J.S. Xue

Synthesis, characterization and superconducting properties of Pb2Sr2(Y/Ca)Cu3O9−δ single crystals

Abstract Single crystals of Pb 2 Sr 2 (Y/Ca)Cu 3 O 9-δ have been grown from a PbO-NaCl flux. The absence of CuO flux enables the crystals to be separated non-destructively from the melt using an organic solvent. This method yields high quality, untwinned, reasonably sized crystals with a sharp transition temperature of 84 K. The crystals exhibit an orthorhombic structure with lattice parameters; deduced from a Guinier camera analysis, a = 5.383 (1) A , b = 5.423 (1) A and c = 15.765 (2) A . The crystals are considerably thicker along the c -direction than typical crystals of YBa 2 Cu 3 O 7-δ and Bi 2 Sr 2 CaCu 2 O 8 which facilitates measurements of anisotropic properties. As an example we present a measurement of the component of the resistivity in the ab -plane, and along the c -axis.

Growth and characterization of superconducting single crystals of layered 1212 PbSrYCaCu oxide

High quality single crystals of Pb1−ySr2Y1−xCaxCu2+yOz have been grown by spontaneous nucleation from a self-flux and by a semiceramic method from partially molten material. Crystals up to 2×2×1 mm3 have tetragonal symmetry with 3.819 < a < 3.826 A and 11.823 < c < 11.851 A. Some of the crystals are partially superconducting “as grown”, others become superconducting after annealing in oxygen with Tonsetc = 40–65 K. These differences are probably caused by inhomogeneous distribution of oxygen and/or calcium in the crystal lattice. The crystals have been characterized by EDAX, EPMA and Guinier powder X-ray diffraction and their uniformity, as well as the influence of annealing on their superconducting properties are discussed.

Anisotropy in selected electrical, magnetic and optical properties of Pb2Sr2(Y/Ca)Cu3O8

Measurements of anisotropy in the dc‐resistivity, magneto‐resistivity, low and intermediate field magnetization and mid infrared reflectance of single crystals of Pb2Sr2(Y/Ca)Cu3O8+δ which have a superconducting transition temperature of ≊76 K are reported. The ab‐plane properties−a high resistivity, large magnetic penetration depth and low plasma edge−suggest that the in‐plane carrier concentration is unusually small in this material given the relatively high value of Tc. The extent of the anisotropy between ab‐plane and c‐axis properties appears to be similar to that observed in YBa2Cu3O7−δ, and is thus less severe than in Bi2Sr2CaCu2O8.

Weak pseudogap in crystals of Pb{sub 2}Sr{sub 2}(Y,Ca)Cu{sub 3}O{sub 8+{delta}}

We report on NMR measurements in underdoped Pb2Sr2(Y,Ca)Cu3O8+d crystals. A pseudogap is observed in the Knight shift and spin-lattice relaxation rate. In contrast to other underdoped compounds, the pseudogap observed in the Knight shift is weak and occurs at a significantly lower temperature. On the other hand, the effect the pseudogap has on spin-lattice relaxation is quite similar to that in other compounds. The contrast between weak and strong pseudogaps is discussed.

Weak pseudogap in crystals of Pb 2 Sr 2 ( Y , C a ) C u 3 O 8 + δ

We report on NMR measurements in underdoped Pb2Sr2(Y,Ca)Cu3O8+d crystals. A pseudogap is observed in the Knight shift and spin-lattice relaxation rate. In contrast to other underdoped compounds, the pseudogap observed in the Knight shift is weak and occurs at a significantly lower temperature. On the other hand, the effect the pseudogap has on spin-lattice relaxation is quite similar to that in other compounds. The contrast between weak and strong pseudogaps is discussed.

Forbidden Frölich-interaction-induced 2-LO Raman scattering in Pb2Sr2RCu3O8 (R=rare earth element)

Abstract The one- and two-phonon Raman scattering spectrum of single crystal Pb 2 Sr 2 RCu 3 O 8 , where R is a rare earth element, is examined under resonance conditions. Strong features appear in the one-phonon spectrum at the positions of longitudinal optical (LO) infrared modes as the laser energy approaches that of the charge transfer (CT) gap. These modes, which we interpret as Frohlich-interaction-induced, are accompanied by a second order spectrum composed of overtones and combinations of the LO and ordinary Raman-allowed phonons. The forbidden LO scattering weakens with increasing metallicity of the samples due to the vanishing of the CT gap and the screening of the Frolich interaction. We examine the dependence on incident energy of both the lineshape and center position of the highest frequency two-phonon feature, which can be unambiguously attributed to a pure overtone of an infrared LO vibration. Due to the two-phonon nature of the scattering process the crystal momentum, Q , is no longer restricted to small wavevectors and one can expect to observe effects of phonon dispersion. We find, indeed, changes in the lineshape as the gap is approached, accompanied by a decrease in the center frequency to a value comparable to twice that of the Q ∼0 one-phonon LO mode.