Effect of He pressure on the superconducting transition temperatures of Na2CsC60 and (NH3)4Na2CsC60

摘要

The Na based mixed alkali doped C(sub 60) superconductors show anomalous behavior with respect to the ''universal'' superconducting transition temperature (Tc) vs lattice constant a(sub 0) relation followed by most of the fcc A(sub 3)C(sub 60) superconductors. We have measured dt(sub c)/dP for Na(sub 2)CsC(sub 60) and (NH(sub 3))(sub 4)Na(sub 2)CsC(sub 60) using solid He as the pressure medium to (approximately) 6 kbar finding dT(sub c)/dP equal to (minus)0.8(plus minus)(0.01) K/kbar and (minus)1.0((plus minus)0.1)K/kbar for Na(sub 2)CsC(sub 60) and Na(sub 2)(NH(sub 3))(sub 4)C(sub 60) respectively. Our value for Na(sub 2)CsC(sub 60) differs markedly from that obtained by Mizuki et al of about (minus)1.3 K/kbar. However, using N(sub 2) or Ar, we obtain values for dT(sub c)/dP in substantial agreement with Mizuki et al who used fluorinert to generate their pressure. This work emphasizes the need for compressibility measurements with the same pressure medium in the appropriate temperature range so that meaningful comparisons can be made between various pressure measurements and models which are based on lattic spacing.