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

TL;DR

This review discusses the experimental detection of Majorana fermions as surface Andreev bound states in superfluid $^3$He-B, highlighting how ion mobility measurements confirm their existence and properties.

Contribution

It provides a comprehensive review of recent experimental and theoretical work demonstrating Majorana bound states at the surface of $^3$He-B, a topological superfluid.

Findings

Ion mobility measurements match theoretical predictions.

Quantitative agreement confirms surface Majorana fermions.

Surface states are well-characterized by temperature and depth dependence.

Abstract

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 is an ideal surface providing a specular boundary condition; The surface Andreev bound states formed there are expected to be Majorana fermions with a well-defined energy spectrum. 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 {\it 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.