Alcalde Bessia, F., M. Pérez, M. Sofo Haro, I. Sidelnik, J. J. Blostein, S. Suárez, P. Pérez, M. Gómez Berisso, and J. Lipovetzky. "Displacement Damage in CMOS Image Sensors After Thermal Neutron Irradiation." IEEE Transactions on Nuclear Science 65, no. 11 (2018): 2793–2801.
Abstract: In this paper, CMOS image sensors were exposed to thermal neutrons observing an increase in the dark signal of many pixels. The effect was found to be similar to the damage caused by alpha particles irradiation. Rutherford backscattering spectroscopy (RBS) and SIMNRA simulation were used to confirm that the sensors contain boron in the insulation layers. The damage produced by thermal neutrons is explained as displacement damage caused by alpha particles and lithium-7 ions in the silicon active volume of the sensors after boron-10 thermal neutron capture.
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Amigó, M. L., N. Haberkorn, P. Pérez, S. Suárez, and G. Nieva. "Vortex dynamics in β -FeSe single crystals: effects of proton irradiation and small inhomogeneous stress." Superconductor Science and Technology 30, no. 12 (2017): 125017.
Abstract: We report on the critical current density J c and the vortex dynamics of pristine and 3 MeV proton irradiated (cumulative dose equal to ##IMG## [http://ej.iop.org/images/0953-2048/30/12/125017/sustaa9518ieqn1.gif] {$2\times {10}^{16}$} cm −2 ) β -FeSe single crystals. We also analyze a remarkable dependence of the superconducting critical temperature T c , J c and the flux creep rate S on the sample mounting method. Free-standing crystals present T c = 8.4(1) K, which increases to 10.5(1) K when they are fixed to the sample holder by embedding them with GE-7031 varnish. On the other hand, the irradiation has a marginal effect on T c . The pinning scenario can be ascribed to twin boundaries and random point defects. We find that the main effect of irradiation is to increase the density of random point defects, while the embedding mainly reduces the density of twin boundaries. Pristine and irradiated crystals present two outstanding features in the temperature dependence of the flux creep rate: ##IMG## [http://ej.iop.org/images/0953-2048/30/12/125017/sustaa9518ieqn2.gif] {$S(T)$} presents large values at low temperatures, which can be attributed to small pinning energies, and a plateau at intermediate temperatures, which can be associated with glassy relaxation. From Maley analysis, we observe that the characteristic glassy exponent μ changes from ∼1.7 to 1.35–1.4 after proton irradiation.
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Haberkorn, H., M. Xu, W. R. Meier, J. Schmidt, S. Suárez, S. L. Bud’ko, and P. C. Canfield. "Substantial reduction of the anisotropy in the critical current densities Jcof Ni-doped CaKFe4As4single crystals by chemical and irradiation-induced disorder." Superconductor Science and Technology 34, no. 3 (2021): 035013.
Abstract: We demonstrate that the anisotropy in the critical current densities, Jc, of CaK(Fe1-xNix)4As4 (x = 0, 0.015, 0.025, and 0.030) single crystals reduces with increasing chemical and irradiation-induced disorder. The magnetic field dependences of Jc are analyzed by performing magnetization measurements with H applied parallel and perpendicular to the crystallographic c-axis. The results show that undoped crystals display large anisotropies in Jc due to an enhancement of the vortex pinning with H applied parallel to the crystallographic ab-planes. This anisotropy reduces substantially as Ni addition increases. Moreover, we found that random disorder introduced by proton irradiation enhances mainly the vortex pinning for H parallel to the c-axis. Consequently, using adequate fluencies, the vortex pinning at low temperatures in both undoped and doped samples becomes close to isotropic. These results make the CaKFe4As4 system promising for applications that require isotropic Jc under magnetic fields.
