The Gutzwiller wave function for a strongly correlated model can, if supplemented with a longrange Jastrow factor, provide a proper variational description of Mott insulators, so far unavailable. We demonstrate this concept in the prototypical one-dimensional t − t ′ Hubbard model, where at half filling we reproduce all known phases, namely the ordinary Mott undimerized insulator with power-law spin correlations at small t ′ /t, the spin-gapped metal above a critical t ′ /t and small U , and the dimerized Mott insulator at large repulsion.PACS numbers: 71.10. Fd, 71.10.Pm, 71.27.+a, 71.30.+h Since Mott's original proposal [1] the correlationdriven metal-Mott insulator transition (MIT) has attracted rising interest, renewed by the discovery of novel strongly correlated materials. On the verge of becoming Mott insulators, many systems display very unusual properties, high-T c superconductivity being one spectacular example. While understanding Mott insulators and MITs is conceptually simple, calculations constitute a hard and long standing problem. Conventional electronic structure methods, such as Hartree-Fock (HF) or density functional theory in the local density approximation (LDA) cannot describe MITs, unless one allows for some kind of symmetry breaking. The standard example is long-range static magnetic order in the unrestricted HF, local-spin-density, or LDA+U approximations. This device works by effectively turning the MIT into a conventional metal-band insulator transition, thus masking the essence of the Mott phenomenon, where a charge gap appears quite independently of spin order. The fact that most known Mott insulators are indeed accompanied at low temperatures by some symmetry breaking, usually of magnetic type, further encourages the (wrong) surmise that it is not possible to describe any Mott insulator without a symmetry breaking.Another useful and popular approximation that may invite the same conclusion is based on the variational Gutzwiller wave function (GWF) and its various generalizations. [2,3,4,5] The GWF is the simplest way to improve a symmetry-unbroken, hence metallic, Slater determinant by partly projecting out the expensive doubleoccupancy charge configurations. In principle, were the projection complete, the GWF would indeed describe a Mott insulator devoid of symmetry breaking. Full projection however means zero band-energy gain, generally incorrect, except for infinite on-site repulsion. For finite projection, appropriate at finite repulsion, the GWF unfortunately describes a metallic state in any finite dimensional lattice, at least so long as the uncorrelated Slater determinant state is metallic. [3] To obtain an insulator, one is forced once again to Gutzwiller project an artificially symmetry-broken determinant wave function (WF). The main drawback of the GWF can be immediately recognized if one recalls Mott's original description of a correlation-driven insulator. Let us consider for simplicity the single-band Hubbard model at half-filling. First of all it is clear that the ...

Motivated by the recently observed pattern of unidirectional domains in high-Tc superconductors [Y. Kohsaka et al., Science 315, 1380], we investigate the emergence of spontaneous modulations in the d-wave superconducting resonating valence bond phase using the t-J model at x = 1/8 doping. Half-filled charge domains separated by four lattice spacings are found to form along one of the crystal axis leading to modulated superconductivity with out-of-phase d-wave order parameters in neighboring domains. Both renormalized mean-field theory and variational Monte Carlo calculations yield that the energies of modulated and uniform phases are very close to each other. 74.20.Mn, 75.40.Mg Puzzling properties of the high-T c superconductors have often been attributed to competing instabilities. 1Indeed, it is believed that doping an antiferromagnetic (AF) Mott insulator (which could be described e.g. by the so-called t-J model 2 ) leads to quantum disordered states with short-ranged magnetic correlations between S = 1/2 spins and exotic properties.3 Among them, the resonating valence bond (RVB) state was the first theoretical proposal supposed to capture the essence of high-T c superconductivity.4 Remarkably, this approach based on the Gutzwiller-projected BCS trial wavefunction, the parameters of which are usually determined either by using renormalized mean-field theory 5 (RMFT) or by variational Monte Carlo (VMC) method, 6 not only correctly predicted the d-wave symmetry of the superconducting (SC) order parameter, 7 but reproduced in addition the experimentally observed doping dependence of a variety of physical observables in the SC regime. 8In fact, exactly at half-filling where a particle-hole SU(2) symmetry applies, the RVB phase is equivalent to the staggered flux (SF) state, 9 a projected Slater determinant build from a tight binding model under a staggered magnetic flux. Remarkably, short-ranged staggered orbital current correlations have been seen in the Gutzwiller-projected d-wave RVB phase 10 and in the exact ground state of a small t-J cluster.11 Upon doping the SU(2) symmetry is broken leading to two distinct phases, a d-wave RVB superconductor and a doped SF phase, a candidate for the pseudogap phase, 12,13 characterized by the opening of an antinodal gap in the excitation spectrum. Indeed, coexistence of sharp nodal quasiparticles and broad antinodal excitations have been found in angle-resolved photoemission spectroscopy (ARPES) studies on an array of underdoped cuprates such as La 2−x Sr x CuO 4 (LSCO), 14 Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212), 15 and Ca 2−x Na x CuO 2 Cl 2 (Na-CCOC). 16However, there are also some low-T properties of the SC state which cannot be simply explained within the original RVB framework. For example, following its theoretical prediction, 17 static charge and spin stripe order has been detected in neutron scattering experiments and resonant soft x-ray scattering in some cuprate compounds as Nd-LSCO 18 and La 2−x Ba x CuO 4 (LBCO). 19More microscopic evidences of inhomogeneities h...

