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.

In this project, we explored how to possibly use a group of substances called "strongly correlated materials" (SCMs) to build next-generation electronics. In ordinary materials, electrons act independently. But in SCMs, they move in a highly coordinated way. This collective behavior gives them important properties, from conducting electricity with zero resistance to quick, sudden changes in how they conduct electricity. First, we studied whether these systems could serve as photovoltaic devices. This was motivated by earlier theoretical and experimental work suggesting that the efficiency of solar cells based on SCM insulators could be boosted by so-called impact ionisation. The idea is that a highly photoexcited electron can use its excess energy to lift additional electrons across the material's energy gap, generating more charge carriers from the same amount of light. In conventional semiconductor solar cells this rarely happens, because the excess energy is instead carried away by the material's microscopic vibrations, known as phonons. The investigations of the present project confirmed that impact ionisation is indeed important in insulating SCM. In particular, we found that it can be enhanced by connecting the material to the device through narrowband, that is weakly conducting, contacts. We also found that lattice vibrations do not significantly suppress the effect. Second, we examined how strong electric fields affect these materials, aiming to understand the so-called resistive switch: an abrupt transition between an insulating and a conducting state. This switching makes such materials promising candidates for next-generation memory (RAM) technology. In both cases, and in device electronics generally, heat transport and dissipation play an important role, and these too are governed by the microscopic vibrations, which carry heat away and cool the device. A surprising result emerged here: contrary to naive expectation, dissipation actually helps a sizeable current develop at the transition. This is in striking contrast to structural irregularities (impurities), which broaden the spectra in a superficially similar way but instead hinder the current. Finally, we explored a question touching one of the foundations of quantum mechanics: the role of measurement. Observing a quantum system usually disturbs it, but, as it turns out, not always. We studied a microscopic electronic device, a kind of single-electron transistor, while continuously monitoring it. We found that a delicate quantum phenomenon known as the Kondo effect can survive being watched, provided one keeps track of the electrons' charge rather than their spin. Monitor the spin instead, and the effect quickly "heats away." These findings may provide a contribution to a broader question: how future quantum technologies might be observed without being destroyed.