63Cu Knight Shift Study in High-TcSuperconductor Tl2Ba2CuO6+ywith a Single CuO2Layer

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Abstract
63 Cu Knight shift in Tl 2 Ba 2 CuO 6+ y with a single CuO 2 plane has been measured by using a partially oriented powder with the c-axis perpendicular to the external field ( K ab ). For the compounds with T c =72 K and 40 K, K ab exclusively connected to the spin susceptibility (χ s ) is T -independent above T c , while it decreases rapidly below T c . By taking into account the T -independent orbital shift of K orb =0.24% and 0.26%, respectively, χ s ( T ) is found to become almost zero below T / T c =0.3, in good agreemt with the isotropic energy gap ( s-wave ) model with a larger energy gap (2Δ=4.4 k B T c ) than the BCS value (2Δ=3.5 k B T c ). For the nonsuperconducting Tl 2 Ba 2 CuO 6+ y , K ab (χ s ) exhibits no T -variation, which is different from the behavior observed in other high- T c oxides except YBa 2 Cu 3 O 7 .