Philip King

Shallow versus deep hydrogen states in ZnO and HgO

The muonium states mimicking interstitial hydrogen in ZnO and HgO are compared. Whereas in ZnO a theoretically predicted shallow donor state is confirmed, in HgO we find a considerably deeper state. The respective ionization temperatures are around 40 K and 150 K and the donor ionization energies are 19±1 and 136±3 meV, deduced from the temperature dependence of the µSR (muon spin-rotation) signal amplitudes. The µSR spectra provide a comprehensive characterization of the undissociated paramagnetic states: the hyperfine parameters, which measure the electron spin density on and near the muon, differ by a factor of ~30. These define a hydrogenic radius of 1.1 nm in ZnO but indicate a much more compact electronic wavefunction in HgO, more akin to those of Mu* and the AA9 centre in Si. These data should largely carry over to hydrogen as a guide to its electrical activity in these materials.

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.

STUDIES OF RANGE AND STRAGGLING OF MUONS IN METALS BY THE PROJECTED RANGE IMAGING (PRI) TECHNIQUE

Abstract A new μSR method for directly imaging the implantation depth distribution of positive muons in metals (Projected Range Imaging: PRI) is presented and the results obtained from several experiments are reported. Strong magnetic field gradients inside a metallic film are produced by driving an electric current in it: spin polarized muons stopped in the film have Larmor precession frequencies proportional to the implantation depth. Generation of field gradients, advantages and features of the method are discussed. For muon energy in the MeV range the results are compared with Monte Carlo simulations and with experimental results obtained by using the conventional moderator curve method. Applications to range and straggling studies with epithermal muons are also envisaged. These future extensions will allow a better understanding of the energy loss mechanisms in the previously inaccessible very low energy range.

Optimising a muon spectrometer for measurements at the ISIS pulsed muon source

This work describes the development of a state-of-the-art muon spectrometer for the ISIS pulsed muon source. Conceived as a major upgrade of the highly successful EMU instrument, emphasis has been placed on making effective use of the enhanced flux now available at the ISIS source. This has been achieved both through the development of a highly segmented detector array and enhanced data acquisition electronics. The pulsed nature of the ISIS beam is particularly suited to the development of novel experiments involving external stimuli, and therefore the ability to sequence external equipment has been added to the acquisition system. Finally, the opportunity has also been taken to improve both the magnetic field and temperature range provided by the spectrometer, to better equip the instrument for running the future ISIS user programme.

The donor nature of muonium in undoped, heavily n-type and p-type InAs

The charge state of muonium has been investigated in p-type doped, nominally undoped (low n-type) and heavily n-type doped InAs. The donor Mu(+) state is shown to be the dominant defect in all cases. Consequently, muonium does not simply counteract the prevailing conductivity in this material. This is consistent with the charge neutrality level lying above the conduction band minimum in InAs.

Synchronous pulsed magnetic fields in muon spin rotation

Abstract One of the advantages offered by pulsed-polarized sources of muons, such as the ISIS Facility at RAL (UK), is the possibility to apply external fields, synchronous to the muon pulse, with a duration as short as a few muons lifetimes ( ∼20 μs ). Here we present a muon spin rotation (μSR) technique in which an external pulsed magnetic field and/or field gradient, generated by a laminar current-loop method, are applied to investigate new properties. Whereas field gradients seem more appropriate for a straightforward imaging of the implantation depth of positive muons in metals, the pulsed uniform magnetic fields are shown suitable for the direct measurement of the sudden-to-adiabatic cross-over. The latter method is also expected to be applicable to the experimental study of the delayed muonium formation.

A Precise Measurement of the Oxygen Isotope Effect on the Néel Temperature in Cuprates

A limiting factor in the ability to interpret isotope effect measurements in cuprates is the absence of sufficiently accurate data for the whole phase diagram; there is precise data for 𝑇𝑐, but not for the antiferromagnetic transition temperature 𝑇𝑁. Extreme sensitivity of 𝑇𝑁 to small changes in the amount of oxygen in the sample is the major problem. This problem is solved here by using the novel compound (Ca0.1La0.9)(Ba1.65La0.35)Cu3O𝑦 for which there is a region where 𝑇𝑁 is independent of oxygen doping. Meticulous measurements of 𝑇𝑁 for samples with 16O and 18O find the absence of an oxygen isotope effect on 𝑇𝑁 with unprecedented accuracy. A possible interpretation of our finding and existing data is that isotope substitution affects the normal state charge carrier density.

Muons for Materials Investigation at ISIS

In addition to neutron production, ISIS is an intense source of pulsed muons for condensed matter investigations. Like neutrons, muons are used to study atomic-level structure and dynamics, and the siting of a muon source within a neutron facility provides opportunities for researchers to exploit both techniques in their investigations. Indeed, in many cases, muons provide complementary information to that obtained using neutrons, and some 30% of researchers who use ISIS muon beams are also regular neutron users. In this brief article, we will explain a little more about the muon technique and its applications, and also describe one of the latest developments at the ISIS muon facility. More detailed descriptions of the technique can be found in various reviews [1-4].

Muon spin rotation with pulsed sources: Fundamental and application-oriented aspects of synchronous pulsed magnetic fields

AbstractsIn a pulsed polarized source of muons, such as the ISIS Facility at the Rutherford Laboratory (UK), external fields with a duration as short as a few muon’s life-times (typically 20 μs) and synchronous to the muon pulse can be applied. Here we present a muon spin rotation (μSR) study in which an external pulsed magnetic field and/or field gradient, generated by a laminar current-loop method, are applied to investigate new properties. Field gradients are exploited for directly imaging the implantation depth distribution of positive muons in metals. Pulsed uniform magnetic fields are shown suitable for the direct measurement of the sudden-to-adiabatic crossover and, when appropriately delayed with respect to the muon pulse, are expected to be crucial for the experimental study of the delayed muonium formation.