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Thermally induced localization of dopants in a magnetic spin ladder

Physical Review Research 2024年第2期6卷 023325-023325页

作者： K. Knakkergaard Nielsen Hans-Kopfermann-Strasse 1 D-85748 Garching Germany

I unveil an emergent localization phenomenon pertaining to the motion of a dopant in a thermal spin lattice. This is rendered localized by thermal spin fluctuations, whereby it is in stark constrast to the intrinsic origin of Anderson localization for quenched disorder. The system of interest consists of spin-1/2 particles organized in a two-leg ladder with nearest-neighbor Ising interactions J. The motion of a hole—the dopant—is initialized by suddenly removing a spin from the thermal spin ensemble, which then moves along the ladder via nearest-neighbor hopping t. I find that the hole remains localized for all values of J/t and for all nonzero temperatures. The origin is an effective disorder potential seen by the hole and induced by thermal spin fluctuations. Its length scale is found to match with the underlying spin-spin correlation length at low temperatures. For ferromagnetic couplings (J<0), the associated localization length of the hole increases with decreasing temperature and becomes proportional to the correlation length at low temperatures, asymptotically delocalizing at low temperatures. For antiferromagnetic couplings (J>0), there is a smooth crossover between thermal localization at high temperatures to localization driven by the antiferromagnetic order at low temperatures. At infinite temperatures, the dynamics becomes independent of the sign of the spin coupling, whereby the localization length is a universal function of |J|/t, diverging as (t/J)2 for |J|≪t. Finally, I analyze a setup with Rydberg-dressed atoms, which naturally realizes finite-range Ising interactions, accessible in current experimental setups. I show that the discovered localization phenomenon can be probed on experimentally accessible length- and timescales, providing a strong testing ground for my predictions.

关键词： Anderson localization Cold gases in optical lattices Dopants Metal-insulator transition Rydberg gases Transport phenomena High-temperature superconductors Exact solutions for many-body systems Holstein-Primakoff method Ising model metropolis algorithm Spin ladders Spin lattice models Transfer matrix method t-J model

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Sampling algorithms for Discrete Markov Random Fields and Related Graphical Models

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JOURNAL OF THE AMERICAN STATISTICAL ASSOCIATION 2021年第536期116卷 2065-2086页

作者： Izenman, Alan Julian Temple Univ Dept Stat Sci 1810 Liacouras Walk Philadelphia PA 19122 USA Temple Univ Data Sci Inst 1810 Liacouras Walk Philadelphia PA 19122 USA

Discrete Markov random fields are undirected graphical models in which the nodes of a graph are discrete random variables with values usually represented by colors. Typically, graphs are taken to be square lattices, although more general graphs are also of interest. Such discrete MRFs have been studied in many disciplines. We describe the two most popular types of discrete MRFs, namely the two-state Ising model and the q-state Potts model, and variations such as the cellular automaton, the cellular Potts model, and the random cluster model, the latter of which is a continuous generalization of both the Ising and Potts models. Research interest is usually focused on providing algorithms for simulating from these models because the partition function is so computationally intractable that statistical inference for the parameters of the appropriate probability distribution becomes very complicated. Substantial improvements to the metropolis algorithm have appeared in the form of cluster algorithms, such as the Swendsen-Wang and Wolff algorithms. We study the simulation processes of these algorithms, which update the color of a cluster of nodes at each iteration.

关键词： Cellular Potts model Ising model metropolis algorithm Potts model Swendsen– Wang algorithm Wolff algorithm

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High-accuracy variational Monte Carlo for frustrated magnets with deep neural networks

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Physical Review B 2023年第5期108卷 054410-054410页

作者： Christopher Roth Attila Szabó Allan H. MacDonald Physics Department University of Texas at Austin Austin 78712-1710 Texas Rudolf Peierls Centre for Theoretical Physics University of Oxford Oxford OX1 3PU United Kingdom ISIS Facility Rutherford Appleton Laboratory Harwell Campus Didcot OX11 0QX United Kingdom

We show that neural quantum states based on very deep (4–16-layered) neural networks can outperform state-of-the-art variational approaches on highly frustrated quantum magnets, including quantum-spin-liquid candidates. We focus on group convolutional neural networks that allow us to efficiently impose space-group symmetries on our ansätze. We achieve state-of-the-art ground-state energies for the J1−J2 Heisenberg models on the square and triangular lattices, in both ordered and spin-liquid phases, and discuss ways to access low-lying excited states in nontrivial symmetry sectors. We also compute spin and dimer correlation functions for the quantum paramagnetic phase on the triangular lattice, which do not indicate either conventional or valence-bond ordering.

