Recent discovery of superconductivity in YbRhSi has raised particular
interest in its pairing mechanism and gap symmetry. Here we propose a
phenomenological theory of its superconductivity and investigate possible gap
structures by solving the multiband Eliashberg equations combining realistic
Fermi surfaces from first-principles calculations and a quantum critical form
of magnetic pairing interactions. The resulting gap symmetry shows sensitive
dependence on the in-plane propagation wave vector of the quantum critical
fluctuations, suggesting that superconductivity in YbRhSi is located on
the border of and -wave solutions. This leads to two
candidate phase diagrams: one has only a spin-triplet -wave
superconducting phase; the other contains multiple phases with a spin-singlet
-wave state at zero field and a field-induced spin-triplet
-wave state. In addition, the electron pairing is found to be
dominated by the jungle-gym' Fermi surface rather than the doughnut’-like
one, in contrast to previous thought. This requests a more elaborate and
renewed understanding of the electronic properties of YbRhSi.

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