Correlated flat bands in the paramagnetic phase of triangular antiferromagnets Na$_2$BaX(PO$_4$)$_2$ (X = Mn, Co, Ni)

TL;DR

This study investigates the electronic and magnetic properties of Na$_2$BaX(PO$_4$)$_2$ (X = Mn, Co, Ni), revealing correlated flat bands and potential quantum spin liquid states in their paramagnetic phases through first-principles and dynamical mean-field calculations.

Contribution

It demonstrates that these structurally similar systems exhibit correlated flat bands and diverse magnetic responses, advancing understanding of flat band physics in magnetic materials.

Findings

Similar electronic structures despite different elements.

Strong intra-plane antiferromagnetic coupling.

Degenerate ground states suggest potential quantum spin liquids.

Abstract

Flat band systems in condensed matter physics are intriguing because they can exhibit exotic phases and unconventional properties. In this work, we studied three correlated magnetic systems, Na$_2$BaX(PO$_4$)$_2$ (X = Mn, Co, Ni), and revealed their unusual electronic structure and magnetic properties. Despite their different effective angular momentum, our first-principles calculations showed a similar electronic structure among them. However, their different valence configurations led to different responses to electronic correlations in the high-temperature paramagnetic phase. Using the dynamical mean-field method, we found that all systems can be understood as a multi-band Hubbard model with Hund'ss coupling. Our calculations of spin susceptibility and the {\it ab-initio} estimation of magnetic exchange coupling indicated strong intra-plane antiferromagnetic coupling and weak inter-plane coupling in all systems. The ground states of these systems are largely degenerate. It is likely that none of these magnetic states would dominate over the others, leading to the possibility of quantum spin liquid states in these systems. Our work unifies the understanding of these three structurally similar systems and opens new avenues for exploring correlated flat bands with distinct electronic and magnetic responses.