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Antiphase domain growth: Correlation functions and structure factors in the scaling regime

Physical Review B 2021年第9期104卷 094101-094101页

作者： Ulrike Zweck Michael Leitner Technische Universität München Physik-Department and Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) Garching 85748 Germany

We study the coarsening of three-dimensional antiphase domain structures via Monte Carlo simulations. Linear lattice dimensions of N=1024 enable us to reach a scaling regime covering about 2.5 orders of magnitude of linear scale. With short-range interactions on cubic lattices at temperatures of 0.75Tc, the resulting antiphase domain structures are isotropic, which allows us to describe the real-space correlation functions by a common function scaled by a time-dependent parameter. We compare abstract Potts models and realistic models of atomic order in compounds and show that those with same ground-state degeneracy q lead to equivalent antiphase domain structures, while the scaling functions for different q show slight but significant deviations. Finally, we quantitatively discuss notions of real-space scale (specific interface area) and reciprocal-space scale (superstructure peak width) and thus give numerically exact values for the corresponding parameter K of the Scherrer equation.

关键词： Domains Growth processes Nonequilibrium statistical mechanics Solid-solid transformations Structural order parameter Ordered compounds metropolis algorithm

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Topological phase transitions in two-dimensional bent-core liquid crystal models

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Physical Review E 2021年第6期104卷 064701-064701页

作者： B. Kamala Latha Surajit Dhara V. S. S. Sastry School of Physics University of Hyderabad Hyderabad 500046 India Centre for Modelling Simulation and Design University of Hyderabad Hyderabad 500046 India

Two-dimensional liquid crystal (LC) models of interacting V-shaped bent-core molecules with two rigid rodlike identical segments connected at a fixed angle (θ=112∘) are investigated. The model assigns equal biquadratic tensor coupling among constituents of the interacting neighboring molecules on a square lattice, allowing for reorientations in three dimensions (d=2, n=3). We find evidence of two temperature-driven topological transitions mediated by point disclinations associated with the three ordering directors, condensing the LC medium successively into uniaxial and biaxial phases. With Monte Carlo simulations, temperature dependencies of the system energy, specific heat, orientational order parameters, topological order parameters, and densities of unbound topological defects of the different ordering directors are computed. The high-temperature transition results in topological ordering of disclinations of the primary director, imparting uniaxial symmetry to the phase. The low-temperature transition precipitates simultaneous topological ordering of defects of the remaining directors, resulting in biaxial symmetry. The correlation functions, quantifying spatial variations of the orientational correlations of the molecular axes show exponential decays in the high-temperature (disordered) phase, and power-law decays in the low-temperature (biaxial) phase. Differing temperature dependencies of the topological parameters point to a significant degree of cross coupling among the uniaxial and biaxial tensors of interacting molecules. This simplified Hamiltonian leaves θ as the only free model parameter, and the system traces a θ-dependent trajectory in a plane of the phenomenological parameter space.

关键词： Critical phenomena Order parameters Phase diagrams Phase transitions Topological phases of matter Biaxial liquid crystals Entropic sampling methods metropolis algorithm

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Influence of finite-size and edge effects on the exchange-bias properties of ferromagnetic/antiferromagnetic nanodots: Granular Monte Carlo investigation

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Physical Review B 2019年第5期99卷 054410-054410页

作者： Haydar Kanso Renaud Patte Vincent Baltz Denis Ledue Normandie Université UNIROUEN INSA Rouen CNRS GPM F-76800 Saint Étienne du Rouvray France Université Grenoble Alpes CNRS CEA Grenoble INP INAC-Spintec F-38000 Grenoble France

In this paper, we investigate exchange-biased square nanodots whose lateral sizes range between 130 and 500 nm, in comparison with continuous films by kinetic Monte Carlo simulations. We use a granular model which takes into account disordered interfacial phases by considering less stable magnetic grains at the interface in the antiferromagnetic (AF) layer. We further model the effect of the nanofabrication process by considering grains with reduced surfaces at the edges, due to grain cutting. Since less stable grains at the nanodot edges in the AF layer have been experimentally evidenced, we assumed a weaker anisotropy for the grains which are in the AF layer at the dot edges. Our results evidence two different mechanisms of the ferromagnetic (F) layer reversal depending on the magnitude of the coupling between F grains. In the weak coupling regime relative to the anisotropy, the exchange field is independent of the coupling and no variability from one nanodot to another is observed. By contrast, in the strong coupling regime, the exchange field depends on the coupling and it shows a high variability from one nanodot to another. Our model also well explain some experimental features observed in NiFe/IrMn nanodots (for various lateral sizes) and continuous films, at various measurement temperatures and various AF thicknesses. Finally, our model explains a long lasting issue about why the exchange field in nanodots can be either smaller or larger than in continuous films.

