Double-stage nematic bond ordering above double stripe magnetism: Application to BaTi2Sb2O

Abstract

Spin-driven nematicity, or the breaking of the point-group symmetry of the lattice without longrange 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 Néel 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 BaTi2Sb2O is a strong candidate for a more exotic form of spin-driven nematic order, in which fluctuations occurring in four Néel 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 BaTi2Sb2O. We then use density functional theory calculations to extract exchange parameters to confirm that the model is applicable to BaTi2Sb2O. arXiv:1610.06622v1 [cond-mat.str-el]

Emergent Ising degrees of freedom above a double-stripe magnetic ground state

“…The black dashed line indicates the diagonal mirror symmetry broken in each state. This figure has been reproduced from Zhang et al [19].…”

“…Indeed, the NNN biquadratic exchange K 2 favors collinear alignment of the four sublattices, but will not distinguish between DS (ϕ) and plaquette (ζ ) orders. However, NNN ring-exchange terms (R 2 ) may be added to the Hamiltonian to select ϕ, and thus the DS ground state [19,37]. In what follows, we will therefore neglect ζ .…”

“…In a recent paper [19], we discussed the two Ising bond orders that arise above double-stripe magnetism, which breaks two distinct discrete symmetries. That paper focused on the bond order degrees of freedom in a two-dimensional calculation.…”

“…Here, we investigate the interplay of these two bond orders with magnetic order in quasi-two-dimensional systems. Here, long-range magnetism develops at a finite temperature, leading to the possibility that thermal fluctuations melt the magnetic order via up to three distinct phase transitions: one magnetic and two Ising bond order transitions associated with the two discrete symmetries [19].…”

“…II, we briefly review the model without magnetism treated in Ref. [19], and develop the effective field theory by deriving an effective action via Hubbard-Stratonovich transformations of the quartic spin terms. Then we discuss the conditions for the emergence of magnetic order and obtain a set of saddle-point equations by minimizing this effective action with respect to all order parameters in both cases.…”