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Colloquium: Is FeSe a nematic quantum paramagnet?

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Dung-Hai Lee, University of California, Berkeley
19 March 2015 from 4:00 PM to 5:00 PM
117 Osmond Laboratory
Contact Name
Moses Chan
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For spin-1/2 systems when quantum fluctuations destroy the N\'eel long-range order and open a spin gap, the resulting state usually breaks the lattice translation and rotation symmetries. This can be understood as reflecting a Berry's phase associated with the magnetic monopole of the N\'eel order parameter.  Here we point out the same Berry phase causes spin-1 systems to spontaneously break the discrete rotational symmetry of the crystal while preserving its translation symmetry. We support this claim by presenting an exactly solvable microscopic model whose ground state is a nematic quantum paramagnet.  In addition we derive a field theory to describe the continuous, Landau-forbidden phase transition between a N\'eel ordered state and a nematic quantum paramagnet, and compare the theory with existing numerical results.  We propose that the nematic quantum paramagnetic state captures essential features of bulk FeSe which becomes nematic around 90K but does not develop any spin long range order down to the the lowest measured temperature.