Supervisor: Dario Daghero
Superconductivity is a quantum phenomenon based on the formation of a coherent state where the charge carriers are paired electrons (Cooper pairs); this state is energetically favorable and is separated from the single-particle states by an energy gap. These characteristics explain why superconductors have zero electrical resistance, expel magnetic fields from their bulk, and feature other peculiar quantum properties.However, the mechanism responsible for the formation of the Cooper pairs is well explained (by the so-called BCS theory) only in a small subset of the existing superconductors.
The ongoing discovery of superconducting materials that are not described by the BCS theory brings up ever new challenges to the identification of their pairing mechanism, the understanding of which would open the way to the engineering of new superconductors with optimal properties. A fundamental step in this research is the determination of the symmetry of the superconducting order parameter, since it is closely related to the wavefunction of the Cooper pairs, to the mechanism responsible for their formation and to the energy gap. The spectroscopic techniques based on the tunnel effect and on the so-called Andreev reflection are a direct tool to investigate the superconducting order parameter.
Our group is internationally renowned for the expertise in tunnel and Andreev-reflection spectroscopy (see references below) in various families of superconductors (cuprates, MgB2, iron-based superconductors, and more exotic non-centrosymmetric systems). Collaborations are established with ETH Zurich and Bern University (Switzerland), Nagoya and Tokyo Universities (Japan), Leibniz Institute for Solid State and Materials Research (Dresden, Germany), Max Planck Institute for Solid State Research (Stuttgart, Germany) and many others.
These collaborations open the possibility for students to spend time abroad for their research. The thesis here proposed envisages a mainly experimental work, which includes:
- electric transport measurements (resistivity, resistance, critical current) as a function of temperature (0.3 K – 350 K) and/or of the magnetic field (up to 9 Tesla)
- tunnel spectroscopy and /or point-contact spectroscopy at cryogenic temperatures and in magnetic field
- data analysis, possibly including the development or the generalization of models for data fitting
- interpretation of the results in light of the existing theories and the established literature.
Selected References
Daghero and R.S. Gonnelli, Supercond. Sci. Technol. 23, 043001 (2010) (topical review)
Daghero, M. Tortello, G.A. Ummarino, R.S. Gonnelli, Rep. Prog. Phys 74, 124509 (2011)
Daghero et al., Nature Comm. 3, 786 (2012)
R.S. Gonnelli et al., Sci. Rep. 6, 26394 (2016)