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A comparison of high-Tc superconductors: Specific heat in magnetic fields at very low temperatures

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Caspary, R., Winkelmann, M., & Steglich, F. (1989). A comparison of high-Tc superconductors: Specific heat in magnetic fields at very low temperatures. Zeitschrift für Physik B, 77(1), 41-45. doi:10.1007/BF01313617.

We discuss specific-heat,C(T), results on YBa2Cu3O7−δ(δ=0.13, 0.99) and Bi2Sr2CaCu2O8. For completion, we compare these with literature data on La1.85Sr0.15CuO4 and La2CuO4 by emphasizing the data at lowest temperatures,T≦0.45K. The strong upturn inC(T)/T found atB=0 is predominantly associated with a splitting of nuclear states by quadrupolar interactions. The shift of this upturn towards higher temperatures, when magnetic fields up to 8 T are applied, indicates additional hyperfine splitting. For YBa2Cu3O6.87 and Bi2Sr2CaCu2O8, subtraction of the nuclear specific heat from the raw data reveals a spin-glass derived contribution γ*(T,B), with γ* being strongly depressed upon both warming and applying externalB-fields. The probable source for γ* T are impurity phases (in YBa2Cu3O7−δ) and/or frustrated Cu2+ moments created by lattice defects. Any intrinsic linear specific heat, γ i , due to, e.g., spinon excitations or normal electronic carriers, would have to imply γ i <1 mJ/K2 molef.u. for each of these two compounds.