Antiferromagnetic Phase Transition in Four-Layered High-TcSuperconductors Ba2Ca3Cu4O8(FyO1-y)2withTc=55–102 K:63Cu- and19F-NMR Studies

Abstract

We report on the magnetic characteristics of four-layered high- T c superconductors Ba 2 Ca 3 Cu 4 O 8 (F y O 1- y ) 2 with apical fluorine through 63 Cu- and 19 F-NMR measurements. The substitution of oxygen for fluorine at the apical site increases carrier density ( N h ) and T c from 55 up to 102 K. The NMR measurements reveal that antiferromagnetic order, which can uniformly coexist with superconductivity, exists up to N h ≃0.15, which is somewhat smaller than N h ≃0.17, the quantum critical point (QCP) for five-layered compounds. The fact that the QCP for the four-layered compounds moves to a region of lower carrier density than for five-layered ones ensures that a decrease in the number of CuO 2 layers makes an interlayer magnetic coupling weaker.