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13:00 | Large Deviations for Fermions at Equilibrium - An Approach to Macroscopic Behavior at Nanoscales ABSTRACT. In 2012, experimental measurements of electric resistance of nanowires in Si doped with phosphorus atoms demonstrate that quantum effects on charge transport almost disappear for nanowires of lengths larger than a few nanometers, even at very low temperature (4.2K). Such experiments suggest an exponentially fast convergence of microscopic quantities towards macroscopic ones in real fermion systems. In Mathematics, it corresponds to the existence of so-called large deviations for the corresponding quantities. We will discuss this mathematical issue for fermion systems on the lattice. |

13:30 | Excitations in complex materials: accuracy, precision, feasibility ABSTRACT. |

14:00 | Recycling results of models to simulate real materials: connector approach ABSTRACT. One important approach for the description of quantum many-body systems is to design functionals of relatively simple quantities, such as the density or a one-body Green’s function, instead of calculating expectation values with many-body wavefunctions. However, many such functionals still lack precision, and some observables cannot be accessed at all in a satisfactory way. One way of designing approximations is to use results of model systems. Here, we will show how model results can be used in an in principle exact way, called “Connector Theory”, in order to describe observables and systems of interest. Within this approach, a quantity of interest is calculated for a model system as function of a parameter once and forever, and the results are stored. Under certain conditions, the model result for an appropriate choice of parameter can then be used to replace a value of interest in a given real system. This choice of parameter is called “connector”. We will discuss the principles and general properties of such an approach, and show that it is a convenient starting point for approximations. [1] M. Vanzini, et al., arXiv:1903.07930v4 [2] A. Aouina, M. Gatti, and L. Reining, Faraday Discussions 224, 27 (2020) |

14:30 | Approximation of optimal transport problem by moment constraints and application to DFT PRESENTER: Rafael Coyaud ABSTRACT. The Strictly Correlated Electrons (SCE) limit of the Levy-Lieb functional in Density Functional Theory (DFT) gives rise to a symmetric multimarginal optimal transport problem with Coulomb cost, where the number of marginal laws is equal to the number of electrons in the system, which can be very large in relevant applications. We investigate a relaxation of optimal transport problems when marginal constraints are replaced by some moment constraints (MCOT problem), and show the convergence of the latter towards the former. Using Tchakaloff's theorem, we show that the MCOT problem is achieved by a finite discrete measure. For multimarginal optimal transport problems, the number of points weighted by this measure scales linearly with the number of marginal law. We leverage the sparsity of those minimizers in the design of a numerical method and prove that any local minimizer to the resulting problem is actually a global one. We illustrate the performance of the proposed method by numerical examples which solves MCOT relaxations of 3D systems with up to 100 electrons. |

13:00 | Towards a Predictive Theory of Grain Growth: Experiments, Simulations and Data Analytics PRESENTER: Katayun Barmak ABSTRACT. A grand challenge problem in engineering of polycrystals is to develop prescriptive process technologies capable of producing an arrangement of grains that provides for a desired set of materials properties. One method by which the grain structure is engineered is through grain growth or coarsening of a starting structure. Grain growth can be viewed as the evolution of a large metastable network, and can be mathematically modeled by a set of deterministic local evolution laws for the growth of an individual grain combined with stochastic models to describe the interaction between grains. Thus, to develop a predictive theory, investigation of a broad range of statistical measures of microstructure are needed and must be obtained using experiments, simulations, data analytics and mathematical modeling. We will present results of both in-situ and ex-situ experiments of grain growth in thin films, which are used as the experimental test bed. One important advantage of thin films is the ability to combine in-situ examination of the motion of boundaries with periodic freezing of the structure to map and quantify statistically significant populations, before reheating to promote continued grain growth. The experimental results will be compared and contrasted with results of sharp and diffuse interface simulations. |

13:30 | Grain Structure, Grain Growth and Evolution of the Grain Boundary Network PRESENTER: Yekaterina Epshteyn ABSTRACT. Cellular networks are ubiquitous in nature. Most technologically useful materials arise as polycrystalline microstructures, composed of a myriad of small monocrystalline cells or grains, separated by interfaces, or grain boundaries. Grain boundaries play an essential role in determining the properties of materials across a wide range of scales. During grain growth (also termed coarsening), an initially random grain boundary arrangement reaches a steady state that is strongly correlated to the interfacial energy density. In this talk, we will discuss recent progress on modeling, simulation and analysis of the evolution of the grain boundary network in polycrystalline materials. |

14:00 | Energetic Variational Approaches: Boundaries and Temperature ABSTRACT. The boundary effects and thermal effects play crucial roles in real life applications. These problem give formidable challenges to the mathematical analysis and numerical simulations. In this talk, I will employ the energetic variational approaches (EnVarA) in rigorously establishing the dynamical systems that are include all the basic laws and constraints of thermodynamics. I will demonstrate the difficulties and subtleties by looking at these effects in various general diffusion dynamics and gradient dynamics. |

14:30 | Exploding, weeping and reversible phase transformations in polycrystals ABSTRACT. The systematic tuning of the lattice parameters to achieve improved kinematic compatibility between phases is an effective strategy for improving the reversibility, and lowering the hysteresis, of solid-solid phase transformations. We present an apparently paradoxical example in which tuning to near perfect compatibility in oxides of ZrHfYNb leads to a high degree of irreversibility, as manifested by violently explosive behavior. Tuning lattice parameters slightly away from near perfect compatibility gives an intermediate “weeping” behavior, in which the polycrystal slowly and steadily falls apart at the grain boundaries. Finally, tuning to satisfy a previously unrecognized condition we term the “equidistance condition” results in reversible behavior with the lowest hysteresis in this system. We give evidence that these observations are explained by a more careful mathematical analysis of compatibility of the polycrystal. These results show an extreme diversity of behavior, from reversible to explosive, is possible in a chemically homogeneous system by manipulating conditions of compatibility. Joint work with Eckhard Quandt, Justin Jetter, Jascha Rohmer, Hanlin Gu |

