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Using four different experimental techniques, we performed comprehensivestudies of the bulk superconductive properties of single crystals of the nearlyoptimally doped $Ba_{1-x}K_xFe_2As_2$ ($T_{c} \approx 36\,K$), a typicalrepresentative of the 122 family. We investigated temperature dependencies ofthe (i) specific heat $C_{el}(T)$, (ii) first critical magnetic field$H_{c1}(T)$, (iii) intrinsic multiple Andreev reflection effect (IMARE), and(iv) infrared reflectivity spectra. All data clearly show the presence of (atleast) two superconducting nodeless gaps. The quantitative data on thesuperconducting spectrum obtained by four different techniques are consistentwith each other: (a) the small energy gap $\Delta_S(0) \approx 1.8 - 2.5\,meV$,and the large gap energy $\Delta_L(0) \approx 9.5 - 11.3\,meV$ thatdemonstrates the signature of an extended s-wave symmetry ($\sim~33 \%$in-plane anisotropy), (b) the characteristic ratio $2\Delta_L/k_BT_C$noticeably exceeds the BCS value.

Probing Bulk Superconducting Order Parameter in Ba(K)Fe$_2$As$_2$ by Four Complementary Techniques