关键词： Exchange bias Magnetic anisotropy Multilayer thin films metropolis algorithm

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Density Depletion and Enhanced Fluctuations in Water near Hydrophobic Solutes: Identifying the Underlying Physics

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Physical Review Letters 2022年第4期128卷 045501-045501页

作者： Mary K. Coe Robert Evans Nigel B. Wilding H. H. Wills Physics Lab University of Bristol Tyndall Avenue Bristol BS8 1TL United Kingdom

We investigate the origin of the density depletion and enhanced density fluctuations that occur in water in the vicinity of an extended hydrophobic solute. We argue that both phenomena are remnants of the critical drying surface phase transition that occurs at liquid-vapor coexistence in the macroscopic planar limit, i.e., as the solute radius Rs→∞. Focusing on the density profile ρ(r) and a sensitive spatial measure of fluctuations, the local compressibility profile χ(r), we develop a scaling theory which expresses the extent of the density depletion and enhancement in compressibility in terms of Rs, the strength of solute-water attraction ϵs, and the deviation from liquid-vapor coexistence δμ. Testing the predictions against results of classical density functional theory for a simple solvent and grand canonical Monte Carlo simulations of a popular water model, we find that the theory provides a firm physical basis for understanding how water behaves at a hydrophobe.

关键词： Hydrophobic interactions Classical fluids Water Density functional calculations metropolis algorithm Monte Carlo methods

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In-plane magnetic field-induced skyrmion crystal in frustrated magnets with easy-plane anisotropy

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Physical Review B 2021年第22期103卷 224418-224418页

作者： Satoru Hayami Department of Applied Physics The University of Tokyo Tokyo 113-8656 Japan

We theoretically investigate the instability toward a skyrmion crystal (SkX) in frustrated triangular magnets. By performing simulated annealing and Monte Carlo simulations, we show that a frustrated spin model with easy-plane single-ion anisotropy exhibits a SkX in an in-plane external magnetic field. The emergent SkX is a counterpart of that induced by easy-axis anisotropy in an out-of-plane magnetic field, both of which show the topological Hall effect. We also find that a variety of multiple-Q states distinct from the SkX are stabilized in the presence of easy-plane anisotropy. Our results provide a possibility to realize a SkX in easy-plane frustrated magnets by applying an in-plane magnetic field to in-plane cycloidal spiral magnets.

关键词： Frustrated magnetism Skyrmions Antiferromagnets Noncollinear magnets Heisenberg model metropolis algorithm Monte Carlo methods Simulated annealing

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Optimal Entanglement Witnesses: A Scalable Data-Driven Approach

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Physical Review Letters 2021年第4期127卷 040401-040401页

作者： Irénée Frérot Tommaso Roscilde ICFO-Institut de Ciencies Fotoniques The Barcelona Institute of Science and Technology Avenida Carl Friedrich Gauss 3 08860 Barcelona Spain Max-Planck-Institut für Quantenoptik D-85748 Garching Germany Laboratoire de Physique CNRS UMR 5672 Ecole Normale Supérieure de Lyon Université de Lyon 46 Allée d’Italie Lyon F-69364 France

Multipartite entanglement is a key resource allowing quantum devices to outperform their classical counterparts, and entanglement certification is fundamental to assess any quantum advantage. The only scalable certification scheme relies on entanglement witnessing, typically effective only for special entangled states. Here, we focus on finite sets of measurements on quantum states (hereafter called quantum data), and we propose an approach which, given a particular spatial partitioning of the system of interest, can effectively ascertain whether or not the dataset is compatible with a separable state. When compatibility is disproven, the approach produces the optimal entanglement witness for the quantum data at hand. Our approach is based on mapping separable states onto equilibrium classical field theories on a lattice and on mapping the compatibility problem onto an inverse statistical problem, whose solution is reached in polynomial time whenever the classical field theory does not describe a glassy system. Our results pave the way for systematic entanglement certification in quantum devices, optimized with respect to the accessible observables.

