Spin Ferroquadrupolar Order in the Nematic Phase of FeSe
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We provide evidence that spin ferroquadrupolar (FQ) order is the likely ground state in the nonmagnetic nematic phase of stoichiometric FeSe. By studying the variational mean-field phase diagram of a bilinear-biquadratic Heisenberg model up to the 2nd nearest neighbor, we find the FQ phase in close proximity to the columnar antiferromagnet commonly realized in iron-based superconductors; the stability of the FQ phase is further verified by the density matrix renormalization group. The dynamical spin structure factor in the FQ state is calculated with flavor-wave theory, which yields a qualitatively consistent result with inelastic neutron scattering experiments on FeSe at both low and high energies. We verify that FQ can coexist with C4 breaking environments in the mean-field calculation, and further discuss the possibility that quantum fluctuations in FQ act as a source of nematicity.
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Wang, Zhentao, Hu, Wen-Jun and Nevidomskyy, Andriy H.. "Spin Ferroquadrupolar Order in the Nematic Phase of FeSe." Physical Review Letters, 116, no. 24 (2016) American Physical Society: https://doi.org/10.1103/PhysRevLett.116.247203.