13:00 | Symmetry breaking for local/nonlocal interaction functionals ABSTRACT. In this talk we review a series of results obtained in collaboration with E. Runa and A. Kerschbaum concerning one-dimensionality and periodicity of minimizers for a class of local/nonlocal interactions functionals in general dimension. The competition between a local attractive and a nonlocal repulsive term is at the base of the emergency of periodic patterns. In particular, while the functionals we consider are symmetric under permutation of coordinates, in a suitable regime minimizers are one-dimensional, thus incurring in the phenomenon of symmetry breaking. We prove such a characterization of minimizers for functionals which model pattern formation in colloidal systems or in generalized antiferromagnetic interactions, both in the sharp and in the diffuse interface setting. |

13:30 | Minimality of polygons in a nonlocal anisotropic isoperimetric problem PRESENTER: Ihsan Topaloglu ABSTRACT. In this talk I will discuss the minimization of an energy functional given by the sum of a crystalline perimeter and a nonlocal interaction of Riesz type, under volume constraint. I will show that, in the small mass regime, if the polygonal Wulff shape of the anisotropic perimeter has certain symmetry properties, then it is the unique global minimizer of the total energy. I will also present a rigidity result for the structure of (local) minimizers in two dimensions. This is a joint work with M. Bonacini and R. Cristoferi. |

14:00 | Discrete isoperimetric inequalities in periodic graphs and semidiscrete Optimal Transport PRESENTER: Mircea Petrache ABSTRACT. We consider the edge-isoperimetric problem in a periodic weighted graph in Euclidean space. Basic motivation is to understand shapes of crystals beyond lattice cases. The aim is to find cases where the precise isoperimetric inequality can be deduced. By this we mean that the inequality must be achieved by specific shapes, which we also aim to determine. To treat this problem we take inspiration from the PDE approach introduced in the continuum isoperimetric inequality by Cabre, based on early results of Trudinger, and from the Optimal Transport approach to the isoperimetric inequality described by Gromov and by Figalli-Maggi-Pratelli. These two approaches become strongly intertwined in our discrete setting, and our problem becomes related to a Semidiscrete Optimal Transport problem. The precise geometry of cells in the optimal transportation are determined based on a result related to Minkowski's theorem. Several interesting questions appear as next steps in the classification. This is work is based on a project in collaboration with Matias Gomez, who recently completed his master at Universidad Tecnica Federico Santa Maria. |

13:00 | Abnormal martensite microstructure in TiNi-based alloys not satisfying cofactor condition PRESENTER: Tomonari Inamura ABSTRACT. Microstructure and shape memory property of TiNi-based alloys not satisfying the cofactor condition were investigated to understand the relation among the incompatibility of martensite microstructure, dislocation structure and shape memory properties. Lattice parameters of the parent and martensite phases were controlled by additional elements. Some alloys exhibited abnormal martensite microstructure in which there is no remnant of parent-martensite interface (habit plane). Shift of martensite transformation temperatures, accumulation of dislocation and shape memory behavior upon cyclic thermal transformation are discussed based on the geometry of the abnormal martensite microstructure. |

13:30 | First-principles calculations for compositional prediction of phase-change materials ABSTRACT. First-principle density functional theory calculations are one of the cornerstones of theoretical condensed matter theory. These first-principles methods allow for the prediction of the physical properties of a wide variety of materials, even those unknown to experiment. Recent advances in first-principle calculations and high-throughput methods allow for the advancement of these calculations as tools for accurately predicting the physical properties of materials. Here, we will summarize recent advances in improving the accuracy of first-principles calculations and describe our high-throughput methods to find the optimal phase-change material. Finally, we will discuss open challenges and questions that arose during our pursuit of our goal and discuss some possible answers to these challenges. |

14:00 | Towards modelling of (20-1) plastic twinning in Ni-Ti martensite PRESENTER: Hanus Seiner ABSTRACT. (20-1) twins are often observed in B19‘ martensite in the Ni-Ti shape memory alloys deformed beyond the reversibility limit. The mechanism of formation of these twins is assumed to be a combination of twinning and shuffle, which was found to be significantly more energy demanding than all other (pseudo)plastic straining mechanisms in NiTi, in particular the transformation twinning and the [100](001) plastic slip. The (20-1) twins are typically found forming quasi-regular wedge-like patterns, clearly resembling kink patterns observed in systems with extremely anistropic plastic behavior. In this contribution, we will show that the (20-1) twinning can be obtained by combining the plastic kinking and the classical martensitic twinning. This mechanism does not require additional shuffle and might be, thus, energetically favorable. We explore the theoretical framework for describing the coupled kink-twin structure (suggesting the name kwink for it) at the continuum mechanics level, and suggest possible approaches for mathematical modelling of kwinks. [1] M. Nishida et al. Scripta Materialia 39 (1998) 1749–1754. [2] T. Ezaz et al. Materials Science & Engineering A 558 (2012) 422-430 |

13:00 | Variational analysis of a two-dimensional frustrated spin system: emergence and rigidity of chirality transitions PRESENTER: Gianluca Orlando ABSTRACT. We examine the discrete-to-continuum variational limit of the $J_1$-$J_3$ spin model on the square lattice in the vicinity of the helimagnet/ferromagnet transition point as the lattice spacing vanishes. Carrying out the Gamma-convergence analysis of proper scalings of the energy, we prove the emergence and characterize the geometric rigidity of the chirality phase transitions. |

