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Voltage-dependent cluster expansion for electrified solid-liquid interfaces: Application to the electrochemical deposition of transition metals

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Physical Review B 2017年第20期96卷 205134-205134页

作者： Stephen E. Weitzner Ismaila Dabo Department of Materials Science and Engineering Materials Research Institute and Penn State Institutes of Energy and the Environment The Pennsylvania State University University Park Pennsylvania 16802 USA

The detailed atomistic modeling of electrochemically deposited metal monolayers is challenging due to the complex structure of the metal-solution interface and the critical effects of surface electrification during electrode polarization. Accurate models of interfacial electrochemical equilibria are further challenged by the need to include entropic effects to obtain accurate surface chemical potentials. We present an embedded quantum-continuum model of the interfacial environment that addresses each of these challenges and study the underpotential deposition of silver on the gold (100) surface. We leverage these results to parametrize a cluster expansion of the electrified interface and show through grand canonical Monte Carlo calculations the crucial need to account for variations in the interfacial dipole when modeling electrodeposited metals under finite-temperature electrochemical conditions.

关键词： Electrochemistry Surface & interfacial phenomena Liquid-solid interfaces Density functional theory metropolis algorithm Monte Carlo methods Multiscale modeling

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Monte Carlo study of magnetic nanoparticles adsorbed on halloysite Al2Si2O5(OH)4 nanotubes

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Physical Review B 2017年第22期96卷 224404-224404页

作者： O. M. Sotnikov V. V. Mazurenko A. A. Katanin Theoretical Physics and Applied Mathematics Department Ural Federal University Mira Street 19 620002 Ekaterinburg Russia Institute of Metal Physics Kovalevskaya Street 18 Ekaterinburg 620990 Russia

We study properties of magnetic nanoparticles adsorbed on the halloysite surface. For that a distinct magnetic Hamiltonian with a random distribution of spins on a cylindrical surface was solved by using a nonequilibrium Monte Carlo method. The parameters for our simulations, the anisotropy constant, nanoparticle size distribution, saturated magnetization, and geometrical characteristics of the halloysite template, were taken from recent experiments. We calculate the hysteresis loops and temperature dependence of the zero-field-cooling (ZFC) susceptibility, the maximum of which determines the blocking temperature. It is shown that the dipole-dipole interaction between nanoparticles moderately increases the blocking temperature and weakly increases the coercive force. The obtained hysteresis loops (e.g., the value of the coercive force) for Ni nanoparticles are in reasonable agreement with the experimental data. We also discuss the sensitivity of the hysteresis loops and ZFC susceptibilities to the change in anisotropy and dipole-dipole interaction, as well as the 3d-shell occupation of the metallic nanoparticles; in particular we predict larger coercive force for Fe than for Ni nanoparticles.

关键词： Dipolar interaction Magnetic anisotropy Magnetism Disordered systems Magnetic nanoparticles Nanotubes Heisenberg model metropolis algorithm Monte Carlo methods

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Monte Carlo calculations of the finite density Thirring model

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Physical Review D 2017年第1期95卷 014502-014502页

作者： Andrei Alexandru Gökçe Başar Paulo F. Bedaque Gregory W. Ridgway Neill C. Warrington Department of Physics The George Washington University Washington D.C. 20052 USA Department of Physics University of Maryland College Park Maryland 20742 USA

We present results of the numerical simulation of the two-dimensional Thirring model at finite density and temperature. The severe sign problem is dealt with by deforming the domain of integration into complex field space. This is the first example where a fermionic sign problem is solved in a quantum field theory by using the holomorphic gradient flow approach, a generalization of the Lefschetz thimble method.

关键词： Finite temperature field theory Lattice QCD Lower-dimensional field theories QCD phase transitions Lattice field theory metropolis algorithm Monte Carlo methods

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Electrically induced structure transition in nematic liquid crystal droplets with conical boundary conditions

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Physical Review E 2017年第5期96卷 052701-052701页

作者： V. Yu. Rudyak M. N. Krakhalev V. S. Sutormin O. O. Prishchepa V. Ya. Zyryanov J.-H. Liu A. V. Emelyanenko A. R. Khokhlov Lomonosov Moscow State University Faculty of Physics Moscow 119991 Russia Kirensky Institute of Physics Federal Research Center - Krasnoyarsk Scientific Center Siberian Branch Russian Academy of Sciences Krasnoyarsk 660036 Russia Institute of Engineering Physics and Radio Electronics Siberian Federal University Krasnoyarsk 660041 Russia Department of Chemical Engineering National Cheng Kung University Tainan Taiwan

Polymer-dispersed liquid crystal composites have been a focus of study for a long time for their unique electro-optical properties and manufacturing by “bottom-up” techniques at large scales. In this paper, nematic liquid crystal oblate droplets with conical boundary conditions (CBCs) under the action of electric field were studied by computer simulations and polarized optical microscopy. Droplets with CBCs were shown to prefer an axial-bipolar structure, which combines a pair of boojums and circular disclinations on a surface. In contrast to droplets with degenerate planar boundary conditions (PBCs), hybridization of the two structure types in droplets with CBCs leads to a two-minima energy profile, resulting in an abrupt structure transition and bistable behavior of the system. The nature of the low-energy barrier in droplets with CBCs makes it highly sensitive to external stimuli, such as electric or magnetic fields, temperature, and light. In particular, the value of the electric field of the structure reorientation in droplets with CBCs was found to be a few times smaller than the one for droplets with PBCs, and the droplet state remained stable after switching off the voltage.

