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Nodal spin liquid with fermionized pi-vortices

Su-Peng Kou, Lan-Feng Liu

Published Oct 12, 2009• 0 citations
OpenAlex

Abstract

In this paper, a new type quantum spin liquid state - nodal spin liquid (NSL) with nodal fermionic excitations explored. A candidate of NSL is the non-magnetic insulator state in 2D pi-flux Hubbard model on square lattice near Mott transition, of which the vortices (half skyrmions) become nodal fermionic excitations. In particular, the U(1) times U(1) mutual--Chern-Simons (MCS) theory is obtained to describe the low energy physics of NSL.

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In this paper, a new type quantum spin liquid state - nodal spin liquid (NSL) with nodal fermionic excitations explored. A candidate of NSL is the non-magnetic insulator state in 2D pi-flux Hubbard model on square lattice near Mott transition, of which the vortices (half skyrmions) become nodal fermionic excitations. In particular, the U(1) times U(1) mutual--Chern-Simons (MCS) theory is obtained to describe the low energy physics of NSL.
In this paper, we study the non-magnetic insulator state near Mott transition of 2D pi-flux Hubbard model on square lattice and find that such non-magnetic insulator state is quantum spin liquid state with nodal fermionic excitations - nodal spin liquid (NSL). When there exists small easy-plane anisotropic energy, the ground state becomes Z2 topological spin liquid (TSL) with full gapped excitations. The U(1)times U(1) mutual--Chern-Simons (MCS) theory is obtained to describe the low energy physics of NSL and TSL.
In this paper, based on the formulation of an O (3) non-linear σ model, we study the two-dimensional π-flux Hubbard model at half-filling. A quantum non-magnetic insulator is explored near the metal-insulator transition that may be a possible candidate of the spin liquid state. Such quantum non-magnetic insulator on square lattice is not induced by frustrations. Instead, it originates from quantum spin fluctuations with relatively small effective spin moments. In the strong-coupling limit, our results of the spin velocity and spin order parameter agree with results obtained from earlier calculations.
We introduce an exactly solvable SU(2)-invariant spin-1/2 model with exotic spin excitations. With time reversal symmetry (TRS), the ground state is a spin liquid with gapless or gapped spin-1 but fermionic excitations. When TRS is broken, the resulting spin liquid exhibits deconfined vortex excitations which carry spin-1/2 and obey non-Abelian statistics. We show that this SU(2) invariant non-Abelian spin liquid exhibits the spin quantum Hall effect with quantized spin Hall conductivity σ(xy)(s)=ℏ/2π, and that the spin response is effectively described by the SO(3) level-1 Chern-Simons theory at low energy. We further propose that a SU(2) level-2 Chern-Simons theory is the effective field theory describing the topological structure of the non-Abelian SU(2) invariant spin liquid.
Motivated by cold atom experiments on Chern insulators, we study the honeycomb lattice Haldane-Hubbard Mott insulator of spin-1/2 fermions using exact diagonalization and density matrix renormalization group methods. We show that this model exhibits various chiral magnetic orders including a wide regime of triple-Q tetrahedral order. Incorporating third-neighbor hopping frustrates and ultimately melts this tetrahedral spin crystal. From analyzing the low energy spectrum, many-body Chern numbers, entanglement spectra, and modular matrices, we identify the molten state as a chiral spin liquid (CSL) with gapped semion excitations. We formulate and study the Chern-Simons-Higgs field theory of the exotic CSL-to-tetrahedral spin crystallization transition.
The guiding conception of vortex-condensation-driven Mott insulating behavior is central to the theory of the nodal liquid. We amplify our earlier description of this idea and show how vortex condensation in two-dimensional (2D) electronic systems is a natural extension of 1D Mott insulating and 2D bosonic Mott insulating behavior. For vortices in an underlying superconducting pair field, there is an important distinction between the condensation of flux hc/2ehc/2ehc/2e and flux hc/ehc/ehc/e vortices. The former case leads to spin-charge confinement, exemplified by the band insulator and the charge-density wave. In the latter case, spin and charge are liberated, leading directly to a 2D Mott insulator exhibiting spin-charge separation. Possible upshots include not only the nodal liquid, but also an undoped antiferromagnetic insulating phase with gapped excitations exhibiting spin-charge separation.