Fragile surface zero-energy flat band in three-dimensional chiral superconductors

Gapless phases of matter have received enormous attention in recent years. In the context of unconventional superconductors (SCs), such gapless phases manifest as nodal excitations in superconducting gaps and have a surface zero-energy flat band due to nontrivial topology of bulk superconducting states. For heavy-fermion SCs such as UPt3 and URu2Si2, time-reversal-breaking gap functions with both line and point nodes have been proposed and this type of gap functions also host surface zero-energy flat bands in spite of the absence of time-reversal symmetry [1]. In this presentation, we show that the surface flat bands in three-dimensional chiral SCs, which include UPt3 and URu2Si2, are fragile against (i) the surface misorientation and (ii) the surface Rashba spin-orbit interaction. The fragility of (i) is specific to chiral SCs, whereas that of (ii) happens for general odd-parity SCs. We demonstrate that these flat-band instabilities vanish or suppress a zero-bias conductance peak in a normal/insulator/superconductor junction, which behavior is clearly different from high-T$_{\mathrm{c}}$ and noncentrosymmetric SCs. By calculating the angle-resolved conductance, we also discuss a topological surface state associated with the coexistence of line and point nodes. [1] P. Goswami and L. Balicas, arXiv:1312.3632; P. Goswami and A. H. Nevidomskyy, Phys. Rev. B 92, 214504 (2015). [2] S. Kobayashi, Y. Tanaka, and M. Sato, Phys. Rev. B 92, 214514 (2015).