Optical Absorption in Weyl Superconductors: Novel Surface-Bulk Response With and Without Disorder*

We study the optical absorbance in a p+ip Weyl superconductor (WSC) by investigating the interplay between bulk and surface states responses. WSCs harbor gapless chiral Majorana surface states that are topologically protected. We show that in the clean limit, a novel topological optical absorbance occurs due entirely to surface-bulk transitions, which manifest a characteristic absorption peak. In the presence of disorder, we study the optical conductivity of the bulk through the Keldysh response theory. For weak disorder, the bulk response is reminiscent of the Mattis-Bardeen result in dirty s-wave superconductors, although it is nonzero at zero temperature even when the frequency falls below twice the pairing gap (due to the gapless Weyl points). For stronger disorder, the bulk response becomes more Drude-like and the p-wave features of the response gradually disappear. In the weak disorder limit, the total absorbance of the system is governed by the combined effects from both the bulk and surface-bulk responses. The additional features stemming from the surface-bulk response can serve as an indicator for the presence of surface states in sufficiently clean systems. We discuss the relevance of our results to the potential WSC UTe2.