Deep learning optimal quantum annealing schedules for random Ising models

A crucial step in the race towards quantum advantage is optimizing quantum annealing using ad-hoc annealing schedules. Motivated by recent progress in the field, we propose to employ long short term memory (LSTM) neural networks to automate the search for optimal annealing schedules for (random) weighted Max-Cut on regular graphs. By training our network using locally adiabatic annealing paths, we are able to predict optimal annealing schedules for unseen instances and even larger graphs than those used for training.Comment: 9 pages, 6 figure

The crucial role of atomic corrugation on the flat bands and energy gaps of twisted bilayer graphene at the "magic angle" θ∼1.08∘\theta\sim 1.08^\circ

We combine state-of-the-art large-scale first principles calculations with a low-energy continuum model to describe the nearly flat bands of twisted bilayer graphene at the first magic angle $\theta =1.08^\circ$. We show that the energy width of the flat band manifold, as well as the energy gap separating it from the valence and conduction bands, can be obtained only if the out-of-plane relaxations are fully taken into account. The results agree both qualitatively and quantitatively with recent experimental outcomes.Comment: Published in Phys. Rev. B 99, 195419 (2019

Size-dependent structural and electronic properties of Bi(111) ultrathin nanofilms from first principles

Few layer bismuth nanofilms with (111) orientation have shown striking electronic properties, especially as building blocks of novel two-dimensional heterostructures. In this paper we present state-of-the-art first principles calculations, based on both density functional theory and maximally localized Wannier functions, that encompass electronic and structural properties of free-standing Bi(111) nanofilms. We accurately evaluate both the in-plane lattice constant and, by including the van der Waals interaction between bismuth bilayers, the intra/interlayer distances. Interestingly and somehow unexpectedly, the in-plane lattice constant is predicted to shrink by about 5% going from the thickest investigated nanofilm (∼80 A ̊ ) to single bilayer Bi(111), entailing a thickness dependent lattice mismatch in complex heterostructures involving ultrathin Bi(111). Moreover, quantum confinement effects, that would be expected to rule the electronic structure at this size range, compete with surface states that appear close to and across the Fermi level. The implication is that not only all but the thinnest films have a metallic band structure but also that such surface states might play a role in either the formation of interfaces with other materials or for sensing applications. Finally, the calculated electronic structure compares extremely well with ARPES measurements

Layer-dependent electronic and magnetic properties of Nb3I8

The discovery of intrinsic two-dimensional magnetism has ignited intense research interest due to the several and attractive applications in spintronics. Nb3I8 is a recently investigated van der Waals material, exhibiting ferromagnetism and extraordinary visible light-harvesting ability in monolayer form, besides useful remarkable features for the realization of future high-performance nanodevices. Here we use density functional theory and classical Monte Carlo simulations to investigate the electronic and magnetic properties of Nb3I8 in bulk, monolayer, and some multilayer (bilayer and trilayer) forms. Two suitable vdW exchange-correlation functionals, vdW-DF2-C09 and rev-vdW-DF2, have been chosen to compare the first-principles calculation predictions. The layer number directly influences the ground-state magnetism, indicating the possibility of using the thickness as a parameter to control the magnetic response of the material. In particular, it is possible to switch on the antiferromagnetism by adding one or two layers to the ferromagnetic monolayer. This makes Nb3I8 an excellent platform for spintronics applications. Monte Carlo simulations based on a third-nearest-neighbor Ising model provide a Curie temperature close to room temperature (similar to 307 K) for the monolayer. The results on the electronic and magnetic properties render the two-dimensional Nb3I8 an ideal and promising candidate for future research and applications

Graphene nanoribbon electrical decoupling from metallic substrates

We address the structural and electronic properties of graphene nanoribbons (GNRs) covalently immobilized on a metallic substrate by means of an organic layer. The GNR–organic layer and organic layer–metal interfaces can be thought of as constituents of a nanodevice and have been accurately studied using large-scale density functional theory calculations. Our results demonstrate the possibility of combining nanopatterned metal–organic layer substrates with selected GNRs to obtain well ordered and stable structures while preserving the GNR energy band gap, an essential requirement for any switching nanodevice

First-Principles Calculations of Clean and Defected ZnO Surfaces

We report on a theoretical study of the nonpolar ZnO (101̅0) and (112̅0) surfaces carried out in the framework of density functional theory, aiming to elucidate the thermodynamic and kinetic stability of the clean surface against the formation and diffusion of oxygen vacancies. At variance with other oxide materials and ZnO surfaces with different orientation, we show that, under exposure to molecular oxygen in the gas phase, no significant amounts of oxygen vacancies can be sustained by the surface, in agreement with recent Scanning Tunnelling Microscope (STM) observations. However, our calculations show also that under ultrahigh vacuum and high-temperature conditions the observation of oxygen vacancies might be possible, as reported in earlier experiments of Göpel and Lampe.(1) We characterize the defected surfaces electronic and structural properties as a function of the position of the defect with respect to the surface and discuss the diffusion paths of such defects both parallel and across the surface