LaFeAsO as a Self Doped Spin-1 Mott Insulator: Quantum String Liquid State and Superconductivity

Abstract

We suggest that LaFeAsO is basically a spin-1 Mott insulator that has become an antiferromagnetically ordered bad metal with reduced sublattice moment, by a self (under) doping of an equal and small density (y<<1) of electron and hole carriers. External doping in LaFeAsO_1−xF_x kills long range antiferromagnetic order and stabilises a liquid of quantum singlet strings (spin-1 Haldane chain). We view this state as two spin-1/2 resonating valence bond (RVB) system, coupled by a weak Hund coupling and bond charge repulsions, leading to an AKLT (Affleck, Kennedy, Lieb, and Tasaki) type spin pairing. A pair of doped holes (electrons) creates an open string and remain at the ends as holon (doublon). Charge ± 2e singlet strings condense to produce high T_c superconductivity. Higher T_c’s are likely in our Mott insulator based scenario, when competing orders are taken care of. As Fe–Fe distance increases from LaFePO to LaFeAsO, we suggest that REFeSb) and REFeBiO (RE: rare earth) and also the chalogenide α-FeTe, if synthesized might be spin-1 Mott insulators, with a prospect for higher superconducting T_c.