Bogoliubov angle, particle-hole mixture, and angle-resolved photoemission spectroscopy in superconductors

Abstract

Superconducting state is characterized by excitations---Bogoliubov quasiparticles that represent a coherent mixture of electron $(\ensuremath{-}e)$ and hole $(+e)$ components. We propose an observable, which we call Bogoliubov angle, which can be extracted using angular resolved photoemission spectroscopy (ARPES) and reveal this particle-hole entanglement by measuring relative weight of particle and hole amplitude of the Bogoliubov quasiparticle. This angle can be measured using the ratio of ARPES spectral intensities at positive and negative energies. Bogoliubov angle could be used to identify variety of possible pairing states and for discussion on possible nature of the pseudogap states that could contain pairing correlations.