关键词： Magnetic phase transitions Spin dynamics Topological materials Artificial neural networks Discrete symmetries in condensed matter metropolis algorithm Quantum Monte Carlo

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Homophily-Based Social Group Formation in a Spin Glass Self-Assembly Framework

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Physical Review Letters 2023年第5期130卷 057401-057401页

作者： Jan Korbel Simon D. Lindner Tuan Minh Pham Rudolf Hanel Stefan Thurner Section for the Science of Complex Systems CeMSIIS Medical University of Vienna Spitalgasse 23 A-1090 Vienna Austria Complexity Science Hub Vienna Josefstädterstrasse 39 A-1080 Vienna Austria Niels Bohr Institute Blegdamsvej 17 2100 Copenhagen Denmark Santa Fe Institute 1399 Hyde Park Road Santa Fe New Mexico 87501 USA

Homophily, the tendency of humans to attract each other when sharing similar features, traits, or opinions, has been identified as one of the main driving forces behind the formation of structured societies. Here we ask to what extent homophily can explain the formation of social groups, particularly their size distribution. We propose a spin-glass-inspired framework of self-assembly, where opinions are represented as multidimensional spins that dynamically self-assemble into groups; individuals within a group tend to share similar opinions (intragroup homophily), and opinions between individuals belonging to different groups tend to be different (intergroup heterophily). We compute the associated nontrivial phase diagram by solving a self-consistency equation for “magnetization” (combined average opinion). Below a critical temperature, there exist two stable phases: one ordered with nonzero magnetization and large clusters, the other disordered with zero magnetization and no clusters. The system exhibits a first-order transition to the disordered phase. We analytically derive the group-size distribution that successfully matches empirical group-size distributions from online communities.

关键词： Classical statistical mechanics Network formation & growth Network phase transitions Phase diagrams Phase transitions Self-assembly Social systems Networks & random structures Social dynamics Social networks Spin glasses Data analysis Ising model Markovian processes Mean field theory metropolis algorithm Simulated annealing

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Liquid-hexatic transition for soft disks

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Physical Review E 2023年第2期108卷 024103-024103页

作者： Yoshihiko Nishikawa Werner Krauth A. C. Maggs Graduate School of Information Sciences Tohoku University Sendai 980-8579 Japan Laboratoire de Physique de l'Ecole normale supérieure ENS Université PSL CNRS Sorbonne Université Université de Paris Cité 75005 Paris France CNRS UMR7083 ESPCI Paris Université PSL 10 rue Vauquelin 75005 Paris France

We study the liquid-hexatic transition of soft disks with massively parallel simulations and determine the equation of state as a function of system size. For systems with interactions decaying as the inverse mth power of the separation, the liquid-hexatic phase transition is continuous for m=12 and m=8, while it is of first order for m=24. The critical power m for the transition between continuous and first-order behavior is larger than previously reported. The continuous transition for m=12 implies that the two-dimensional Lennard-Jones model has a continuous liquid-hexatic transition at high temperatures. We also study the Weeks-Chandler-Andersen model and find a continuous transition at high temperatures that is consistent with the soft-disk case for m=12. Pressure data as well as our implementation are available from an open-source repository.

关键词： BKT transition Equations of state Phase transitions 2-dimensional systems metropolis algorithm

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Magnetic phase recognition of artificial kagome spin ice through initial magnetization curve

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Physical Review B 2023年第1期108卷 014404-014404页

作者： Breno Malvezzi Cecchi Nathan Cruz Marcelo Knobel Kleber Roberto Pirota Gleb Wataghin Institute of Physics State University of Campinas 13083-859 Campinas São Paulo Brazil

Artificial spin ices (ASIs) are designable arrays of interacting nanomagnets that span a wide range of magnetic phases. Here, we demonstrate that the phase of an artificial kagome spin ice can be identified through its initial magnetization curve. As a proof of concept, micromagnetic simulations of these curves were performed starting from representative microstates of different phases of the system. We show that the curves are characterized by phase-specific features in such a way that a supervised classification algorithm predicts the phase of the initial microstate with good reliability. Moreover, most curves associated with paramagnetic and spin ice 1 phases are recognizable simply by visual inspection. This achievement represents a different strategy for identifying phases in ASIs that is easier and more accessible than magnetic imaging techniques normally used for this task.

