Monte Carlo simulation of boson lattices

Boson lattices are theoretically well described by the Hubbard model. The basic model and its variants can be effectively simulated using Monte Carlo techniques. We describe two newly developed approaches, the Stochastic Series Expansion (SSE) with directed loop updates and continuous--time Diffusion Monte Carlo (CTDMC). SSE is a formulation of the finite temperature partition function as a stochastic sampling over product terms. Directed loops is a general framework to implement this stochastic sampling in a non--local fashion while maintaining detailed balance. CTDMC is well suited to finding exact ground--state properties, applicable to any lattice model not suffering from the sign problem; for a lattice model the evolution of the wave function can be performed in continuous time without any time discretization error. Both the directed loop algorithm and the CTDMC are important recent advances in development of computational methods. Here we present results for a Hubbard model for anti--ferromagnetic spin--1 bosons in one dimensions, and show evidence for a dimerized ground state in the lowest Mott lobe.Comment: 3 pages, 5 figur

Flat bands, Dirac cones, and atom dynamics in an optical lattice

We study atoms trapped with a harmonic confinement in an optical lattice characterized by a flat band and Dirac cones. We show that such an optical lattice can be constructed which can be accurately described with the tight-binding or Hubbard models. In the case of fermions the release of the harmonic confinement removes fast atoms occupying the Dirac cones while those occupying the flat band remain immobile. Using exact diagonalization and dynamics we demonstrate that a similar strong occupation of the flat band does not happen in the bosonic case and furthermore that the mean-field model is not capable of describing the dynamics of the boson cloud.peerReviewe

Roton-roton crossover in strongly correlated dipolar Bose-nonstnon condensates

We study the pair correlations and excitations of a dipolar Bose gas layer. The anisotropy of the dipole-dipole interaction allows us to tune the strength of pair correlations from strong to weak perpendicular and weak to strong parallel to the layer by increasing the perpendicular trap frequency. This change is accompanied by a roton-roton crossover in the spectrum of collective excitations, from a roton caused by the head-to-tail attraction of dipoles to a roton caused by the side-by-side repulsion, while there is no roton excitation for intermediate trap frequencies. We discuss the nature of these two kinds of rotons and the relation to instabilities of dipolar Bose gases. In both regimes of trap frequencies where rotons occur, we observe strong damping of collective excitations by decay into two rotons.peerReviewe

NO Electrochemical Reduction on Pt Electrocatalysts: A DFT Approach

Electrochemical denitrification is a promising technology for the removal of toxic nitrate and nitrite species from groundwater due to the process’s environmental compatibility, energy efficiency, safety, and product selectivity[1]-[3]. The adsorbed NO is generally considered to be a selectivity-determining species during this electrochemical potential-dependent reaction[4], [5]. Atomic-scale studies using density functional theory (DFT), in turn, can provide powerful molecular-level information about the elementary mechanisms of reductive NO electrocatalysis, and these basic building blocks can further serve as a starting point for future trends-based analysis on different transition metal surfaces. In this work, we employ periodic, self-consistent DFT calculations to clarify the adsorption structures and thermochemistry of NO and its related reaction intermediate species on different Pt single crystal surfaces ((111) and (100)). Kinetics are further determined by calculating the activation barriers for N-O dissociation, protonation, and N-N bond formation. We begin by describing results for the reaction at low NO coverage (ca. 0.11 ML). After describing the thermodynamics (the adsorption) and kinetics (N-O bond breaking and protonation barrier) for the dentirification reaction scheme, a plausible mechanistic reaction pathway is proposed. Water-assisted protonation barriers are generally shown to be lower than barriers for non-electrochemical reactions, such as surface hydrogenation or N-O bond breaking[6]. Consistent with available experimental evidence, from the most probable reaction pathway, we conclude that ammonium ion would be the most favorable product that would evolve. There is no evidence for the formation of hydroxylamine (H2NOH) at low coverage. At higher NO coverages (ca. 0.45 ML), the relative energy states of the intermediates show a similar trend for the energetics when it is compared with the study with lower NO coverage. Next, N2O formation is examined. We find that trans-(NO)2 species could be a precursor state for the N­2O formation due to its lower kinetic barrier than barriers that are found for cis-type NO dimers. The simulation demonstrates that N2O could be formed at higher potentials through two protonations of trans-(NO)2 species, and the reaction is thermodynamically more favorable at higher NO coverage. This barrier could be surmountable at room temperature, and these mechanistic conclusions hold on both the (111) and (100) surfaces. Finally, using the DFT results to determined rate constants, we construct a detailed kinetic Monte Carlo model of the overall NO electroreduction network. The model accurately tracks experimental NO stripping curves and provides estimates of the degrees of rate control of elementary reaction steps.</jats:p

Atomistic Insights into Nitrogen-Cycle Electrochemistry : A Combined DFT and Kinetic Monte Carlo Analysis of NO Electrochemical Reduction on Pt(100)

