Gravité artificielle

La gravité artificielle est une accélération résultant de l'application d'une force et mise dans l'espace pour simuler la gravité. Ce terme peut également recouvrir différents concepts de science-fiction permettant de manipuler la gravité. Dans le contexte du vol spatial habité, la mise en œuvre de la gravité artificielle doit permettre de combattre les effets de l'impesanteur sur le corps humain durant une période prolongée. Plusieurs méthodes permettent de générer une gravité artificielle mais aucune n'a été mise en œuvre car toutes imposent des vaisseaux de très grande taille et présentent des inconvénients qui dans le cadre des missions spatiales programmées jusque-là ne sont pas compensées par leurs avantages. Les problèmes soulevés par l'absence de gravité L'absence de gravité subie par un astronaute dans l'espace est à l'origine de problèmes de santé dont la gravité dépend de la durée de l'exposition, il s'agit de l'un des problèmes principaux de l'adaptation de

Tensor network investigation of frustrated Ising models

Jeanne Colbois

In spin systems, geometrical frustration describes the impossibility of minimizing simultaneously all the interactions in a Hamiltonian, often giving rise to macroscopic ground-state degeneracies and emergent low-temperature physics. In this thesis, combining tensor network (TN) methods to Monte Carlo (MC) methods and ground-state energy lower bound approaches, we study two-dimensional frustrated classical Ising models. In particular, we focus on the determination of the residual entropy in the presence of farther-neighbor interactions in kagome lattice Ising antiferromagnets (KIAFM).In general, using MC to determine the residual entropy is a significant challenge requiring ad-hoc updates, a precise evaluation of the energy at all temperatures to allow for thermodynamic integration, and a good control of the finite-size scaling behavior. As an alternative, we turn to TNs; however, we argue that, in the presence of frustration and macroscopic ground-state degeneracy, standard algorithms fail to converge at low temperatures on the usual TN formulation of partition functions. Inspired by methods for constructing ground-state energy lower bounds, we propose a systematic way to find the ground-state local rule using linear programming. Characterizing the rules as tiles that can be tessellated to form ground states of the model gives rise to a natural contractible TN formulation of the partition function. This method provides a direct access to the ground-state properties of frustrated models and, in particular, allows an extremely precise determination of their residual entropy.We then study two models inspired by artificial spin systems on the kagome lattice with out-of-plane (OOP) anisotropy. The first model is motivated by experiments on an array of chirally coupled nanomagnets. We argue that the farther-neighbor to nearest-neighbor couplings ratios in this system are much smaller than in the dipolar case, J2/J1 being of the order of 2%. A comparison of the experimental correlations with the results of extensive TN and MC simulations shows that (1) the experimental second- and third-neighbor correlations are inverted as compared to those of a pure nearest-neighbor model at equilibrium (even with a magnetic field), and (2) second-neighbor couplings as small as 1% of the nearest-neighbor couplings will affect the spin-spin correlations even at fairly high temperatures.Motivated by dipolar coupled artificial spin systems, we turn to the progressive lifting of the ground-state degeneracy of the KIAFM. We provide a detailed study of the ground-state phases of this model with up to third neighbor interactions, for arbitrary J2, J3 such that J1 >> J2, J3, obtaining exact results for the ground-state energies. When all couplings are antiferromagnetic, we exhibit three macroscopically degenerate ground-state phases and establish their residual entropy using our TN approach. Furthermore, in the phase corresponding to the dipolar KIAFM truncated to third neighbors, we use the ground-state tiles to establish the existence of a mapping to the ground-state manifold of the triangular Ising antiferromagnet.

EPFL2022

Low Damped Cloth Simulation

Junghyun Ahn

This paper proposes a new implicit integration technique that reproduces a stable cloth without introducing excessive damping forces. Semi-implicit integration methods have been widely used in cloth simulations because of their high stability and speed. Artificial internal damping forces are generated during the linearization process of the semi-implicit integration. The simulations become extremely stable due to the artificial forces; however, the forces significantly degrade the realism of cloth simulations, since they are generated with respect to rotational rigid motions, as well as internal deformations. Hence, we propose a new method to decrease the damping artifacts. The artificial internal damping forces are computed solely for pure internal deformations, and are stably incorporated into the dynamical system. Experiments show that our simulator can reproduce various cloth materials without excessive damping artifacts even in real-time.

2006

Tensor network investigation of frustrated Ising models

Jeanne Colbois

EPFL2022

Tensor network investigation of frustrated Ising models

Low Damped Cloth Simulation

Tensiometer measurements under variable gravity conditions

Tensor network investigation of frustrated Ising models

Tensiometer measurements under variable gravity conditions

Low Damped Cloth Simulation