T. G. Uvarova

Thermal conductivity of layered superconductors

Abstract The thermal conductivity K(T) of the superconducting 90-KYBCO, 60-KYBCO, BSCCO (2212) and non-superconducting Pr2CuO4, PrBa2Cu3O7−x, Bi2Sr2YCu2O8 single crystals have been measured. All investigated superconducting crystals demonstrate an upturn in K(T) at the superconducting critical temperature Tc. The thermal conductivity of the non-superconducting oxides varies strongly from a common dielectric-like behavior (Pr2CuO4) to a glassy-like temperature dependence (BSYCO).

Thermal conductivity of high-T c crystals: Dependence on magnitude and orientation of magnetic field, and effect of Zn doping

We present precise measurements of in-plane thermal conductivity for superconducting single crystals of YBa2Cu3O7−x (YBCO) withTc=92 and 60 K, Bi2Sr2CaCu2O8 (BSCCO), and of Zn-doped YBCO. Magnetization and thermal conductivity data obtained with the same 90-K YBCO crystal demonstrate a close relationship between the magnetic thermal resistivity and the internal magnetic fieldB in a superconductor in the mixed state. For all superconductors studied here, the magnetic thermal resistivity is a sublinear function of magnetic field. The origins of the nonlinearity are discussed.Angular dependences of the magnetic thermal resistivity have been shown to depart from the anisotropic 3D superconductor model and are in quantitative agreement with a quasi-2D model. Implications for spatial modulation of the order parameter are made.

Thermal conductivity of doped YBCO single crystals

Abstract Thermal conductivity K of the three systems: oxygen deficient YBa 2 Cu 3 O x ( x =6.05−6.95), Y 1−z Pr z Ba 2 Cu 3 O x ( z =0−1, x =6.05−6.95), and YBa 2 (Cu 1−y Zn y ) 3 O 6.95 ( y =0.0 −0.05) in a-b plane have been systematically studied as function of temperature and magnetic field. The evolution of the thermal conductivity upon doping depends on the origin of metal-insulator transition. The unusual temperature dependence K(T) of non-metallic crystals suggests that some unconventional mechanism of phonon relaxation dominates in these crystals. Analysis of the experimental data confirms the phononic explanation of the K(T) for YBa 2 Cu 3 O 7 .

Study of the Y1−xErxBa2Cu3O7−δ system: electrical and fluctuation conductivity

Abstract The temperature dependence of in-plane electric resistance have been measured for a series of Y1−xErxBa2Cu3O6.94 single crystals with 0⩽x⩽0.48. The data for different samples are identical suggesting that Er substitution for Y does not produce a sizable scattering of charge carriers and does not effect the fluctuation conductivity.

Effects of impurities on the in-plane thermal conductivity of insulating YBa2Cu3O6

We have measured the in-plane thermal conductivity K(T) for insulating single crystals of YBa2Cu3O6 doped with Er atoms in the Y site and Ni and Zn in the Cu(2) site. Thermal conductivity of pure YBa2Cu3O6 indicates a significant additional heat resistance at temperatures near the antiferromagnetic transition and below 200 K. Erbium impurities increase the phonon scattering, the effect being most pronounced at T<150 K. The 3-d impurities strongly suppress the thermal conductivity at T>15 K even for concentrations less than 2.5 at %. The experimental data suggest that acoustic phonons are strongly coupled with spin excitations in CuO2 planes.

Magnetic and superconducting properties of Gd1−xPrxBa2Cu3O7−y single crystals

Abstract Magnetic and superconducting properties were studied for high quality Al-free orthorhombic Gd 1− x Pr x Ba 2 Cu 3 O 7− y single crystals (0≤ x ≤ 1.0) grown by the flux method. An indication of the interaction between Pr and Cu(2) magnetic sublattices was found for nonsuperconducting Pr123. Different sign of magnetic anisotropy was established for the Pr and Gd ions at low T for crystals with x =0.6 and 0.44. It was also shown that the superconductivity reported by Zou et al. [Phys. Rev. Lett. 80, 1074 (1998)] for Pr123 single crystals grown by TSFZ method seems to be connected with partial substitution of Ba for the Pr-sites.

Magnetic properties of Gd1−xPrxBa2Cu3O7−y single crystals

Magnetic properties were studied for the high quality Al-free orthorhombic Gd_{1-x}Pr_xBa_2Cu_3O_{7-y} single crystals (0<=x<=1.0) grown by the flux method. An indication on the interaction between the Pr and Cu(2) magnetic sublattices was found for Pr123. Different signs of magnetic anisotropy were established for the Pr and Gd ions at low temperatures. It was also shown that superconductivity reported by Zou et al. [Phys. Rev. Lett. 80 (1998) 1074] for Pr123 single crystals grown by TSZF method seems to be connected with partial substitution of Ba for the Pr-sites.

Thermal conductivity of oxygen-deficient YBCO: Evidence for the spin-phonon coupling

We have studied the in-plane thermal conductivity in oxygen-deficient single crystals of YBa2Cu3O6+x (0<x<1). Strong temperature dependence of K(T) observed in antiferromagnetic YBa2Cu3O6 near the Neel temperature indicates the strong coupling of the acoustic phonons with localized Cu spins. In the underdoped superconducting crystals the systematic evolution of the metallic normal state K(T) correlates with pseudo-gap behavior of the spin excitation spectrum. This suggests that the upturn in K(T) below Tc may arise from the spin-gap opening.