Hubbard model on triangular N -leg cylinders: Chiral and nonchiral spin liquids

The existence of a gapped chiral spin liquid has been recently suggested in the vicinity of the metal-insulator transition of the Hubbard model on the triangular lattice, by intensive density-matrix renormalization group (DMRG) simulations [A. Szasz, J. Motruk, M. P. Zaletel, and J. E. Moore, Phys. Rev. X 10, 021042 (2020)10.1103/PhysRevX.10.021042]. Here, we report the results obtained within the variational Monte Carlo technique based upon Jastrow-Slater wave functions, implemented with backflow correlations. As in DMRG calculations, we consider N-leg cylinders. For N=4 and in the presence of a next-nearest-neighbor hopping, a chiral spin liquid emerges between the metal and the insulator with magnetic quasi-long-range order. Within our approach, the chiral state is gapped and breaks the reflection symmetry. By contrast, for both N=5 and 6, the chiral spin liquid is not the state with the lowest variational energy: in the former case, a nematic spin liquid is found in the entire insulating regime, while for the less frustrated case with N=6 the results are very similar to that obtained on two-dimensional clusters [L. F. Tocchio, A. Montorsi, and F. Becca, Phys. Rev. B 102, 115150 (2020)2469-995010.1103/PhysRevB.102.115150], with an antiferromagnetic phase close to the metal-insulator transition and a nematic spin liquid in the strong-coupling regime

Magnetic and spin-liquid phases in the frustrated t-t′ Hubbard model on the triangular lattice

The Hubbard model and its strong-coupling version, the Heisenberg one, have been widely studied on the triangular lattice to capture the essential low-temperature properties of different materials. One example is given by transition metal dichalcogenides, as 1T-TaS$_2$, where a large unit cell with 13 Ta atoms forms weakly coupled layers with an isotropic triangular lattice. By using accurate variational Monte Carlo calculations, we report the phase diagram of the $t-t′$ Hubbard model on the triangular lattice, highlighting the differences between positive and negative values of $t′/t$; this result can be captured only by including the charge fluctuations that are always present for a finite electron-electron repulsion. Two spin-liquid regions are detected: one for $t′/t0$. The spin-liquid phase appears to be gapless, though the variational wave function has a nematic character, in contrast to the Heisenberg limit. We do not find any evidence for nonmagnetic Mott phases in the proximity of the metal-insulator transition, at variance with the predictions (mainly based upon strong-coupling expansions in $t/U$) that suggest the existence of a weak-Mott phase that intrudes between the metal and the magnetically ordered insulator

Variational Monte Carlo Study of Spin-Gapped Normal State and BCS-BEC Crossover in Two-Dimensional Attractive Hubbard Model

We study properties of normal, superconducting (SC) and CDW states for an attractive Hubbard model on the square lattice, using a variational Monte Carlo method. In trial wave functions, we introduce an interspinon binding factor, indispensable to induce a spin-gap transition in the normal state, in addition to the onsite attractive and intersite repulsive factors. It is found that, in the normal state, as the interaction strength $|U|/t$ increases, a first-order spin-gap transition arises at $|U_{\rm c}|\sim W$ ($W$: band width) from a Fermi liquid to a spin-gapped state, which is conductive through hopping of doublons. In the SC state, we confirm by analysis of various quantities that the mechanism of superconductivity undergoes a smooth crossover at around |U_{\ma{co}}|\sim |U_{\rm c}| from a BCS type to a Bose-Einstein condensation (BEC) type, as $|U|/t$ increases. For |U|<|U_{\ma{co}}|, quantities such as the condensation energy, a SC correlation function and the condensate fraction of onsite pairs exhibit behavior of $\sim \exp(-t/|U|)$, as expected from the BCS theory. For |U|>|U_{\ma{co}}|, quantities such as the energy gain in the SC transition and superfluid stiffness, which is related to the cost of phase coherence, behave as $\sim t^2/|U|\propto T_{\rm c}$, as expected in a bosonic scheme. In this regime, the SC transition is induced by a gain in kinetic energy, in contrast with the BCS theory. We refer to the relevance to the pseudogap in cuprate superconductors.Comment: 14 pages, 22 figures, submitted to Journal of the Physical Society of Japa

