Spin-triplet paired phases inside ferromagnet induced by Hund's rule coupling and electronic correlations: Application to $\mathrm{UGe}_2$

TL;DR

This paper proposes a real-space spin-triplet pairing mechanism driven by Hund's rule coupling and electronic correlations, successfully explaining the phase diagram and magnetic properties of UGe₂, including coexistent ferromagnetism and superconductivity.

Contribution

It introduces a novel pairing mechanism based on Hund's rule and correlations, applied within the Anderson lattice model to explain UGe₂'s phases, matching experimental observations.

Findings

Theoretical phase diagram matches experimental data for UGe₂.

Weak A₂- and A-type phases appear near phase transition borders.

Magnetic moments are reproduced semi-quantitatively.

Abstract

We discuss a mechanism of real-space spin-triplet pairing, alternative to that due to quantum paramagnon excitations, and demonstrate its applicability to $\mathrm{UGe_2}$. 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 $\mathrm{UGe_2}$ with the coexistent ferromagnetic (FM1) and superconducting ($A_1$-type) phase. Additionally, weak $A_2$- and $A$-type paired phases appear in very narrow regions near the metamaganetic (FM2 $\rightarrow$ FM1) and FM1 $\rightarrow$ 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.