13:30 | Stochastic homogenization of high-contrast media PRESENTER: Igor Velcic ABSTRACT. We are analyzing the influence of stochastic perturbations on the spectrum of the diffusion operator. The operator has certain defficiencies (there are holes in the domain of radius $\varepsilon$ where the coefficients are of order $\varepsilon^2$). This is in the literature known as high-contrast and the purely periodic case is analyzed by V.V. Zhikov. We are separately analyzing finite and infinite domains, which show different behavior with respect to stochastic homogenization. This is a joint work with M. Cherdantsev (University of Cardiff) and Kirill Cherednichenko (University of Bath). |

14:00 | Invertibility properties of limits of Sobolev homeomorphisms ABSTRACT. We study weak limits of Sobolev homeomorphisms. It turns out that these mappings are injective almost everywhere if Sobolev exponent p>n−1 both in the image and in the domain. For p≤n−1 we construct a strong limit of homeomorphisms such that the preimage of a point is a continuum for every point in a set of positive measure in the image and a topological image of a point is a continuum for every point in a set of positive measure in the domain. In the critical case p=n−1 injectivity almost everywhere for limits of Sobolev homeomorphisms follows under certain additional assumptions on the distortion of mappings. The first part of the research is joint research with O. Bouchala and S. Hencl (Charles University, Prague, Czech Republic) and the second part is joint research with S. Vodopyanov (Sobolev Institute of Mathematics, Novosibirsk, Russia). |

13:00 | Three-dimensional solitons in flexoelectric nematic liquid crystals ABSTRACT. This presentation deals with experimentally observed three dimensional solitons that develop in flexoelectric nematic liquid crystals, with negative dielectric and conduction anisotropies, when subject to an alternating electric field. The liquid crystal is confined in a thin region between two plates, perpendicular to the applied field. The horizontal, uniformly aligned director field is at equilibrium due to the negative anisotropy of the media. However, this state is unstable to perturbations that manifest themselves as confined, bullet-like, director distortions traveling up and down the sample at a speed of several hundred microns per second. We develop a variational model that couples the Ericksen-Leslie equation of director field distortion and the Poisson-Nearnst-Planck equations governing the diffusion and transport of electric charge, and the electrostatic potential. We perform a stability analysis of the equilibrium state to determine the threshold wave numbers and speed of the disturbance. The linear system obtained by taking the Fourier transform of the original one is analyzed by combining methods of the Floquet theory, together with time and space averaging tools as well as arguments based on asymptotic analysis. The predictions based on the size, phase-shift and speed of the soliton show good agreement with the experimentally measured ones. |

13:30 | Bifurcation analysis of a new Q-tensor model for Smectic-A liquid crystals PRESENTER: Patrick Farrell ABSTRACT. We propose a novel free energy functional for modelling Smectic-A liquid crystals. The functional is written in terms of a Q-tensor and a real-valued density variable. The model can reproduce important and partially-understood structures in smectics, such as focal conic domains and oily streaks. The model is nonconvex and supports many solutions; calculating the ground state is therefore a difficult numerical challenge. We discuss the computation of multiple solutions using a nonlinear deflation technique. |

14:00 | Debonding of a gel from a rigid substrate PRESENTER: Duvan Henao ABSTRACT. A variational model for the delamination of polymer gel thin films from rigid substrates is presented. A formal asymptotic analysis of a simplified 2D version of the underlying governing equations show that, as the film grows thinner, the absorption of the moisture of its surroundings tends to produce a homogeneous vertical stretch in the part of the film that remains bonded to the substrate and a state of free swelling in the debonded part. A transition layer, with a width comparable to the film thickness, is developed in order to connect the two swelling modes. This work is joint with Carme Calderer (U. Minnesota), Carlos Garavito-Garzon (U. Minnesota), Suping Lyu (Medtronic, Inc.), and Lorenzo Tapia (PUC - Chile). |

14:30 | Liquid crystals on deformable surfaces PRESENTER: Axel Voigt ABSTRACT. Liquid crystals with molecules constrained to the tangent bundle of a curved surface show interesting phenomena resulting from the tight coupling of the elastic and bulk-free energies of the liquid crystal with geometric properties of the surface. We derive a thermodynamically consistent Landau-de Gennes- Helfrich model which considers the simultaneous relaxation of the Q-tensor field and the surface. The resulting system of tensor-valued surface partial differential equation and geometric evolution laws is numerically solved to tackle the rich dynamics of this system and to compute the resulting equilibrium shape. The results strongly depend on the intrinsic and extrinsic curvature contributions and lead to unexpected asymmetric shapes. |

13:00 | A stochastic algorithm for Born-Oppenheimer Molecular dynamics ABSTRACT. In direct simulations using Born-Oppenheimer Molecular dynamics (BOMD), the underlying electron structures have to be computed at each time step, which involves the computation of the eigenvalues and eigenstates. In addition, the calculation of the forces on the nuclei also requires access to the density-matrix and the energy density-matrix. We present a reformulation of the BOMD with a stochastic relaxation, which is free of the diagonalization steps. In addition, we show the convergence of the relaxed model. |

