Research

The carrier density in solids plays an important role for the electronic and optical properties. It determines the position up to which electronic bands are filled. It also fixes the inter-particle distance and hence the carrier-carrier interaction strength. For two dimensional materials, the carrier density can be tuned by the field effect. Accessing a wide density range represents a powerful approach to explore interaction induced physics. Recently, ionic liquid and polymer electrolyte gating have emerged, which offer unprecedented control over the carrier density. Here, we study the electronic properties of thin oxides, oxide heterostructures and exfoliated two-dimensional crystals such as graphene, boron-nitride, molybdenum-disulfide and iron-based superconductors in the field effect geometry with electrolyte gating. We address, among other questions, whether it is possible to induce, destroy or modify superconducting behavior. The use of the field effect to change the density is superior to doping or intercalation as structural changes are avoided. Density dependent studies can be performed on one and the same sample and difficulties related to the incorporation of a large or varying number of dopants are absent.

Bose-Einstein condensation is a state of matter with tremendous theoretical and experimental interest. The semiconductor community with its impressive capabilities to manipulate charge carriers with gating and optical techniques has attempted to generate particle systems in which this phenomenon occurs. In a system composed of adjacent two-dimensional electron layers separated by a thin barrier, large enough to suppress tunneling but thin enough to ensure strong interlayer Coulomb interaction, evidence for Bose Einstein Condensation was finally found when a sufficiently strong perpendicular magnetic field was applied. The electrons arrange in a new state which extends across both layers and with properties that can only be understood if one assumes the condensation of charge neutral excitons shared between the two layers. Techniques have been developed to separately contact both layers and they are used to explore the many intriguing properties of this state of matter in transport and with local probe methods.