Doped Bi-2212 Single Crystals Research Articles

THZ Phonon Spectroscopy of Bi-2223 and Bi-2212: Evidence for Phonon Pairing

Abstract Facts are presented evidencing the strong electron–phonon interaction and the scaling of a superconducting gap and a critical temperature in doped Bi-2212 single crystals. A sharp extra structure in the current-voltage characteristics (CVC's) of Bi-2212 contacts is attributed to the presence of the extended van Hove singularity (EVHS) close to the Fermi level in slightly overdoped and slightly underdoped samples. THZ phonon spectroscopy studies of Bi-2223 and Bi-2212 are overviewed. An observed giant instability in I(V) – characteristics of Bi-2223 nanosteps is probably caused by a resonant emission of 2Δ – optical phonons in a process of recombination of nonequilibrium quasiparticles (Krasnov model).

Improved Flux Pinning Properties of Bi-Based Superconductors by Dilute RE-Doping

Flux pinning properties of lead doped Bi2212 single crystals and Bi2223 sintered bulks were improved by low level light rare earth (LRE) doping to the Sr-site. The dilute LRE doping effects were prominent below ~30 K for Bi(Pb)2212 single crystals and below ~60 K for Bi(Pb)2223 sintered bulks. These results suggest that effective point-defect-type pinning sites were successfully introduced by small amount of LRE-doping. The improvement of flux pinning properties by dilute LRE-doping was the most remarkable when the carrier doping state was nearly optimal and the LRE doping level was ~0.5% for Sr-site.

Direct observation of vortex lattice in Bi 2Sr 2CaCu 2O 8+ δ by low temperature STM/STS

In order to investigate the correlation between vortex distribution and inherent electronic inhomogeneity, we have performed scanning tunneling microscopy/spectroscopy (STM/STS) of optimally doped Bi2212 single crystals. Vortices have been successfully visualized by mapping differential tunneling conductance at 12 meV, which was substantially enhanced around the vortex cores. We found that most of the vortices were located in the regions displaying relatively large gaps and rounded gap edges. However, some vortices penetrated smaller gap regions, giving evidence that the non-superconducting large Δ regions originally existing in the crystals favor to pin down vortices.

Magnetic hysteresis and relaxation in Bi2212 single crystals doped with Fe and Pb

Magnetic hysteresis and magnetic relaxation measurements have been performed to study vortex pinning behaviors for pure, Fe doped and heavily Pb doped Bi2212 single crystals. Unlike pure and Fe doped Bi2212 crystals, heavily Pb doped crystal showed strong vortex pinning behavior. We interpret the strong pinning in heavily Pb doped Bi2212 single crystals as arising from the improved Josephson coupling in Bi2212 single crystal after heavy Pb doping. In heavily Pb doped single Bi2212 crystals, H/sub dis/(T) was observed to decrease with increasing T. Here, H/sub dis/(T) is an order-disorder field that separates a weakly elastically disordered vortex lattice from a plastically disordered vortex solid. However, in pure and Fe doped Bi2212 single crystals, H/sub dis/(T) was observed to be temperature independent. We also report a significant shift of T/sub CR/, a crossover temperature separating two pinning regimes, toward higher temperatures with heavy Pb doping of Bi2212 single crystals. On the other hand T/sub CR/ did not shift with Fe doping of Bi2212 single crystals. It is argued that the temperature dependence of H/sub dis/(T) and the shift of T/sub CR/ in heavily Pb doped Bi2212 crystals was related to the enhanced c-axis conductivity caused by the Pb situated between the CuO/sub 2/ layers and imposing a 3D characteristic on the vortex lattice.

Characteristic properties of doped Bi2212 single crystals studied by magneto optical measurement

Local J/sub c/ distribution under H /spl par/ c was carefully investigated for lightly Pb-doped Bi2212 single crystals having uniform microstructure in order to clarify essential effect of Pb-doping on intra-grain J/sub c/ properties. An abrupt change of internal field gradient was observed at a constant field, H/sup */, for each Bi(Pb)2212 single crystal below 25 K. J/sub c/ is apparently higher above H/sup */. The H/sup */ was observed at several hundred Oe which was much lower than the second peak field, H/sub pk/, in the magnetization hysteresis loops. This suggested that dimensionality of vortex changed at H/sup */ from 3D to quasi-2D state with an increase of internal magnetic field, possibly due to an inhomogeneous distribution of lead ions in the crystal. Higher J/sub c/ above H/sup */ and temperature dependent H/sub pk/ indicated generation of field-induced pinning sites in the present system. Doping effects of cobalt for heavily Pb-doped Bi2212 single crystals on the flux pinning properties were also studied. In-plane J/sub c/ was found to become isotropic with increasing the Co-doping level.

