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 021042 (2020)]. 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)], with an antiferromagnetic phase close to the metal-insulator transition and a nematic spin liquid in the strong-coupling regime

DESIGN OF VASCULARIZABLE SCAFFOLDS FOR LARGE TISSUE ENGINEERING

The emerging field of tissue engineering is dedicated to restore, maintain or improve the functions of damaged or lost human tissues. However, despite significant successes have been achieved over the last 20 years, several challenges still remain, preventing a pervasive clinical application of tissue engineering. One of the main challenges lies in the development of scaffolding materials able to mimic the complex organization of the in vivo milieu and provide tailored stimuli for tissue growth and maturation. A fundamental aspect of this problem resides in the design of scaffolds having three-dimensional vascular architecture, able to provide optimal nutrients diffusion, supporting and maintaining viable tissue in vitro, and capable to promote vascularization after implantation. The lack of proper vascularization is currently limiting the size of the engineered tissues to smaller than clinically relevant dimensions. The aim of this PhD work, merging the study of novel biomaterials and the development of original microfabrication methods, is to create enabling technologies towards the design of innovative scaffolds for large tissues engineering. For this purpose, a library of RGD-mimetic hydrogels with controlled chemical, mechanical and biological features has been developed. The obtained hydrogels have been combined with foaming and sacrificial molding techniques to engineer customizable scaffolds with hierarchical three-dimensional architectures. These novel hydrogel scaffolds supported optimal three-dimensional cell growth and promoted in vitro vascularization in large constructs. The reported results suggest that the presented approach could represent a viable solution to scale engineered tissue to clinically relevant dimensions releasing the full potential of regenerative medicine

In-Plane and Out-of-Plane MEMS Motion Sensors Based on Fringe Capacitances

Abstract New MEMS motion sensors have been developed. These prototypes are based on a sensing technique that exploits the fringe capacitance between two co-planar electrodes designed over a thin oxide layer covering a grounded wafer substrate. A relevant fraction of the electric-field streamlines, generated by the readout voltage applied between the electrodes, develops in the air (or vacuum) volume over the electrodes. A grounded suspended mass moving within this volume modifies the streamlines configuration, causing relative changes in the capacitance between the electrodes as large as the ∼80% of the initial value. Two types of devices based on the described concept have been designed and built in an industrial surface micromachining process, to sense acceleration in the direction both parallel and orthogonal to the substrate surface. The realized devices have been tested and a sensitivity of ∼0.9 fF/g and ∼0.2 fF/g has been obtained for the in plane and for the out-of-plane structures respectively

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

DALICA: intelligent agents for user profile deduction.

In this paper we are going to discuss the potential contributions that agent technology can bring into an Ambient Intelligence scenario, related to the fruition of cultural assets. The users are located in an area which is known to the agents: in the application, the users are the visitors of Villa Adriana, an archaeological site in Tivoli, near Rome (Italy). Agents are aware of user moves by means of Galileo satellite signal, i.e., the proposed application is based on a blend of different technologies. The agents, developed in the DALI logic programming language, pro-actively learn and/or enhance users profiles and are thus capable to competently assist the users during their visit, to elicit habits and preferences and to propose cultural assets to the users according to the learned profile

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

Solutions of the Two-Dimensional Hubbard Model: Benchmarks and Results from a Wide Range of Numerical Algorithms

Numerical results for ground-state and excited-state properties (energies, double occupancies, and Matsubara-axis self-energies) of the single-orbital Hubbard model on a two-dimensional square lattice are presented, in order to provide an assessment of our ability to compute accurate results in the thermodynamic limit. Many methods are employed, including auxiliary-field quantum Monte Carlo, bare and bold-line diagrammatic Monte Carlo, method of dual fermions, density matrix embedding theory, density matrix renormalization group, dynamical cluster approximation, diffusion Monte Carlo within a fixed-node approximation, unrestricted coupled cluster theory, and multireference projected Hartree-Fock methods. Comparison of results obtained by different methods allows for the identification of uncertainties and systematic errors. The importance of extrapolation to converged thermodynamic-limit values is emphasized. Cases where agreement between different methods is obtained establish benchmark results that may be useful in the validation of new approaches and the improvement of existing methods

Which Factors Determine Spatial Segregation in the South American Opossums (Didelphis aurita and D. albiventris)? An Ecological Niche Modelling and Geometric Morphometrics Approach

Didelphis albiventris and D. aurita are Neotropical marsupials that share a unique evolutionary history and both are largely distributed throughout South America, being primarily allopatric throughout their ranges. In the Araucaria moist forest of Southern Brazil these species are sympatric and they might potentially compete having similar ecology. For this reason, they are ideal biological models to address questions about ecological character displacement and how closely related species might share their geographic space. Little is known about how two morphologically similar species of marsupials may affect each other through competition, if by competitive exclusion and competitive release. We combined ecological niche modeling and geometric morphometrics to explore the possible effects of competition on their distributional ranges and skull morphology. Ecological niche modeling was used to predict their potential distribution and this method enabled us to identify a case of biotic exclusion where the habit generalist D. albiventris is excluded by the presence of the specialist D. aurita. The morphometric analyses show that a degree of shape discrimination occurs between the species, strengthened by allometric differences, which possibly allowed them to occupy marginally different feeding niches supplemented by behavioral shift in contact areas. Overlap in skull morphology is shown between sympatric and allopatric specimens and a significant, but weak, shift in shape occurs only in D. aurita in sympatric areas. This could be a residual evidence of a higher past competition between both species, when contact zones were possibly larger than today. Therefore, the specialist D. aurita acts a biotic barrier to D. albiventris when niche diversity is not available for coexistence. On the other hand, when there is niche diversification (e.g. habitat mosaic), both species are capable to coexist with a minimal competitive effect on the morphology of D. aurita