Strong correlations in quantum systems: from exotic phases in frustrated magnets to ultracold atoms

Matthias Punk, Innsbruck University

17.07.2014 at 11:00 

Studying large systems of strongly interacting particles taught us that many properties of interesting materials can hardly be understood by investigating the fundamental degrees of freedom and the laws governing their interactions. Rather, the properties of such materials are determined by emergent collective degrees of freedom which often bear no resemblance to the electrons and atoms forming a solid. In this talk I will discuss two examples of strongly correlated many-body systems where emergent degrees of freedom play a crucial role.
As a first example we discuss the nature of exotic spin-liquid phases in frustrated magnets. Recent neutron scattering experiments on a layered spin-1/2 kagome lattice antiferromagnet called Herbertsmithite revealed the first striking signature of fractionalized spinon excitations in a quantum spin liquid state. I'm going to argue that many experimentally observed features can be understood from the presence of vortex excitations of an emergent Z2 gauge field in this material.
The superfluid-Mott insulator transition of interacting bosons serves as a second example. In the vicinity of such a critical point no well-defined quasiparticle excitations exist, which turns the computation of transport properties into a major theoretical challenge. Here I'm going to show how transport properties of a mobile impurity depend on properties of the bosons in the quantum critical regime. This setup is inspired by recent experiments with so-called quantum gas microscopes, which allow for a direct optical tracking of the impurity atom coupled to bosons at the critical point.