13:30 | Stochastic Peierls-Nabarro Model for Dislocations in High Entropy Alloys PRESENTER: Tianpeng Jiang ABSTRACT. High entropy alloys (HEAs) are single phase crystals that consist of random solid solutions of multiple elements in approximately equal proportions. This class of novel materials have exhibited superb mechanical properties, such as high strength combined with other desired features. The strength of crystalline materials is associated with the motion of dislocations. In this talk, we present a stochastic continuum model based on the Peierls--Nabarro framework for interlayer dislocations in a bilayer HEA from an atomistic model that incorporates the atomic level randomness. We use asymptotic analysis and limit theorem in the convergence from the atomistic model to the continuum model. The total energy in the continuum model consists of a stochastic elastic energy in the two layers, and a stochastic misfit energy that accounts for the interlayer nonlinear interaction. The obtained continuum model can be considered as a stochastic generalization of the classical, deterministic Peierls--Nabarro model for the dislocation core and related properties. This derivation also validates the stochastic model adopted by Zhang et al. [Acta Mater., 166 (2019), pp. 424--434]. |

14:00 | Plane wave methods for quantum eigenvalue problems of incommensurate systems ABSTRACT. We propose a novel numerical algorithm for computing the electronic structure related eigenvalue problems of incommensurate systems. Unlike the conventional practice that approximates the system by a large commensurate supercell, our algorithm directly discretizes the eigenvalue problems under the framework of a plane wave method. The emerging ergodicity and the interpretation from higher dimensions give rise to many unique features compared to what we have been familiar with in the periodic systems. The numerical results of 1D and 2D quantum eigenvalue problems are presented to show the reliability and efficiency of our algorithm. Furthermore, the extension of our algorithm to full Kohn-Sham density functional theory calculations is discussed. |

17:30 | A Scalable Matrix-Free Eigensolver for Studying Many-Body Localization ABSTRACT. We present a scalable and matrix-free eigensolver for studying nearest-neighbor Heisenberg spin chain plus random on-site disorder models that undergo a many-body localization (MBL) transition. This type of problem is computationally challenging because its dimension grows exponentially with the physical system size, and the solve must be iterated many times to average over different configurations of the random disorder. For each eigenvalue problem, eigenvalues from different regions of the spectrum and their corresponding eigenvectors need to be computed. Traditionally, the interior eigenstates for a single eigenvalue problem are computed via the shift-and-invert Lanczos algorithm. Due to the extremely high memory footprint of the LU factorizations, this technique is not well suited for large number of spins, e.g., one needs thousands of compute nodes on modern high performance computing infrastructures to go beyond 24 spins. We propose a new matrix-free approach that does not suffer from this memory bottleneck and even allows for simulating spin chains up to 24 spins on a single compute node. The efficiency and effectiveness of the proposed algorithm is demonstrated by computing eigenstates in a massively parallel fashion, and analyzing their entanglement entropy to gain insight into the MBL transition beyond 26 spins. |

18:00 | Scalable variational embedding for quantum many-body problems ABSTRACT. In quantum embedding theories, a quantum many-body system is divided into localized clusters of sites which are treated with an accurate `high-level' theory and glued together self-consistently by a less accurate `low-level' theory at the global scale. We introduce variational embedding, which combines the insights of semidefinite relaxation and quantum embedding theory to provide a lower bound on the ground-state energy that improves as the cluster size is increased. The variational embedding method is formulated as a semidefinite program (SDP), which suffers from suboptimal computational scaling when treated with black-box solvers. We exploit the interpretation of this SDP as an embedding method to develop an algorithm which alternates parallelizable local updates of the high-level quantities with updates that enforce the low-level global constraints. |

18:30 | Tensor approximations of electronic wavefunctions and the curse of dimension ABSTRACT. Solving the Schroedinger equation numerically for N electrons suffers from the curse of dimension as an electronic wavefunction is a function on R^3N. So for just a single water molecule you are already dealing with a function on R^30. I discuss rigorous mathematical results on sparsity of electronic wavefunctions in special model systems or asymptotic scaling regimes which are either weakly or strongly correlated. Both the Tucker format as associated with the CI (Configuration-Interaction) and MCSCF (Multi-Configuration Self Consistent Field) method and in the more recent tensor-train/matrix product state format associated with the QC-DMRG (Quantum Chemistry Density Matrix Renormalization Group) method are considered. The talk is based on joint work with various co-authors over the years: Benjamin Goddard (Edinburgh), Huajie Chen (Beijing Normal University), Mi-Song Dupuy (TU Munich), and Benedikt Graswald (TU Munich). |

19:00 | Some Recent Advances in Quantum Monte Carlo Methods PRESENTER: Cyrus Umrigar ABSTRACT. Part of the high computational cost of quantum Monte Carlo methods comes from the necessity of using small time steps to keep the error coming from the Trotter breakup of the projectos small. Some recent developments have enabled the use of considerably larger time steps than previously possible. One development pertains to the use of pseudopotentials. One of two approximations are usually made when using pseudopotentials in diffusion Monte Carlo, the locality approximation and the T-moves approximation. Each has its advantages and disadvantages relative to the other. We describe a method that combines the advantages of both methods. The other development enables large time steps in both pseudpotential and all-electron calculations. |

17:30 | Towards a gradient flow for microstructure ABSTRACT. Cellular networks are ubiquitous in nature. Most engineered materials are polycrystalline microstructures composed of a myriad of small grains separated by grain boundaries, thus comprising cellular networks. A central problem is to develop technologies capable of producing an arrangement, or ordering, of the material, in terms of mesoscopic parameters like geometry and crystallography, appropriate for a given application. Is there such an order in the first place? We describe the emergence of the grain boundary character distribution (GBCD), a statistic that details texture evolution, and illustrate why it should be considered a material property. Its identification as a gradient flow by our method is tantamount to exhibiting the harvested statistic as the iterates in a mass transport JKO implicit scheme, which we found astonishing. Consequently the GBCD is the solution of a Fokker-Planck Equation. The development exposes the question of how to understand the circumstances under which a harvested empirical statistic is a property of the underlying process. (joint work with P. Bardsley, K. Barmak, E. Eggeling, M. Emelianenko, Y. Epshteyn and S. Ta'asan) |

