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 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.
Bibliographical noteFunding Information:
I.I.M. acknowledges funding from the Office of Naval Research (ONR) through the Naval Research Laboratory's Basic Research Program. J.K.G. acknowledges the support of the NRC program at NRL. R.M.F. is supported by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, under Award No. DE-SC0012336. This research was supported in part by Ames Laboratory Royalty Funds and Iowa State University startup funds (G.Z. and R.A.F.). The Ames Laboratory is operated for the U. S. Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358. R.A.F. and R.M.F. also acknowledge the hospitality of the Aspen Center for Physics, supported by National Science Foundation Grant No. PHYS-1066293 where we initiated this project.
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