Structure and order induced by defects

by Dr Markus Müller (PSI/LTC)

Wednesday 21 Jan 2026, 15:00 → 16:00 Europe/Zurich

OVGA/200

The introduction of defects in an otherwise homogeneous material often generates new degrees of freedom that live on the background of a non-trivial vacuum. Especially if defects introduce local frustration the resulting degrees of freedom may exhibit degenerate configurations that live a life of their own. A finite density of such defects may then undergo ordering and act back on the ‘vacuum’ that supports them and/or reflect some its properties . 

I will review this paradigm in the case of magnetic spiral order induced in YBaCuFeO₅ by defects due to chemical disorder. While the formation of spirals has been well understood, their detailed nature – interpolating between helicoidal and cycloidal, rendering them multiferroically active – awaits a complete theory. This is a problem where ab initio calculations and statistical physics and condensed matter theory might work together very fruitfully. 

Finally we will discuss a few other examples of interesting defects, especially in 2d magnetic materials, and outline the conceptual and ab initio challenges in ongoing and future projects.