18:00 | Entropy decay in grain growth ABSTRACT. It has been argued for quite a while that there is one more law of thermodynamics controlling the transition from mesoscale to macroscale: entropy of mesoscale microstructures decays in isolated systems. The talk aims to describe this concept and present recent experimental results confirming entropy decay for the process of grain growth [1]. It will be discussed also the problem of constitutive relations in grain growth. 1. P. Vedanti, X. Wu, V. Berdichevsky, 2020, Entropy decay during grain growth, Scientific Reports, 10 :11912 |

18:30 | Evolution of grain boundaries with dynamic lattice orientations and triple junction drag PRESENTER: Masashi Mizuno ABSTRACT. Mathematical analysis of the evolution of grain boundaries is one of the main topics in material sciences. In my talk, I explain the modeling of planar grain boundaries together with dynamic lattice orientations and triple junction drag. By the maximal dissipation principle, we obtain three equations; a curve shortening equation, the evolution of the orientations, and the triple junctions. We take the relaxation limit for the curve shortening equation in order to study the effect of the misorientations and the triple junction drag. Next, I show mathematical analysis of the system, such as the existence and long time asymptotics of a solution. Finally, we discuss the extension of the model to grain boundary networks and show numerical calculations. |

19:00 | Numerical Analysis of a Continuous Interior Penalty Method for the Phase Field Crystal Equation PRESENTER: Natasha Sharma ABSTRACT. We present a sixth-order phase field crystal equation which is a phase field model describing the dynamical formation of crystalline structures. The numerical simulation of this equation presents challenges such as the spatial discretization of the nonlinear higher-order differential operators that appear in the equation and the approximation of dynamic interfaces that travel over the entire material. To cope with these challenges, we propose an unconditionally stable and uniquely solvable convergent numerical scheme and prove these theoretical properties. We close the talk by presenting the numerical results of some benchmark problems to verify the practical performance of the proposed approach. |

17:30 | Stochastic Gradient Descent for Solving Partial Differential Equations with Uncertainty PRESENTER: Ting Wang ABSTRACT. Randomness is ubiquitous in natural sciences and modern engineering. The uncertainty is often modeled as a random coefficient in the differential equations that describe the physics. The resulted stochastic problems are often solved by the polynomial chaos (PC) approach that expands the weak solution in the polynomial chaos basis. In this work, we recast the stochastic problem as a Dirichlet energy minimization problem using the variational characterization. This formulation naturally leads us to apply the stochastic gradient descent (SGD) to solve the stochastic optimization problem. Since the SGD often has very slow convergence rate, we propose a control variate type variance reduction method to speed up the convergence of SGD. We provide numerical evidence to demonstrate that our SGD based solver can significantly outperform the vanilla PC solver in some cases. |

18:00 | Variational system identification of the partial differential equations governing microstructure evolution in materials: Inference over sparse and spatially unrelated data PRESENTER: Krishna Garikipati ABSTRACT. Pattern formation is a widely observed phenomenon in diverse fields including materials physics, developmental biology and many others. The physics underlying the patterns is specific to the mechanisms, and is encoded by partial differential equations (PDEs). With the aim of discovering hidden physics, we have previously presented a variational approach to identifying such systems of PDEs in the face of noisy data at varying fidelities (Comp Meth App Mech Eng, 353:201 2019). We extend our methods to address the challenges presented by image data on microstructures in materials physics. PDEs are posed as initial and boundary value problems over combinations of time intervals and spatial domains whose evolution is either fixed or can be tracked. However, micrographs of pattern evolution in materials are over domains that are unrelated at different time instants, and come from different physical specimens. The temporal resolution can rarely capture the fastest time scales, and noise abounds. We exploit the variational framework to choose weighting functions and identify PDE operators from such dynamics. A consistency condition arises for parsimonious inference of spatial operators at steady state, complemented by a confirmation test for acceptance of the inferred operators. The framework is demonstrated on synthetic and real data. |

18:30 | Data-Driven Physics Discovery PRESENTER: Philippe Hawi ABSTRACT. We demonstrate a new data-driven approach to physics discovery with predictive abilities on a variety of multiscale material problems. We rely on a probabilistic learning on manifolds (PLoM) framework. First, we infer an intrinsic structure that describes the physical quantities using diffusions maps (DMAPS). Data used for the physics discovery is sampled from the observation fields using stencils adapted to each application. The shape and size of the stencils encode our beliefs about physics and relationships that govern the different quantities. Once a low-dimensional manifold embedding is discovered, we augment the training dataset by using a sampling on manifolds technique that combines the solution to an Îto stochastic differential equation with a high-dimensional Gaussian Kernel Density Estimate. Finally, non-parametric density estimation and conditioning are performed on the augmented dataset to predict physical quantities of interest. We also explore the sensitivity of the learning and prediction to various DMAPS parameters, as well as the structure and size of the stencil used to draw samples from the observation data. |

17:30 | Twisted Trilayer Graphene: a Precisely Tunable Platform for Correlated Electrons ABSTRACT. We introduce twisted trilayer graphene (tTLG) with two independent twist angles as an ideal system for the precise tuning of electronic interlayer coupling to maximize the effect of correlated behaviors. As established by experiment and theory in the related twisted bilayer graphene system, van Hove singularities (VHS) in the density of states can be used as a proxy of the tendency for correlated behaviors. To explore the evolution of VHS in the twist-angle phase space of tTLG, we present a general low-energy electronic structure model for any pair of twist angles. We show that the basis of the model has infinite dimensions even at a finite energy cutoff and that no Brillouin zone exists even in the continuum limit. Using this model, we demonstrate that the tTLG system exhibits a wide range of magic angles at which VHS merge at the charge neutrality point through two distinct mechanisms: the incommensurate perturbation of twisted bilayer graphene's flat bands or the equal hybridization between two bilayer moiré superlattices. Joint work with Ziyan Zhu, Stephen Carr, Daniel Massatt, and Efthimios Kaxiras |