Magnetic and spin-liquid phases in the frustrated t-t\u2032 Hubbard model on the triangular lattice

The Hubbard model and its strong-coupling version, the Heisenberg one, have been widely studied on the triangular lattice to capture the essential low-temperature properties of different materials. One example is given by transition metal dichalcogenides, as 1T-TaS2, where a large unit cell with 13 Ta atoms forms weakly coupled layers with an isotropic triangular lattice. By using accurate variational Monte Carlo calculations, we report the phase diagram of the t-t\u2032 Hubbard model on the triangular lattice, highlighting the differences between positive and negative values of t\u2032/t; this result can be captured only by including the charge fluctuations that are always present for a finite electron-electron repulsion. Two spin-liquid regions are detected: one for t\u2032/t0. The spin-liquid phase appears to be gapless, though the variational wave function has a nematic character, in contrast to the Heisenberg limit. We do not find any evidence for nonmagnetic Mott phases in the proximity of the metal-insulator transition, at variance with the predictions (mainly based upon strong-coupling expansions in t/U) that suggest the existence of a weak-Mott phase that intrudes between the metal and the magnetically ordered insulator

One-dimensional spin liquid, collinear, and spiral phases from uncoupled chains to the triangular lattice

We investigate the Hubbard model on the anisotropic triangular lattice with two hopping parameters t and t' in different spatial directions, interpolating between decoupled chains (t = 0) and the isotropic triangular lattice (t = t'). Variational wave functions that include both Jastrow and backflow terms are used to compare spin-liquid and magnetic phases with different pitch vectors describing both collinear and coplanar ( spiral) order. For relatively large values of the on-site interaction U/t' greater than or similar to 10 and substantial frustration, i.e., 0.3 less than or similar to t/t' less than or similar to 0.8, the spin-liquid state is clearly favored over magnetic states. Spiral magnetic order is only stable in the vicinity of the isotropic point, while collinear order is obtained in a wide range of interchain hoppings from small to intermediate frustration

Phase Diagram of the Triangular Extended Hubbard Model

We study the extended Hubbard model on the triangular lattice as a function of filling and interaction strength. The complex interplay of kinetic frustration and strong interactions on the triangular lattice leads to exotic phases where long-range charge order, antiferromagnetic order, and metallic conductivity can coexist. Variational Monte Carlo simulations show that three kinds of ordered metallic states are stable as a function of nearest neighbor interaction and filling. The coexistence of conductivity and order is explained by a separation into two functional classes of particles: part of them contributes to the stable order, while the other part forms a partially filled band on the remaining substructure. The relation to charge ordering in charge transfer salts is discussed

Spin-liquid versus spiral-order phases in the anisotropic triangular lattice

We study the competition between magnetic and spin-liquid phases in the Hubbard model on the anisotropic triangular lattice, which is described by two hopping parameters t and t' in different spatial directions and is relevant for layered organic charge-transfer salts. By using a variational approach that includes spiral magnetic order, we provide solid evidence that a spin-liquid phase is stabilized in the strongly correlated regime and close to the isotropic limit t'/t = 1. Otherwise, amagnetically ordered spiral state is found, connecting the (collinear) Neel and the (coplanar) 120 degrees phases. The pitch vector of the spiral phase obtained from the unrestricted Hartree-Fock approximation is substantially renormalized in the presence of electronic correlations, and the Neel phase is stabilized in a wide regime of the phase diagram, i.e., for t'/t < 0.75. We discuss these results in the context of organic charge-transfer salts. DOI: 10.1103/PhysRevB.87.03514