关键词： Boojums Disclinations & dislocations First order phase transitions Line defects Point defects Second order phase transitions Structural order parameter Topological defects Liquid crystals Nematic liquid crystals metropolis algorithm Polarized optical microscopy Simulated annealing

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Deep learning beyond Lefschetz thimbles

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Physical Review D 2017年第9期96卷 094505-094505页

作者： Andrei Alexandru Paulo F. Bedaque Henry Lamm Scott Lawrence Department of Physics The George Washington University Washington D.C. 20052 USA Department of Physics University of Maryland College Park Maryland 20742 USA Albert Einstein Center for Fundamental Physics Institute for Theoretical Physics University of Bern Sidlerstrasse 5 CH-3012 Bern Switzerland

The generalized thimble method to treat field theories with sign problems requires repeatedly solving the computationally expensive holomorphic flow equations. We present a machine learning technique to bypass this problem. The central idea is to obtain a few field configurations via the flow equations to train a feed-forward neural network. The trained network defines a new manifold of integration which reduces the sign problem and can be rapidly sampled. We present results for the 1+1 dimensional Thirring model with Wilson fermions on sizable lattices. In addition to the gain in speed, the parametrization of the integration manifold we use avoids the “trapping” of Monte Carlo chains which plagues large-flow calculations, a considerable shortcoming of the previous attempts.

关键词： Finite temperature field theory Lattice gases Lattice models in statistical physics Lattice field theory Lattice gauge theory Many-body techniques metropolis algorithm Monte Carlo methods

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Cohesive stress heterogeneities and the transition from intrinsic ductility to brittleness

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Physical Review B 2017年第17期96卷 174115-174115页

作者： D. Tanguy Univ Lyon Université Claude Bernard Lyon 1 CNRS Institut Lumière Matière F-69622 Villeurbanne France

The influence of nanoscale cavities on the fracture of the Σ33{554}[110] symmetrical tilt grain boundary is studied by atomistic simulations. The crack crystallography is chosen such that dislocation emission is easy. A transition from a ductile behavior of the tip to a brittle one is obtained for a dense (coverage beyond 15% and intercavity spacing smaller than 4 nm) distribution of small cavities (sizes in-between 1 and 2 nm). The results are in good agreement with recent experiments from the literature. Even at the highest coverage, the character of the crack is highly sensitive to the initial position of the tip and a mixture of ductile and brittle responses is found. This complexity is beyond the usual criterion based on the drop of the work of separation with the amount of damage in the structure. It is shown that a heterogeneous cohesive zone model, with parameters extracted from the simulations and enriched with a criterion for plasticity, can explain the simulations and reproduce the transition. Additional simulations show that outside this range of small sizes and dense packing, which gives essentially a two-dimensional response (either crack opening or infinite straight dislocation emission), dislocation half-loops appear for intercavity spacing starting at about 4 nm. They constitute, together with regions of low coverage/small cavities, efficient obstacles to brittle cracking. These results could be guidelines to designing interfaces more resistant to solute embrittlement, in general. The cohesive zone model is generic. Furthermore, the {554} single crystal was used to determine to which extent the results depend on the details of the core structure versus the cavity distribution. These elements show that the conclusions reached have a generic character.

关键词： Embrittlement Fracture Grain boundaries Plasticity Point defects metropolis algorithm Molecular dynamics

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Phase diagram of boron carbide with variable carbon composition

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Physical Review B 2017年第5期95卷 054101-054101页

作者： Sanxi Yao Qin Gao Michael Widom Department of Physics Carnegie Mellon University Pittsburgh Pennsylvania 15213 USA

Boron carbide exhibits intrinsic substitutional disorder over a broad composition range. The structure consists of 12-atom icosahedra placed at the vertices of a rhombohedral lattice, together with a 3-atom chain along the threefold axis. In the high-carbon limit, one or two carbon atoms can replace boron atoms on the icosahedra while the chains are primarily of type C-B-C. We fit an interatomic pair interaction model to density-functional-theory total energies to investigate the substitutional carbon disorder. Monte Carlo simulations with sampling improved by replica exchange and augmented by two-dimensional multiple histogram analysis predict three phases. The low-temperature, high-carbon-composition monoclinic Cm structure disorders through a pair of phase transitions, first via an Ising-like transition to a monoclinic centrosymmetric state with space group C2/m, then via a first-order three-state Potts-like transition to the experimentally observed rhombohedral R3¯m symmetry.

