Doping and critical-temperature dependence of the energy gaps in Ba(Fe1-xCox)(2)As-2 thin films

作者：Pecchio P^*; Daghero D; Ummarino G A; Gonnelli R S; Kurth F; Holzapfel B; Iida K

来源：Physical Review B, 2013, 88(17): 174506.

DOI：10.1103/PhysRevB.88.174506

The dependence of the superconducting gaps in epitaxial Ba(Fe1-xCox)(2)As-2 thin films on the nominal doping x (0.04 <= x <= 0.15) was studied by means of point-contact Andreev-reflection spectroscopy. The normalized conductance curves were well fitted by using the two-dimensional Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps-although the possible presence of gap anisotropies cannot be completely excluded. The amplitudes of the two gaps Delta(S) and Delta(L) show similar monotonic trends as a function of the local critical temperature T-c(A) (measured in the same point contacts) from 25 K down to 8 K. The dependence of the gaps on x is well correlated to the trend of the critical temperature, i.e., to the shape of the superconducting region in the phase diagram. When analyzed within a simple three-band Eliashberg model, this trend turns out to be compatible with a mechanism of superconducting coupling mediated by spin fluctuations, whose characteristic energy scales with T-c according to the empirical law Omega(0) = 4.65k(B)T(c), and with a total electron-boson coupling strength lambda(tot) = 2.22 for x <= 0.10 (i.e., up to optimal doping) that slightly decreases to lambda(tot) = 1.82 in the overdoped samples (x = 0.15).

• 出版日期2013-11-11