Electronic multipoles and multiplet pairs induced by Pomeranchuk and Cooper instabilities of Bogoliubov Fermi surfaces

Abstract

It was recently pointed out that Fermi surfaces can survive even in superconductors. Here, the authors study the instability of such systems toward electronic ordering. The ordered states are classified into diagonal and off-diagonal ones, which, respectively, indicate the Pomeranchuk instability and Cooper pairing not of original electrons but of Bogoliubov particles (bogolons). The corresponding order parameters are expanded by multipole moments and multiplet pair amplitudes of original electrons. These results provide insights into superconductors with multiple phase transitions.

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