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Abelian flux induced magnetic frustrations of spinor boson superfluids on a square lattice

Fadi Sun, Junsen Wang, Jinwu Ye, Youjin Deng

Published Nov 16, 2017• 0 citations
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Abstract

Inspired by recent experimental advances to generate Abelian flux for neutral cold atoms and photons moving in a lattice, we investigate the possible effects of the $ \pi $ flux through a unit cell in the pseudo-spin 1/2 spinor boson Hubbard model in a square lattice. We find that the $ \pi $ flux induces a dramatic interplay between the charge and the spin which leads to a frustrated superfluid. We develop a new and systematic order from quantum disorder analysis to determine not only the true quantum ground state, but also the excitation spectrum. The superfluid ground state has a 4 sublattice $ 90^{\circ} $ coplanar spin structure which supports 4 linear gapless modes with 3 different velocities. We speculate the transition from the weak coupling frustrated SF to the strong coupling Ferromagnetic Mott state to be in a new universality class of non-Ginsburg Landau type. These novel phenomena may be observed in these recent cold atom and photonic experiments.

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Abstract Ground-state phases of the ultracold bosons in 2D optical lattices subject to artificial magnetic flux are studied by the inhomogeneous Gutzwiller variational method. Results show that the density of a vortex increases linearly with the magnetic flux per plaquette. For a smaller magnetic flux, no supersolid (SS) states are present. The superfluid (SF) states are vortex liquids of either particle-type or hole-type, depending on whether the average filling is greater or less than the integer number. For a larger magnetic flux, regular density-wave states, which also exhibit superfluidity of either particle-type or hole-type, appear at the boundary between the Mott insulator and the SF. Moreover, the uniform SF phases differ to the conventional SF by carrying convection super-current and coreless vorticity. The convection super-current or vorticity in the SS phases assemble in pairs or stripes, as well as checkerboards, according to the flux strength per unit cell.

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Superfluid-insulator transitions on the triangular lattice

A. A. Burkov, Leon Balents

We report on a phenomenological study of superfluid to Mott insulator transitions of bosons on the triangular lattice, focusing primarily on the interplay between Mott localization and geometrical charge frustration at $1∕2$-filling. A general dual-vortex field theory is developed for arbitrary rational filling factors $f$, based on the appropriate projective symmetry group. At the simple nonfrustrated density $f=1∕3$, we uncover an example of a deconfined quantum critical point very similar to that found on the half-filled square lattice. Turning to $f=1∕2$, the behavior is quite different. Here, we find that the low-energy action describing the Mott transition has an emergent non-Abelian $SU(2)\ifmmode\times\else\texttimes\fi{}U(1)$ symmetry, which is not present at the microscopic level. This large non-Abelian symmetry is directly related to the frustration-induced quasidegeneracy between many charge-ordered states not related by microscopic symmetries. Through this ``pseudospin'' $SU(2)$ symmetry, the charged excitations in the insulator close to the Mott transition develop a Skyrmion-like character. This leads to an understanding of the supersolid phase of the triangular lattice $XXZ$ model as a ``partially melted'' Mott insulator, for more information see Melko et al. [Phys. Rev. Lett. 95, 127207 (2005)], Heidarian and Damle [Phys. Rev. Lett. 95, 127206 (2005)], and Wessel and Troyer [Phys. Rev. Lett. 95, 127205 (2005)]. The latter picture naturally explains a number of puzzling numerical observations of the properties of this supersolid. Moreover, we predict that the nearby quantum phase transition from this supersolid to the Mott insulator is in the noncompact ${\mathrm{CP}}^{1}$ critical universality class as presented in Motrunich and Vishwanath [Phys. Rev. B. 70, 075104 (2004)]. A description of a broad range of other Mott and supersolid states, and a diverse set of quantum critical points between them, is also provided.