Charge orders in organic charge-transfer salts

Motivated by recent experimental suggestions of charge-order-driven ferroelectricity in organic charge-transfer salts, such as $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl, we investigate magnetic and charge-ordered phases that emerge in an extended two-orbital Hubbard model on the anisotropic triangular lattice at $3/4$ filling. This model takes into account the presence of two organic BEDT-TTF molecules, which form a dimer on each site of the lattice, and includes short-range intramolecular and intermolecular interactions and hoppings. By using variational wave functions and quantum Monte Carlo techniques, we find two polar states with charge disproportionation inside the dimer, hinting to ferroelectricity. These charge-ordered insulating phases are stabilized in the strongly correlated limit and their actual charge pattern is determined by the relative strength of intradimer to interdimer couplings. Our results suggest that ferroelectricity is not driven by magnetism, since these polar phases can be stabilized also without antiferromagnetic order and provide a possible microscopic explanation of the experimental observations. In addition, a conventional dimer-Mott state (with uniform density and antiferromagnetic order) and a nonpolar charge-ordered state (with charge-rich and charge-poor dimers forming a checkerboard pattern) can be stabilized in the strong-coupling regime. Finally, when electron-electron interactions are weak, metallic states appear, with either uniform charge distribution or a peculiar $12$-site periodicity that generates honeycomb-like charge order.Comment: 20 pages, 15 figure

DANTE Digital Pulse Processor for XRF and XAS experiments

DANTE is a new Digital Pulse Processor (DPP) developed for fluorescence detectors, like silicon drift detectors (SDDs) or High Purity Germanium detectors (HPGe), used in X-ray Fluorescence (XRF) and X-ray Absorption Spectroscopy (XAS) experiments at synchrotron facilities. Its main features are its optimal energy resolution and peak stability for detector count rate values up to 1-2 Mcps, and its enhanced rejection of pile-up events. In this paper, we present the first complete evaluation of DANTE performance in SOLEIL synchrotron facility. DANTE has been tested in laboratory with an X-ray generator source and in different experiments at LUCIA and PUMA beamlines at SOLEIL.Comment: Manuscript submitted to JINST, 22 pages, 20 figures. v02: Corrected version. v03: Referee comments include

Tunable anisotropic networks for 3-D oriented neural tissue models

Organized networks are common in nature showing specific tissue micro-architecture, where cells can be found isotropically or anisotropically distributed in characteristic arrangements and tissue stiffness. However, when addressing an in vitro tissue model, it is challenging to grant control over mechanical properties while achieving anisotropic porosity of polymeric networks, especially in three-dimensional systems (3-D). While progress was achieved organizing cells in two-dimension (2-D), fabrication methods for aligned networks in 3-D are limited. Here, we describe the use of a biomimetic extra-cellular matrix system allowing programming of anisotropic structures into precisely advancing pore diameters in 3-D. Using control over polymeric composition, crosslinking directionality and freezing gradient dynamics, we revealed a mechanism to top-down biofabricate 3-D structures with tunable micro-porosity capable of directing cellular responses at millimeter scale such as axonal anisotropic outgrowth that is a unique characteristic of the brain cortex. Further, we showed the unique integration of this method with a microfluidic system establishing a neural-endothelial heterotypic conjugation, which can potentially be broadly applied to multiple organ systems.The authors are grateful for the FCT distinctions attributed to Raphael F. Canadas (SFRH/BD/92565/2013), who was awarded a PhD scholarship, and to J. M. Oliveira (IF/00423/2012 and IF/01285/2015). The authors acknowledge that this material and collaboration is based in part upon work supported by FLAD. This work was in part supported by European Research Council Grant agreement ERC-2012-ADG 20120216-321266 for project ComplexiTE. The authors also thank to Morteza Miansari (previously at BAMM Lab, currently at Babol Noshirvani University of Technology, Iran) for the assistance in the design and fabrication of the microfluidic device and to Esra Karaca (previously at BAMM Lab, currently at Stanford Cardiovascular Institute, USA) for the assistance in mice primary neurons isolation.info:eu-repo/semantics/publishedVersio