"Nematicity and quantum paramagnetism in FeSe"

Fa Wang: Steven A. Kivelson & Dung-Hai Lee; nature physics, 09/07/15.

Additional Authors: Steven A. Kivelson & Dung-Hai Lee

Abstract:

In common with other iron-based high-temperature superconductors, FeSe exhibits a transition to a ‘nematic phase below 90K in which the crystal rotation symmetry is spontaneously broken. However, the absence of strong low-frequency magnetic fluctuations near or above the transition has been interpreted as implying the primacy of orbital ordering. In contrast, we establish that quantum fluctuations of spin-1 local moments with strongly frustrated exchange interactions can lead to a nematic quantum paramagnetic phase consistent with the observations in FeSe. We show that this phase is a fundamental expression of the existence of a Berrys phase associated with the topological defects of a Néel antiferromagnet, in a manner analogous to that which gives rise to valence bond crystal order for spin-1/2 systems. We present an exactly solvable model realizing the nematic quantum paramagnetic phase, discuss its relation with the spin-1  J1J2       model, and construct a field theory of the Landau-forbidden transition between the Néel state and this nematic quantum paramagnet.