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Haberkorn, N., S. Bengio, S. Suárez, P. D. Pérez, J. A. Hofer, and M. Sirena. "Effect of thermal annealing and irradiation damage on the superconducting critical temperature of nanocrystalline γ-Mo2N thin films." Materials Letters 236 (2019): 252–255.
Abstract: We report on the influence of the disorder and stoichiometry in the resulting superconducting critical temperature of γ-Mo2N thin films. Initially, three films (with Tc values of 7.6 K, 6.8 K and 6 K) were grown at room temperature by reactive sputtering, on Si (1 0 0) using different N2/(Ar+N2) mixtures. The influence of the thermal annealing up to 973 K and irradiation damage produced by 1 MeV Zr+(fluence up 2 × 1014 cm−2) is analyzed. The Tc of pristine films remains unchanged for increasing irradiation doses up 2 × 1014 cm−2. The Tc for annealed films decreases close to the value expected for bulk samples (≈5 K) for increasing the annealing temperature. Successive irradiations of the annealed films tend to increase their Tc up to its initial values (before annealing). The results indicate that the Tc in nanometric grain size γ-Mo2N thin films is affected by both nitrogen stoichiometry and disorder at the atomic scale.
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Haberkorn, N., S. Bengio, S. Suárez, P. D. Pérez, M. Sirena, and J. Guimpel. "Effect of the nitrogen-argon gas mixtures on the superconductivity properties of reactively sputtered molybdenum nitride thin films." Materials Letters 215 (2018): 15–18.
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Haberkorn, N., S. Bengio, H. Troiani, S. Suárez, P. D. Pérez, P. Granell, F. Golmar, M. Sirena, and J. Guimpel. "Thickness dependence of the superconducting properties of γ- Mo<inf>2</inf>N thin films on Si (001) grown by DC sputtering at room temperature." Materials Chemistry and Physics 204 (2018): 48–57.
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Haberkorn, N., J. Guimpel, S. Suárez, H. E. Troiani, P. Granell, F. Golmar, J. - H. Lee, S. H. Moon, and H. Lee. "Strong influence of the oxygen stoichiometry on the vortex bundle size and critical current densities J c of GdBa 2 Cu 3 O x -coated conductors grown by co-evaporation." Superconductor Science and Technology 30, no. 9 (2017): 095009.
Abstract: We report on the influence of oxygen stoichiometry on the vortex creep mechanism of GdBa 2 Cu 3 O x -coated conductors produced by co-evaporation. The oxygen stoichiometry of the films, x , was modified in a controlled way between 6.85 and 7, which resulted in systematic and reversible control of the superconducting critical temperature between about 78 and 93 K. The change in the oxygen stoichiometry produces a strong reduction in the self-field critical current densities J c without significantly modifying the power-law dependence at intermediate magnetic fields, which indicates a negligible contribution of oxygen vacancies to the pinning. In addition, the characteristic glassy exponent μ shows a systematic diminution from about 1.63 at x = 7 to about 1.12 at x = 6.85. The results are compared with those obtained for proton- and oxygen-irradiated films, in which the vortex dynamics is determined by a balance between the improved pinning, originating from nanocluster inclusion, and the suppressed superconducting properties due to disorder in the nanoscale.
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Haberkorn, N., S. Suárez, S. L. Bud'ko, and P. C. Canfield. "Strong pinning and slow flux creep relaxation in Co-doped CaFe2As2 single crystals." Solid State Communications 318 (2020): 113963.
