Wing-Ho Ko

Doped Kagome System as Exotic Superconductor

A Chern-Simons theory for the doped spin-1/2 kagome system is constructed, from which it is shown that the system is an exotic superconductor that breaks time-reversal symmetry. It is also shown that the system carries minimal vortices of flux

Extended Gutzwiller approximation for inhomogeneous systems

hc/4e

Spontaneous Spin Ordering of a Dirac Spin Liquid in a Magnetic Field

(as opposed to the usual

Magnetism and Mott transition: A slave-rotor study

hc/2e

Proposal for detecting spin-chirality terms in Mott insulators via resonant inelastic x-ray scattering

in conventional superconductors) and contains fractional quasiparticles (including fermionic quasiparticles with semionic mutual statistics and spin-1/2 quasiparticles with bosonic self-statistics) in addition to the usual spin-1/2 fermionic Bogoliubov quasiparticles. Two Chern-Simons theories\char22{}one with an auxiliary gauge field kept and one with the auxiliary field and a redundant matter field directly eliminated\char22{}are presented and shown to be consistent with each other.

Raman Signature of the U(1) Dirac Spin-Liquid State in the Spin-1/2 Kagome System

The generalization of the Gutzwiller approximation to inhomogeneous systems is considered, with extra spin-and-site-dependent fugacity factors included. It is found that the inclusion of fugacity factors reconciles the seemingly contradictory choices of Gutzwiller factors used in the literature. Moreover, from the derivation of the Gutzwiller factors, it is shown that the Gutzwiller approximation breaks the rotational symmetry of the trial wave functions and that different components of the spin-spin interaction need to be renormalized differently under the approximation. Various schemes to restore the rotational symmetry are discussed and are compared with results from variational Monte Carlo calculations for the two-dimensional square-lattice antiferromagnet. Results along different paths within the full parameter space, which correspond to different choices of fugacity factors in the literature, are also compared.

Ordering and criticality in an underscreened Kondo chain

The Dirac spin liquid was proposed to be the ground state of the spin-1/2 kagomé antiferromagnets. In a magnetic field B, we show that the state with Fermi pocket is unstable to the Landau level (LL) state. The LL state breaks the spin rotation around the axis of the magnetic field. We find that the LL state has an in-plane 120 degrees q=0 magnetization M which scales with the external field M approximately B;{alpha}, where alpha is a universal number of the Dirac spin liquid. We discuss the related experimental implications which can be used to detect the possible Dirac spin liquid phase in herbertsmithite ZnCu3(OH)6Cl2.

Theoretical study of a one-dimensional chain of alternating spin-1 and electron sites with spin-mediated hopping

Motivated by the debate of spin-density-wave (SDW) versus local-moment (LM) picture in the iron-based superconducting (FeSC) materials, we consider a two-band orbital-symmetric Hubbard model in which there is robust Fermi surface nesting at

A Proposal for Detecting Spin-Chirality Terms in Mott Insulators via RIXS

(\pi,0)

Chern-Simons Theory of Hole Doped Kagome System

. We obtain the phase diagram of such system by a mean-field slave-rotor approach, in which the Fermi surface nesting and the SDW order are explicitly taken into account via a natural separation of scale between the Hunds coupling and the Coulomb interaction. We find that for a sizable range of Hunds coupling the Mott transition acquires a strong first-order character, but there also exists a small range of stronger Hunds coupling in which an enhancement of magnetization can be observed on the SDW side. We interpret the former scenario as one in which a sharp distinction can be drawn between LM and the SDW picture, and the latter scenario as one in which signs of LM physics begin to develop in the metallic phase. It is tempting to suggest that some FeSC materials are in the vicinity of the latter scenario.