This Materials World Network project will synthesize, characterize and study experimentally, properties of high temperature superconductors such as the cuprates, whose carrier concentrations will be electrostatic altered in field effect transistor geometries employing an electronic double layer transistor (EDLT) technique that uses ionic liquids as gate dielectrics. In effect electrostatic doping would replace chemical doping. Since carrier concentration is a control parameter that governs the properties of these systems, this approach will facilitate the investigation of systems through continuous and reversible changes of carrier concentration. The program includes studies of X-ray scattering and superfluid density to supplement the usual measurements of electrical transport and magneto-transport. Electronic double layer transistors contain a gate electrode, a layer into which charge can be accumulated or depleted, source and drain electrodes, as well as electrodes for measuring longitudinal and transverse voltages. Ionic liquids are molten salts consisting of large ions such that their Coulomb interaction is sufficiently small to make them room temperature liquids. Upon applying a gate voltage, ions move to the surface of the layer to be doped, forming an electric double layer such that with the charge induced, acts as a capacitor of nanoscale thickness. The charge accumulation or depletion layers formed using EDLTs may be the order of just a few unit cells in thickness and will contain high electric fields and electric field gradients. The doped layer may be a two-dimensional rather than a three-dimensional superconductor. It is thus not a given that electrostatic doping is completely equivalent to chemical doping to the same charge level. On the other hand, when the microscopic process of electric field doping becomes well understood, considerable insight into the rich physics of the cuprates may be gained.
NON-TECHNICAL SUMMARY This Materials World Network project will employ ionic liquid field effect transistor configurations to electrostatically alter charge carrier densities, and study the resultant changes in the physical properties of high temperature superconductors. The usual approach to this is to prepare samples with different chemical compositions. This new approach eliminates the need to do this, speeding the rate of exploration. Ionic liquids, which are molten salts at room temperature, replace the gate insulator in the field effect transistor configuration. Their use can facilitate charge transfers more than 100 times greater than achievable with conventional insulators, because of the formation of an electronic double layer, which is in effect a capacitor with a nanometer scale gap. The broader impacts of the proposed program include training of graduate students in a traditional physics Ph.D. program and providing a venue for useful and positive research experiences for undergraduates from both Minnesota and elsewhere. In addition, this international effort will allow for strong interaction of personnel from the US and Spain, which will broaden the experience of young scientists in the global scientific community. Finally, the work described in this proposal may have an impact on the search for new superconductors, which is one of the grand challenges of contemporary condensed matter and materials physics. It could demonstrate that electrostatic doping is a viable alternative to traditional chemical doping in this quest.
This project is supported by the Condensed Matter Physics program and the Office of Special Programs, Division of Materials Research.