关键词： Dipolar interaction Magnetic phase transitions Kagome lattice Data analysis Machine learning metropolis algorithm Micromagnetic modeling

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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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Atomic-Scale Surface Segregation in Copper-Gold Nanoparticles

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Physical Review Letters 2023年第23期130卷 236201-236201页

作者： Grégoire Breyton Hakim Amara Jaysen Nelayah Jérôme Creuze Hazar Guesmi Damien Alloyeau Guillaume Wang Christian Ricolleau Université Paris Cité CNRS Laboratoire Matériaux et Phénomènes Quantiques (MPQ) 75013 Paris France Laboratoire d’Etude des Microstructures ONERA-CNRS UMR104 Université Paris-Saclay BP 72 Châtillon Cedex 92322 France ICMMO/ESP2M Université Paris-Saclay UMR 8182 17 avenue des sciences 91405 Orsay cedex France ICGM ICMMM—Institut Charles Gerhardt Montpellier—Institut de Chimie Moléculaire et des Matériaux de Montpellier Montpellier France

We combine electron microscopy measurements of the surface compositions in Cu-Au nanoparticles and atomistic simulations to investigate the effect of gold segregation. While this mechanism has been extensively investigated within Cu-Au in the bulk state, it was never studied at the atomic level in nanoparticles. By using energy dispersive x-ray analysis across the (100) and (111) facets of nanoparticles, we provide evidence of gold segregation in Cu3Au and CuAu3 nanoparticles in the 10 nm size range grown by epitaxy on a salt surface with high control of the nanoparticles morphology. To get atomic-scale insights into the segregation properties in Cu-Au nanoparticles on the whole composition range, we perform Monte Carlo calculations employing N-body interatomic potentials highlighting a complete segregation of Au in the (100) and (111) facets for gold nominal composition above 70% and 60%, respectively. Furthermore, we show that there is no size effect on the segregation behavior since we evidence the same oscillating concentration profile from the surface to the nanoparticle’s core as in the bulk. These results shed new light on the interpretation of the enhanced reactivity, selectivity, and stability of Cu-Au nanoparticles in various catalytic reactions.

关键词： Surface reconstruction Alloys Face-centered cubic Nanoparticles Electron diffraction Energy-dispersive x-ray spectroscopy High-resolution electron microscopy metropolis algorithm Scanning transmission electron microscopy Tight-binding model

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MCMC conditional maximum likelihood for the two-way fixed-effects logit

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ECONOMETRIC REVIEWS 2024年第6期43卷 379-404页

作者： Bartolucci, Francesco Pigini, Claudia Valentini, Francesco Univ Perugia IT Perugia Italy Marche Polytech Univ & MoFiR IT Ancona Italy Marche Polytech Univ IT Ancona Italy

We propose a Markov Chain Monte Carlo Conditional Maximum Likelihood (MCMC-CML) estimator for the two-way fixed-effects logit model for dyadic data, typically used in network analyses. The proposed MCMC approach, based on a metropolis algorithm, allows us to overcome the computational issues of evaluating the probability of the outcome conditional on nodes in- and out-degrees, which are sufficient statistics for the incidental parameters. Under mild regularity conditions, the MCMC-CML estimator converges to the exact CML one and is asymptotically normal. Moreover, it is more efficient than the existing pairwise CML estimator. We study the finite sample properties of the proposed approach by means of an extensive simulation study and three empirical applications, where we also show that the MCMC-CML estimator can be applied to logit models for binary panel data with both subject and time-fixed effects. Results confirm the expected theoretical advantage of the proposed approach, especially with small, concentrated, and sparse networks or with rare events in panel data.

关键词： Directed network fixed effects link formation metropolis algorithm panel data C23 C25 C63

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Scale and conformal invariance in rotating interacting few-fermion systems

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Physical Review Research 2024年第2期6卷 023279-023279页

作者： Viktor Bekassy Johannes Hofmann Department of Microtechnology and Nanoscience (MC2) Department of Physics

We show that rotating two-dimensional Fermi gases possess a nonrelativistic scale and conformal invariance at weak but nonzero interactions, where the scale invariance of universal short-range interactions is not yet broken by quantum effects. We demonstrate the symmetry in the excitation spectrum of few-fermion ensembles in a harmonic trap obtained by exact diagonalization. The excitation spectrum is shown to split in a set of primary states and derived excited states that consist of breathing modes as well as two different center-of-mass excitations, which describe cyclotron and guiding-center excitations of the total particle cloud. Furthermore, the conformal symmetry is manifest in the many-body wave function, where it dictates the form of the hyperradial component, which we demonstrate using Monte Carlo sampling of few-body wave functions.

关键词： Few-body systems Mesoscopics Synthetic gauge fields 2-dimensional systems Fermi gases Conformal symmetry metropolis algorithm Perturbation theory

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