Loading...

Oxygen and Rare Earth Doping Effects on REn 2BanCu2n 2Om Superconductor Compounds

Sandoghchi, Mohammad | 2020

713 Viewed
  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 52689 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Akhavan, Mohammad
  7. Abstract:
  8. There are two challenging problems with the physical properties of the recently discovered cuprate, Y3Ba5Cu8O19 δ (Y358): (1) the value of its transition temperature (Tc) and (2) its unknown crystal structure. While some reports argue that Tc of Y358 and its crystal structure are completely different from those of the YBa2Cu3O7 δʹ (Y123) compound, other reports indicate the similarity of both transition temperatures and XRD patterns (or crystal structure) of these materials. It has been suggested that these similarities are due to oxygen deficiency (δ) of Y3Ba5Cu8O19 δ or the decomposition of Y358 to Y123. In this thesis, we address these issues. At first, we have tried to answer the question of whether oxygen deficiency (δ) can be the cause of the reported differences in the transition temperatures of Y358 in the literature. So, several Y358 specimens have been prepared by the standard solid state reaction and the sol gel methods by controlling the oxygen flow during the preparation of the samples. It seems that the oxygen deficiency (δ<0.5 or 2.6%) cannot explain the existence of ~10 K difference in the reported Tc's of the Y358 compound in the literature. Also, the oxygen ions substitution by the fluorine ions does not show any improvement in Tc of Y358; although a small amount of F doping decreases the resistance of the compound. Therefore, it is concluded that the transition temperature of the Y358 compound is ~ 90 K. Then, to investigate why the XRD patterns of 358 and 123 compounds are similar, we have studied the analogous Sm3Ba5Cu8O19 δ (Sm358) compound. Since the difference in crystal radius of Ba and Sm are less than that of Ba and Y, it is possible to prepare Sm1+xBa2 xCu3O7+δ solid solutions which also includes the Sm358 compound. Therefore, the similarity of XRD patterns for Sm358 and Sm123 compounds can be understood on the basis of the Sm1+xBa2 xCu3O7+δ formula. Also, the obtained transition temperatures of the Sm358 and Sm123 compounds were similar, ~ 93 K. Then, we tried to investigate the effects of doping on the transition temperature of the Sm358 compound by preparation of Sm3+xBa5 xCu8O19 δ and Sm3 x yPrxCayBa5Cu8O19 δ compounds. The Sm substitution for Ba, or Pr and Ca substitution for Sm, indirectly change the oxygen content of Sm358, and Tc of the compound. These changes of Tc have been discussed based on the Abrikosov Gorkov pair braking theory, hole filling due to the extra valence of Pr ions, and the creation of holes by Ca ions. None of these theories can individually explain these changes in Tc. Also, the observed 60 K and 17 K plateaus in the Tc versus Pr curve of the Sm3 xPrxBa5Cu8O19 δ compound respectively indicates the importance of the structural and magnetic phase transitions in Tc(x). Therefore, it seems that the behavior of Tc under Pr and Ca doping is due to collective mechanisms. Also, according to the application of the Abrikosov Gorkov pair braking and hole filling theory, a Tc(x,y) equation has been obtained for the Sm3 x yPrxCayBa5Cu8O19 δ system with x>0.3. This equation predicates that the highest transition temperature of Sm3 x yPrxCayBa5Cu8O19 δ compound is ~ 95 K.
  9. Keywords:
  10. Cuprates ; Transition Temperature ; Calcium ; Y358 Supperconductor ; Praseodymium Substitution ; Hole Filling ; Pair Braking ; Rare Earth Element

 Digital Object List

 Bookmark