ICQM faculty member Fa Wang publishes an article in Nature Physics reporting ‘Nematicity and Quantum Paramagnetism in FeSe’

FeSe however shows very different phenomenology. It has a nematic phase transition at 90Kelvin but no magnetic order down to the lowest temperature. And recent nuclear magnetic resonance studies do not see enhancement of low energy spin fluctuation across the nematic transition in FeSe. People thus speculate that the nematic transition in FeSe is not driven by magnetic correlation but possibly orbital order.

Prof. Fa Wang of ICQM at Peking University, in collaboration with Prof. Kivelson of Stanford University and Prof. Dung-Hai Lee of UC Berkeley, challenged this perception of non-magnetic mechanism for nematicity in FeSe. They established by theoretical arguments and numerical results that a “nematic quantum paramagnet” phase can exist for frustrated spin-1 models on square lattice. This phase spontaneously breaks 4-fold crystal rotation symmetry but has no magnetic order and no low-energy spin excitations. They argued that the nematic quantum paramagnet phase can explain the unusual phase diagram of FeSe, and predicted the existence of finite energy spin fluctuations at both stripe and Neel ordering wavevectors. This work has been published in Nature Physics [Nature Physics 11, 959 (2015)].

The work done in PKU was supported by the National Science Foundation of China(Grant No. 11374018) and National Key Basic Research Program of China (Grant No. 2014CB920902).

Left: Schematic depiction of two prototypical wavefunctions of nematic quantum paramagnet states. The spin-1 moments on solid blue lines form AKLT chains. Right: Spin excitation gap(S=1) and spin-singlet excitation gap(S=0) for 4x4 square lattice J₁-J₂ Heisenberg model. Around J₂/J₁=0.6 the spin gap is large but the spin-singlet gap is very small, suggesting nematic quantum paramagnet ground states in the thermodynamic limit.