Ferro-orbital ordering transition in iron telluride Fe$_{1+y}$Te

TL;DR

This study investigates the complex phase transitions in Fe$_{1+y}$Te with higher interstitial Fe content, revealing a ferro-orbital ordering transition linked to bond-order wave formation and metallic behavior.

Contribution

It uncovers a distinct ferro-orbital ordering transition associated with bond-order wave and metallicity in Fe$_{1+y}$Te with high interstitial Fe, expanding understanding of its phase behavior.

Findings

Increased interstitial Fe decouples magnetic and structural transitions.

Ferro-orbital order correlates with bond-order wave and metallic conduction.

Distinct hysteretic transition to BOW state at lower temperature.

Abstract

Fe$_{1+y}$Te with $y \lesssim 0.05$ exhibits a first-order phase transition on cooling to a state with a lowered structural symmetry, bicollinear antiferromagnetic order, and metallic conductivity, $d\rho/dT > 0$. Here, we study samples with $y = 0.09(1)$, where the frustration effects of the interstitial Fe decouple different orders, leading to a sequence of transitions. While the lattice distortion is closely followed by \emph{incommensurate} magnetic order, the development of \emph{bicollinear} order and metallic electronic coherence is uniquely associated with a separate hysteretic first-order transition, at a markedly lower temperature, to a phase with dramatically enhanced bond-order wave (BOW) order. The BOW state suggests ferro-orbital ordering, where electronic delocalization in ferromagnetic zigzag chains decreases local spin and results in metallic transport.