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

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 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 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 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 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.

18 pages, 10 figures

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• Broken mirror symmetry, incommensurate spin correlations, and $B_{2g}$ nematic order in iron pnictides
• Cascade of vestigial orders in two-component superconductors: nematic, ferromagnetic, s-wave charge-4e, and d-wave charge-4e states
• Intertwined spin-orbital coupled orders in the iron-based superconductors
• Hidden antiferro-nematic order in Fe-based superconductor BaFe${}_2$As${}_2$ and NaFeAs above $T_S$
• Microscopic theory of magnetic detwinning in iron-based superconductors with large-spin rare earths
• Competing magnetic interactions in spin-1/2 square lattice: hidden order in Sr${}_2$VO${}_4$
• Possible Bicollinear Nematic State with Monoclinic Lattice Distortions in Iron Telluride Compounds
• Nematic Fluctuations in an Orbital Selective Superconductor Fe${}_{1+y}$Te${}_{1-x}$Se${}_x$
• Exotic phase transition and superconductivity in layered titanium-oxypnictides implied by a computational phonon analysis
• Revealing the hidden order in BaTi2As2O via nuclear magnetic resonance
• Theory for doping trends in titanium oxypnictide superconductors
• Emergent Ising degrees of freedom above double-stripe magnetism