We provide the first systematic characterization of the structural and photoluminescence properties of optically active centers fabricated upon implantation of 30–100 keV Mg+ ions in synthetic diamond. The structural configurations of Mg-related defects were studied by the electron emission channeling technique for short-lived, radioactive 27Mg implantations at the CERN-ISOLDE facility, performed both at room temperature and 800 °C, which allowed the identification of a major fraction of Mg atoms (∼30 to 42%) in sites which are compatible with the split-vacancy structure of the MgV complex. A smaller fraction of Mg atoms (∼13 to 17%) was found on substitutional sites. The photoluminescence emission was investigated both at the ensemble and individual defect level in the 5–300 K temperature range, offering a detailed picture of the MgV-related emission properties and revealing the occurrence of previously unreported spectral features. The optical excitability of the MgV center was also studied as a function of the optical excitation wavelength to identify the optimal conditions for photostable and intense emission. The results are discussed in the context of the preliminary experimental data and the theoretical models available in the literature, with appealing perspectives for the utilization of the tunable properties of the MgV center for quantum information processing applications.

Radioactive27 Mg (t 1/2 =9.5 min) was implanted into GaN of different doping types at CERN's ISOLDE facility and its lattice site determined via β − emission channeling. Following implantations between room temperature and 800°C, the majority of 27 Mg occupies the substitutional Ga sites, however, below 350°C significant fractions were also found on interstitial positions ~0.6 Å from ideal octahedral sites. The interstitial fraction of Mg was correlated with the GaN doping character, being highest (up to 31%) in samples doped p-type with 2×1019 cm −3 stable Mg during epilayer growth, and lowest in Si-doped n-GaN, thus giving direct evidence for the amphoteric character of Mg. Implanting above 350°C converts interstitial 27 Mg to substitutional Ga sites, which allows estimating the activation energy for migration of interstitial Mg as between 1.3 and 2.0 eV.

The surface resistance of state-of-the-art REBa2Cu3O7−x coated conductors has been measured at 8 GHz versus temperature and magnetic field. We show that the surface resistance of REBa2Cu3O7−x strongly depends on the microstructure of the material. We have compared our results to those determined by the rigid fluxon model. The model gives a very good qualitative description of our data, opening the door to unravel the effect of material microstructure and vortex interactions on the surface resistance of high temperature superconductors. Moreover, it provides a powerful tool to design the best coated conductor architecture that minimizes the in-field surface resistance. We have found that the surface resistance of REBa2Cu3O7−x at 50 K and up to 9 T is lower than that of copper. This fact poses coated conductors as strong candidate to substitute copper as a beamscreen coating in CERN’s future circular collider. To this end we have also analyzed the secondary electron yield (SEY) of REBa2Cu3O7−x and found a compatible coating made of sputtered Ti and amorphous carbon that decreases the SEY close to unity, a mandatory requirement for the beamscreen chamber of a circular collider in order to prevent the electron-cloud phenomenon.

Solid state physics (SSP) research at ISOLDE has been running since the mid-1970s and accounts for about 10%–15% of the overall physics programme. ISOLDE is the world flagship for the on-line production of exotic radioactive isotopes, with high yields, high elemental selectivity and isotopic purity. Consequently, it hosts a panoply of state-of-the-art nuclear techniques which apply nuclear methods to research on life sciences, material science and bio-chemical physics. The ease of detecting radioactivity—<1 ppm concentrations—is one of the features which distinguishes the use of radioisotopes for materials science research. The manner in which nuclear momenta of excited nuclear states interact with their local electronic and magnetic environment, or how charged emitted particles interact with the crystalline lattices allow the determination of the location, its action and the role of the selected impurity element at the nanoscopic state. ISOLDE offers an unrivalled range of available radioactive elements and this is attracting an increasing user community in the field of nuclear SSP research and brings together a community of materials scientists and specialists in nuclear solid state techniques. This article describes the current status of this programme along with recent illustrative results, predicting a bright future for these unique research methods and collaborations.

We have investigated the lattice location of implanted transition metal (TM) 56 Mn, 59 Fe and 65 Ni ions in undoped single-crystalline cubic 3C-SiC by means of the emission channeling technique using radioactive isotopes produced at the CERN-ISOLDE facility. We find that in the room temperature as-implanted state, most Mn, Fe and Ni atoms occupy carbon-coordinated tetrahedral interstitial sites (T C). Smaller TM fractions were also found on Si substitutional (S Si) sites. The TM atoms partially disappear from ideal-T C positions during annealing at temperatures between 500 °C and 700 °C, which is accompanied by an increase in the TM fraction occupying both S Si sites and random sites. An explanation is given according to what is known about the annealing mechanisms of silicon vacancies in silicon carbide. The origin of the observed lattice sites and their changes with thermal annealing are discussed and compared to the case of Si, highlighting the feature that the interstitial migration of TMs in SiC is much slower than in Si.

