Review: Observation of Majorana Bound States at a Free Surface of $^3$He-B.

The p-wave superfluid $^3$He is a textbook example of topological superfluids. Among its multiple superfluid phases, the B phase ($^3$He-B) is known as a topological state protected by time-reversal symmetry. One of the important topological features of $^3$He-B is the formation of bound states at its surface. Notably, such surface Andreev bound states are predicted to be Majorana fermions, i.e., their antiparticles are identical to their particles. Because of the well-elucidated bulk properties of the superfluid $^3$He owing to its cleanliness, $^3$He-B provides an ideal platform to pursue Majorana fermions in condensed matter systems. In this article, we review recent investigations of surface Andreev bound states by the mobility of ions trapped below a free surface of $^3$He-B. The free surface provides a specular boundary condition, where the surface Andreev bound states are expected to show a well-defined Majorana nature owing to the absence of microscopic roughness on the surface. We show that the temperature and depth dependences of the experimentally obtained mobility of negative ions [H. Ikegami {\it et al.,} {\it J. Phys. Soc. Jpn.} {\bf 82} 124607 (2013)] are quantitatively reproduced by a theoretical study that includes scattering of the surface Andreev bound states [Y. Tsutsumi, {\it Phys. Rev. Lett.} {\bf 118} 145301 (2017)]. This quantitative agreement unambiguously demonstrates the experimental detection of surface Andreev bound states. We also discuss the future prospects of the Majorana physics in $^3$He-B.