arXiv:cond-mat/0506457•Physical Review B•Oct 3, 2005•38 citations
Inspired by recent experimental advances to generate Abelian flux for neutral cold atoms and photons moving in a lattice, we investigate the possible effects of the π\piπ flux through a unit cell in the pseudo-spin 1/2 spinor boson Hubbard model in a square lattice.
We find that the π\piπ flux induces a dramatic interplay between the charge and the spin which leads to a frustrated superfluid.
We develop a new and systematic order from quantum disorder analysis to determine not only the true quantum ground state, but also the excitation spectrum.
The superfluid ground state has a 4 sublattice 90∘90^{\circ}90∘ coplanar spin structure which supports 4 linear gapless modes with 3 different velocities.
We speculate the transition from the weak coupling frustrated SF to the strong coupling Ferromagnetic Mott state to be in a new universality class of non-Ginsburg Landau type.
These novel phenomena may be observed in these recent cold atom and photonic experiments.
We study possible superfluid states of the Rashba spin-orbit coupled (SOC) spinor bosons with the spin anisotropic interaction $ λ$ hopping in a square lattice. The frustrations from the non-abelian flux due to the SOC leads to novel spin-bond correlated superfluids. By using a recently developed systematic "order from quantum disorder" analysis, we not only determine the true quantum ground state, but also evaluate the mass gap in the spin sector at $ λ< 1 $, especially compute the the excitation spectrum of the Goldstone mode in the spin sector at $ λ=1 $ which would be quadratic without the analysis. The analysis also leads to different critical exponents on the two sides of the 2nd order transition driven by a roton touchdown at $ λ=1 $. The intimate analogy at $ λ=1 $ with the charge neutral Goldstone mode in the pseudo-spin sector in the Bilayer quantum Hall systems at the total filling factor $ ν_T=1 $ are stressed. The experimental implications and detections of these novel phenomena in cold atoms loaded on a optical lattice are presented.
We report that a possible Z2 quantum spin liquid (QSL) can be observed in a new class of frustrated system: spinor bosons subject to a pi flux in a square lattice. We construct a new class of Ginsburg-Landau (GL) type of effective action to classify possible quantum or topological phases at any coupling strengths. It can be used to reproduce the frustrated SF with the 4 sublattice $ 90^{\circ} $ coplanar spin structure plus its excitations in the weak coupling limit and the FM Mott plus its excitations in the strong coupling limit achieved in our previous work. It also establishes deep and intrinsic connections between the GL effective action and the order from quantum disorder (OFQD) phenomena in the weak coupling limit. Most importantly, it predicts two possible new phases at intermediate couplings: a FM SF phase or a frustrated magnetic Mott phase. We argue that the latter one is more likely and melts into a $ Z_2 $ quantum spin liquid (QSL) phase. If the heating issue can be under a reasonable control at intermediate couplings $ U/t \sim 1 $, the topological order of the $ Z_2 $ QSL maybe uniquely probed by the current cold atom or photonic experimental techniques.
Abstract Ground-state phases of the ultracold bosons in 2D optical lattices subject to artificial magnetic flux are studied by the inhomogeneous Gutzwiller variational method. Results show that the density of a vortex increases linearly with the magnetic flux per plaquette. For a smaller magnetic flux, no supersolid (SS) states are present. The superfluid (SF) states are vortex liquids of either particle-type or hole-type, depending on whether the average filling is greater or less than the integer number. For a larger magnetic flux, regular density-wave states, which also exhibit superfluidity of either particle-type or hole-type, appear at the boundary between the Mott insulator and the SF. Moreover, the uniform SF phases differ to the conventional SF by carrying convection super-current and coreless vorticity. The convection super-current or vorticity in the SS phases assemble in pairs or stripes, as well as checkerboards, according to the flux strength per unit cell.
