The origin of the 90-K nematic transition in the chalcogenide FeSe, which displays no magnetic order down to T=0, remains a major puzzle for a unifying theory for the iron-based superconductors. We analyze this problem in light of recent experimental data which reveal very small Fermi pockets in this material. We show that the smallness of the Fermi energy leads to a near degeneracy between magnetic fluctuations and fluctuations in the charge-current density-wave channel. Although the two fluctuation modes cooperate to promote the same preemptive Ising-nematic order, they compete for primary order. We argue that this explains why in FeSe the nematic order emerges when the magnetic correlation length is smaller than in other Fe-based materials. We argue that pressure lifts this near degeneracy and causes nonmonotonic behavior of the nematic transition.
|Physical Review B - Condensed Matter and Materials Physics
|Published - May 20 2015
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© 2015 American Physical Society.