18:00 | Recent progress towards large-scale quantum accuracy materials simulations PRESENTER: Vikram Gavini ABSTRACT. This talk will discuss two recent developments that can pave the way for large-scale quantum accuracy materials simulations using density functional theory (DFT). Firstly, we present the development of computational methods and numerical algorithms for conducting fast and accurate large-scale DFT calculations using adaptive higher-order finite-element discretization, which forms the basis for the recently released DFT-FE open-source code. The computational efficiency, scalability and performance of DFT-FE will be presented, which will demonstrate a significant outperformance of widely used plane-wave DFT codes. Secondly, we will discuss the ongoing efforts in improving the exchange-correlation description in DFT, leveraging the recent breakthrough in computing exact exchange-correlation potentials from ab-initio correlated densities for polyatomic systems. |

18:30 | Responsive materials and integrated functional devices ABSTRACT. Applications in energy harvesting, soft robotics, wearable and prosthetic devices call for the development of new soft responsive materials that can be easily synthesized and integrated into functional devices. One approach to doing so is to incorporate responsive molecules or particles into a solid matrix, but this requires a careful balance to optimize the overall response. We provide a variational formulation for this problem, and this naturally leads to phase transforming solids. We then turn to the problem of integrated functional devices, and discuss the mathematical difficulties of optimal design of such devices. We illustrate both topics with selected examples |

17:30 | Asymptotic self-similarity in a model of shape-memory alloys ABSTRACT. We study energy-driven pattern formation in shape memory alloys. No matter how shape memoryalloys are deformed at a low temperature, they return to their original shape by heating them above acritical temperature. The cause of this phenomenon is a first order solid-to-solid phase transformation.Microstructures close to phase boundaries of martensitic nuclei can be modeled variationally. Weconsider a model by Kohn and M ̈uller (1992 & ’94), and prove asymptotic self-similarity of minimizers.This generalizes results by Conti (2000) to various physically relevant boundary conditions, moregeneral domains, and arbitrary volume fractions, including low-hysteresis shape memory alloys. The proof relies on pointwise estimates and local energy scaling laws for minimizers. Based on a joint work with Sergio Conti, Johannes Diermeier and Barbara Zwicknagl. |

18:00 | Constrained Energy Minimization for Bent-Core Liquid Crystals ABSTRACT. Situated between liquid and crystalline phases, liquid crystals are phases with various physical applications including their utilization in optical and display devices. The molecular shape of a liquid crystal material has profound effects on the macroscopic physics of the phase, causing some material to produce more efficient and less expensive optical devices than others. One example is a recently discovered phase made up of bow-shaped molecules, a characteristic that endows them with spontaneous ferroelectricity. Under the effect of an applied electric field, two competing mechanisms of switching can be detected in the tilted structure of these materials. An important question in this setup is how switching can be affected by specific system parameters. We formulate the model as an energy minimization problem allowing us to use several variational tools in its analysis. We emphasize how we can deal with challenges that arise from mixed constraints and nonlinearities peculiar to this problem. Our results address existence and uniqueness of solutions to the ensuing partial differential equations, which in turn shed light on the physical mechanisms observed. |

18:30 | Variational methods to determine bounds on the response of viscoelastic composites in the time domain PRESENTER: Ornella Mattei ABSTRACT. The determination of the effective properties of viscoelastic composites is a long-standing problem. Many of the results proposed in the literature have been obtained by applying variational methods and by considering the response of the material to cyclic loadings. On the other hand, very few results have been provided for the case of transient loadings. In this talk, we will highlight the challenges of using variational formulations and we will provide some alternative solutions. |

19:00 | Asymptotic rigidity and homogenization of variational models for anisotropic high-contrast composites PRESENTER: Carolin Kreisbeck ABSTRACT. Rigidity results in elasticity are powerful statements that allow deriving global properties of a deformation from local ones. The classical Liouville theorem states that every local isometry of a domain corresponds to a rigid body motion. If the connectedness of the set fails, clearly, global rigidity can no longer be true. In this talk, we will discuss two new asymptotic rigidity theorems, showing that if an elastic body contains sufficiently stiff connected components arranged into fine parallel layers or fibers, then strict global constraints of anisotropic nature occur in the limit of vanishing layer or cross-section thickness. Besides their theoretical interest, these findings facilitate the homogenization of variational problems modeling high-contrast bilayered and fiber-reinforced composite materials. We will address two types of models in nonlinear elasticity and finite crystal plasticity and show how to determine their homogenized Gamma-limits in terms of explicit formulas. |

17:30 | Ion-dependent DNA Configuration in Bacteriophage Capsids ABSTRACT. Bacteriophages densely pack their long dsDNA genome inside a proteinic capsid. The conformation of the viral genome inside the capsid is consistent with a hexagonal liquid crystalline structure, and experimental results have confirmed that it depends on environmental ionic conditions. In this work, we propose a biophysical model to describe the dependence of DNA configurations inside bacteriophage capsids on ions types and concentrations. The total free energy of the system combines the liquid crystal free energy, the electrostatic energy and the Lennard--Jones energy. The equilibrium points of this energy solve a highly nonlinear, second order partial differential equation (PDE) that defines the distributions of DNA and the ions inside the capsid. We develop a computational approach to simulate predictions of our model. The numerical results show good agreement with existing experiments and molecular dynamics simulations. |

