In 1960, Luttinger proposed a universal principle connecting the total capacity of a system for particles with its response to low-energy excitations. Although easily confirmed in systems with independent particles, this theorem remains applicable in correlated quantum systems characterized by intense inter-particle interactions.

However, and quite surprisingly, Luttinger’s theorem has been shown to fail in very specific and exotic instances of strongly correlated phases of matter. The failure of Luttinger’s theorem and its consequences on the behavior of quantum matter are at the core of intense research in condensed matter physics.

#### The Ishikawa-Matsuyama Invariant and Correlated Insulators

Independently of these developments, important efforts have been dedicated to the classification and characterization of correlated insulating states of matter. In this context, it was shown that a broad class of topological insulators can be labeled by a single integer, known as the Ishikawa-Matsuyama invariant, which fully captures its transport properties.

This result constitutes a milestone as it offers a simple prescription for classifying insulating states in the presence of strong interactions. Very recently, however, theorists identified exotic models of correlated insulators that mysteriously elude this appealing classification: corrections to the Ishikawa-Matsuyama invariant are thus required in peculiar settings.

#### Connection Between Luttinger’s Theorem and Insulating State Classification

Writing in the prestigious *Physical Review Letters*, Lucila Peralta Gavensky and Nathan Goldman (ULB), together with Subir Sachdev (Harvard), reveal that the failure of Luttinger’s theorem and the classification of insulating states of matter are connected by a fundamental relation. In essence, these authors demonstrate that the Ishikawa-Matsuyama invariant fully characterizes correlated insulators whenever Luttinger’s theorem is satisfied.

In contrast, this topological invariant is shown to be insufficient to label correlated phases as soon as Luttinger’s theorem is violated, and the authors provide explicit expressions for the required corrections in terms of relevant physical quantities.

This important connection between Luttinger’s theorem and the topological classification of quantum matter sheds light on the emergence of exotic phenomena in strongly correlated quantum matter.

Reference: “Connecting the Many-Body Chern Number to Luttinger’s Theorem through Středa’s Formula” by Lucila Peralta Gavensky, Subir Sachdev and Nathan Goldman, 4 December 2023, *Physical Review Letters*.

DOI: 10.1103/PhysRevLett.131.236601

because science!

Writing in the prestigious Physical Review Letters (PRL), pseudoscience is rampant. They firmly believe that two objects (such as two sets of cobalt-60) of high-dimensional spacetime rotating in opposite directions can be transformed into two objects that mirror each other.