Triplet pairing mechanisms from Hund's-Kondo models - applications to heavy fermion superconductors

The family of heavy fermion materials, with active f-electrons,
hosts a large variety of candidate-triplet superconductors, with upper
critical fields often exceeding the Pauli limit by an order of magnitude.
Some, like UTe2, remain superconducting in fields over 60T indicating
tightly bound pairs with coherence lengths shorter than 2nm. Notably,
almost every triplet heavy fermion superconductor shares a common
structural motif – two or more f-shell atoms in the primitive unit cell
related to each other by inversion, with only two exceptions UAu2 and
YbRh2Si2. I will present a triplet pairing mechanism driven by Hund’s and
Kondo coupling and enabled by this structural motif. In essence, Hund's
coupling leads to pre-formed triplet pairs between the electrons trapped
inside local moments. Kondo hybridization imbues these triplet
correlations onto the conduction electron cloud that screens the moment.
As these heavy electron pairs delocalize on a lattice of such moments,
they remember the triplet entanglement of their parent configuration. In
heavy fermion superconductors as diverse as UPt3, UBe13, UTe2, CeRh2As2,
UGe2,
U(Co,Rh)Ge, CeSb2 as well as PrTi2Al20 and analogues, the moments are
situated away from the inversion center, so that their pre-formed triplet
pairs can be odd under inversion. As they delocalize, these heavy fermion
pairs then have finite overlap with odd-parity triplet pairs on the same
Fermi surface, leading to a triplet pairing instability. This pairing
mechanism is demonstrated by a two-channel Kondo model, in which Hund’s
coupling modifies the Kondo interaction into a triplet superexchange
between local moment and conduction spins. This unifies the triplet
superconductivity and the local moment physics in a coherent framework,
and
we discuss experimental consequences and existing support for this pairing
mechanism. The near-universal correlation with the structural motif
suggests a common origin of heavy fermion triplet superconductivity in
Hund’s-coupled local moments.