Implantación de K en el superconductor FeSe como medio de aumento de la temperatura crítica superconductora / Implementation of K in the suoerconductors FeSe to increase the superconducting critical temperature

Mogensen , Gonzalo A. (2022) Implantación de K en el superconductor FeSe como medio de aumento de la temperatura crítica superconductora / Implementation of K in the suoerconductors FeSe to increase the superconducting critical temperature. Maestría en Ciencias Físicas, Universidad Nacional de Cuyo, Instituto Balseiro.

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En este trabajo se propone la implantación de potasio por irradiación en monocristales de β-FeSe y posterior recocido como un método novedoso de producción del compuesto K_xFe_2−ySe_2. A partir de mediciones de magnetización en función del campo y de la temperatura, se caracterizaron las temperaturas críticas T_c, las densidades de corrientes críticas superconductoras J_c y los campos magnéticos H_c1 correspondientes a la penetración de vórtices en el sistema. Se analizaron las diferencias entre las distintas etapas de la muestra: antes de irradiar, después de irradiar y luego del recocido. Se encontró que la implantación de K generó una señal diamagnética superconductora con una temperatura crítica T_cK. Si bien la Tc decrece por los daños de la irradiación, este valor, al igual que T_cK, aumenta con el recocido con respecto al estado prístino. Se vio que se empobrecen las propiedades superconductoras con un segundo recocido, entre ellos la intensidad de la señal magnética superconductora asociada a la presencia de potasio decae de 0.5% al 0.2% del primer al segundo recocido. Se demostró que esta metodología genera un aumento en la T_c ∼8 K a ∼10 K, pero que además surge una segunda temperatura crítica T_cK entre 20 K y 45 K que concuerda con los valores reportados para K_xFe_2−ySe_2. Se comparó cómo decae la señal magnética superconductora de una curva de magnetización ZFC en función de la temperatura con un modelo realizado en este trabajo. Este modelo calculaba la proporción de muestra superconductora en función de la temperatura utilizando la distribución de potasio que se obtuvo utilizando el código SRIM. La irradiación generó dislocaciones de átomos de hierro, entre otros daños estructurales, que generaron una señal magnética histerética la cual se intentó aislar de la magnetización superconductora. Se vio que se requieren modelos macroscópicos que analicen la interacción entre el ferromagnetismo y el diamagnetismo superconductor para que el procedimiento sea más preciso. De todas formas se logró aislar la mayor parte de la señal superconductora. De esta manera, a partir de las ramas superconductoras se pudo calcular la densidad de corriente crítica superconductora J_c utilizando el modelo de Bean. Se analizó la dependencia de J_c con la temperatura y de las etapas de la muestra. J_c mejora notablemente con el recocido, sin embargo decae luego de la irradiación y de un segundo recocido con respecto al estado prístino. También se observó que para temperaturas cada vez más altas, como en ∼7 K, la señal ferromagnética predominaba frente a la señal diamagnética superconductora, lo que implicaría un mal cálculo de J_c si no se hubiesen separado estas señales magnéticas. Como se mencionó anteriormente, se caracterizó el campo crítico H_c1 de la muestra en función de la temperatura. Se vio que su valor aumenta luego de la implantación de potasio y prácticamente se mantiene con los recocidos. También se realizó una caracterización estructural y composicional, utilizando las técnicas de difracción de rayos X (XRD), microscopía electrónica de barrido (SEM) junto con espectros de dispersión de energía (EDS), microscopía de fuerza atómica (AFM) y espectroscopía Raman. La XRD fue una medición de bulto que permitió ver el desorden generado por la implantación de K, complementando lo visto en el magnetómetro. En el SEM se observó que la irradiación generó fragilidad sobre la zona irradiada y permitió detectar la presencia de potasio. Por otro lado, en AFM se mostró que la irradiación generó una mayor rugosidad en las muestras. Por último, se reportan las mediciones preliminares de espectroscopía Raman.

Resumen en inglés

In this work, the implantation of potassium by irradiation in β-FeSe single-crystals and subsequent annealing is proposed as a novel method of producing the compound K_xFe_2−ySe_2. The critical temperatures T_c, the critical superconducting current density Jc and the magnetic fields H_c1 corresponding to the penetration of vortices in the system were characterized from measurements of magnetization as a function of the field and the temperature. The differences between the different stages of the sample were analyzed: before irradiation, after irradiation and after annealing. It was found that K implantation generated a superconducting diamagnetic signal with a critical temperature T_cK. Although T_c decreases due to irradiation damage, this value and TcK increase with annealing regarding the pristine state. It was seen that the superconducting properties are deteriorated with a second annealing, among them the intensity of the superconducting magnetic signal associated with the presence of potassium decreases from 0.5% of the first annealing to 0.2% of the second. It was shown that this methodology generates an increase in T_c ∼8 K to ∼10 K, but also that a second critical temperature T_cK arises between 20 K and 45 K that agrees with the values reported for K_xFe_2−ySe_2. How the superconducting magnetic signal of a ZFC magnetization curve decays as a function of temperature was compared with a model developed in this work. This model calculated the proportion of superconducting sample as a function of temperature using the potassium distribution that was obtained using the SRIM code. The irradiation generated dislocations of iron atoms, among other structural damages, that generated a hysteretic magnetic signal which was tried to be isolated from the superconducting magnetization. It was seen that microscopic models that analyze the interaction between ferromagnetism and superconducting diamagnetism are required for the procedure to be more precise. In any case, it was possible to isolate most of the superconducting signal. In this way, from the superconducting branches it was possible to calculate the critical superconducting current density J_c using the Bean model. The dependence of J_c with temperature and with sample stages was analyzed. J_c improves remarkably with annealing, however it decays after irradiation and a second annealing with respect to the pristine state. It was also observed that for increasingly higher temperatures, such as ∼7 K, the ferromagnetic signal predominated over the superconducting diamagnetic signal, which would imply a miscalculation of J_c if these magnetic signals had not been separated. As mentioned above, the critical field H_c1 of the sample as a function of temperature was characterized. It was seen that its value increases after potassium implantation and is practically maintained with annealing. A structural and compositional characterization was also carried out, using X-ray diffraction (XRD), scanning electron microscopy (SEM) together with energy dispersion spectra (EDS), atomic force microscopy (AFM) and Raman spectroscopy. The XRD was a bulk measurement that allowed seeing the disorder generated by the implantation of K, complementing what was seen in the magnetometer. In SEM it was observed that the irradiation generated fragility on the irradiated area and allowed to detect the presence of potassium. On the other hand, in AFM it was shown that the irradiation generated a greater roughness in the samples. Finally, preliminary Raman spectroscopy measurements are reported.

Tipo de objeto:Tesis (Maestría en Ciencias Físicas)
Palabras Clave:Chalcogenides; Calcogenuros; Irradiation; Irradiación; Superconductors; Superconductores
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Materias:Física > Materia condensada
Física > Superconductividad
Divisiones:Gcia. de área de Investigación y aplicaciones no nucleares > Gcia. de Física > Materia condensada > Bajas temperaturas
Código ID:1066
Depositado Por:Tamara Cárcamo
Depositado En:12 Jul 2022 10:58
Última Modificación:12 Jul 2022 11:01

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