The Non-Commutative Brillouin Torus, a Non-Commutative Geometry perspective

Abstract

Non-commutative geometry has significantly contributed to quantum mechanics by providing mathematical tools to extract topological and geometrical information from these systems. This thesis explores the methods used by Jean Bellissard and collaborators for analyzing homogeneous materials. The focus is on two topological algebras that extend Fourier analysis over $\mathbb{T}^d$ to study tight-binding models for homogeneous materials. These algebras, generalizations of $C(\mathbb{T}^d)$ and $C^{\infty}(\mathbb{T}^d)$, are considered anon-commutative smooth manifold, referred to as the Non-Commutative Brillouin Torus. Techniques from Fourier analysis, such as Fourier coefficients and Fejér summation, are adapted to this context, capturing the topological and smooth structure of the non-commutative manifold. The topological structure is represented by a C* algebra, while the smooth structure is captured by a smooth sub-algebra, forming the basis for constructing topological invariants of Hamiltonians through continuous cyclic cohomology. Additionally, the K theory of C* algebras and smooth sub-algebras is a crucial tool for studying topological invariants of Hamiltonians. Smooth sub-algebras share a functional calculus with C* algebras, providing sufficient information to study their K theory. This relationship is vital for identifying topological invariants of Hamiltonians, particularly under conditions like low temperature and electron density, which contribute to the quantization of transversal conductivity in homogeneous materials. This thesis aims to be a valuable resource for newcomers to the field.