Momentum-resolved superconducting gap in the bulk of Ba_1−xK_xFe₂As₂ from combined ARPES and μSR measurements

Here we present a calculation of the temperature-dependent London penetration depth, λ(T), in Ba_1-xK_xFe₂As₂ (BKFA) on the basis of the electronic band structure (Zabolotnyy et al 2009 Nature 457 569, Zabolotnyy et al 2009 Physica C 469 448) and momentum-dependent superconducting gap (Evtushinsky et al 2009 Phys. Rev. B 79 054517) extracted from angle-resolved photoemission spectroscopy (ARPES) data. The results are compared to the direct measurements of λ(T) by muon spin rotation (μSR) (Khasanov et al 2009 Phys. Rev. Lett. 102 187005). The value of λ(T=0), calculated with no adjustable parameters, equals 270 nm, while the directly measured one is 320 nm; the temperature dependence λ(T) is also easily reproduced. Such agreement between the two completely different approaches allows us to conclude that ARPES studies of BKFA are bulk-representative. Our review of the available experimental studies of the superconducting gap in the new iron-based superconductors in general allows us to state that most of them bear two nearly isotropic gaps with coupling constants 2Δ/k_BT_c=2.5±1.5 and 7±2.