Hole Compensation Effect And A-site Radius Effect Studies On La-based Cuprate Superconductors

The cuprate oxides with high-temperature superconductivity have attracted much renewed attention in condensed matter physics due to the discovery of superconductivity in cuprate oxides in 1986.The materials have been studied comprehensively and thorougthly not only because of the potential applications,but also because of many intriguing physical properties observed in these materials,such as presudogap,charging ordering,etc.Therefore,the investigation to the mechanisms and normal state transport properties of high-temperature superconductors will surely give impetus to the progress in many fields of the condensed matter physics.In the past years,continuous efforts have been devoted to the study on the effects of substitution for planar Cu.Above all these experiment results,we found that the suppression of superconductivity is due to the fact that the impurities induce local electrons to CuO₂ planes regardless of magnetic characteristics of the impurities.So in this dissertation,we substituted Sr for La in B-site doped cuprate superconductors in which the superconductivity is suppressed.We investigated the hole compensation effect on superconductivity though magnetic properties,the transport properties and the optical response.Also,through change of the A-site average radius on La-based superconductors,we investigated the the superconductivity,the magnetic propertis,the optical response and the normal state transport properties of these materials.The results have been published in international journals.The whole dissertation consists of five chapters.The chapter one:a brief overview of the history,progress,and the current status of the high-tmperature superconductivity.In detail,the crystal structure,electronic structure,the superconductivity,the magnetic properties and the transport properties of copper oxide superconductors are disscused.A brief review on the explanation of the mechanism of high-temperature superconductvity is given in this chapter.Also we give a brief overview of the history and progress of new iron-based high-temperature superconductors.The chapter two:we focus the discussion on the research progress on La-based copper oxide superconductors.The crystal structure,phase diagram,magnetic properties,overdoped region,and stripe phase are discussed in this chapter.We focus on the doping effect in CuO₂ planes,and discuss mechanism of the doping effect. The chapter three:hole compensation effect on La_1.85-xSr_0.15+xCu_0.94M_0.06O₄(M= Mn and Ru) was studied.Through dc magnetization measurement,we found that with the increase of induced holes,the superconductivity reappears.And with the increase of hole carriers,Meissner volune and T_c^onsetfirst increase,and then decrease,form a new phase diagram The Mn⁴⁺ doping system exhibits twice superconductive phase transitions while Ru⁴⁺ doping system exhibits only one superconductive phase transition.Coexistence and competition between superconductivity and magnetism are observed.We suggest that these surprising phenomena should be attributed to the compensation effect that is the electrons on high valence impurities are compensated by holes introduced through Sr doping.This compensation effect is confirmed by our IR spectra results. The compensation effect also gives reasonalbe explanations to twice superconductive phase transitions and one superconductive phase transition.The chapter four:we studied the compensation effert on Ni and Zn doped La_1.85Sr_0.15CuO₄ system.It is found that the holes introduced by Sr substitution for La have compensated the local electrons and result in the reappearance of the superconductivity in LSCNO system,while no trace for the superconductivity is observed in LSCZO system.We ascribe this phoemena to the fact that Zn²⁺ ions cannot form octahedron.So the d orbits of Zn²⁺ ions cannot form bonds with p orbits of the surrounding oxygens.Consequently,the local electrons of Zn impurities are isolated from the pdσhybrdization paths and cannot be compensated by hole carriers in the pdσhybridzation paths.The chapter five:we carefully studied the A-site average radius effect on superconductive phase transition temperature T_c of La_1.85M_0.15CuO₄(M=Ca,Sr and Ba) system.With the increase of the substitution for Sr by Ca/Ba,the T_c of the system deceases gradualy.The pressure independence of T_c for La_1.85Ca_0.15CuO₄ can not be explained by previous model.We reconsider the possible explanation for all results now and before,and suggests that the superconductive phase transition temperature is determined by the degree of the pdσhybridization between the copperd_x²-y² and oxygen P_x,y orbits and the overlap of the wave function.The changes of A-site cation average radius affects the degree of the pdσhybridization.Samples with small A-site cation average radius is hard to be compressed and T_c does not shift under external pressure.Samples with large A-site cation average radius can be easily compressed and T_c shifts with external pressure.