A continuous metal-insulator transition driven by spin correlations

TL;DR

This paper presents a clear example of a metal-insulator transition driven solely by spin correlations in Cd2Os2O7, where the transition occurs at a temperature much lower than the magnetic ordering temperature, challenging conventional theories.

Contribution

It demonstrates a spin-correlation-driven metal-insulator transition with a gap opening below the Néel temperature, distinct from Mott or spin density wave mechanisms, in a fully symmetric pyrochlore antiferromagnet.

Findings

Hall coefficient reveals four Fermi surfaces below T_N

Charge gap opens only at ~10K, below T_N=227K

Transition mechanism resembles Slater insulator without Brillouin zone folding

Abstract

Metal-insulator transitions involve a mix of charge, spin, and structural degrees of freedom, and when strongly-correlated, can underlay the emergence of exotic quantum states. Mott insulators induced by the opening of a Coulomb gap are an important and well-recognized class of transitions, but insulators purely driven by spin correlations are much less common, as the reduced energy scale often invites competition from other degrees of freedom. Here we demonstrate a clean example of a spin-correlation-driven metal-insulator transition in the all-in-all-out pyrochlore antiferromagnet Cd2Os2O7, where the lattice symmetry is fully preserved by the antiferromagnetism. After the antisymmetric linear magnetoresistance from conductive, ferromagnetic domain walls is carefully removed experimentally, the Hall coefficient of the bulk reveals four Fermi surfaces, two of electron type and two of hole type, sequentially departing the Fermi level with decreasing temperature below the N\'eel temperature, T_N. Contrary to the common belief of concurrent magnetic and metal-insulator transitions in Cd2Os2O7, the charge gap of a continuous metal-insulator transition opens only at T~10K, well below T_N=227K. The insulating mechanism resolved by the Hall coefficient parallels the Slater picture, but without a folded Brillouin zone, and contrasts sharply with the behavior of Mott insulators and spin density waves, where the electronic gap opens above and at T_N, respectively.