Increasing catch rates are considered the main impact of dynamic fisheries practices on marine ecosystems, but other effects can be equally important and are often ignored. Here we quantify a major, previously unknown source of shark mortality: entanglement in drifting fish aggregating devices, now widely used in the global tropical tuna purse‐seine fishery. Using satellite tagging and underwater observational data, we developed two novel, independent, and complementary approaches, which quantify and highlight the scale of this problem. Entanglement mortality of silky sharks (Carcharhinus falciformis) in the Indian Ocean was 5–10 times that of the known bycatch of this imperiled species from the region's purse‐seine fleet. More importantly, these estimates from a single ocean (480 000–960 000 silky sharks) mirror those from all world fisheries combined (400 000–2 million silky sharks), a situation that clearly requires immediate management intervention and extensive monitoring.

We study the superfluid-insulator transition in Bose-Hubbard models in one-, two-, and threedimensional cubic lattices by means of a recently proposed variational wave function. In one dimension, the variational results agree with the expected Berezinskii-Kosterlitz-Thouless scenario of the interaction-driven Mott transition. In two and three dimensions, we find evidences that, across the transition, most of the spectral weight is concentrated at high energies, suggestive of pre-formed Mott-Hubbard side-bands. This result is compatible with the experimental data by Stoferle et al. [Phys. Rev. Lett. 92, 130403 (2004)].

We show, by using a correlated Jastrow wave function and a mapping onto a classical model, that the two-dimensional Mott transition in a simple half-filled one-band model can be unconventional and very similar to the binding-unbinding Kosterlitz-Thouless transition of vortices and anti-vortices, here identified by empty and doubly occupied sites. Within this framework, electrons strongly interact with collective plasmon excitations that induce anomalous critical properties on both sides of the transition. In particular, the insulating phase is characterized by a singular power law behavior in the photoemission spectrum, that can be continuously connected to the fully-projected insulating state, relevant to strongly correlated low-energy models.Comment: Reviewed and enlarged version, 6 pages, 7 figure

Characterizing the vulnerability of both target and non-target (bycatch) species to a fishing gear is a key step towards an ecosystem-based fisheries management approach. This study addresses this issue for the tropical tuna purse seine fishery that uses fish aggregating devices (FADs). We used passive acoustic telemetry to characterize, on a 24 h scale, the associative patterns and the vertical distribution of skipjack (Katsuwonus pelamis), yellowfin (Thunnus albacares), and bigeye tuna (Thunnus obesus) (target species), as well as silky shark (Carcharhinus falciformis), oceanic triggerfish (Canthidermis maculata), and rainbow runner (Elagatis bipinnulata) (major non-target species). Distinct diel associative patterns were observed; the tunas and the silky sharks were more closely associated with FADs during daytime, while the rainbow runner and the oceanic triggerfish were more closely associated during the night. Minor changes in bycatch to catch ratio of rainbow runner and oceanic triggerfish could possibly be achieved by fishing at FADs after sunrise. However, as silky sharks display a similar associative pattern as tunas, no specific change in fishing time could mitigate the vulnerability of this more sensitive species. For the vertical distribution, there was no particular time of the day when any species occurred beyond the depth of a typical purse seine net. While this study does not provide an immediate solution to reduce the bycatch to catch ratios of the FAD-based fishery in the western Indian Ocean, the method described here could be applied to other regions where similar fisheries exist so as to evaluate potential solutions to reducing fishing mortality of non-target species.

Indications of density-wave states in underdoped cuprates, coming from recent STM (scanning tunneling microscopy) and Hall-resistance measurements, have raised new concerns whether stripes could be stabilized in the superconducting phase of cuprate materials, even in the absence of antiferromagnetism. Here, we investigate this issue using state-of-the-art quantum Monte Carlo calculations of a t − J model. In particular we consider the stability of unidirectional hole domains in a modulated superconducting background, by taking into account the effect of tetragonal-lattice distortions, next-nearest neighbor hopping and long-range Coulomb repulsion.

Anchored fish aggregating devices (FADs) are deployed by fishermen worldwide to facilitate the capture of pelagic fish. We investigated the associative behavior of yellowfin Thunnus albacares, skipjack Katsuwonus pelamis, and bigeye tuna T. obesus in an array of anchored FADs off the coast of Mauritius (southwestern Indian Ocean) using passive acoustic telemetry. Our results suggest that yellowfin and bigeye tuna have longer FAD residence times than skipjack tuna. The survival curves based on the continuous residence times for bigeye and skipjack tuna were best explained by single exponential models, indicating time-independent associative processes and characteristic timescales of 4.3 and 0.9 d, respectively. Continuous residence times of yellowfin tuna were best explained by time-dependent power law models, but the single exponential model (characteristic timescale of 6.5 d) also fit the data well. The analysis of absence times (time between 2 FAD associations) revealed that single exponential models fit the data for all 3 species (characteristic timescales of 1.3, 5, and 2.7 d for yellowfin, skipjack, and bigeye, respectively), with a time-dependent sigmoidal component at short timescales for skipjack and bigeye tuna, ascribed to diel behavior and the short inter-FAD distances of the array. Our results are consistent with those of previous studies but also reveal common behavioral patterns among species and suggest that inter-FAD distances affect absence times but not residence times. In other words, high densities of FADs tend to decrease the amount of time tuna spend unassociated with FADs.