18:00 | A structure-preserving finite element method for uniaxial nematic liquid crystals PRESENTER: Juan Pablo Borthagaray ABSTRACT. In the Landau-de Gennes $\mathbf{Q}$-tensor model for uniaxial nematic liquid crystals with variable degree of orientation, molecule distribution is given by a rank-one tensor and its degree of orientation by a scalar field. We present a structure-preserving discretization of the liquid crystal energy with piecewise linear finite elements that can handle the degenerate elliptic resulting problem without regularization, and show that it is consistent and stable. We present simulations in two and three dimensions to illustrate the method's ability to handle non-trivial defects as well as colloidal and electric field effects. |

18:30 | Debonding waves in gel thin films PRESENTER: Xianmin Xu ABSTRACT. The sliding friction is important for the motion of soft materials like rubber and gel on substrates. We develop a mathematical model for the sliding of a gel sheet adhered to a moving substrate. The sliding takes place by the motion of detached region between the gel sheet and the substrates, i.e., the propagation of a Schallamach wave. The model is based on a reduced elastic energy for a thin film and the Onsager Principle for its dynamical sliding. Efficient numerical methods are developed to solve the problem. Numerical examples illustrate that the model can describe the Schallamach wave and are consistent with the existing experiments qualitatively. |

19:00 | Knotted liquid crystals in bacteriophage DNA ABSTRACT. The genome of some viruses, such as bacteriophages or human herpes, is a double stranded DNA (dsDNA) molecule that is stored inside a viral protein capsid at a concentration of 200 mg/ml-800mg/ml and an osmotic pressure of 60 atmospheres. The organization of the viral genome under these extreme physical conditions is believed to be liquid crystalline. Cryoelectron microscopy experiments suggest that dsDNA near the viral capsid is in a chromonic state, and it is found in an isotropic state at the center of the capsid. Topological experiments of DNA extracted from viral capsids show a wide variety of knotted molecules and a knot distribution suggests a chiral organization of the genome. In this talk we will discuss the emergent picture that suggests that DNA is a chirally organized liquid crystalline phase in which knots may be the product of liquid crystal defects. |

17:30 | Collisions of monopoles and disclinations in nematics PRESENTER: Pawel Pieranski ABSTRACT. Monopoles, dubbed dowsons, are point singularities of the unitary complex order parameter exp(iphi) characterizing the so-called dowser texture in a thin nematic layer with homeotropic boundary conditions. Using setups called dowsons colliders, pairs of dowsons are generated and set into motion on counter-rotating trajectories leading to collisions. In a first approximation, the velocity of dowsons is orthogonal and proportional to the local phase gradient. The outcome of collisions depends on the distance of dowsons' trajectories in terms of the phase: for Dphi< Pi, a collision of a pair of dowsons leads to their annihilation while for Dphi > Pi the colliding dowsons are passing by. This rule is valid only for quasi-static stationary wound up phase fields and can be easily broken by application of a Poiseuille flow in an appropriate direction. Pairs of disclinations are generated in a reproducible manner by the isotropic-nematic transition in twisted nematic cells submitted to magnetic fields of appropriate geometries. Their collision, driven by an electric field, leads to a rewiring process that can be fully controlled by means of the magnetic and electric fields. |

18:00 | Nematic Liquid Crystal Defects for Landau-de Gennes Energy Minimizers in Three Dimensions PRESENTER: Patricia Bauman ABSTRACT. We investigate minimizers for the Landau-de Gennes energy for liquid crystals in a slab, D x (-a,a) where the cross section D is a bounded simply connected domain in two dimensions. We require that the liquid crystal is tangential on the top and bottom and prescribe boundary conditions on the lateral surfaces. Our results describe the defect structures and provide estimates on the energies of minimizers. We also show how these depend on the relative sizes of the Landau-de Gennes order parameter and the slab thickness 2a. This is joint work with Dan Phillips. |

18:30 | Defects in thin nematic liquid-crystalline films PRESENTER: Daniel Phillips ABSTRACT. We investigate the structure of nematic liquid crystal thin films described by the Ball Majumdar, tensor-valued order parameter model with Dirichlet boundary conditions on the sides of nonzero degree. We prove that as the elastic modulus goes to zero in the energy a limiting uniaxial nematic texture forms, locally minimizing a planar Frank energy trapping a finite number of defects, all of degree 1/2 or all of degree -1/2, corresponding to vertical disclination lines at those locations. |

19:00 | A variational model for smectics in SBV PRESENTER: Bianca Stroffolini ABSTRACT. In smectic liquid crystals, rod-like molecules tend to align parallel to each other along a common director n and form parallel layers with equal thickness of the order of a few nm. These materials may exhibit defects, i.e., regions of rapid changes in the molecular orientation, that occur along a surface. Thus, it might be reasonable to consider variational models which, unlike most classical theories for liquid crystals, are based on discontinuous order parameters, as proposed in a recent paper by Ball and Bedford. In this talk, I'll discuss some preliminary results towards the mathematical analysis of defect patterns in thin film of smectic A phases. |

17:30 | Crumples as a Generic Stress-Focusing Instability in Confined Sheets ABSTRACT. The last decade has seen a renaissance in the geometrically-nonlinear buckling of sheets, which is central to the mechanics of synthetic skins, biological tissues, textiles, and 2D materials like graphene. Significant effort has been devoted to understanding and exploiting buckled structures on nearly-planar substrates. We show that smooth wrinkles do not survive to large curvatures in general geometries, giving way to a generic “crumpled” [1] response that we characterize in a suite of experiments across scales [2]. We probe the wrinkle-to-crumple transition using polymer films confined on spherical, hyperbolic, and cylindrical surfaces, and we reproduce the same behaviors in macroscopic membranes inflated with gas. These varied setups reveal robust morphological features of the crumpled phase, and they allow us to disentangle the competing effects of curvature and compression. Our work highlights the need for a theoretical understanding of this ubiquitous elastic building block, and we lay out concrete directions for such studies. [1] King et al., PNAS 109, 9716 (2012). [2] Timounay et al., PRX 10, 021008 (2020). |

