Supersolidity in the triangular lattice spin-1/2 XXZ model: A variational perspective

We study the spin-1/2 XXZ model on the triangular lattice with a nearest neighbor antiferromagnetic Ising coupling $J_z>0$ and unfrustrated ($J_\perp0$) kinetic terms in zero magnetic field. Incorporating long-range Jastrow correlations over a mean field spin state, we obtain the variational phase diagram of this model on large lattices for arbitrary $J_z$ and either sign of $J_\perp$. For $J_\perp<0$, we find a $\sqrt{3}\times\sqrt{3}$ supersolid for $J_z/|J_\perp| \gtrsim 4.7$, in excellent agreement with quantum Monte Carlo data. For $J_\perp >0$, a distinct $\sqrt{3}\times\sqrt{3}$ supersolid is found to emerge for $J_z/J_\perp \geq 1$. Both supersolids exhibit a spontaneous density deviation from half-filling. At $J_z/J_\perp=\infty$, the crystalline order parameters of these two supersolids are nearly identical, consistent with exact results.Comment: 4 pages, 4 figures, 1 table, published versio

From magnetism to one-dimensional spin liquid in the anisotropic triangular lattice

We investigate the anisotropic triangular lattice that interpolates from decoupled one-dimensional chains to the isotropic triangular lattice and has been suggested to be relevant for various quasi-two-dimensional materials, such as Cs$_2$CuCl$_4$ or $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$, an organic material that shows intriguing magnetic properties. We obtain an excellent accuracy by means of a novel representation for the resonating valence bond wave function with both singlet and triplet pairing. This approach allows us to establish that the magnetic order is rapidly destroyed away from the pure triangular lattice and incommensurate spin correlations are short range. A non-magnetic spin liquid naturally emerges in a wide range of the phase diagram, with strong one-dimensional character. The relevance of the triplet pairing for $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ is also discussed.Comment: 4+epsilon pages, 6 figure

Inhomogeneous metallic phase upon disordering a two dimensional Mott insulator

We find that isoelectronic disorder destroys the spectral gap in a Mott-Hubbard insulator in 2D leading, most unexpectedly, to a new metallic phase. This phase is spatially inhomogeneous with metallic behavior coexisting with antiferromagnetic long range order. Even though the Mott gap in the pure system is much larger than antiferromagnetic exchange, the spectral gap is destroyed locally in regions where the disorder potential is high enough to overcome the inter-electron repulsion thereby generating puddles where charge fluctuations are enhanced. With increasing disorder, these puddles expand and concomitantly the states at the Fermi energy get extended leading to a metallic phase. We discuss the implications of our results for experiments.Comment: (4 pages, 5 figures

Persistent supersolid phase of hard-core bosons on the triangular lattice

We study hard-core bosons with unfrustrated hopping ($t$) and nearest neighbour repulsion ($U$) on the triangular lattice. At half-filling, the system undergoes a zero temperature ($T$) quantum phase transition from a superfluid phase at small $U$ to a supersolid at $U_c \approx 4.45$ in units of $2t$. This supersolid phase breaks the lattice translation symmetry in a characteristic $\sqrt{3} \times \sqrt{3}$ pattern, and is remarkably stable--indeed, a smooth extrapolation of our results indicates that the supersolid phase persists for arbitrarily large $U/t$.Comment: 4 pages, 5 figures, two column forma

Electric field control of spins in bilayer graphene: Local moment formation and local moment interactions

We study local moment formation for adatoms on bilayer graphene (BLG) within a mean-field theory of the Anderson impurity model. The wavefunctions of the BLG electrons induce strong particle-hole asymmetry and band dependence of the hybridization, which is shown to result in unusual features in the impurity model phase diagram. We also study the effect of varying the chemical potential, as well as varying an electric field perpendicular to the bilayer; the latter modifies the density of states of electrons in BLG and, more significantly, shifts the impurity energy. We show that this leads to regimes in the impurity phase diagram where local moments can be turned on or off by applying modest external electric fields. Finally, we show that the RKKY interaction between local moments can be varied by tuning the chemical potential (as has also been suggested in monolayer graphene) or, more interestingly, by tuning the electric field so that it induces changes in the band structure of BLG.Comment: Revised discussion and figures, 17 page

Two-step melting of three-sublattice order in

We consider S = 1 triangular lattice Heisenberg antiferromagnets with a strong single-ion anisotropy D that dominates over the nearest-neighbour antiferromagnetic exchange J. In this limit of small J∕D, we study low temperature (T ~ J ≪ D) properties of such magnets by employing a low-energy description in terms of hard-core bosons with nearest neighbour repulsion V ≈ 4J + J2∕D and nearest neighbour unfrustrated hopping t ≈ J2∕2D. Using a cluster Stochastic Series Expansion (SSE) algorithm to perform sign-problem-free quantum Monte Carlo (QMC) simulations of this effective model, we establish that the ground-state three-sublattice order of the easy-axis spin-density Sz(r) melts in zero field (B = 0) in a two-step manner via an intermediate temperature phase characterized by power-law three-sublattice order with a temperature dependent exponent η(T)∈[1/9,1/4] η(T)∈[19,14] . For η(T)<2/9 η(T)<29 in this phase, we find that the uniform easy-axis susceptibility of an L × L sample diverges as χL ~ L2−9η at B = 0, consistent with recent predictions that the thermodynamic susceptibility to a uniform field B along the easy axis diverges at small B as χeasy-axis(B)~B−4−18η/4−9η χeasy-axis(B)~B-4-18η4-9η in this regime