A unified theory of ferromagnetic quantum phase transitions in heavy fermion metals

TL;DR

This paper presents a unified theoretical framework explaining the different types of ferromagnetic quantum phase transitions in heavy fermion metals, emphasizing the role of magnetic anisotropy and predicting various phases and Fermi surface changes.

Contribution

It introduces an anisotropic ferromagnetic Kondo-Heisenberg model analyzed via an improved large-N Schwinger boson approach, unifying the understanding of ferromagnetic quantum phase transitions in heavy fermion systems.

Findings

Predicted a phase diagram with both continuous and first-order ferromagnetic quantum phase transitions.

Identified three ferromagnetic phases, including a half-metallic spin selective Kondo insulator.

Found abrupt Fermi surface topology changes consistent with experimental observations in UGe2.

Abstract

Motivated by the recent discovery of a continuous ferromagnetic quantum phase transition in CeRh$_6$Ge$_4$ and its distinction from other U-based heavy fermion metals such as UGe$_2$, we develop a unified explanation of their different ground state properties based on an anisotropic ferromagnetic Kondo-Heisenberg model. We employ an improved large-$N$ Schwinger boson approach and predict a full phase diagram containing both a continuous ferromagnetic quantum phase transition for large magnetic anisotropy and first-order transitions for relatively small anisotropy. Our calculations reveal three different ferromagnetic phases including a half-metallic spin selective Kondo insulator with a constant magnetization. The Fermi surface topologies are found to change abruptly between different phases, consistent with that observed in UGe$_2$. At finite temperatures, we predict the development of Kondo hybridization well above the ferromagnetic long-range order and its relocalization near the phase transition, in good agreement with band measurements in CeRh$_6$Ge$_4$. Our results highlight the importance of magnetic anisotropy and provide a unified theory for understanding the ferromagnetic quantum phase transitions in heavy fermion metals.