Composite pairing in a mixed valent two channel Anderson model

TL;DR

This paper develops a theory of composite pairing in heavy fermion superconductors using a two-channel Anderson model with symplectic Hubbard operators, revealing charge redistribution and predicting a measurable NQR frequency shift.

Contribution

It introduces symplectic Hubbard operators for a unified treatment of pairing and valence fluctuations in heavy fermion systems, advancing theoretical understanding.

Findings

Composite pairing involves mixing of empty and doubly occupied states.

Charge redistribution occurs within the unit cell due to pairing.

A sharp NQR frequency shift is predicted in CeCoIn_5.

Abstract

Using a two-channel Anderson model, we develop a theory of composite pairing in the 115 family of heavy fermion superconductors that incorporates the effects of f-electron valence fluctuations. Our calculations introduce "symplectic Hubbard operators": an extension of the slave boson Hubbard operators that preserves both spin rotation and time-reversal symmetry in a large N expansion, permitting a unified treatment of anisotropic singlet pairing and valence fluctuations. We find that the development of composite pairing in the presence of valence fluctuations manifests itself as a phase-coherent mixing of the empty and doubly occupied configurations of the mixed valent ion. This effect redistributes the f-electron charge within the unit cell. Our theory predicts a sharp superconducting shift in the nuclear quadrupole resonance frequency associated with this redistribution. We calculate the magnitude and sign of the predicted shift expected in CeCoIn_5.