The pressure effect on the superconducting transition temperature (dTc/dP) has been examined for the hole-high temperature superconductors (HTSCs) Tl2Can-1Ba2CunO2n+4 for n=1, 2 and 3. A negative dTc/dP was observed in this family of HTSCs with n=1, in strong contrast to other hole-HTSCs, raising a question concerning the charge asymmetry in HTSCs previously proposed. By examining all existing α(≡ dTc/TcdP)-results, it was found that hole HTSCs and electron-HTSCs fall into two distinct regions in the α-Tc plot. A universal α-Tc behavior also emerges for the hole-HTSCs. The implications of these observations are discussed.
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