João Campos Gil

Cu(In,Ga)Se2 (CIGS) and Cu2ZnSnS4 (CZTS) are potential absorber materials for solar cell applications. We report an investigation of these materials using muon spin spectroscopy. In these experiments, positive muons produced at accelerator facilities (here the ISIS Facility, Rutherford Appleton Laboratory, U.K.) are implanted into the material and come to rest at interstitial sites in the host lattice. The muon is a sensitive local probe to study materials properties on an atomistic level. An advantage of the method is that interface properties can be studied by placing the probe particles at different depths in the sample. Muonium, the positive muon with an electron, can be considered as a light isotope of hydrogen (mass ratio 1/9) with almost identical electronic properties to hydrogen. Thus, muon spectroscopy provides also information about hydrogen in the host material. The aim of the present experiment is to obtain information about the muonium/hydrogen states formed in CIGS and CZTS solar cell materials. A major goal of the experiment is to obtain information about the physical embedding process of the implanted particle into the host lattice. The present study combines experimental measurements with total energy calculations in the framework of density functional theory. We obtain the final configurations of muonium in CZTS, that we discuss in parallel to those in CIGS. This allows us to deepen our understanding of the influence of the hydrogen impurity on the properties of these materials. We also discuss the final steps in the process of muon implantation in these materials.

The local binding and migration behavior of the proton defect in cubic yttria-stabilized zirconia (YSZ) is studied by first-principles calculations and muon-spin spectroscopy (μSR) measurements. The calculations are based on density-functional theory (DFT) supplemented with a hybrid-functional approach with the proton defect embedded in quasi-random supercells of 10.3 mol% yttria content, where the yttrium–zirconium substitutional defects are charge compensated by oxygen vacancies. Representative migration pathways for the proton comprising both transfer and bond reorientation modes are analysed and linked to the underlying microstructure of the YSZ lattice. The μSR data show the evolution of the diamagnetic fraction corresponding to the muon-isotope analogue with an activation energy of diffusion equal to 0.17 eV. Comparisons between the calculations and the experiment allow an assessment of the character of the short-range migration of the proton particle in cubic YSZ.

Age-related macular degeneration (AMD) is a leading cause of blindness worldwide, with a complex pathophysiology and phenotypic diversity. Here, we apply Similarity Network Fusion (SNF) to cluster AMD patients into putative metabolomics-derived endotypes. Using a discovery cohort of 163 AMD patients from Boston, US, and a validation cohort of 214 patients from Coimbra, Portugal, we identified four distinct metabolomics-derived endotypes with varying retinal structural and functional characteristics, confirmed across both cohorts. Patients clustered into Endotype 1 exhibited a milder form of AMD and were characterized by low levels of amino acids in specific metabolic pathways. Meanwhile, patients clustered into both Endotype 3 and 4 were associated with more severe AMD and exhibited low levels of fatty acid metabolites and elevated levels of sphingomyelins and fatty acid metabolites, respectively. These preliminary findings indicate that metabolomics-derived endotyping may offer a refined strategy for categorizing AMD patients based on their specific pathophysiological underpinnings, rather than relying solely on traditional observational clinical indicators.

The transition state model proposed for muonium formation in solids is critically discussed with respect to the delayed capture model. The two models differ mainly in how the electron capture at the muon is treated. In the delayed capture model the electron stems from the ionization track of the implanted muon. Important electron mobility information is derived in several papers from the time the electron needs to arrive at the muon. In our transition state model, the electron is picked up in the charge-exchange regime during slowing down and is present already when the muon stops in the target. Thus, no information about electron mobility can be obtained from such measurements.

Implanted positive muons with low energies (in the range 1–30 keV) are extremely useful local probes in the study of thin films and multi-layer structures. The average muon stopping depth, typically in the order of tens of nanometers, is a function of the muon implantation energy and of the density of the material, but the stopping range extends over a broad region, which is also in the order of tens of nanometers. Therefore, an adequate simulation procedure is required in order to extract the depth dependence of the experimental parameters. Here, we present a method to extract depth-resolved information from the implantation energy dependence of the experimental parameters in a low-energy muon spin spectroscopy experiment. The method and corresponding results are exemplified for a semiconductor film, Cu(In,Ga)Se₂, covered with a thin layer of Al₂O₃, but can be applied to any heterostructure studied with low-energy muons. It is shown that if an effect is present in the experimental data, this method is an important tool to identify its location and depth extent.

In muon spin rotation experiments, the diamagnetic fraction observed at low temperatures is a consequence of the muon interaction with the lattice during the incorporation process and depends on the properties of the material. An increase in the diamagnetic fraction with decreasing temperature is observed in some cases, which we attribute to a thermal spike at the end of the muon trajectory. The thermal spike is due to energy liberated not only during the muon stopping process but also as a consequence of the stress release when the electron bound to a muon in an unrelaxed configuration jumps to a neighboring lattice site. The magnitude and time extension of this effect are more significant for highly disturbed lattices which exhibit very low thermal conductivities at low temperatures. A phenomenological description of some selected examples is presented, together with a discussion of heat transport, using existing models for the thermal spike.