Absence of zero-energy surface bound states in Cu_xBi ₂Se₃ studied via Andreev reflection spectroscopy

Cu_xBi₂Se₃ has been proposed as a potential topological superconductor characterized by an odd-parity full bulk superconducting gap and zero-energy surface Andreev bound states (Majorana fermions). A predicted consequence of such Majorana fermions is a peak in the zero-energy density of states, which should lead to a persistent zero bias conductance peak (ZBCP) in Andreev reflection (AR) or tunneling experiments. Here, we employ a newly developed nanoscale AR spectroscopy method to study normal metal/superconductor (N-S) devices featuring Cu_xBi ₂Se₃. The results show that a ZBCP can be tuned in or out from Cu_xBi₂Se₃ samples depending on the N-S barrier strength. While the appearance of ZBCP may be traced to different origins, its absence under finite barrier strength represents the absence of zero-energy Majorana fermions. The present observations thus call for a reexamination of the intriguing superconductivity in Cu_xBi ₂Se₃.