September 30: PhD defence Licciardello on Quantum criticality and superconductivity in the electron nematic FeSe1-xSx
In order to understand superconductivity, you must get rid of it
The electronic properties of a superconductor at very low temperatures are believed to contain important information on the mechanisms that lead to superconductivity itself. A better understanding of those mechanisms might provide the key to reach room temperature superconductivity which, if ever achieved, would revolutionize our world. Paradoxically however, to study the electrical properties of a superconductor by means of transport experiments, the superconducting state must first be destroyed.
‘Critical’ electrons that move along preferential directions
During his PhD, Salvo performed electrical transport experiments on various samples of the iron chalcogenide family FeSe1-xSx by injecting current into each sample and measuring their resistance while varying the temperature. Below a certain temperature (the critical temperature), resistance became zero, meaning that the material had become superconducting. Then, using the high magnetic fields available at the HFML, Salvo was able to destroy this superconducting state and continue measuring the resistance of FeSe1-xSx down to even lower temperatures (0.3 degrees above absolute zero). During this investigations, Salvo identified a very special value of sulphur concentration, at which the resistance displayed an unexpected temperature dependence all the way down to the zero temperature limit. These unexpected properties are described in a framework called quantum criticality, and that particular point in the phase diagram is called a quantum critical point. This point appears to coincide with the end point of the nematic phase of FeSe1-xSx, a state in which electrons develop a preferential spatial direction rather like the molecules in a liquid crystal. What Salvo discovered was therefore a rare example of a nematic quantum critical point. At this point, and close to it, the transport properties were found to be profoundly modified though the superconducting critical temperature of the material was hardly affected. By understanding the reason behind this puzzle, i.e. why such profound changes in the electronic transport do not seem to boost, or weaken, the superconductivity, we will perhaps one day get one step closer to the dream of a room temperature superconductor.
Personal and professional contributions
“Before starting my PhD, I thought that characterizing the behavior of a material at the absolute zero, the lowest temperature theoretically allowed by the laws of thermodynamics and experimentally unreachable, was impossible. In an advanced scientific facility and with a good dose of perseverance, it can actually be done. Now I can look back to my old self and realize that my work of the past four years not only gave a concrete contribution to the scientific community, but also helped me grow as a person.”
Salvatore Licciardello will defend his doctoral thesis on Monday September 30, 4.30 pm
Supervisor: prof. dr. N.E. Hussey