S.P. Cottrell

The NeXus data format

A description is presented of the NeXus data format for X-ray and neutron scattering and muon spectroscopy.

Oxide muonics: II. Modelling the electrical activity of hydrogen in wide-gap and high-permittivity dielectrics

Following the prediction and confirmation that interstitial hydrogen forms shallow donors in zinc oxide, inducing electronic conductivity, the question arises as to whether it could do so in other oxides, not least in those under consideration as thin-film insulators or high-permittivity gate dielectrics. We have screened a wide selection of binary oxides for this behaviour, therefore, using muonium as an accessible experimental model for hydrogen. New examples of the shallow-donor states that are required for n-type doping are inferred from hyperfine broadening or splitting of the muon spin rotation spectra. Electron effective masses are estimated (for several materials where they are not previously reported) although polaronic rather than hydrogenic models appear in some cases to be appropriate. Deep states are characterized by hyperfine decoupling methods, with new examples found of the neutral interstitial atom even in materials where hydrogen is predicted to have negative-U character, as well as a highly anisotropic deep-donor state assigned to a muonium–vacancy complex. Comprehensive data on the thermal stability of the various neutral states are given, with effective ionization temperatures ranging from 10 K for the shallow to over 1000 K for the deep states, and corresponding activation energies between tens of meV and several eV. A striking feature of the systematics, rationalized in a new model, is the preponderance of shallow states in materials with band-gaps less below 5 eV, atomic states above 7 eV, and their coexistence in the intervening threshold range, 5–7 eV.

Evidence for weak itinerant long-range magnetic correlations in UGe2

Positive muon spin relaxation measurements performed on the ferromagnet UGe2 reveal, in addition to the well-known localized 5f-electron density responsible for the bulk magnetic properties, the existence of itinerant quasistatic magnetic correlations. Their critical dynamics is well described by the conventional dipolar Heisenberg model. These correlations involve small magnetic moments.

Radio-frequency μSR experiments at the ISIS pulsed muon facility

This paper explores the use of pulsed radio-frequency (RF) techniques to remove the frequency limitations imposed on conventional transverse muon spin rotation (μSR) experiments at a pulsed muon source by the finite muon pulse width. The implementation of the 90° pulse technique is demonstrated by observing the free precession signal of diamagnetic muons implanted in polythene, the change in signal amplitude as a function of RF pulse length is plotted and the precise condition for a 90° pulse determined. The technique is evaluated by comparing measurements made using conventional spin rotation experiments to those employing pulsed RF methods. The potential for applying standard NMR multiple-pulse methods to the μSR experiment is considered and the use of two-pulse RF sequences (90°x−τ−90°x and 90°x−τ−180°x) to form a muon spin echo demonstrated.

Muonium states and dynamics in phosphorus and sulphur

The various states formed by positive muons implanted into phosphorus and sulphur have been characterized as a model for interstitial hydrogen, of which little is known in these elements. Repolarization studies reveal muonium‐like states in each case, giving estimates of the hyperfine parameters and, for sulphur at least, an indication of the coexistence of a molecular radical state. The longitudinal‐field relaxation functions suggest conversion of the paramagnetic states to diamagnetic, i.e. ionization or chemical reaction, in competition with strong spin–lattice relaxation.

MUON SPIN RELAXATION AS A PROBE OF MOLECULAR DYNAMICS OF ORGANOMETALLIC COMPOUNDS

LF‐Muon Spin Relaxation data are reported for the organometallic compounds Pb(C6H5)4, (C6H6)Cr(CO)3 and (C5H5)2Ru. In each case the change in relaxation rate with temperature shows a peak analogous to the T_1 minimum in NMR. The activation parameters were calculated, and the mechanism of muon spin relaxation in the case of (C6H6)Cr(CO)3 is shown to be the reorientation motion of the benzene ring.

ISIS muons for materials and molecular science studies

This paper marks the first 25 years of muon production at ISIS and the creation in that time of a facility dedicated to the use of these elementary particles as unique microscopic probes in condensed matter and molecular science. It introduces the basic techniques of muon spin rotation, relaxation and resonance, collectively known as μSR, that were already in use by specialist groups at other accelerator labs by the mid-1980s. It describes how these techniques have been implemented and made available at ISIS, beginning in 1987, and how they have evolved and improved since then. Ever widening applications embrace magnetism, superconductivity, interstitial diffusion and charge transport, semiconductors and dielectrics, chemical physics and radical chemistry. Over these first 25 years, a fully supported user facility has been established, open to all academic and industrial users. It presently comprises four scheduled instruments, optimized for different types of measurement, together with auxiliary equipment for radiofrequency or microwave spin manipulation and future plans for pump–probe laser excitation.

Design and commissioning of a high magnetic field muon spin relaxation spectrometer at the ISIS pulsed neutron and muon source

The high magnetic field (HiFi) muon instrument at the ISIS pulsed neutron and muon source is a state-of-the-art spectrometer designed to provide applied magnetic fields up to 5 T for muon studies of condensed matter and molecular systems. The spectrometer is optimised for time-differential muon spin relaxation studies at a pulsed muon source. We describe the challenges involved in its design and construction, detailing, in particular, the magnet and detector performance. Commissioning experiments have been conducted and the results are presented to demonstrate the scientific capabilities of the new instrument.

Laser-muon spin spectroscopy in liquids—A technique to study the excited state chemistry of transients

This study introduces laser-muon spin spectroscopy in the liquid phase, which extends muonium chemistry in liquids to the realm of excited states and enables the detection of muoniated molecules by their spin evolution after laser excitation. This leads to new opportunities to study the Kinetic Isotope Effects (KIEs) of muonium/atomic hydrogen reactions and to probe transient chemistry in radiolysis processes involved in muonium formation, as well as muoniated intermediates in excited states.

Muon implantation in inert gases studied by radio frequency spectroscopy

Diamagnetic and muonium (Mu) fractions formed in low-pressure inert gases, by energetic muon implantation, have been studied using the technique of time-delayed radio frequency muon spin resonance (RF-μSR). Results obtained establish the validity of the long-held view that formation of these species is due only to prompt processes, and in turn confirms that the diamagnetic environment is due to a muon molecular ion, MMu+, and not a bare μ+. In addition, polarization fractions for the diamagnetic and Mu environments have been determined at different pressures, thereby complementing earlier data, and demonstrating that the RF-μSR technique provides polarization fractions in good accord with those obtained using conventional transverse-field muon spin resonance measurements.