Inhomogeneity, competing orders, and topological excitations in two-dimensional superconductors
We focus on two-dimensional (2D) or layered superconductors (SC), namely high-temperature superconductors (HTSC), like cuprates and pnictides, oxide interfaces like LaAlO3/SrTiO3 (LAO/STO), and thin disordered superconducting films (TDSCF) like InO, TiN, NbN, and transition metal dichalcogenides, which all show a superconductor-to-insulator transition (SIT) when their electron density is varied. In these systems superconductivity often competes with other phases (insulating, magnetic, charge-ordered, ...) giving rise to rich phase diagrams, new phenomena and possible device engineering with strongly different responses to small stimuli. A major issue in these systems is the common formation of an intrinsically inhomogeneous state: In HTSC cuprates this takes the form of (dynamical or static) charge ordering, which is ubiquitously found in these systems. Its formation mechanism and role in determining the high critical temperature and the anomalous properties is a central issue of this field and will be investigated here. Regarding the SIT in thin films, the role of inhomogeneity on the optical properties of collective phase and amplitude excitations of the superconducting order parameter will be studied, comparing the results with the outcomes in arrays of Al nanograins and other structurally inhomogeneous systems.
The two-dimensional electron gas formed at oxide interfaces gives rise to an inhomogeneous superconducting state possibly realising the long-sought quantum Griffiths phase with rare regions markedly affecting the properties of the system. These (inhomogeneous) systems also have interesting properties for spintronics like spin-Hall and spin-Galvanic effects, with intriguing applicative consequences, The simultaneous presence of SC and Rashba Spin-Orbit Coupling also opens the way to the occurrence of topologically non-trivial phases and Majorana fermions, whose stability in the presence of disorder and electron-phonon coupling will be studied.