University of St Andrews

I will give a overview of recent efforts in my group to understand low temperature persistent photoconductivity (PPC) in SrTiO3 (STO). I will discuss several different cases, including nominally stoichiometric STO where PPC switches on below 1 K, and STO with oxygen vacancies. The latter can be induced using Ar ion irradiation, giving rise to PPC and additionally to transient negative photoconductivity. Vacancies can also be optically induced, providing the possibility of tuning their depth and density. I will discuss how these various phenomena can be understand in an overall framework, and how these techniques give us a powerful way to tune this fascinating quantum material.

Exciton-polaritons, are mixed light-matter quasiparticles resulting from the strong coupling of photons confined in a microcavity and quantum well excitons. Being bosons, polaritons can condense into macroscopically coherent many-body state and have thus emerged as prime candidates for the study of non-equilibrium systems of interacting bosons. Our recent  studies, exploit non-equilibrium nature of polariton condensates, showing that polariton condensates can spontaneously magnetize, and how their spin can be controlled both optically and electrically. We employ spatially patterned external laser excitation to create arbitrary potential landscapes for polaritons and demonstrate ferromagnetic and antiferromagnetic coupling between neighbouring condensates. Polariton condensates can be imprinted into arbitrary two dimensional lattices with tunable intra- and inter-site interactions providing exciting opportunities for devising novel and versatile quantum simulation platforms.Financial support from Bilateral Greek-Russian "Polisimulator" project co-financed by Greece and the EU Regional Development Fund is acknowledged.

There has been a huge surge in interest recently in manipulating the properties of 2D materials since the isolation of graphene in 2004. From this, a class of 2D materials called the transition metal dichalcogenides (TMD), which consist of a transition metal (M), a chalcogen (X) and have the formula MX2, have shown to have a whole array of interesting electronic properties including semiconducting, metallic and superconductivity. Manipulation of these properties has taken many forms including chemical surface functionalisation, intercalation of molecules in between layers and electric field doping. The latter of these techniques allows precise control of the electronic state of the 2D material meaning a semiconducting insulating 2D material can be doped into the conducting state.An ionic liquid field effect transistor is one example of electric field doping and one which is growing in popularity to tune the electronic properties of 2D materials. An ionic liquid is placed on top of the 2D material and a voltage is applied to an electrode in the liquid so as to induce charge separation within the liquid. The compact electrical double layer formed at the interface between the liquid and the 2D material induces a high level of doping and in this talk I will discuss the electrochemical effects of using ionic liquids as a gate dielectric as well as the novel electronic states which are induced in the 2D material.

In the last decade, a deep understanding about the topological properties of single particle systems has been achieved by considering their relation with symmetry. This has led to a complete classification of non-interacting Hamiltonians. On the other hand, a full understanding of the role of interactions in these systems is still lacking. In this talk I will review general aspects of the topological properties of non-interacting Hamiltonians, and present a system where interactions open up the unexpected possibility of generating fractional quasiparticles.