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arXiv:cond-mat/0306198 · Physical Review B

Microscopic wave functions of spin-singlet and nematic Mott states of spin-one bosons in high-dimensional bipartite lattices

Michiel Snoek, Fei Zhou

Published Mar 16, 2004• 67 citations
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Abstract

We present microscopic wave functions of spin-singlet Mott insulating states and nematic Mott insulating states. We also investigate quantum phase transitions between the spin-singlet Mott phase and the nematic Mott phase in both large-$N$ limit and small-$N$ limit $(N$ being the number of particles per site) in high-dimensional bipartite lattices. In the mean-field approximation employed in this article we find that phase transitions are generally weakly first order.

DOI
10.1103/physrevb.69.094410

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We present microscopic wave functions of spin-singlet Mott insulating states and nematic Mott insulating states. We also investigate quantum phase transitions between the spin-singlet Mott phase and the nematic Mott phase in both large-NNN limit and small-NNN limit (N(N(N being the number of particles per site) in high-dimensional bipartite lattices. In the mean-field approximation employed in this article we find that phase transitions are generally weakly first order.
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We study magnetically stabilized nematic order for spin-one bosons in optical lattices. We show that the Zeeman field-driven quantum phase transitions between non-nematic Mott states and quantum spin nematic Mott states in the weak hopping limit are in the universality class of the ferromagnetic XXZXXZXXZ (S=1∕2)(S=1∕2)(S=1∕2) spin model. We further discuss these transitions as condensation of interacting magnons. The development of O(2) nematic order when external fields are applied corresponds to condensation of magnons, which breaks a U(1) symmetry. Microscopically, this results from a coherent superposition of two non-nematic states at each individual site. Nematic order and spin-wave excitations around critical points are studied and critical behaviors are obtained in a dilute gas approximation. We also find that spin-singlet states are unstable with respect to quadratic Zeeman effects and Ising nematic order appears in the presence of any finite quadratic Zeeman coupling. All discussions are carried out for states in three-dimensional bipartite lattices.
We consider insulating phases of cold spin-1 bosonic particles with antiferromagnetic interactions, such as 23Na,{}^{23}\mathrm{Na},23Na, in optical lattices. We show that spin-exchange interactions give rise to several distinct phases, which differ in their spin correlations. In two- and three-dimensional lattices, insulating phases with an odd number of particles per site are always nematic. For insulating states with an even number of particles per site, there is always a spin-singlet phase, and there may also be a first-order transition into the nematic phase. The nematic phase breaks spin rotational symmetry but preserves time reversal symmetry, and has gapless spin-wave excitations. The spin-singlet phase does not break spin symmetry and has a gap to all excitations. In one-dimensional lattices, insulating phases with an odd number of particles per site always have a regime where translational symmetry is broken and the ground state is dimerized. We discuss signatures of various phases in Bragg scattering and time-of-flight measurements.
We consider the effective model of H=−J1∑Si⋅Sj−J2∑(Si⋅Sj)2H=-J_{1}\sum_{}\mathbf{S}_{i}\cdot \mathbf{S}_{j}-J_{2}\sum_{}(\mathbf{S}_{i}\cdot \mathbf{S}_{j}) ^{2}H=−J1​∑​Si​⋅Sj​−J2​∑​(Si​⋅Sj​)2, describing the Mott insulating phase with odd number of spin-1 bosons in optical lattices (J2>J1>0J_{2}>J_{1}>0J2​>J1​>0). In terms of an SU(3) boson representation, a valence bond mean field theory is developed. In 1D, a \textit{first-order} quantum phase transition from a spin singlet to a spin nematic phase with \textit{gapful} excitations is identified at $ J_{1}/J_{2}=0.19833$, while on a 2D square lattice a spin nematic ordered phase with gapless excitations prevails. In both 1D and 2D cases, we predict that the spin structure factor displays dominant antiferromagnetic fluctuations, while the quadrupole structure factor exhibits strong ferroquadrupolar correlations.
We study the superfluid--Mott insulator (SF-MI) transition of spin-1 bosons interacting antiferromagnetically in an optical lattice. Starting from a Bose-Hubbard tight-binding model for spin-1 bosons, we obtain the zero-temperature phase diagram by a mean-field approximation. We find that the MI phase with an even number of atoms per site is a spin singlet state, while the MI phase with an odd number of atoms per site has spin 1 at each site in the limit of t=0t=0t=0, where ttt is the hopping matrix element. We also show that the superfluid phase is a polar state as in the case for a spin-1 Bose condensate in a harmonic trap. It is found that the MI phase is strongly stabilized against the SF-MI transition when the number of atoms per site is even, due to the formation of singlet pairs. We derive the effective spin Hamiltonian for the MI phase with one atom per site and briefly discuss the spin order in the MI phase.
We study the ground state properties of spin-1 bosons in a two-dimensional optical lattice, by applying a variational Monte Carlo method to the S=1 Bose-Hubbard model on a square lattice at unit filling. A doublon-holon binding factor introduced in the trial state provides a noticeable improvement in the variational energy over the conventional Gutzwiller wave function and allows us to deal effectively with the inter-site correlations of particle densities and spins. We systematically show how spin-dependent interactions modify the superfluid-Mott insulator transitions in the S=1 Bose-Hubbard model due to the interplay between the density and spin fluctuations of bosons. Furthermore, regarding the magnetic phases in the Mott region, the calculated spin structure factor elucidates the emergence of nematic and ferromagnetic spin orders for antiferromagnetic (U2>0U_2>0U2​>0) and ferromagnetic (U2<0U_2<0U2​<0) couplings, respectively.