FLATOPS · Flat bands and topology in superconductive materials

Flat bands and topology in superconductive materials

A superconductor has the property that below a certain temperature, called the critical temperature, the electric resistivity vanishes and the material becomes a perfect conductor, that is the current flow is not accompanied by the dissipation of energy. Thanks to this unique property, superconductivity has found important applications, for example it is used to create the large and uniform magnetic fields required for Magnetic Resonance Imaging, nowadays an essential medical tool. However many more applications on a much larger scale would likely open up if materials were found whose critical temperature is higher than the current records. Unfortunately our understanding of the origin of superconductivity in currently known high-temperature superconductors is still rather incomplete and there is no clear established receipt for engineering materials with even higher critical temperatures. One approach is to increase the density of states, a quantity which measures the amount of quantum states available for electrons in a given energy range since this has the effect of increasing the critical temperature. The density of states is proportional to the effective mass of the electrons in a crystal. Therefore to increase the critical temperature one should find a material where electrons have an effective mass as large as possible. In the limit of infinite effective mass the electronic states form a so-called flat band. A crucial question is what are the transport properties in the presence a flat band. At the classical level an electron with an infinite effective mass is stuck in place and cannot be moved by any external force, no matter how large. This means that the material cannot conduct any current and is an insulator rather than a superconductor. Thus it may appear that the idea of using the high density of states of a flat band to increase the critical temperature is bound to fail. The central result of FLATOPS is that the situation is completely different if quantum effects are taken into account. Whereas in a flat band single particles are localized due to the infinite effective mass, in the presence of interactions the effective mass of composite particles, such as Cooper pairs, can be finite and thus allow for the transport of current. This quantum effect where two single particles, which are immobile due to the diverging effective mass of the flat band, are combined into a mobile Cooper pair with finite effective mass has no classical analogue and is the key to achieve high-temperature superconductivity. In this project it has also been established that the effective mass of Cooper pairs is controlled by a geometric property of the flat band quantum wave functions, the so-called quantum metric, which essentially measures the overlap between the flat band wave functions. The higher the overlap the higher the chance for a Cooper pair to jump from one wave function to the other as a consequence of interactions and thus the mobility of the Cooper pair is higher. This corresponds to a higher critical temperature. Therefore, using flat bands with large quantum metric is a new promising route to engineer high-temperature superconducting materials. This new route to high-temperature superconductivity has been established in FLATOPS and has been proved to be physically sound using a number of different approaches. These results will be important in the effort of engineering novel materials with increasingly higher critical temperatures.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз