Revealing the hidden order in BaTi2As2O via nuclear magnetic resonance

TL;DR

This study uncovers a hidden bond order in BaTi2As2O using nuclear magnetic resonance, revealing a complex electronic reconstruction that involves lattice and nematic instabilities, which may be key to understanding superconductivity in this material.

Contribution

The paper provides direct experimental evidence of bond order in BaTi2As2O and links it to lattice and nematic instabilities, advancing understanding of electronic reconstructions in layered metals.

Findings

Detection of p-d bond order between Ti and As atoms

Bond order breaks lattice rotational and translational symmetry

Enhanced spin fluctuations and reduced density of states below Ts

Abstract

In low-dimensional metallic systems, lattice distortion is usually coupled to a density-wave-like electronic instability due to Fermi surface nesting (FSN) and strong electron-phonon coupling. However, the ordering of other electronic degrees of freedom can also occur simultaneously with the lattice distortion thus challenges the aforementioned prevailing scenario. Recently, a hidden electronic reconstruction beyond FSN was revealed in a layered metallic compound BaTi2As2O below the structural transition temperature Ts ~ 200 K. The nature of this hidden electronic instability is under strong debate. Here, by measuring the local orbital polarization through 75As nuclear magnetic resonance experiment, we observe a p-d bond order between Ti and As atoms in BaTi2As2O single crystal. Below Ts, the bond order breaks both rotational and translational symmetry of the lattice. Meanwhile, the spin-lattice relaxation measurement indicates a substantial loss of density of states and an enhanced spin fluctuation in the bond-order state. Further first-principles calculations suggest that the mechanism of the bond order is due to the coupling of lattice and nematic instabilities. Our results strongly support a bond-order driven electronic reconstruction in BaTi2As2O and shed light on the mechanism of superconductivity in this family.