关键词： First-principles calculations Phase diagrams Phase transitions by order metropolis algorithm

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Long-range interactions in antiferromagnetic quantum spin chains

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Physical Review B 2017年第5期96卷 054441-054441页

作者： B. Bravo D. C. Cabra F. A. Gómez Albarracín G. L. Rossini IFLP-CONICET and Departamento de Física Universidad Nacional de La Plata Casilla de Correos 67 1900 La Plata Argentina

We study the role of long-range dipolar interactions on antiferromagnetic spin chains, from the classical S→∞ limit to the deep quantum case S=1/2, including a transverse magnetic field. To this end, we combine different techniques such as classical energy minima, classical Monte Carlo, linear spin waves, bosonization, and density matrix renormalization group (DMRG). We find a phase transition from the already reported dipolar ferromagnetic region to an antiferromagnetic region for high enough antiferromagnetic exchange. Thermal and quantum fluctuations destabilize the classical order before reaching magnetic saturation in both phases, and also close to zero field in the antiferromagnetic phase. In the extreme quantum limit S=1/2, extensive DMRG computations show that the main phases remain present with transition lines to saturation significatively shifted to lower fields, in agreement with the bosonization analysis. The overall picture maintains a close analogy with the phase diagram of the anisotropic XXZ spin chain in a transverse field.

关键词： Antiferromagnetism Dipolar interaction Magnetism 1-dimensional spin chains Bosonization Density matrix renormalization group metropolis algorithm

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Giant magnetoelectric effect in pure manganite-manganite heterostructures

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Physical Review B 2017年第19期96卷 195130-195130页

作者： Sanjukta Paul Ravindra Pankaj Sudhakar Yarlagadda Pinaki Majumdar Peter B. Littlewood CMP Division Saha Institute of Nuclear Physics HBNI Kolkata 700064 India Harish-Chandra Research Institute HBNI Allahabad 211019 India Argonne National Laboratory Argonne Illinois 60439 USA James Franck Institute University of Chicago Chicago Illinois 60637 USA

Obtaining strong magnetoelectric couplings in bulk materials and heterostructures is an ongoing challenge. We demonstrate that manganite heterostructures of the form (Insulator) /(LaMnO3)n/Interface/(CaMnO3)n/(Insulator) show strong multiferroicity in magnetic manganites where ferroelectric polarization is realized by charges leaking from LaMnO3 to CaMnO3 due to repulsion. Here, an effective nearest-neighbor electron-electron (electron-hole) repulsion (attraction) is generated by cooperative electron-phonon interaction. Double exchange, when a particle virtually hops to its unoccupied neighboring site and back, produces magnetic polarons that polarize antiferromagnetic regions. Thus a striking giant magnetoelectric effect ensues when an external electrical field enhances the electron leakage across the interface.

关键词： Antiferromagnetism Charge density waves Charge order Electric polarization Exchange interaction Ferroelectricity Ferromagnetism Insulators Jahn-Teller effect Magnetic domains Magnetic order Magnetoelectric effect Optical phonons Polarons Functional materials Heterostructures Manganites Mott insulators Multiferroics Perovskites metropolis algorithm Simulated annealing

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Critical behavior of the two-dimensional Coulomb glass at zero temperature

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Physical Review B 2017年第18期95卷 184203-184203页

作者： Preeti Bhandari Vikas Malik Syed Rashid Ahmad Department of Physics Jamia Millia Islamia New Delhi 110025 India Department of Physics and Material Science Jaypee Institute of Information Technology Noida 201309 UP India

The lattice model of the Coulomb glass in two dimensions with box-type random field distribution is studied at zero temperature for system size up to 962. To obtain the minimum energy state we annealed the system using Monte Carlo simulation followed by further minimization using cluster flipping. The values of the critical exponents are determined using the standard finite size scaling. We found that the correlation length ξ diverges with an exponent ν=1.0 at the critical disorder Wc=0.2253 and that χdis≈ξ4−η¯ with η¯=2 for the disconnected susceptibility. The staggered magnetization behaves discontinuously around the transition and the critical exponent of magnetization β=0. The probability distribution of the staggered magnetization shows a three peak structure which is a characteristic feature for the phase coexistence at first-order phase transition. In addition to this, at the critical disorder we have also studied the properties of the domain for different system sizes. In contradiction with the Imry-Ma arguments, we found pinned and noncompact domains where most of the random field energy was contained in the domain wall. Our results are also inconsistent with Binder's roughening picture.

关键词： Disordered systems Doped semiconductors Finite-size scaling metropolis algorithm Monte Carlo methods Simulated annealing

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