We study possible many body phenomena in the Quantum Anomalous Hall system of weakly interacting spinor bosons in a square lattice. There are various novel spin-bond correlated superfluids (SF) and quantum or topological phase transitions among these SF phases. One transition is a first order one driven by roton droppings ( but with non-zero gaps $ Δ_R $ ) tuned by the Zeeman field $ h $. Another is a second order bosonic Lifshitz transition with the dynamic exponents $ z_x=z_y=2 $ and an accompanying $ [C_4 \times C_4]_D $ symmetry breaking. It is driven by the softening of the superfluid Goldstone mode tuned by the ratio of spin-orbit coupled (SOC) strength over the hopping strength. The two phase boundaries meet at a topological tri-critical (TT) point which separates the $ h=0 $ line into two SF phases with $ N=2 $ and $ N=4 $ condensation momenta respectively. At the $ h=0 $ line where the system has an anti-unitary $ Z_2 $ Reflection symmetry, there are infinite number of classically degenerate family of states on both sides. We perform a systematic order from quantum disorder analysis to find the quantum ground states, also calculate the roton gaps $ Δ_R $ generated by the order from disorder mechanism on both sides of the TT point. The $ N=2 $ and $ N=4 $ SF phases have the same spin-orbital XY-AFM spin structure, respect the anti-unitary symmetry and break the $ [C_4 \times C_4]_D $ symmetry, so they be distinguished only by the different topology of the BEC condensation momenta instead of by any differences in the symmetry breaking patterns. All these novel quantum or topological phenomena can be probed in the recent experimentally realized weakly interacting Quantum Anomalous Hall (QAH) model of $ ^87 Rb $ by Wu, {\sl et.al}, Science 354, 83-88 (2016).
Neutral-atom quantum simulators provide a powerful platform for realizing strongly correlated phases, enabling access to dynamical signatures of quasiparticles and symmetry breaking processes. Motivated by recent observations of quantum phases in flux-frustrated ladders with non-vanishing ground state currents, we investigate interacting bosons on the dimerized BBH lattice in two dimensions-originally introduced in the context of higher-order topology. After mapping out the phase diagram, which includes vortex superfluid (V-SF), vortex Mott insulator (V-MI), and featureless Mott insulator (MI) phases, we focus on the integer filling case. There, the MI/V-SF transition simultaneously breaks the Z2T\mathbb Z_2^{T}Z2T​ and U(1) symmetries, where Z2T\mathbb Z_2^{T}Z2T​ corresponds to time-reversal symmetry (TRS). Using a slave-boson description, we resolve the excitation spectrum across the transition and uncover a chiral Higgs mode whose mass softens at criticality, providing a dynamical hallmark of emergent chirality that manifests via the oscillation of the loop currents in quench dynamics. Our results establish an experimentally realistic setting for probing unconventional TRS-broken phases and quasiparticles with intrinsic chirality in strongly interacting quantum matter.
We report on a phenomenological study of superfluid to Mott insulator transitions of bosons on the triangular lattice, focusing primarily on the interplay between Mott localization and geometrical charge frustration at 1∕21∕21∕2-filling. A general dual-vortex field theory is developed for arbitrary rational filling factors fff, based on the appropriate projective symmetry group. At the simple nonfrustrated density f=1∕3f=1∕3f=1∕3, we uncover an example of a deconfined quantum critical point very similar to that found on the half-filled square lattice. Turning to f=1∕2f=1∕2f=1∕2, the behavior is quite different. Here, we find that the low-energy action describing the Mott transition has an emergent non-Abelian $SU(2)\ifmmode\times\else\texttimes\fi{}U(1)$ symmetry, which is not present at the microscopic level. This large non-Abelian symmetry is directly related to the frustration-induced quasidegeneracy between many charge-ordered states not related by microscopic symmetries. Through this ``pseudospin'' SU(2)SU(2)SU(2) symmetry, the charged excitations in the insulator close to the Mott transition develop a Skyrmion-like character. This leads to an understanding of the supersolid phase of the triangular lattice XXZXXZXXZ model as a ``partially melted'' Mott insulator, for more information see Melko et al. [Phys. Rev. Lett. 95, 127207 (2005)], Heidarian and Damle [Phys. Rev. Lett. 95, 127206 (2005)], and Wessel and Troyer [Phys. Rev. Lett. 95, 127205 (2005)]. The latter picture naturally explains a number of puzzling numerical observations of the properties of this supersolid. Moreover, we predict that the nearby quantum phase transition from this supersolid to the Mott insulator is in the noncompact CP1{\mathrm{CP}}^{1}CP1 critical universality class as presented in Motrunich and Vishwanath [Phys. Rev. B. 70, 075104 (2004)]. A description of a broad range of other Mott and supersolid states, and a diverse set of quantum critical points between them, is also provided.