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arXiv:cond-mat/0406061 · Physical Review Letters

Weak First-Order Superfluid-Solid Quantum Phase Transitions

Anatoly B. Kuklov, Nikolay V. Prokof'ev, Boris V. Svistunov

Published Nov 30, 2004• 47 citations
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Abstract

We study superfluid-solid zero-temperature transitions in two-dimensional lattice boson-spin models using worm-algorithm Monte Carlo simulations. We observe that such transitions are typically first order with the exception of special high-symmetry points which require fine-tuning in the Hamiltonian parameter space. We present evidence that the superfluid-checkerboard solid and superfluid-valence-bond solid transitions at half-integer filling factor are extremely weak first-order transitions and in small systems can be confused with continuous or high-symmetry points.

DOI
10.1103/physrevlett.93.230402

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We study superfluid-solid zero-temperature transitions in two-dimensional lattice boson-spin models using worm-algorithm Monte Carlo simulations. We observe that such transitions are typically first order with the exception of special high-symmetry points which require fine-tuning in the Hamiltonian parameter space. We present evidence that the superfluid-checkerboard solid and superfluid-valence-bond solid transitions at half-integer filling factor are extremely weak first-order transitions and in small systems can be confused with continuous or high-symmetry points.
We study the quantum phase transition between the superfluid and valence bond solid in easy-plane J-Q models on the square lattice. The Hamiltonian we study is a linear combination of two model Hamiltonians: (1) an SU(2) symmetric model, which is the well known J-Q model that does not show any direct signs of a discontinuous transition on the largest lattices and is presumed continuous and (2) an easy-plane version of the J-Q model, which shows clear evidence for a first-order transition even on rather small lattices of size $L\ensuremath{\approx}16$. A parameter $0\ensuremath{\le}\ensuremath{\lambda}\ensuremath{\le}1$ [$\ensuremath{\lambda}=0$ being the easy-plane model and $\ensuremath{\lambda}=1$ being the SU(2) symmetric J-Q model] allows us to smoothly interpolate between these two limiting models. We use stochastic series expansion (SSE) quantum Monte Carlo (QMC) to investigate the nature of this transition as $\ensuremath{\lambda}$ is varied---here we present studies for $\ensuremath{\lambda}=0,0.5,0.75,0.85,0.95$, and 1. While we find that the first-order transition weakens as $\ensuremath{\lambda}$ is increased from 0 to 1, we find no evidence that the transition becomes continuous until the SU(2) symmetric point, $\ensuremath{\lambda}=1$. We thus conclude that the square-lattice superfluid-VBS transition in the two-component easy-plane model is generically first order.
We use two quantum Monte Carlo algorithms to map out the phase diagram of the two-dimensional hardcore boson Hubbard model with near (V1){(V}_{1})(V1​) and next near (V2){(V}_{2})(V2​) neighbor repulsion. At half filling we find three phases: superfluid (SF), checkerboard solid, and striped solid depending on the relative values of V1,V2,{V}_{1}{,V}_{2},V1​,V2​, and the kinetic energy. Doping away from half filling, the checkerboard solid undergoes phase separation: The superfluid and solid phases coexist but not as a single thermodynamic phase. As a function of doping, the transition from the checkerboard solid is therefore first order. In contrast, doping the striped solid away from half filling instead produces a striped supersolid phase: coexistence of density order with superfluidity as a single phase. One surprising result is that the entire line of transitions between the SF and checkerboard solid phases at half filling appears to exhibit dynamical O(3)O(3)O(3) symmetry restoration. The transitions appear to be in the same universality class as the special Heisenberg point even though this symmetry is explicitly broken by the V2{V}_{2}V2​ interaction.
We study hard-core bosons on the checkerboard lattice with nearest-neighbor unfrustrated hopping ttt and ``tetrahedral'' plaquette charging energy UUU. Analytical arguments and quantum Monte Carlo simulations lead us to the conclusion that the system undergoes a zero temperature (T)(T)(T) quantum phase transition from a superfluid phase at small U∕tU∕tU∕t to a large U∕tU∕tU∕t Mott insulator phase with $\ensuremath{\rho}=1∕4$ for a range of values of the chemical potential $\ensuremath{\mu}$. Further, the quarter-filled insulator breaks lattice translation symmetry in a characteristic fourfold ordering pattern and occupies a lobe of finite extent in the $\ensuremath{\mu}\text{\ensuremath{-}}U∕t$ phase diagram. A quantum Monte Carlo study slightly away from the tip of the lobe provides evidence for a direct weakly first-order superfluid insulator transition away from the tip of the lobe. While analytical arguments lead us to conclude that the transition at the tip of the lobe belongs to a different Landau-forbidden second-order universality class, an extrapolation of our numerical results suggests that the size of the first-order jump does not go to zero even at the tip of the lobe.
The stability of the supersolid state of bosons reported recently on the checkerboard lattice in the vicinity of a valence-bond crystal phase is assessed. In contrast to previously identified supersolid phases of lattice bosons, this state would---if stable---provide superfluidity atop a solid with inherent quantum dynamics, restricted to the particular filling fraction of one quarter. Using large-scale quantum Monte Carlo simulations based on the stochastic series expansion method constrained to the canonical ensemble, the supersolid state is, however, found to be unstable in the thermodynamic limit. Converging crossing points in both the winding number fluctuations and the Binder ratio related to the order parameter of the valence-bond crystal instead exhibit a direct quantum melting transition of the crystal into the superfluid regime.
We examine recent theoretical predictions for two-dimensional lattice bosonic/spin systems that superfluid-solid (SF-S) quantum phase transitions are often continuous and do not fit the canonical Ginzburg-Landau-Wilson (GLW) picture.We show that the GLW theory based on extended broken symmetry is consistent with existing numerical data, which report weak first-order transitions or problems with the finite-size scaling.This GLW theory excludes generic continuous SF-S transitions and quantum disordered phases.Remarkably, the phase diagram of the deconfined critical action itself features a weak first-order phase transition with spinon deconfinement, in accordance with the conventional theory.Problems with data collapse even in the regime of weak interactions between the spinons strongly suggest that the first-order deconfinement transition is the only possible outcome. * )Since the first study of the SF-VBS point within the J-current model, we have observed numerous first-order SF-VBS transitions for different sets of Hamiltonian parameters.
We consider the easy-plane limit of bipartite SU(N\mathrm{SU}(NSU(N) Heisenberg Hamiltonians, which have a fundamental representation on one sublattice and the conjugate to fundamental on the other sublattice. For N=2N=2N=2 the easy plane limit of the SU(2) Heisenberg model is the well-known quantum XYXYXY model of a lattice superfluid. We introduce a logical method to generalize the quantum XYXYXY model to arbitrary NNN, which keeps the Hamiltonian sign-free. We show that these quantum Hamiltonians have a world-line representation as the statistical mechanics of certain tightly packed loop models of NNN colors in which neighboring loops are disallowed from having the same color. In this loop representation we design an efficient Monte Carlo cluster algorithm for our model. We present extensive numerical results for these models on the two dimensional square lattice, where we find the nearest neighbor model has superfluid order for $N\ensuremath{\le}5$ and valence-bond order for N>5N>5N>5. By introducing SU(N\mathrm{SU}(NSU(N) easy-plane symmetric four-spin couplings we are able to tune across the superfluid-VBS phase boundary for all $N\ensuremath{\le}5$. We present clear evidence that this quantum phase transition is first order for N=2N=2N=2 and N=5N=5N=5, suggesting that easy-plane deconfined criticality runs away generically to a first-order transition for small NNN.