Fermi sea and sky in the Bogoliubov-de Gennes equation

Abstract: We develop a comprehensive logical framework for effectively handling the overcomplete basis set in the Bogoliubov-de Gennes equation that contains two orthonormal basis sets conjugate with each other, such as particle and hole orthonormal basis sets. We highlight the significant implications of our logical framework from theoretical concepts and experimental predictions. Firstly, we rigorously derive all many-body eigenfunctions of arbitrary nonuniform superconductors and uncover that the many-body eigenstates are full of superconducting spin clouds-the electron configuration within the Cooper-like pair of an arbitrary nonuniform superconductor. Secondly, we demonstrate a conjugate loop formed by the effective vacuum states of two orthonormal basis sets conjugate with each other, such as the Fermi sea and sky-the effective vacuum states of positive and negative orthonormal basis sets, respectively. Thirdly, we present a gate-, field-, and phase-tunable tunnel spectroscopy asymmetry arising from the imbalanced particle-hole distribution of the subgap quasiparticles in a quantum-dot Josephson junction. These findings underscore the power of our logical framework and its implications for advancing our understanding and utilization of solid-state devices based on superconductivity.