Low temperature thermal properties of orthorhombic and tetragonal YBa₂Cu₃O_x

Abstract

pecific heat and thermal conductivity measurements 1.8–30 K are reported on the orthorhombic (x ≃ 6.8) nd tetragonal (x ≃ 6.4) forms of ceramic YBa₂Cu₃O _x . Both forms show nearly identical results. Schottky erms dominate the specific heats below 6 K, and Einstein terms above 10 K. Excellent data fits are btained, yielding Debye temperatures of 357 K (orthorhombic) and 330 K (tetragonal). The ratios of the mplitudes of the Schottky and Einstein terms for the two forms agree very closely and increase with the atio of the densities of oxygen vacancies in the two forms. The fitted Einstein frequencies (72–81 cm⁻¹) gree well with the calculated first rapid rise in the phonon density of states for the La compound, 88 cm⁻¹. The orthorhombic data are examined for a linear term with the result that γ≃ 0.4 ± 0.6 mJ mole⁻¹ K⁻².

Thermal conductivities for both forms follow a glasslike K ≃ 5 × 10⁻¹ T² (W cm⁻¹ K⁻¹) behavior elow 5 K, but no plateau is seen up to 30 K. Electronic contributions to both the specific heats and iermal conductivities appear negligible.

Key Words:

• Specific heat
• thermal conductivity
• YBaCuO
• high T_c superconductivity.