Abstract: We report on measurements of critical current densities Jc and flux creep rates S of freestanding Ca(Fe1−xCox)2As2 (x ≈ 0.033) single crystals with Tc ≈ 15.7 K by performing magnetization measurements. The magnetic field dependences of Jc at low temperature display features related to strong pinning. In addition, we find that the system displays small flux creep rates. The characteristic glassy exponent, μ, and the pinning energy, U0, display exceptional high values for pristine crystals. We find that for magnetic fields between 0.3 T and 1 T, μ decreases from ≈ 2.8 to ≈ 2 and U0 remains ≈ 300 K. Analysis of the pinning force indicates that the mechanism is similar to the observed in polycrystalline systems in which grain boundaries and random disorder produce the vortex pinning. Considering the large U0 observed in the single crystal, we attempt to improve the pinning by adding random point disorder by 3 MeV proton irradiation with a fluence of 2 × 1016 proton/cm2. The results show that, unlike other iron-based superconductors, the superconducting fraction is sharply reduced by irradiation. This fact indicates that the superconductivity in the system is extremely fragile to an increment in the disorder. The superconducting volume fraction in the irradiated crystal systematically recovers after removal disorder by thermal annealing, which evidences as to the observation of critical state in curves of magnetization versus magnetic field. No features related to a reentrant antiferromagnetic transition are observed for the irradiated sample.
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Haberkorn, N., S. Suárez, P. D. Pérez, H. Troiani, P. Granell, F. Golmar, J. - H. Lee, and S. H. Moon. "Effect of mixed pinning landscapes produced by 6 MeV oxygen irradiation on the resulting critical current densities Jc in 1.3 µm thick GdBa2Cu3O7-d coated conductors grown by co-evaporation." Physica C: Superconductivity and its Applications 542 (2017): 6–11.
Abstract: We report the influence of crystalline defects introduced by 6 MeV 16O3+ irradiation on the critical current densities Jc and flux creep rates in 1.3 µm thick GdBa2Cu3O7-δ coated conductor produced by co-evaporation. Pristine films with pinning produced mainly by random nanoparticles with diameter close to 50 nm were irradiated with doses between 2 × 1013 cm−2 and 4 × 1014 cm−2. The irradiations were performed with the ion beam perpendicular to the surface of the samples. The Jc and the flux creep rates were analyzed for two magnetic field configurations: magnetic field applied parallel (H║c) and at 45° (H║45°) to the c-axis. The results show that at temperatures below 40 K the in-field Jc dependences can be significantly improved by irradiation. For doses of 1 × 1014 cm−2 the Jc values at μ0H = 5 T are doubled without affecting significantly the Jc at small fields. Analyzing the flux creep rates as function of the temperature in both magnetic field configurations, it can be observed that the irradiation suppresses the peak associated with double-kink relaxation and increases the flux creep rates at intermediate and high temperatures. Under 0.5 T, the flux relaxation for H‖c and H||45° in pristine films presents characteristic glassy exponents μ = 1.63 and μ = 1.45, respectively. For samples irradiated with 1 × 1014 cm−2, these values drop to μ = 1.45 and μ = 1.24, respectively
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Haberkorn, N., M. Xu, W. R. Meier, S. Suárez, S. L. Bud’ko, and P. C. Canfield. "Enhancement of critical current density in CaKFe4As4 single crystals through 3 MeV proton irradiation." Superconductor Science and Technology 33, no. 2 (2020): 025008.
Abstract: We study the influence of random point disorder on the vortex dynamics and critical current densities Jc of CaKFe4As4 single crystals by performing magnetization measurements. Different samples were irradiated with a proton (p) beam at constant energy of 3 MeV to fluencies from 2 × 1015 p cm−2 to 4 × 1016 p cm−2. The results show the addition of extrinsic random point disorder enhances the Jc values at low and intermediate temperatures over the entire range of magnetic fields applied. The optimum pinning enhancement is achieved with a proton fluence of 3 × 1016 p cm−2, increasing Jc at 5 K by factors ≈5 and 14 at self-field and μ0H = 3 T, respectively. We analyze the vortex dynamics using the collective creep theory. The enhancement in Jc matches with a systematic reduction in the flux creep relaxation rates as a consequence of a gradual increase in the collective pinning energy U0. The substantial increment in Jc produced by random point disorder, reaching values of 9 MA cm−2 at 5 K and self-field, makes CaKFe4As4 a promising material for applications based on current carrying capacity at high magnetic fields.
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