M. Reedyk

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.

Infrared properties of YBa2Cu3O7-δ

Abstract We review recent experimental data on the infrared properties of YBa 2 Cu 3 O 7-δ ceramics and single crystals. Kramers-Kronig analysis shows that the normal state far-infrared optical conductivity is dominated by a Drude response from free carriers with a plasma frequency of the order of 1.0 eV and a relaxation rate of 0.37 eV (300 cm -1 at 100 K. A strong mid-infrared band centered between 0.2 and 0.5 eV is seen in all the samples. Below the superconducting transition temperatures the far-infrared conductivity is depressed below 400 cm -1 but this is not a true energy gap since there remains, even in the best oriented crystal samples, a residual absorption.

Infrared studies of high Tc superconductors

The infrared properties of the high temperature superconductors are dominated by the strong temperature dependent absorption of the free carriers at low frequency and a temperature independent midinfrared band. Dramatic changes in reflectance do occur as the material becomes superconducting but these changes cannot be used to determine the energy gap, since as a result of the development of the superconducting condensate, changes in the real part of the dielectric function dominate. Also, since the relaxation rate of the free carriers is of the order of 200 cm−1 at 100 K the materials are near the clean limit below this temperature and we do not expect to see structure at the energy of the superconducting gap. The reflectance edge observed in many experiments at 450 cm−1 is caused by the onset of the midinfrared absorption which has the character of a direct particle-hole excitation rather than the Holstein scattering of the free carriers.

Far-infrared ab plane reflectance and transmission of Bi2Sr2CaCu2O8

Abstract We review recent experimental data on the infrared thin film transmission and single crystal reflection spectra of Bi 2 Sr 2 CaCu 2 O 8 (T c ∼ 85 K). Both show that the low frequency response can be described by a Drude absorption with a relaxation rate for the carriers of the order of 100 cm −1 . This places the materials in the clean limit and no superconducting gap structure is evident. A temperature independent midinfrared absorption is present with a sharp onset at 300 cm −1 .

Infrared studies of textured ceramic high-Tc superconductors

The surfaces of ceramic samples of high-temperature superconductors can have a high degree of crystalline orientation, depending of the way in which they are prepared. (In particular it is essential that the sample not be polished in an attempt to “improve” the optical quality.) The surfaces of these textured ceramic samples consist of small crystallites, with about 80% orientation of the crystallographic c axis normal to the surface. The infrared and optical properties of these samples have been measured. We observe that the frequency-dependent conductivity can be decomposed into the sum of a far-infrared Drude part (which at zero frequency equals the ordinary dc conductivity) and a broad, temperature-independent mid-infrared band. The mid-infrared band has about four times the oscillator strength of the low-frequency Drude part.

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.