Electrocatalytic denitrification is a promising technology for the removal of NOx species in groundwater. However, a lack of understanding of the molecular pathways that control the overpotential and product distribution have limited the development of practical electrocatalysts, and additional atomic-level insights are needed to advance this field. Adsorbed NO has been identified as a key intermediate in the NOx electroreduction network, and the elementary steps by which it decomposes to NH4+, N2, NH3OH+, or N2O remain a subject of debate. Herein, we report a combined density functional theory (DFT) and kinetic Monte Carlo (kMC) study of this reaction on Pt(100), a catalytic surface that is known to be suitable for the activation of strong covalent bonds, in acidic electrolytes. This approach describes the effects of coverage-dependent adsorbate–adsorbate interactions, water-mediated protonation kinetics and thermodynamics, and transient potential sweeps, on reaction rates and selectivities. The results predict NO stripping curves in excellent agreement with experiments while, at the same time, providing a mechanistic interpretation of observed current peaks. Furthermore, production of NH4+ products is traced to the rapid kinetics of N–O bond breaking in reactive intermediates, whereas rapid hydrogenation of surface N* species prevent competing pathways from forming either N2 or N2O. The combined DFT-kMC methodology thus provides a unique tool to describe the mechanism and energetics of platinum-catalyzed electroreduction in the nitrogen cycle, and this approach should also find application to related electrocatalytic processes that are of technological and environmental interest.peerReviewe

Structure Sensitivity of Electrochemical Reactions from First Principles: Applications to Nitrogen and Water Cycles

Nitrogen cycle electrochemistry is an emerging area of interest in the electrocatalysis community, with applications ranging from removal of nitrates from wastewater streams to the development of fundamental understanding of NO electrochemistry. In spite of a significant amount of fundamental research for this chemistry on single crystal surfaces, however, the mechanistic details of the reactions are not fully known, and even such basic information as the nature of the rate-limiting step is not understood. In this work, we apply some of the innovations in theoretical electrochemistry that have emerged over the past 5-10 years to the study of NO and nitrate reduction on transition metal surfaces. We begin with a simple Density Functional Theory analysis of direct NO reduction in acid on Pt(111), Pt(100), and Pt(211) surfaces, and we identify potential- and rate-determining steps on each surface, using extensive ab-initio molecular dynamics simulations to estimate the impact of the aqueous environment on surface thermodynamics and kinetics (Ang. Chem. Int. Ed. 2015). We next introduce a novel approach that couples DFT calculations with rigorous kinetic Monte Carlo (kMC) simulations to understand, in unprecedented detail, the effect of higher adsorbed NO coverages on these chemistries, and we demonstrate that these coverage-dependent effects are essential for describing experimentally measured polarization curves (ACS Catalysis 2017). We subsequently generalize our analysis by developing universal Brønsted-Evans-Polanyi relationships that rigorously relate barriers for proton-coupled electron transfer to surface species to the corresponding thermodynamics across a large space of different transition metals and potentials. These relationships, combined with the potential- and coverage-dependent energetics determined on platinum, provide a basis to extrapolate our mechanistic analyses to other transition metals, including PtSn and related alloys. We employ these techniques to propose design strategies to tune selectivity of NO electroreduction to desired products, including N2 and hydroxylamine, on these different transition metals and alloys. If time permits, we will briefly describe some additional work on the development of structural and catalytic reactivity models at the interface between thin hydroxyoxide films and precious metal platinum substrates. We will examine the kinetics of the hydrogen evolution reaction as a function of the structure and oxidation state of these interfaces and will demonstrate that the bifunctional nature of the interfaces plays a key role in their electrocatalytic properties. </jats:p

Observation of a superfluid component within solid helium

We demonstrate by neutron scattering that a localized superfluid component exists at high pressures within solid helium in aerogel. Its existence is deduced from the observation of two sharp phonon-roton spectra which are clearly distinguishable from modes in bulk superfluid helium. These roton excitations exhibit different roton gap parameters than the roton observed in the bulk fluid at freezing pressure. One of the roton modes disappears after annealing the samples. Comparison with theoretical calculations suggests that the model that reproduces the observed data best is that of superfluid double layers within the solid and at the helium-substrate interface.peerReviewe

Addressing Dynamics at Catalytic Heterogeneous Interfaces with DFT-MD : Anomalous Temperature Distributions from Commonly Used Thermostats

Density functional theory-based molecular dynamics (DFT-MD) has been widely used for studying the chemistry of heterogeneous interfacial systems under operational conditions. We report frequently overlooked errors in thermostated or constant-temperature DFT-MD simulations applied to study (electro)catalytic chemistry. Our results demonstrate that commonly used thermostats such as Nose−Hoover, Berendsen, and simple velocity rescaling methods fail to provide are liable temperature description for systems considered. Instead, nonconstant temperatures and large temperature gradients within the different parts of the system are observed. The errors are not a “feature” of any particular code but a represent in several ab initio molecular dynamics implementations. This uneven temperature distribution, due to inadequate thermostatting, is well-known in the classical MD community, where it is ascribed to the failure in kinetic energy equipartition among different degrees of freedom in heterogeneous systems (Harvey et al. J. Comput. Chem. 1998, 726−740) and termed the flying ice cube effect. We provide tantamount evidence that interfacial systems are susceptible to substantial flying ice cube effects and demonstrate that the traditional Nose−Hoover and Berendsen thermostats should be applied with care when simulating, for example, catalytic properties or structures of solvated interfaces and supported clusters. We conclude that the flying ice cube effect in these systems can be conveniently avoided using Langevin dynamics.peerReviewe