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Spontaneous formation of a non-uniform chiral spin liquid in a "moat" lattice

Tigran Sedrakyan, L. I. Glazman, Alex Kamenev

Published Sep 25, 2014• 0 citations
OpenAlex

Abstract

A number of lattices exhibit moat-like band structures, i.e. a band with infinitely degenerate energy minima attained along a closed line in the Brillouin zone. If such a lattice is populated with hard-core bosons, the degeneracy prevents their condensation. At half-filling, the system is equivalent to $s=1/2$ XY model at zero magnetic field, while absence of condensation translates into the absence of magnetic order in the XY plane. Here we show that the ground state breaks the time-reversal as well as inversion symmetries. This state, which may be identified with the chiral spin liquid, has a bulk gap and chiral gapless edge excitations. The applications of the developed analytical theory include an explanation of recent numerical findings and a suggestion for the chiral spin liquid realizations in experiments with cold atoms in optical lattices.

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A number of lattices exhibit moat-like band structures, i.e. a band with infinitely degenerate energy minima attained along a closed line in the Brillouin zone. If such a lattice is populated with hard-core bosons, the degeneracy prevents their condensation. At half-filling, the system is equivalent to s=1/2s=1/2s=1/2 XY model at zero magnetic field, while absence of condensation translates into the absence of magnetic order in the XY plane. Here we show that the ground state breaks the time-reversal as well as inversion symmetries. This state, which may be identified with the chiral spin liquid, has a bulk gap and chiral gapless edge excitations. The applications of the developed analytical theory include an explanation of recent numerical findings and a suggestion for the chiral spin liquid realizations in experiments with cold atoms in optical lattices.
A number of lattices exhibit moatlike band structures, i.e., a band with infinitely degenerate energy minima attained along a closed line in the Brillouin zone. If such a lattice is populated with hard-core bosons, the degeneracy prevents their condensation. At half-filling, the system is equivalent to the s=1/2 XY model at a zero magnetic field, while the absence of condensation translates into the absence of magnetic order in the XY plane. Here, we show that the ground state breaks time reversal as well as inversion symmetries. This state, which may be identified with the chiral spin liquid, has a bulk gap and chiral gapless edge excitations. The applications of the developed analytical theory include an explanation of recent numerical findings and a suggestion for the chiral spin liquid realizations in experiments with cold atoms in optical lattices.
Strongly interacting bosons that live in a lattice with degeneracy in its lowest-energy band experience frustration that can prevent the formation of a Bose-Einstein condensate. Such systems form an ideal playground to investigate spin-liquid behavior. We use the variational principle and the Chern-Simons technique of fermionization of hard-core bosons on the kagome lattice to find that below lattice filling fraction $\ensuremath{\nu}=\frac{1}{3}$ the system favors a topologically ordered chiral spin-liquid state that is gapped in bulk, spontaneously breaks time-reversal symmetry, and supports massless chiral bosonic edge mode. We construct the many-body variational wave function of the state and show that the corresponding energy coincides with the energy of the flat band. This result proves that the ground state of the system cannot stabilize a Bose condensate below $\ensuremath{\nu}=\frac{1}{3}$. The fermionization and variational scheme we outline apply to any non-Bravais lattice. We distinguish between the roles played by the Chern-Simons gauge field in lattices with a flat band and those exhibiting a moatlike dispersion (which is degenerate along a closed contour in the reciprocal space). We also suggest experimental probes to differentiate the proposed ground state from a condensate.
We use the tensor network algorithm to show evidences of a non-uniform chiral spin-liquid (CSL) ground state in a frustrated spin-1/2 model on a square lattice, in the regime of moat-like band structure of the lattice, i.e., a band with infinitely degenerate energy minima attained along a closed contour in the Brillouin zone. The analytical description of the state is given by the effective field theory of a topological square-lattice fermionic Chern insulator coupled to the Chern-Simons gauge field. The observed non-uniform CSL has a substantial effect on a nearby free-fermion environment. We show that, in the presence of arbitrarily small spin exchange interaction, the CSL can endow a gauge-field-modulated effective interaction between the environmental fermions. The induced effective interaction can be attractive within a significant parameter region, leading to an instability towards d-wave superconductivity in the fermionic bath.
We study hard-core bosons on a class of frustrated lattices with the lowest Bloch band having a degenerate minimum along a closed contour, the moat, in the reciprocal space. We show that at small density the ground state of the system is given by a noncondensed state, which may be viewed as a state of fermions subject to Chern-Simons gauge field. At fixed density of bosons, such a state exhibits domains of incompressible liquids. Their fixed densities are given by fractions of the reciprocal-space area enclosed by the moat.
We present a mapping of a two-dimensional system of interacting bosons in a strong perpendicular magnetic field to an equivalent system of interacting bosons on the square lattice in the absence of the field. The mapping utilizes a magnetic Bloch and the corresponding magnetic Wannier single-particle basis in the lowest Landau level. By construction, the ground states of the resulting model of interacting bosons on the square lattice are gapped fractionalized liquids or gapless Bose metal states with broken time-reversal symmetry at specific rational filling fractions.
Frustrated magnets with highly degenerate ground states are at the heart of hunting exotic states of matter. Recent studies in spin-orbit coupled honeycomb magnets have generated immense interest in bond-dependent interactions, appreciating a symmetric off-diagonal $\mathrm{\ensuremath{\Gamma}}$ interaction which exhibits a macroscopic degeneracy in the classical limit. Here, we study a generic spin model and discover a chiral-spin (CS) ordering with spontaneously broken time-reversal symmetry near the dominant $\mathrm{\ensuremath{\Gamma}}$ region. The CS phase is demonstrated to possess a staggered chirality relation in different sublattices, and it exhibits gapless excitations, as revealed by the vanishing energy gap and the finite central charge on cylinders. Although there is a vestige of a tiny peak in the corner of the second Brillouin zone, the magnetic order is likely to vanish as the system size increases. Finally, we also attempt to gain insight into the possible topological signature of the CS phase by calculating the dynamic structure factor and the modular S\mathcal{S}S matrix.