18:00 | Effect of the thickness on the curvature-induced rigidity of flat sheets PRESENTER: Matteo Taffetani ABSTRACT. Gauss' Theorema Egregium implies that changes in Gaussian curvature of a surface must involve stretching of the surface. For thin elastic sheets, bending deformations are favourable over stretching and so Gaussian curvature is generally preserved. As a consequence, curvature imposed in the transverse direction induces a resistance to bending in the longitudinal direction: the sheet would have to stretch to accommodate a deformation that requires a change of the Gaussian curvature, which is prohibitively expensive. The inhibition of bending in one principal direction that results from an imposed curvature in the orthogonal direction is referred to as 'curvature-induced rigidity'. Here, we investigate the behaviour of a rectangular strip of finite thickness held horizontally in a gravitational field, with a transverse curvature imposed at one end. There are two ways that the finite thickness of the sheet affects the efficacy of curvature-induced rigidity: the finite bending stiffness acts to `uncurve' the sheet, at the expense of some stretching energy; the finite weight deforms the strip downwards, eventually causing buckling. We identify the dimensionless parameters that control this problem and determine the critical imposed curvature required to prevent buckling before describing the buckled shape of the strip well beyond the buckling threshold. |

18:30 | Revisiting the buckling of cylindrical shells under axial compression ABSTRACT. We revisit the problem of buckling of cylindrical shells under axial compression, considering cylinders with convex cross sections. We prove that if the cross section has a uniformly apart from zero curvature, then the critical buckling load scales like the thickness to the power of 1, and if it contains at least one zero curvature point, then the buckling load scales at most like the thickness to the power 1.5. This provides another evidence to the famous sensitivity to imperfections phenomenon for the cylindrical shell buckling problem. This is joint work with Andre Martins Rodrigues (UCSB). |

19:00 | Energy scaling for a conically constrained sector ABSTRACT. We consider a thin elastic sheet in the shape of a sector that is clamped along the curved part of the boundary, and left free at the remainder. On the curved part, the boundary conditions agree with those of a conical deformation. We prove upper and lower bounds for the Föppl-von-Kármán energy under the assumption that the out-of-plane component of the deformation is convex. The lower bound is optimal in the sense that it matches the upper bound in the leading order with respect to the thickness of the sheet. As a corollary, we obtain a new estimate for the Monge-Ampère equation in two dimensions. Joint work with Peter Gladbach (Bonn). |

17:30 | Nonlocal operators on bounded and unbounded domains ABSTRACT. Nonlocal theories have been developed around successful implementation of nonlocal models in different areas of science (continuum mechanics, biology, image processing). Theoretical advances have been made by introducing and investigating integral operators and associated systems of equations. In this talk I will present some recent results on nonlocal frameworks systems based on some existing, as well as some newly introduced, nonlocal operators. An in-depth study of properties of the operators includes a series of results on nonlocal versions of integration by parts theorems, Helmholtz-Hodge type decompositions, as well as convergence of operators to their classical equivalents as the interaction horizon vanishes. |

18:00 | Treecode-Accelerated Green Iteration for Kohn-Sham Density Functional Theory PRESENTER: Robert Krasny ABSTRACT. We present a real-space computational method called treecode-accelerated Green Iteration (TAGI) for all-electron Kohn-Sham Density Functional Theory. In this approach the Kohn-Sham differential eigenvalue problem is converted into an integral fixed-point problem by convolution with the modified Helmholtz Green's function. The fixed-point problem is solved by self-consistent field iteration and Green Iteration, where the discrete convolution sums are evaluated by a GPU-accelerated barycentric Lagrange treecode. TAGI also uses adaptive mesh refinement, Fejer quadrature, singularity subtraction, gradient-free eigenvalue update, and Anderson mixing. Ground state energy computations for several atoms and small molecules demonstrate TAGI's ability to achieve chemical accuracy. |

18:30 | Axisymmetric penny-shaped fracture problem at nanoscale ABSTRACT. A problem for a nanosized penny-shaped fracture in an infinite homogeneous isotropic elastic medium is considered. The fracture is opened by applying an axisymmetric normal traction to its surface. The surface energy in the Steigmann-Ogden form is acting on the boundary of the fracture. The problem is solved by using the Boussinesq potentials represented by the Hankel transforms of certain unknown functions. With the help of these functions, the problem can be reduced to a system of two singular integro-differential equations. The numerical solution of this system can be obtained by expanding the unknown functions into the Fourier-Bessel series. Then the approximations of the unknown functions can be obtained by solving a system of linear algebraic equations. Accuracy of the numerical procedure is studied. Various numerical examples for different values of the surface energy parameters are considered. Parametric studies of the dependence of the solutions on the mechanical and the geometric parameters of the system are undertaken. It is shown that the surface parameters have a significant influence on the behavior of the material system. In particular, presence of surface energy leads to the size-dependency of the solutions and increased toughness of the material. |

19:00 | Homogenization for a Variational Problem witha Slip Boundary Condition PRESENTER: Silvia Jimenez Bolanos ABSTRACT. In this talk, we discuss the homogenization of a variational boundary value problem that governs the small vibrations of a periodic mixture of an elastic solid and a slightly viscous ﬂuid with a slip boundary condition on their interface. We use two-scale convergence to obtain the macroscopic behaviour of the solution and identify the role played by the connectedness of the phases and the interface boundary condition. |