Probing superconducting gap of the high-entropy alloy Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 via Andreev reflection spectroscopy

High-entropy alloy (HEA) superconductors characterized as having a high mixing entropy have attracted substantial interest because of their remarkable potential for technological applications. However, the superconducting (SC) gap of HEA superconductors, which is one of the important factors for designing SC devices, has scarcely been studied. Herein, we report the direct observation of the fully gapped pairing symmetry of the HEA superconductor ${\mathrm{Ta}}_1/6{\mathrm{Nb}}_2/6{\mathrm{Hf}}_1/6{\mathrm{Zr}}_1/6{\mathrm{Ti}}_1/6$, probed using Andreev reflection spectroscopy. The Andreev reflection, a pronounced signature of superconductivity, is observed in the differential conductance ($\mathrm{d}I/\mathrm{d}V$) spectra below the SC transition temperature ${T}_{\mathrm{c}}$ of 7.9 K, which is well explained by the modified Blonder-Tinkham-Klapwijk model. The SC energy gap (\ensuremath{\Delta}) as a function of temperature and magnetic field follows the Bardeen-Cooper-Schrieffer (BCS) theory with $\mathrm{\ensuremath{\Delta}}(0)=1.37\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$. However, the gap to ${T}_{\mathrm{c}}$ ratio, $2\mathrm{\ensuremath{\Delta}}(0)/{k}_B{T}_c=4.03$, is larger than the value of 3.54 predicted by the weak-coupling BCS theory, indicating that ${\mathrm{Ta}}_1/6{\mathrm{Nb}}_2/6{\mathrm{Hf}}_1/6{\mathrm{Zr}}_1/6{\mathrm{Ti}}_1/6$ belongs to the class of moderately coupled nodeless superconductors.