The research below are key projects carried out at previous laboratories with a number of colleagues and collaborators (see references).
Search for new phases –
Max Planck Institute for Solid State Research / IFMQ University of Stuttgart
The discovery of new phenomena and phases with novel order parameters is one of the key drivers in condensed matter physics. In the last years a strong focus was therefor on new material research. This included several strands of phenomena.
One of them was the investigation of a putative excitonic insulator phase in Ta2NiSe5. This long-conjectured phase in ‘zero-gap’ semiconductors is driven by the Coulomb interactions between electrons and holes. This can give rise to an elctron-hole pairing which can stabilise a new many-body ground state – the excitonic insulator – that is purely driven by electronic correlations. We carefully studied the thermodynamic (specific heat, magnetisation) and transport properties for the pure compound and explored the phase diagram as function of effective single particle gap by chemical and physical pressure with all data being fully consistent with the predictions for such a phase. This research has triggered several other experiments such as time resolved optics to further investigate the electronic order in the low temperature phase.
 Y.F. Lu et al., Nat. Comm. 8, 14408 (2017)
 T. I. Larkin et al., PRB 95, 195144 (2017)
 D. Werdehausen et al., arXiv:1611.01053
A second thread of experiments has focused on topological semimetals with broken time reversal symmetry / magnetism. Here a key focus is on the question on how the topological surface states can be controlled via magnetic tuning. By switching from topologically trivial to non-trivial phases surface states and thereby surface transport can in principle be switched by orders of magnitude. Materials in which we were involved include magnetic inverse perovskites (see figure) and CeSbTe (see reference).
 L.M. Schoop et al., arXiv:1707.03408
Unconventional Superconductivity studied by Spectroscopic Imaging STM – Cornell University / J. C. Seamus Davis Group
Strongly correlated electron materials are often dominated by local on-site correlations giving rise to sometimes dramatic real space variations of the electronic structure on the nanoscale. These local properties make strongly correlated electron materials ideal materials for studies by spectroscopic imaging scanning tunnelling microscopy (SISTM). Experiments on LiFeAs were one of the first momentum-space resolved determinations of the superconducting gap of an iron-based superconductor.
 M.P. Allan, A.W. Rost, Science 336 563 (2012)
 M.P. Allan, K. Lee, A.W. Rost et al., Nat. Phys. 11, 177 (2015)
Thermodynamics of Strongly Correlated Electron Systems –
University of St Andrews / Oxide Physics Group
Thermodynamic experiments play a key role in discovering and understanding strongly correlated electron phases. Specific heat in metals is a benchmark quantity for the role of correlation effects beyond the single particle picture, while magnetic susceptibility is for example crucial in establishing or excluding magnetic order in spin ice or spin liquids. Thermodynamic studies of the transition metal oxide Sr3Ru2O7 are an example of such a study.
Sr3Ru2O7 is a strongly correlated electron material with an intriguing low temperature magnetic phase diagram. Its thermodynamic properties are controlled by a metamagnetic quantum critical end point and the associated quantum fluctuations. The latter are conjectured to be driving the destabilization of the critical state allowing a novel broken symmetry phase to emerge at low temperatures. The entropic properties of both the quantum critical state and the new low temperature phase are key to understanding the underlying physicsal phenomena.
 A.W. Rost et al., Science 325, 1360 (2009)
 A.W. Rost et al., PNAS 108, 16549 (2011)