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| Author(s): |
Thaís Victa Trevisan
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | Campinas, SP. |
| Institution: | Universidade Estadual de Campinas (UNICAMP). Instituto de Física Gleb Wataghin |
| Defense date: | 2019-09-11 |
| Examining board members: |
Amir Ordacgi Caldeira;
Pascoal José Giglio Pagliuso;
Eduardo Miranda;
Mucio Amado Continentino;
Álvaro Ferraz
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| Advisor: | Amir Ordacgi Caldeira |
| Abstract | |
This thesis is organized in two big blocks, where we investigate two distinct exotic phenomena in low-dimensional systems of interacting electrons. In the first half of this thesis, we address the experimentally observed magnetic anisotropy of aromatic molecules. Our goal is to formulate a microscopic minimal model to describe the fundamental physics behind this curious and controversial phenomenon. We argue that, on the contrary of the main idea of the Ring Current Model, the degrees of freedom of the ? -electrons (i.e., those occupying the pz orbitals of the aromatic ring) are not enough to properly describe the magnetic properties of aromatic molecules. We derive an extension of the Hubbard model where a momentum-momentum effective attractive interaction between the ? -electrons is mediated by virtual excitations of the ?-electrons (i.e., those occupying the sp2 hybridized orbitals in the molecule's plane). In the second half of this thesis, we investigate the suppression of superconductivity in a two-band superconductor in a regime where one of the bands is incipient, i.e., in the limit where its bottom is just above (or below) the Fermi level. In a multiband superconductor, multiple conduction bands can cross the Fermi level simultaneously, originating, at a temperature below the superconducting transition temperature Tc), multiple superconducting gaps, one in each of the bands. By increasing the system's electronic density (n), such that a new band becomes populates, another Fermi pocket emerges in the Fermi surface, signaling a Lifshitz transition. Such a transition leaves a signature in the behavior of Tc (n). Contrary to what is expected, it was recently observed a suppression of Tc close to a Lifshitz transition in two paradigmatic examples of multiband superconductors, the SrTiO3 (STO) and the LaAlO3/SrTiO3 (LAO/STO) interfaces. Using a mean-field approach, we explained this counter-intuitive result as an effect of non-magnetic impurities, which, as evidenced by resistivity data, cannot be neglected in these systems. We show that there is a competition between two opposite effects in the vicinity of the Lifshitz transition of a two-band superconductor with dominant intraband pairing interaction and subleading interband pairing interaction: on the one hand, Tc tends to increase as a result of the enhancement of the system's density of states as the second band appears. On the other hand, interband electronic scattering processes due to the presence of disorder start to happen as the second band becomes populated, which breaks the Cooper pairs and, therefore, has a detrimental effect on superconductivity. When disorder is strong enough, the second effect wins, resulting in a suppression of Tc. Our model also suggests an unconventional nature for superconductivity in both STO and LAO/STO interfaces, a topic that remains open and highly debated in the literature. Furthermore, our model also predicts a change in the symmetry, from s+- to s++, of the superconducting state as a function of n, which can be experimentally verified. This work was done at the University of Minnesota, under the supervision of Professor Rafael Fernandes and in collaboration with the Postdoctoral researcher Michael Schütt, during the one-year scholarship supported by Fapesp BEPE fellowship No. 2016/12874-3 (AU) | |
| FAPESP's process: | 15/21349-7 - Current-current interaction in low dimensional electronic systems |
| Grantee: | Thaís Victa Trevisan |
| Support Opportunities: | Scholarships in Brazil - Doctorate |