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 $2\Delta/k_{\rm B}T_{\rm c}=2.5\pm1.5$and$7\pm2$.