Study of the peak effect in pure, Pb and Pb+Y doped Bi-2212 single crystals

The peak effect (PE) in pure, Pb and Pb+Y doped Bi2212 single crystals with different oxygen doping levels was studied by measuring M– H loops over a wide temperature range. The PE in pure Bi2212 crystals was obtained only for crystals with optimal oxygen doping and over-doping but not observed for oxygen under-doped crystals. For Pb doped Bi2212 crystals, the PE appeared at a higher field than in the pure crystals and persisted up to T c. For (Bi 1.64Pb 0.36)Sr 2Ca 1− x Y x Cu 2O 8+ y ( x=0, 0.05, 0.11, 0.33) single crystals, results show that at low temperatures, the peak field is smaller than in solely Pb doped crystals and decreases as x increases ( x>0.1). However, the peak field at high temperature for the x=0.05 sample is higher than for heavily Pb doped Bi2212 crystals, indicative of strong pinning due to the co-doping. The formation of Bi 5+ rich clusters, which cause the reduction of the c-axis lattice parameter and ρ c , is proposed to be responsible for the appearance of the PE in the undoped crystals. The co-existence of Pb 4+- and Bi 5+-rich clusters causes the strong PE in Pb doped Bi2212 crystals. Y 3+ is proposed to be an effective dopant at low doping levels for flux pinning at high temperatures. The PE for all the crystals was characterised by plotting ( H max− H min)/ H max vs T/ T c, where H min and H max represent the fields at which the magnetisation starts to increase ( H min) and reaches a maximum ( H max) at the peak position. Results showed that the evolution of the peak effect with temperature in the Pb and Pb+Y doped crystals was similar to that seen in Y123.

Field and temperature dependence of critical current density of Fe doped Bi2212 single crystals

Critical current density Jc as a function of magnetic field and temperature has been studied in Fe-doped and pure Bi2212 single crystals. The nominal iron content in the Bi 2.1Sr 1.9Ca 1.0(Cu 1-yFe y) 2O x single crystal was y = 0.000, 0.005, 0.016 and 0.022. A strong field and temperature dependence of J c has been observed for the samples with y = 0.022 and 0.016 doping. At zero field, large J c was observed for these samples up to 40K, as compared to the samples with y = 0.005 and y = 0.000. However, for T > 40K, samples with y = 0.016 and 0.022 had lower values of J c than samples with y = 0.000 and 0.005. Similarly, for H < 500Oe samples with y = 0.000 and 0.005 had lower Jc than the more strongly doped samples. For H > 500Oe, it was the opposite. The samples with y = 0.005 were found to show large Jc over a wider range of temperatures and fields than any other samples studied This indicated that the iron in the CuO 2 planes acted as an effective pinning centre. The effectiveness of the pinning centres was strongly influenced by the field and thermal excitations.

Origin and characterisation of peak effect in pure and Pb doped Bi-2212 single crystals

The peak effect in pure and heavily Pb doped Bi2212 single crystals with different oxygen doping levels were studied by measuring M-H loops. The peak effect in pure Bi2212 crystals was obtained only for crystals with optimum oxygen doping and overdoping but not for oxygen underdoping. For Pb-doped Bi2212 crystals, a strong peak effect appeared in higher field than in pure crystals and it persisted up to T c. Formation and decomposition of Bi 5+ and/or Pb 4+ rich clusters, which caused the reduction of c-axis and ϱ c, were proposed to be responsible for the appearance and disappearance of the peak effect in pure and Pb doped crystals. The peak effect was characterised by plotting (H max-H min)/H max vs T/T c, where H max and H min represents the field in which the magnetisation reaches maximum and starts to increase around secondary peak position, respectively. Results showed that the evolution of the peak effect with temperature in the Pb doped crystals was similar to that in Y123.

TEM analysis of planar defects induced by Ti doping in Bi-2212 single crystals

An electron microscopy study is performed on Ti doped Bi-2212 single crystals. High resolution electron microscopy reveals that the Ti doping leads to incorporation of (010) planar defects in the Bi-2212 lattice. These defects consist of pairs of antiphase boundaries, which are separated along the b-axis by one modulation distance. The atomic shift at the boundary is such that one of the two CuO 2 planes is continuous throughout the defect. Energy dispersive X-ray analysis clearly reveals the presence of Ti in the planar defects, whereas no Ti could be detected in defect free areas. This evidences the fact that these defects are induced by the Ti doping.

Morphology and characterization studies of Y doped Bi-2212 single crystals

Superconducting Bi-2212 single crystals of nominal composition Bi 2.2Sr 1.9CaCu 2O 8, Bi 2.1Y 0.1Sr 1.9CaCu 2O 8 and Bi 2.2Sr 1.9Ca 0.9Y 0.1Cu 2O 8 have been grown by flux method using a conical shaped Al 2O 3 crucible. Effect of Y doping on the crystal morphology has been studied. From the X-ray diffraction studies, the cell parameters were calculated. AC susceptibility showed that Y doping gives rise to highest T c,onset at 92 K and the effect of annealing was also studied. The crystal composition at different places of the crucible and the Al incorporation were studied. The Sr/Ca ratio has been correlated with Y substitution. Chemical etching was also carried out on the grown crystals in order to reveals the nature of twinning and dislocations.