Probing the Fermi surface by positron annihilation and Compton scattering

Positron annihilation and Compton scattering are important probes of the Fermi surface. Relying on conservation of energy and momentum, being bulk sensitive and not limited by short electronic mean-free-paths, they can provide unique information in circumstances when other methods fail. Using a variety of examples, their contribution to knowledge about the electronic structure of a wide range of materials is demonstrated

Fermi surface of an important nano-sized metastable phase: Al3_3Li

Nanoscale particles embedded in a metallic matrix are of considerable interest as a route towards identifying and tailoring material properties. We present a detailed investigation of the electronic structure, and in particular the Fermi surface, of a nanoscale phase ($L1_2$ Al$_3$Li) that has so far been inaccessible with conventional techniques, despite playing a key role in determining the favorable material properties of the alloy (Al\nobreakdash-9 at. %\nobreakdash-Li). The ordered precipitates only form within the stabilizing Al matrix and do not exist in the bulk; here, we take advantage of the strong positron affinity of Li to directly probe the Fermi surface of Al$_3$Li. Through comparison with band structure calculations, we demonstrate that the positron uniquely probes these precipitates, and present a 'tuned' Fermi surface for this elusive phase

Experimental determination of the state-dependent enhancement of the electron-positron momentum density in solids

The state-dependence of the enhancement of the electron-positron momentum density is investigated for some transition and simple metals (Cr, V, Ag and Al). Quantitative comparison with linearized muffin-tin orbital calculations of the corresponding quantity in the first Brillouin zone is shown to yield a measurement of the enhancement of the s, p and d states, independent of any parameterizations in terms of the electron density local to the positron. An empirical correction that can be applied to a first-principles state-dependent model is proposed that reproduces the measured state-dependence very well, yielding a general, predictive model for the enhancement of the momentum distribution of positron annihilation measurements, including those of angular correlation and coincidence Doppler broadening techniques

Fermi surface of the colossal magnetoresistance perovskite La_{0.7}Sr_{0.3}MnO_{3}

Materials that exhibit colossal magnetoresistance (CMR) are currently the focus of an intense research effort, driven by the technological applications that their sensitivity lends them to. Using the angular correlation of photons from electron-positron annihilation, we present a first glimpse of the Fermi surface of a material that exhibits CMR, supported by ``virtual crystal'' electronic structure calculations. The Fermi surface is shown to be sufficiently cubic in nature that it is likely to support nesting.Comment: 5 pages, 5 PS figure

A qualitative exploration of UK prisoners’ experiences of substance misuse and mental health difficulties, and the Breaking Free Health and Justice interventions

This qualitative study explored prisoners’ lived experiences of substance use and mental health difficulties and aimed to examine perceived links between these two areas and how they might be associated with recovery during engagement with the Breaking Free Health and Justice (BFHJ) treatment programs. Interviews were conducted with 32 prisoners receiving treatment for substance use in North-West England. Emerging from prisoners’ interviews were themes relating to difficult life experiences from childhood into adulthood, how these experiences played a role in the emergence of their multiple and complex difficulties, their treatment experiences, and how their current involvement with the criminal justice system acted as a catalyst for positive change, including engagement with the BFHJ programs. This study identified the roles of substance use and mental health difficulties in the lives of participants, identified how their multiple and complex difficulties might be addressed, and provided insights into prisoners’ interpretations of their life experiences

Using the ‘recovery’ and ‘rehabilitation’ paradigms to support desistence of substance-involved offenders: Exploration of dual and multi-focus interventions (Invited Paper)

Purpose The links between substance use and offending are well evidenced in the literature, and increasingly, substance misuse recovery is being seen as a central component of the process of rehabilitation from offending, with substance use identified as a key criminogenic risk factor. In recent years, research has demonstrated the commonalities between recovery and rehabilitation, and the possible merits of providing interventions to substance-involved offenders that address both problematic sets of behaviours. The purpose of this paper is to provide an overview of the links between substance use and offending, and the burgeoning literature around the parallel processes of recovery and rehabilitation. Design/methodology/approach This is provided as a rationale for a new treatment approach for substance-involved offenders, Breaking Free Online (BFO), which has recently been provided as part of the “Gateways” throughcare pathfinder in a number of prisons in North-West England. The BFO programme contains specific behaviour change techniques that are generic enough to be applied to change a wide range of behaviours, and so is able to support substance-involved offenders to address their substance use and offending simultaneously. Findings This dual and multi-target intervention approach has the potential to address multiple, associated areas of need simultaneously, streamlining services and providing more holistic support for individuals, such as substance-involved offenders, who may have multiple and complex needs. Practical implications Given the links between substance use and offending, it may be beneficial to provide multi-focussed interventions that address both these behaviours simultaneously, in addition to other areas of multiple and complex needs. Specifically, digital technologies may provide an opportunity to widen access to such multi-focussed interventions, through computer-assisted therapy delivery modalities. Additionally, using digital technologies to deliver such interventions can provide opportunities for joined-up care by making interventions available across both prison and community settings, following offenders on their journey through the criminal justice system. Originality/value Recommendations are provided to other intervention developers who may wish to further contribute to widening access to such dual- and multi-focus programmes for substance-involved offenders, based on the experiences developing and evidencing the BFO programme

Fermi Surface as the Driving Mechanism for Helical Antiferromagnetic Ordering in Gd-Y Alloys

The first direct experimental evidence for the Fermi surface (FS) driving the helical antiferromagnetic ordering in a gadolinium-yttrium alloy is reported. The presence of a FS sheet capable of nesting is revealed, and the nesting vector associated with the sheet is found to be in excellent agreement with the periodicity of the helical ordering.Comment: 4 pages, 4 figure

The electronic structure of {\em R}NiC2_2 intermetallic compounds

First-principles calculations of the electronic structure of members of the $R$NiC$_2$ series are presented, and their Fermi surfaces investigated for nesting propensities which might be linked to the charge-density waves exhibited by certain members of the series ($R$ = Sm, Gd and Nd). Calculations of the generalized susceptibility, $\chi_{0}({\bf q},\omega)$, show strong peaks at the same ${\bf q}$-vector in both the real and imaginary parts for these compounds. Moreover, this peak occurs at a wavevector which is very close to that experimentally observed in SmNiC$_2$. In contrast, for LaNiC$_2$ (which is a superconductor below 2.7K) as well as for ferromagnetic SmNiC$_2$, there is no such sharp peak. This could explain the absence of a charge-density wave transition in the former, and the destruction of the charge-density wave that has been observed to accompany the onset of ferromagnetic order in the latter.Comment: 5 pages, 7 figures. Accepted for publication in Phys. Rev.