Underlying electronic orders of the phase diagram of cuprates probed by Raman spectroscopy

Abstract

Superconductivity is a state of matter in which materials stop displaying electrical resistance. This property forms the basis to numerous technological advances and it could revolutionize the way we carry and store energy. Unfortunately, this state only reveals itself at very low temperatures.In order to avoid this limitation, a better understanding of the microscopic processes giving rise to superconductivity is needed. Unfortunately, the theories that were devised in the 50s do not apply to a certain class of superconductors, dubbed as non-conventional.Among those non-conventional superconductors, cuprates, discovered in 1986, form the subject of this thesis. They possess a rather high superconducting transition temperature, and they display numerous electronic orders which interact or compete with superconductivity.Thanks to electronic Raman spectroscopy, an optical measurement technique allowing one to probe the electronic excitations of a material and to determine their symmetry, we study several orders of the cuprate phase diagram.In Bi-2212, a bismuth-based cuprate, we study the nematic fluctuations that were recently discovered in the normal state, and we try to assess whether a nematic order is behind the pseudogap phase of cuprates. Our results seem to indicate that it is not the case, though nematic fluctuations are enhanced at the closing of the pseudogap phase, which coincides with a Lifshitz transition in Bi-2212.In the mercury-based cuprates Hg-1201 and Hg-1223, we measure for the first time the evolution of the energy of the superconducting gap under hydrostatic pressure. We highlight an unexpected collapse of this energy at the anti-nodes. Our measurements shed light on the relationship between doping and pressure in cuprates, as well as on the doping control in Hg-1223, and on the effect of pressure upon the modes of mechanic vibration in mercurates.