Kurt Norgaard Clausen

Added flexibility in triple axis spectrometers: the two RITAs at Riso

The cold-neutron triple-axis spectrometer RITA-I at Rise has been operational for about three years, and in the near future an improved version, RITA-2 will replace the existing triple-axis instrument TAS7. We review the performance of RITA-1 and the operation modes of its flexible secondary spectrometer, giving examples of a few key experiments, and describe the software developed for running it. Further, the design of the new RITA-2 instrument is presented. The two RITA spectrometers are compared with their sister instrument SPINS at NIST and with similar instruments planned elsewhere.

The RITA spectrometer at Risø — design considerations and recent results

Abstract In the new cold triple axis spectrometer RITA at Riso we have improved the front end by extensive use of state of the art optical elements and better shielding material. The analyser system is situated in one large open tank and consists of seven individually orientable PG(002) crystal analyser blades, and a 170 mm by 120 mm 2 areas sensitive micro-strip detector. In comparison to the old TAS6 spectrometer we have obtained a factor 2–5 gain in intensity on the sample, a substantially lower background, much more flexibility for optimising the instrument and the possibility to go beyond simple point-by-point inspection. The excellent performance of RITA in the standard TAS mode is exemplified by a study of the dynamics of ≈ 15 nm hematite (α-Fe 2 O 3 ) nano-particles, and utilisation of horizontal and vertical focusing modes to study two dimensional systems is illustrated by a study of the spin fluctuations in the high T c superconductor La 1.83 Sr 0.17 CuO 4 .

Magnetic fluctuations in heavy-fermion metals

Elastic and inelastic neutron scattering have been used to study the antiferromagnetic ordering and magnetic excitations of the U heavy-fermion superconductors UPd2Al3 and URu2Si2 above and below TN. While both materials exhibit the coexistence of superconductivity and antiferromagnetic order, the nature of the antiferromagnetic order and magnetic fluctuations is qualitatively quite different. UPd2Al3 resembles a rare earth magnetic system with coupling of the 4f electrons to the conduction electrons manifested in a broadening of otherwise conventional spin wave excitations. This is in marked contrast to the unconventional antiferromagnetism found in URu2Si2.

From insulator to metal with hot and cold neutrons

Abstract This paper is a short review of magnetic neutron scattering experiments on La2-x(Sr, Ba)x CuO4. Special emphasis is given to measurements of the spin waves using hot-source triple-axis and spallation-source time-of-flight spectroscopies, magnetic fluctuations near the metal-insulator transition. Fermi surface effects and the dramatic impact of superconductivity on the magnetic response function.

The RITA spectrometer at ris

Abstract During the last eight months, two of the neutron scattering instruments TAS3 and TAS6 at the DR3 reactor at Riser National Laboratory have been out of routine operation for mechanical upgrading. The air cushion floor was, at the same time, enlarged and lowered to make room for larger sample environments.

Neutron diffraction from the vortex lattice in the heavy fermion superconductor UPt3 (invited) (abstract)

The heavy fermion superconductor UPt3 is thought to have a d‐wave pairing ground state. The principal experimental evidence for this consists of the anisotropy of the power‐law behavior observed in transverse ultrasound and μ+ SR measurements. The observation of a complex phase diagram in the superconducting state in ultrasound, torsional oscillator, and specific heat measurements may be a further indication of an unconventional pairing state. Theoretical investigations suggest the possibility of vortex lattices that are unconventional in their symmetry, their quantization, or the structure of their composite vortex cores. Transitions between such exotic vortex lattices are in principle allowed and could explain the observed features at H≊0.6 Hc2 (for H∥c) and H≊0.3Hc2 (for H⊥c). Neutron diffraction is an ideal bulk probe of the microscopic properties of the vortex lattice. We have studied the vortex lattice with H⊥c and T≊50 mK in the field range 0.75

The phase diagram and the magnetic structure of nuclear spins in elemental copper below 60 nK

Abstract The phase diagram for nuclear magnetic order is elemental copper and the corresponding ordering vectors were investigated by neutron diffraction at nanokelvin temperatures. The intermediate phase is characterized by an ordering vector (O 2 3 2/3 . This is the first time that this type of order is observed in an fcc antiferromagnet.

