Microstructures Of Cuprate Superconductor At Low Temperatures Studied By Positron Annihilation

Coincidence Doppler broadening spectra of positron annihilation radiationmeasurements have been performed in Y- and Bi-series superconductors dopedwith Zn and Ga. Positron lifetime spectra for Bi-series superconductors dopedwith Zn and Ga at temperatures from 295k to 25K have been also measured. Thefollowing experimental results have been obtained.In the Y-series superconductors doped with Zn, Zn atoms prefer to occupythe Cu(2) sites on the CuO₂ planes. The W parametcrs and the probability ofpositron-3d electron annihilations increase with Zn content. In the Y-seriessuperconductors doped with Ga, Ga atoms occupy the Cu(1) sites on the Cu-Ochains and result in the decreases of the W parameters and the probability ofpositron-3d electron annihilations.The critical temperatures (Tc) of YBa₂Cu_3-xZn_xO₇ decrease with theincrease of Zn content, the Tc for x=0, x=0.12 and x=0.24 samples are 90K, 76K and 30K, respectively. The Tc of YBa₂Cu_3-xGa_xO₇ also decrease with theincrease of Ga content, the Tc for x=0, x=0.12 and x=0.24 samples are 90K,80K and 60K, respectively. The decreasing magnitude of Tc in YBa₂Cu_3-xZn_xO₇with the increase of Zn content is larger than that in YBa₂Cu_3-xGa_xO₇ with theincrease of Ga content.In the Bi-series superconductors, both Ga and Zn occupy the Cu sites onthe CuO₂ planes. In Bi_1.6Pb_0.8Sr₂Ca₂Cu_3-xZn_xO_y samples, the W parameters andthe probability of positron-3d electron annihilations increase with Zn content, InBi_1.6Pb_0.8Sr₂Ca₂Cu_3-xGa_xO_y samples, the W parameters and the probability ofpositron-3d electron annihilations decrease with Ga content.The critical temperatures (Tc) of Bi_1.6Pb_0.8Sr₂Ca₂Cu_3-xGa_xO_y decrease withthe increase of Ga content, the Tc for x=0, x=0.30 and x=0.60 samples are 110K,65K and 60K, respectively. The Tc of Bi_1.6Pb_0.8Sr₂Ca₂Cu_3-xZn_xO_y also decreasewith the increase of Zn content, the Tc for x=0, x=0.15 and x=0.60 samples are110K, 105K and 95K, respectively. The decreasing magnitude of Tc inBi_1.6Pb_0.8Sr₂Ca₂Cu_3-xZn_xO_y with the increase of Zn content is less than that inBi_1.6Pb_0.8Sr₂Ca₂Cu_3-xGa_xO_y with the increase of Ga content.In analyzing the positron lifetime spectra of Bi-series superconductorsdoped with Ga and Zn at temperature from 295K to 30K, we can conclude that:As the temperature of the Bi-series superconductors decreasing, the freeelectronic density of both the bulk and the defects increase due to the effects ofcooling constriction. The transformation from normal- to superconducting- state occurs nearly Tc, which changes the electronic state of superconductorsapparently. As the temperature of the superconductors near to Tc, the freeelectrons tend to form Cooper pairs, which give rise to the decrease of electrondensity and the increase of the values ofÏ„₁,Ï„₂ andÏ„_m. At Tc,Ï„₁,Ï„₂ andÏ„_m reachmaximum values, while the electron density of the superconductor reaches aminimum value.The critical temperature of undoped Bi_1.6Pb_0.8Sr₂Ca₂Cu₃O_y superconductoris relatively high (Tc=110K). However, it decreases with the increase of dopantcontent, whether Bi_1.6Pb_0.8Sr₂Ca₂Cu₃O_y is doped with Zn or Ga. This may be dueto the dopant atoms tend to occupy Cu sites on CuO₂ planes ofBi_1.6Pb_0.8Sr₂Ca₂Cu₃O_y superconductor, and thus destroy the channels ofsuperconductivity and lead to some extent of lattice distortion.The atomic radius of Zn is approximately the same as that of Cu, but theatomic radius of Ga is smaller than that of Cu. So the degree of lattice distortionby Ga doping is more serious than that by Zn doping. As Bi_1.6Pb_0.8Sr₂Ca₂Cu₃O_yis doped with Ga, it will destroy the inner symmetry of the superconductor andthe transportation of the superconducting electron pairs. Therefore, thedecreasing magnitude of Tc in Bi_1.6Pb_0.8Sr₂Ca₂Cu_3-xGa_xO_y with the increase ofGa content is larger than that in Bi_1.6Pb_0.8Sr₂Ca₂Cu_3-xZn_xO_y with the increase ofZn content.The free electron density of Bi_1.6Pb_0.8Sr₂Ca₂Cu_3-xZn_xO_y decreases with theincrease of Zn content, this is because that Zn atom provides less electrons than Cu atom when it substitutes for Cu. The free electron density ofBi_1.6Pb_0.8Sr₂Ca₂Cu_3-xGa_xO_y also decreases with-the increase of Ga content.The crystal structure of Bi_1.6Pb_0.8Sr₂Ca₂Cu₃O_y superconductor doped withZn is different from that of doped with Ga. The crystal structure ofBi_1.6Pb_0.8Sr₂Ca₂Cu_3-xGa_xO_y superconductor is tetragonal when X≥0.15. Whilethe crystal structure of Bi_1.6Pb_0.8Sr₂Ca₂Cu_3-xZn_xO_y superconductor isorthorhombic. In generally, the Tc of the superconductor with an orthorhombiccrystal structure is higher than that with a tetragonal crystal structure.