Despite the renewed interest in ion implantation doping of GaN, efficient electrical activation remains a challenge. The lattice location of 27Mg is investigated in GaN of different doping types as a function of implantation temperature and fluence at CERN's ISOLDE facility. The amphoteric nature of Mg is elucidated, i.e., the concurrent occupation of substitutional Ga and interstitial sites: following room temperature ultra‐low fluence (≈2 × 1010 cm−2) implantation, the interstitial fraction of Mg is highest (20–24%) in GaN pre‐doped with stable Mg during growth, and lowest (2–6%) in n‐GaN:Si, while undoped GaN shows an intermediate interstitial fraction of 10–12%. Both for p‐ and n‐GaN prolonged implantations cause interstitial 27Mg to approach the levels found for undoped GaN. Implanting above 400 °C progressively converts interstitial Mg to substitutional Ga sites due to the onset of Mg interstitial migration (estimated activation energy 1.5–2.3 eV) and combination with Ga vacancies. In all sample types, implantations above a fluence of 1014 cm−2 result in >95% substitutional Mg. Ion implantation is hence a very efficient method to introduce Mg into substitutional Ga sites, i.e., challenges toward high electrical activation of implanted Mg are not related to lack of substitutional incorporation.

We determined the lattice location of Mn in ferromagnetic (Ga,Mn)As using the electron emission channeling technique. We show that interstitial Mn occupies the tetrahedral site with As nearest neighbors (T As ) both before and after thermal annealing at 200 °C, whereas the occupancy of the tetrahedral site with Ga nearest neighbors (T Ga ) is negligible. T As is therefore the energetically favorable site for interstitial Mn in isolated form as well as when forming complexes with substitutional Mn. These results shed new light on the long standing controversy regarding T As versus T Ga occupancy of interstitial Mn in (Ga,Mn)As.[http://dx.doi.org/10.1063/1.4905556] a lino.pereira@fys.kuleuven.be (Ga,Mn)As has become the model system, in which to explore the physics of carrier-mediated ferromagnetism in semiconductors and the associated spintronic phenomena. 1,2In particular, as the most widely studied dilute magnetic semiconductor (DMS), (Ga,Mn)As is the perfect example of how the magnetic behavior of DMS materials is strongly influenced by local structure. In typical high Curie temperature (T C ) (Ga,Mn)As thin films (several % Mn regime), the majority of the Mn atoms substitute for Ga (Mn s ), while a minority fraction (several % of all Mn) occupies interstitial sites (Mn i ).3,4 Mn s provides both the localized magnetic moment and the itinerant hole that mediates the magnetic coupling, whereas Mn i has a twofold compensating effect: (i) magnetically, as Mn i -Mn s pairs couple antiferromagnetically and (ii) electrically, since double donor Mn i compensates Mn s acceptors.3 For a given Mn s concentration, Mn i therefore determines the hole concentration, the Fermi level and the effective Mn s concentration (of non-compensated Mn s moments), all of which define the magneto-electronic behavior of (Ga,Mn)As. The existence of such a crucial role of Mn i is clearly reflected in the effect of the Mn i concentration on the two relevant figures of merit: T C and magnetization. 3−5 Despite this central role in the understanding of (Ga,Mn)As, and, consequently, of Mn-doped III-V DMS materials, interstitial Mn is far from being a well understood defect. The presence of Mn i in ferromagnetic (Ga,Mn)As was first reported based on ion channeling measurements. 6Although consistent with Mn i occupying tetrahedral (T) interstitial sites, the measurements did not allow to discriminate between the two nonequivalent T sites: coordinated by four Ga atoms (T Ga ) or by four As atoms (T As ). Transmission electron microscopy measurements using the (002) diffracted beam indicated that Mn i predominantly occupies the T As site.7 X-ray absorption fine structure (XAFS) techniques were later applied, suggesting T Ga occupancy (e.g., Refs. 8 and 9). However, XAFS is not well suited to distinguish neighboring elements with similar atomic numbers, as is the case for Ga and As, especially in such cases of multi-site occupancy (substitutional and interstitial), where the site to be identified is in fact the minority one (interstitial). In pi...

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