Double stage nematic bond-ordering above double stripe magnetism: application to BaTi$_2$Sb$_2$O

TL;DR

This paper proposes a novel spin-driven nematic order involving four Ne9el sublattices in BaTib2Sbb2O, predicting multiple bond-order phase transitions and explaining experimental observations.

Contribution

It introduces a new theoretical model for exotic nematic order with multiple phase transitions, supported by density functional theory calculations for BaTib2Sbb2O.

Findings

Up to two separate bond-order phase transitions as temperature varies.

The low-temperature phase exhibits orthorhombic distortion and intra-unit-cell charge density wave.

Model aligns with experimental measurements of BaTib2Sbb2O's low-temperature phase.

Abstract

Spin-driven nematicity, or the breaking of the point-group symmetry of the lattice without long-range magnetic order, is clearly quite important in iron-based superconductors. From a symmetry point of view, nematic order can be described as a coherent locking of spin fluctuations in two interpenetrating N\'eel sublattices with ensuing nearest-neighbor bond order and an absence of static magnetism. Here, we argue that the low-temperature state of the recently discovered superconductor BaTi$_2$Sb$_2$O is a strong candidate for a more exotic form of spin-driven nematic order, in which fluctuations occurring in four N\'eel sublattices promote both nearest- and next-nearest neighbor bond order. We develop a low-energy field theory of this state and show that it can have, as a function of temperature, up to two separate bond-order phase transitions -- namely, one that breaks rotation symmetry and one that breaks reflection and translation symmetries of the lattice. The resulting state has an orthorhombic lattice distortion, an intra-unit-cell charge density wave, and no long-range magnetic order, all consistent with reported measurements of the low-temperature phase of BaTi$_2$Sb$_2$O. We then use density functional theory calculations to extract exchange parameters to confirm that the model is applicable to BaTi$_2$Sb$_2$O.