关键词： Entanglement detection Entanglement measures Machine learning Quantum entanglement Resource theories Equilibrium lattice models Quantum spin models metropolis algorithm Monte Carlo methods Statistical methods

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Exact equilibrium properties of square-well and square-shoulder disks in single-file confinement

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Physical Review E 2024年第2期110卷 024601-024601页

作者： Ana M. Montero Andrés Santos Departamento de Física Universidad de Extremadura E-06006 Badajoz Spain Instituto de Computación Científica Avanzada (ICCAEx) Universidad de Extremadura E-06006 Badajoz Spain

This study investigates the (longitudinal) thermodynamic and structural characteristics of single-file confined square-well and square-shoulder disks by employing a mapping technique that transforms the original system into a one-dimensional polydisperse mixture of nonadditive rods. Leveraging standard statistical-mechanical techniques, exact results are derived for key properties, including the equation of state, internal energy, radial distribution function, and structure factor. The asymptotic behavior of the radial distribution function is explored, revealing structural changes in the spatial correlations. Additionally, exact analytical expressions for the second virial coefficient are presented. The theoretical results for the thermodynamic and structural properties are validated by our Monte Carlo simulations.

关键词： Confinement Hard sphere colloids metropolis algorithm

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Finite-temperature equilibrium density profiles of integrable systems in confining potentials

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Physical Review E 2023年第4期107卷 044101-044101页

作者： Jitendra Kethepalli Debarshee Bagchi Abhishek Dhar Manas Kulkarni Anupam Kundu International Centre for Theoretical Sciences Tata Institute of Fundamental Research Bengaluru–560089 India

We study the equilibrium density profile of particles in two one-dimensional classical integrable models, namely hard rods and the hyperbolic Calogero model, placed in confining potentials. For both of these models the interparticle repulsion is strong enough to prevent particle trajectories from intersecting. We use field theoretic techniques to compute the density profile and their scaling with system size and temperature, and we compare them with results from Monte Carlo simulations. In both cases we find good agreement between the field theory and simulations. We also consider the case of the Toda model in which interparticle repulsion is weak and particle trajectories can cross. In this case, we find that a field theoretic description is ill-suited and instead, in certain parameter regimes, we present an approximate Hessian theory to understand the density profile. Our work provides an analytical approach toward understanding the equilibrium properties for interacting integrable systems in confining traps.

关键词： Classical statistical mechanics Classical fluids Finite-size scaling Integrable systems metropolis algorithm Series expansions & exact enumeration

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Field-induced freezing in the unfrustrated Ising antiferromagnet

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Physical Review E 2020年第3期102卷 032112-032112页

作者： Adam Iaizzi Department of Physics National Taiwan University No. 1 Section 4 Roosevelt Road Taipei 10607 Taiwan

We study instantaneous quenches from infinite temperature to well below Tc in the two-dimensional square lattice Ising antiferromagnet in the presence of a longitudinal external magnetic field. Under single-spin-flip metropolis algorithm Monte Carlo dynamics, this protocol produces a pair of magnetization plateaus that prevent the system from reaching the equilibrium ground state except for some special values of the field. We explain the plateaus in terms of local spin configurations that are stable under the dynamics.

关键词： Nonequilibrium statistical mechanics Percolation Spin dynamics Ising model metropolis algorithm

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Universal scaling for the permeability of random packs of overlapping and nonoverlapping particles

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Physical Review E 2022年第4期105卷 L043301-L043301页

作者： Jérémie Vasseur Fabian B. Wadsworth Eloïse Bretagne Donald B. Dingwell Earth and Environmental Science Ludwig-Maximilians-Universität Theresienstrasse 41 80333 Munich Germany Department of Earth Sciences Science Laboratories Durham University Durham DL1 3LE United Kingdom

Constraining fluid permeability in porous media is central to a wide range of theoretical, industrial, and natural processes. In this Letter, we validate a scaling for fluid permeability in random and lattice packs of spheres and show that the permeability of packs of both hard and overlapping spheres of any sphere size or size distribution collapse to a universal curve across all porosity ϕ in the range of ϕc<ϕ<1, where ϕc is the percolation threshold. We use this universality to demonstrate that permeability can be predicted using percolation theory at ϕc<ϕ≲0.30, Kozeny-Carman models at 0.30≲ϕ≲0.40, and dilute expansions of Stokes theory for lattice models at ϕ≳0.40. This result leads us to conclude that the inverse specific surface area, rather than an effective sphere size or pore size is a universal controlling length scale for hydraulic properties of packs of spheres. Finally, we extend this result to predict the permeability for some packs of concave nonspherical particles.

关键词： Continuous percolation transition Critical exponents Flows in porous media Geophysical fluid dynamics Granular materials Granular packing Jamming Percolation Percolation phase transition Continuum particle models Packing & jamming problems Porous materials Porous media Lattice-Boltzmann methods metropolis algorithm Molecular dynamics Monte Carlo methods

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