Nuclear order in silver at pico-Kelvin temperature

Abstract Nuclear order in silver is observed by neutron diffraction at pico-Kelvin temperatures. The structure is a type-I antiferromagnet with critical field of 100 μT. The entropy-field phase diagram was determined using the spin-dependent absorption.

Neutron studies of nuclear magnetism at ultralow temperature

Abstract Nuclear magnetic order in copper and silver has been investigated by neutron diffraction. Antiferromagnetic order is observed in these simple, diamagnetic metals at temperatures below 50 nK and 560 pK, respectively. Both crystallize in the FCC-symmetry which is fully frustrated for nearest-neighbour interactions. Silveris a good example for a frustrated, spin 1 2 Heisenberg system. The antioferromagnetic structure is of type I with a very simple phase diagram. In copper the exchange interaction is similar to silver, but there the dipolar interaction has comparable strength. This leads to a very complex phase diagram in applied fields. The low- and the high-field structures are of type I, while in intermediate fields the unconventional ordering vector q - 2π(1, 1 3 ), 1 3 ) is observed. Strong hysteresis effects indicate first-order phase boundaries in copper.

“Imaging” at the 11th PSI Summer School – for the first time

Neutron News Volume 24 • Number 1 • 2013 9 T Institut Montana in Zugerberg, Switzerland, hosted the 11th edition of the PSI Summer School on Condensed Matter Research, entitled “Imaging Life and Matter – using photons, neutrons and muons” on August 11–17, 2012. As in previous years the school accommodated PhD students and postdoctoral scientists without prior knowledge of neutron, muon, and X-ray techniques, but with an interest in learning how these techniques can be used (at PSI, say) for their different research fi elds. For the fi rst time imaging as a research tool was the focus of interest, indicating its growing attractiveness in particular at large scale facilities. Lectures by Jose Baruchel, Marco Stampanoni, Pierre Boillat and Anders Kaestner gave the base lines for the imaging techniques available at SLS, SINQ and SμS addressing also the image processing needs and capabilities. The talks by Manuel Guizar-Sicairos and Christian Grünzweig, respectively, presented phase contrast imaging with X-rays and neutrons, in particular for the visualization of magnetic domain structures, whereas Frithjof Nolting showed how magnetic domain structures behave on the micro and nano-level and in high temporal dynamics. Rajmond Mokso demonstrated how the high beam intensity at the SLS TOMCAT beamline can be used for extremely high frequent tomography sequences in order to study biologic objects in 4D. As Eberhard Lehmann stated in his talk, the neutron beam intensities cannot compete with this performance, but neutron imaging has its strength in the stroboscopic approach, the delivery of alternative imaging contrasts and the suitable energy selectivity – mainly focused on the macro-scale. Ian Robinson and Helen Walker presented diffraction-based imaging methods with X-rays, where the beam coherence and magnetic effects played the major role. Diffraction with neutrons and X-rays was presented by Julia Repper, mainly used in material research. Gerd Schneider and Ralph Müller demonstrated how modern X-ray techniques can provide microscopy features in 2 (on cellular level) and 3 dimensions for the study of biological objects and samples. This was complemented by Takahashi Ishikawa’s talk about electron microscopy. The link between imaging and neutron diffraction was established by talks from Joachim Kohlbrecher (SANS) and Jochen Stahn (refl ectometry), whereas Thomas Prokscha gave insight to muon studies of magnetism on the nano scale. Bill Pedrini and Rafael Abela provided the outlook to the next PSI large scale facility “Swiss-FEL” (Xray free electron laser), where imaging on extremely short time-scales will certainly play an important role in the future. The three evening sessions were dedicated to special applications of imaging methods for the study of Older Master paints (Jaap Boon),