the electron impact ionization of helium has been measured and calculated for incident energies of 150 eV and 488 eV, and an ejected electron energy 34.5 eV. At both incident energies coplanar (e, 2e) experiments were carried out for a fixed pair of ejected directions +90°, − 90° and a range of scattering angles −30° → +30°, and also for a fixed pair of ejected directions +75°, − 105° and a range of scattering angles −34° → +18°. The kinematics of these experiments cover a momentum transfer range 0.38 → 3 au. The data are presented directly as pairs of (e, 2e) scattered electron angular distributions, and in the manipulated form of their sum, difference and the ratio difference/sum. These are compared with up to five types of theoretical calculations. Good, but not perfect, agreement is found between experiment and calculations that include significant post-collision interaction effects. All calculations predict an almost identical oscillatory feature, in the difference and ratio of the cross sections, which is observed in the experimental data for incident energy 150 eV and is due to the fact that the cross section passes through zero close to, but not exactly at, 0° scattering.

International Symposium on (e,2e), Double Photoionization and Related Topics & 15th International Symposium on Polarization and Correlation in Electronic and Atomic Collisions

From 30 July to 1 August 2009, over a hundred scientists from 18 countries attended the International Symposium on (e,2e), Double Photoionization and Related Topics and the 15th International Symposium on Polarization and Correlation in Electronic and Atomic Collisions which were held at the W T Young Library of the University of Kentucky, USA. Both conferences were satellite meetings of the XXVI International Conference on Photonic, Electronic and Atomic Collisions (ICPEAC) held in Kalamazoo, Michigan, USA, 21–28 July 2009. These symposia covered a broad range of experimental and theoretical topics involving excitation, ionization (single and multiple), and molecular fragmentation, of a wide range of targets by photons and charged particles (polarized and unpolarized). Atomic targets ranged from hydrogen to the heavy elements and ions, while molecular targets ranged from H2 to large molecules of biological interest. On the experimental front, cold target recoil ion momentum spectroscopy (COLTRIMS), also known as the Reaction Microscope because of the complete information it gives about a wide variety of reactions, is becoming commonplace and has greatly expanded the ability of researchers to perform previously inaccessible coincidence experiments. Meanwhile, more conventional spectrometers are also advancing and have been used for increasingly sophisticated and exacting measurements. On the theoretical front great progress has been made in the description of target states, and in the scattering calculations used to describe both simple and complex reactions. The international nature of collaborations between theorists and experimentalists is exemplified by, for example, the paper by Ren et al which has a total of 13 authors of whom the experimental group of six is from Heidelberg, Germany, one theoretical group is from Australia, with the remainder of the theoreticians coming from several different institutions in the United States. A total of 52 invited talks and 44 submitted posters covered recent advances in these topics. These proceedings present papers on 35 of the invited talks. The Local Organizers gratefully acknowledge the financial support of the University of Kentucky College of Arts and Sciences, and the University of Kentucky Department of Physics and Astronomy. We also thank Carol Cotrill, Eva Ellis, Diane Yates, Sarah Crowe, and John Nichols, of the Department of Physics and Astronomy, University of Kentucky for their invaluable assistance in the smooth running of the conferences; Oleksandr Korneta for taking the group photograph; and Emily Martin for helping accompanying persons. Nicholas L S Martin University of Kentucky Bruno A deHarak Illinois Wesleyan University International Scientific Organizing Committee Co-Chairs Don Madison (USA)Klaus Bartschat (USA) Members Lorenzo Avaldi (Italy)Nils Andersen (Denmark) Jamal Berakdar (Germany)Uwe Becker (Germany) Michael Brunger (Australia)Igor Bray (Australia) Greg Childers (USA)Nikolay Cherepkov (Russia) JingKang Deng (China)Albert Crowe (UK) Alexander Dorn (Germany)Danielle Dowek (France) Jim Feagin (USA)Oscar Fojon (Argentina) Nikolay Kabachnik (Russia)Tim Gay (USA) Anatoli Kheifets (Australia)Alexei Grum-Grzhimailo (Russia) George King (UK)Friedrich Hanne (Germany) Tom Kirchner (Germany)Alan Huetz (France) Azzedine Lahmam-Bennani (France)Morty Khakoo (USA) Julian Lower (Australia)Birgit Lohmann (Australia) William McCurdy (USA)Bill McConkey (Canada) Andrew Murray (UK)Rajesh Srivastava (India) Bernard Piraux (Belgium)Al Stauffer (Canada) Tim Reddish (Canada)Jim Williams (Australia) Roberto Rivarola (Argentina)Akira Yagishita (Japan) Michael Schulz (USA)Peter Zetner (Canada) Anthony Starace (USA)Joachim Ullrich (Germany) Giovanni Stefani (Italy)Erich Weigold (Australia) Masahiko Takahashi (Japan)

Out-of-plane (e,2e) experiments on helium L = 0, 1, 2 autoionizing levels

We present out-of-plane (e, 2e) measurements (using an incident electron energy of 488 eV) and calculations for the helium L = 0, 1, and 2 autoionizing levels and for direct ionization [1]. While the recoil peak almost vanishes in the angular distribution for direct ionization, it remains significant for the autoionizing levels and exhibits a characteristic shape for each orbital angular momentum L = 0, 1, 2. These findings can qualitatively be explained by an L-dependent addition to the ionization amplitude, but only a second-order model in the projectile?target interaction can quantitatively reproduce the observed magnitudes of the recoil peaks. We present the data as both angular distributions and energy spectra for the resonances. Preliminary experimental out-of-plane (e, 2e) (using incident electron of 150 eV) results will also be shown. The experimental part of this work is being carried out using an existing (e,2e) apparatus modified to allow the electron gun (see figure 2) to move on the surface of a (mathematical) cone [2]. This permits the measurement of out-of-plane (e,2e) angular distributions, for a full 360?, using a special geometry that allows out-of-plane conditions to be combined with the binary peak in a single measurement [2].

Complex q parameters for helium L = 0,1,2 autoionizing levels

First-order and second-order hybrid distorted-wave + convergent R-matrix with pseudo-states (close-coupling) calculations for electron impact ionization in the n = 2 autoionizing region of helium are analyzed in terms of complex Fano q parameters.

Recent out-of-plane (e,2e) experiments on autoionizing levels of helium

Out-of-plane (e, 2e) measurements and calculations are reported for the helium autoionizing levels (2s2)1S, (2p2)1D, (2s2p)1P, and for direct ionization [1]. While the recoil peak almost vanishes in the angular distribution for direct ionization, it remains significant for the autoionizing levels and exhibits a characteristic shape for each orbital angular momentum L = 0, 1, 2. These findings can qualitatively be explained by an L-dependent addition to the ionization amplitude, but only a second-order model in the projectile-target interaction can quantitatively reproduce the observed magnitudes of the recoil peaks. We present the data as both angular distributions and energy spectra for the resonances. The experimental part of this work is being carried out using an existing (e,2e) apparatus modied to allow the electron gun (see figure 1) to move on the surface of a (mathematical) cone [2]. This permits the measurement of out-of-plane (e,2e) angular distributions, for a full 360, using a special geometry that allows out-of-plane conditions to be combined with the binary peak in a single measurement. Details of the apparatus will be presented.