Mott insulators out of equilibrium: phonons and screening

Current-conducting materials consist of negatively charged electrons, moving in the background of positively charged, heavier particles, the ions. In a first approximation, ions can be regarded as fixed at certain positions altogether building a regular crystal lattice. It is a major theoretical challenge to simulate the motion of this huge (N~10 23) number of particles interacting with each other via the Coulomb force. In fact, accounting for all these forces exactly would be an impossible task even for t he most powerful computers. Fortunately, in most materials, each electron can be regarded as an independent particle moving in the background of the other electrons and ions. This considerably simplifies computational costs and allows theoretical predictio ns about the properties of these materials. This is the great success of the so -called band theory of solids. On the other hand, a number of so-called strongly correlated materials (abbreviated with SCM) exists for which this scheme does not work. Obviously, numerical simulation of SCM is much more challenging. Nevertheless, these materials often display a variety of peculiar electronic and magnetic properties, such as superconductivity at high temperatures, or a huge response of resistivity to applied magnetic fields and other, which are not observed in ordinary materials. More dramatically, in many cases, while band theory would predict a conducting behavior, strong correlation produces a so-called Mott gap which makes these SCM insulators or semiconductors. All these properties make SCM interesting candidates for electronic components of the future to replace or complement modern silicon technology. In this project, we will investigate two effects that could be useful for the application of these materials as electronic devices. First, we will study the possibility to use these systems as photovoltaic devices. This is motivated by preliminary theoretical and experimental studies suggesting that the efficiency of such devices based on Mott systems could be enhanced by so- called impact ionisation processes. The idea is that highly photoexcited electrons could use their extra energy to excite additional electrons across the Mott gap. Second, we shall investigate a transition, the so-called resistive switch, between an insulating and a conducting state which is induced by applying a large voltage to the system. Such a transition, observed experimentally in some SCM, makes them interesting candidates as components for RAM technology. In both these effects, but in general in device electronics, heat transport and dissipation play an important role. Here, we will particularly concentrate on the interplay of the Mott gap with ion vibration modes, so called phonons, which are important for transporting heat away and, thus, for cooling down the device. The project is carried out in collaboration with a group at TU Vienna and one at the university of Erlangen-Nürnberg.