Momentum-resolved superconducting gap in the bulk of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ from combined ARPES and $mu$SR measurements

Here we present a calculation of the temperature-dependent London penetration depth, $lambda(T)$, in Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ (BKFA) on the basis of the electronic band structure [1,2] and momentum-dependent superconducting gap [3] extracted from angle-resolved photoemission spectroscopy (ARPES) data. The results are compared to the direct measurements of $lambda(T)$ by muon spin rotation ($mu$SR) [4]. The value of $lambda(T=0)$, calculated with emph{no} adjustable parameters, equals 270 nm, while the directly measured one is 320 nm; the temperature dependence $lambda(T)$ 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 all hole-doped of them bear two nearly isotropic gaps with coupling constants $2Delta/k_{rm B}T_{rm c}=2.5pm1.5$ and $7pm2$.