Spin-orbit-induced quantum chiral phases

초록

Scalar spin chirality (SSC), whose nonzero value (S-i center dot (S-j x S-k)) not equal 0 implies the breaking of time-reversal and certain point-group symmetries in the ground state, is a key quantity characterizing chiral magnetism in both classical and quantum settings. Classical SSC is manifested, for instance, in the skyrmion crystal phase, while quantum SSC is still highly sought after in various frustrated spin-1/2 models. An interesting possibility that has not been explored so far is the case in which SSC is symmetrywise allowed, yet remains zero classically due to the collinear or coplanar arrangement of spins, but is generated by virtue of quantum fluctuation. We demonstrate the existence of precisely such a phase in a spin-1/2 triangular-lattice model with XXZ interaction, spin-orbit-induced exchange interactions, and an external magnetic field. Using the infinite density matrix renormalization group (iDMRG) method, we thoroughly map out the phase diagram of the model and identify several phases with coexisting magnetic order and SSC. Nonzero SSC arises despite the classical magnetic order being collinear or coplanar. We provide detailed magnon analysis to ascribe its origin to quantum fluctuations around classical magnetic order. The magnon analysis reproduces the sign and characteristic magnitude of the iDMRG SSC and identifies its microscopic origin in quantum fluctuations. We map out the magnon spectrum and its Berry curvature, culminating in the prediction of finite thermal Hall conductivity in these phases with SSC.