Spin-triplet paired phases inside a ferromagnet induced by Hund's rule coupling and electronic correlations: Application to UGe2

Abstract

We discuss a mechanism of real-space spin-triplet pairing, alternative to that due to quantum paramagnon excitations, and demonstrate its applicability to UGe2. Both the Hund's rule ferromagnetic exchange and inter-electronic correlations contribute to the same extent to the equal-spin pairing, particularly in the regime in which the weak-coupling solution does not provide any. The theoretical results, obtained within the orbitally-degenerate Anderson lattice model, match excellently the observed phase diagram for UGe2 with the coexistent ferromagnetic (FM1) and superconducting (A1-type) phase. Additionally, weak A2-and A-type paired phases appear in very narrow regions near the metamaganetic (FM2 → FM1) and FM1 → paramagnetic first-order phase-transition borders, respectively. The values of magnetic moments in the FM2 and FM1 states are also reproduced correctly in a semiquantitative manner. The Hund's metal regime is also singled out as appearing near FM1-FM2 boundary.

Review of U-based Ferromagnetic Superconductors: Comparison between UGe2, URhGe, and UCoGe

“…Recently, a mechanism of spin-triplet pairing was proposed, 95) as an alternative to FM spin-fluctuation pairing. Robust SC is predicted to exist only in the FM1 phase by the combination of the FM exchange based on Hund's rule and the interelectronic Coulomb interaction (see Fig.…”

Coexistent spin-triplet superconducting and ferromagnetic phases induced by Hund's rule coupling and electronic correlations: Effect of the applied magnetic field

“…Such a choice yields, at zero field, the sequence of magnetic and SC states that match the experimental phase diagram of UGe 2 . 12 The model (1) is solved within the statisticallyconsistent Gutzwiller approximation (SGA) 12,17,19,20 that, at zero-temperature, is equivalent to approximate (see below) minimization of the ground-state-energy functional of the form…”

“…Hence, the simplified solution detailed in Appendix A of Ref. [12] represents, to a good accuracy, the full solution. The same type of picture is used throughout the present paper for the case of nonzero applied field.…”

“…Recently, we have proposed that pairing in UGe 2 emerges due to the combined effect of FM exchange interaction (the Hund's-rule coupling) combined with interelectronic correlations. 12 Ref. 12 is regarded as Part I of our analysis of UGe 2 properties (hereinafter referred to as I).…”

“…12 Ref. 12 is regarded as Part I of our analysis of UGe 2 properties (hereinafter referred to as I). The spin-paired A 1 state proved to be the dominant phase there with the pair spins opposite to those of average spin polarization, a natural feature appearing in the half-metallic phase.…”