His PhD thesis focused on the analysis of long time correlations of the lattice dynamics of crystals close to structural phase transitions and attempts to explain the central peak phenomenon observed e.g. in SrTiO3 and BaTiO3.

His host was Prof. Chuck Willis and Flach worked on the observation and properties of discrete breathers – generic exact localized solutions of broad classes of nonlinear lattice wave systems.

In 1994, he was a guest scientist at Max Planck Institute for the Physics of Complex Systems in Dresden and later his position changed to become the head of visitors program in 1997.

The goals of the center include being a renowned laboratory for quantum dynamics and nonlinear classical nano-structured systems and research the interfaces of applied and computational theoretical condensed matter physics and optics.

In addition to writing academic journal publications, Flach has contributed as an editorial board member of Chaos (since 2016) and Physical Review E (2009–2011).