Superconductivity in Chromium-pnictides: A Matrix Random Phase Approximation analysis

Abstract

Unconventional superconductivity has been an important topic of research since its dis- covery in CeCu2 Si2 (with Tc = 0.7K), by Frank Steglich and collaborators in 1979. Its importance greatly increased in 1986 when Bednorz and Müller discovered superconduc- tivity with Tc = 35K in a cuprate ceramic material. Since then, several different families of unconventional superconductors have been discovered. In all of them, the pairing mechanism is believed to not be via phonons, thus the ‘unconventional’ sobriquet. All of these families clearly present electronic correlations, which are universally believed to be involved in the pairing mechanism. The fact that the many-body problem involved has no known solution explains the longevity of this problem. In this dissertation, we apply the Matrix Random Phase Approximation to study superconductivity in LaCrAsO. We vary the chemical potential to induce a Lifshitz transition, i.e., a change in the topology of the Fermi surface, and study superconductivity across this transition. We find that spin singlet pairing is induced by spin fluctuations of ferromagnetic character, which results in the triplet pairing being competitive with the singlet pairing, although not dominant. In addition, the dxy (B2g ) symmetry of the gap function (with nodes along the coordinate axes)does not vary across the Lifshitz transition. Finally, across the Lifshitz transition, the outer edges of the electron pockets, close to the Brillouin zone boundary, accumulate the majority of the amplitude of the gap function, while the hole pocket (located around the Brillouin zone center) presents almost no gap function amplitude.