Low-temperature creation of Frenkel defects via hot electron-hole recombination in highly pure NaCl single crystals

The creation spectrum of stable F centres (being part of F-H pairs of Frenkel defects) by synchrotron radiation of 7–40 eV has been measured for highly pure NaCl single crystals at 12 K using a highly sensitive luminescent method. It is shown that the efficiency of F centre creation in a closely packed NaCl is low at the decay of anion or cation excitons (7.8–8.4 and 33.4 eV, respectively) or at the recombination of relaxed conduction electrons and valence holes. Only the recombination of nonrelaxed (hot) electrons with holes provides the energy exceeding threshold value EFD, which is sufficient for the creation of Frenkel defects at low temperature

Location of the Energy Levels of the Rare-Earth Ion in BaF2 and CdF2

The location of the energy levels of rare-earth (RE) elements in the energy band diagram of BaF2 and CdF2 crystals is determined. The role of RE3+ and RE2+ ions in the capture of charge carriers, luminescence, and the formation of radiation defects is evaluated. It is shown that the substantial difference in the luminescence properties of BaF2:RE and CdF2:RE is associated with the location of the excited energy levels in the band diagram of the crystals

Feshbach resonances and mesoscopic phase separation near a quantum critical point in multiband FeAs-based superconductors

High Tc superconductivity in FeAs-based multilayers (pnictides), evading temperature decoherence effects in a quantum condensate, is assigned to a Feshbach resonance (called also shape resonance) in the exchange-like interband pairing. The resonance is switched on by tuning the chemical potential at an electronic topological transition (ETT) near a band edge, where the Fermi surface topology of one of the subbands changes from 1D to 2D topology. We show that the tuning is realized by changing i) the misfit strain between the superconducting planes and the spacers ii) the charge density and iii) the disorder. The system is at the verge of a catastrophe i.e. near a structural and magnetic phase transition associated with the stripes (analogous to the 1/8 stripe phase in cuprates) order to disorder phase transition. Fine tuning of both the chemical potential and the disorder pushes the critical temperature Ts of this phase transition to zero giving a quantum critical point. Here the quantum lattice and magnetic fluctuations promote the Feshbach resonance of the exchange-like anisotropic pairing. This superconducting phase that resists to the attacks of temperature is shown to be controlled by the interplay of the hopping energy between stripes and the quantum fluctuations. The superconducting gaps in the multiple Fermi surface spots reported by the recent ARPES experiment of D. V. Evtushinsky et al. arXiv:0809.4455 are shown to support the Feshbach scenario.Comment: 31 pages, 7 figure

Radiation creation of cation defects in alkali halide crystals: Review and today’s concept

Irradiation of alkali halide crystals creates pairs of Frenkel defects both in anion and cation sublattices. How-ever, the particular nonimpact creation mechanisms (related to the decay of different electronic excitations) of cation Frenkel pairs are still unclear. At helium temperatures, there is yet no direct evidences of the creation of stable (long-lived) elemental cation defects. On the other hand, a number of complex structural defects containing cation vacancies and/or interstitials, were detected after irradiation of alkali halides at higher temperatures. Besides already proved mechanism related to the association of H and VK centers into trihalide molecules, the following possibilities of cation interstitial-vacancy pair creation are analyzed as well: (i) a direct decay of cation or anion excitons, (ii) the transformation of anion Frenkel pairs, formed at the decay of anion excitons or e-h recombination, into cation ones

Influence of complex impurity centres on radiation damage in wide-gap metal oxides

Different mechanisms of radiation damage of wide-gap metal oxides as well as a dual influence of impurity ions on the efficiency of radiation damage have been considered on the example of binary ionic MgO and complex ionic–covalent $Lu_{3}Al_{5}O_{12}$ single crystals. Particular emphasis has been placed on irradiation with $\sim$2 GeV heavy ions ($^{197}Au, ^{209}Bi, ^{238}U$, fluence of 10$^{12}$ ions/cm$^{2}$) providing extremely high density of electronic excitations within ion tracks. Besides knock-out mechanism for Frenkel pair formation, the additional mechanism through the collapse of mobile discrete breathers at certain lattice places (e.g., complex impurity centres) leads to the creation of complex defects that involve a large number of host atoms. The experimental manifestations of the radiation creation of intrinsic and impurity antisite defects (Lu|$_{Al}$ or Ce|$_{Al}$ – a heavy ion in a wrong cation site) have been detected in LuAG and $LuAG:Ce^{3+}$ single crystals. Light doping of LuAG causes a small enhancement of radiation resistance, while pair impurity centres (for instance, $Ce|_{Lu}–Ce|_{Al}$ or $Cr^{3+}–Cr^{3+}$ in MgO) are formed with a rise of impurity concentration. These complex impurity centres as well as radiation-induced intrinsic antisite defects (Lu|$_{Al}$ strongly interacting with Lu in a regular site) tentatively serve as the places for breathers collapse, thus decreasing the material resistance against dense irradiation

Evidence for the formation of two types of oxygen interstitials in neutron-irradiated α-Al2O3 single crystals

Authors are indebted to R. Vila for stimulating discussions. Tis work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under Grant agreement No 633053. The views and opinions expressed herein do not necessarily refect those of the European Commission. In addition, the research leading to these results has received funding from the Latvian grant LZP-2018/1-0147 (EV). Institute of Solid State Physics, University of Latvia as the Center of Excellence is supported through the Framework Program for European universities Union Horizon 2020, H2020-WIDESPREAD-01–2016–2017-TeamingPhase2 under Grant Agreement No. 739508, CAMART2 project.Due to unique optical/mechanical properties and significant resistance to harsh radiation environments, corundum (α-Al2O3) is considered as a promising candidate material for windows and diagnostics in forthcoming fusion reactors. However, its properties are affected by radiation-induced (predominantly, by fast neutrons) structural defects. In this paper, we analyze thermal stability and recombination kinetics of primary Frenkel defects in anion sublattice − the F-type electronic centers and complementary oxygen interstitials in fast-neutron-irradiated corundum single crystals. Combining precisely measured thermal annealing kinetics for four types of primary radiation defects (neutral and charged Frenkel pairs) and the advanced model of chemical reactions, we have demonstrated for the first time a co-existence of the two types of interstitial defects – neutral O atoms and negatively charged O- ions (with attributed optical absorption bands peaked at energies of 6.5 eV and 5.6 eV, respectively). From detailed analysis of interrelated kinetics of four oxygen-related defects, we extracted their diffusion parameters (interstitials serve as mobile recombination partners) required for the future prediction of secondary defect-induced reactions and, eventually, material radiation tolerance.--//-- The article Lushchik, A., Kuzovkov, V.N., Kotomin, E.A. et al. Evidence for the formation of two types of oxygen interstitials in neutron-irradiated α-Al2O3 single crystals. Sci Rep 11, 20909 (2021). https://doi.org/10.1038/s41598-021-00336-0 published under CC BY 4.0 licence.EURATOM 633053, LZP-2018/1-0147; Institute of Solid State Physics, University of Latvia as the Center of Excellence is supported through the Framework Program for European universities Union Horizon 2020, H2020-WIDESPREAD-01–2016–2017-TeamingPhase2 under Grant Agreement No. 739508, CAMART2 project