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.
The research proposed in the Project has a fundamental character as well as important applicative implications. SC in general is now experiencing a renewed interest, particularly in the field of HTSC, where recent experiments unambiguously demonstrate the ubiquitous presence of CDW, thereby boosting the theoretical debate about the SC-CDW interplay. In the more recent field of oxide interfaces, the relevance of inhomogeneity and of strong RSOC have raised several questions and proposals. Also the field of TDSCF is attracting a renewed interest for the many unsolved fundamental issues and for the electronic applicative potentialities.
Our project is at the core of these hot and intense activities, as it mainly deals with the interplay between SC and electron inhomogeneities (or charge ordering) giving rise to density fluctuations (both dynamical or static), and investigates the inhomogeneity as an intrinsic emergent character in low-dimensional superconducting electron systems. The idea that inhomogeneity is not an extrinsic epiphenomenon, but may be a main physical actor of low-dimensional SC is a new perspective, which has emerged only recently and in various forms in the different systems mentioned above. The timeliness and hotness of the proposed research is naturally witnessed by the reference list below, which mostly contains very recent papers.
In HTSC, although the idea of fluctuating charge ordering as a particular form of electron softness was proposed by our group long ago, but the recent experimental confirmations, the new experimental collaborations (in RIXS and transport) allow to make more precise and rich the theoretical scenario. There is a huge activity in this field and our group is in a favourable position to answer and systematise the several open issues raised by experiments. This would bring important progress in a field that has always been broadly investigated.
The field of oxide interfaces is instead more recent and it attracted great attention in the last decade. Also in this field our group played a relevant role pointing out the intrinsically inhomogeneous character of LAO/STO interfaces[5-10] and providing possible microscopic mechanisms to account for it. Again, the understanding of the electron softness mechanisms would entail the possibility of practical exploitation. In particular a high degree of innovation would lay in the possibility of designing devices with tailored gating and RSOC inhomogeneity[11]. This could open the way to spintronic devices and to the formation of Majorana fermions in these interfaces. In this latter regard, we also aim at exploring the possibility that electron softness may induce self-generated interfaces hosting a random gas of Majorana fermions. Manipulating, displacing and braiding them would be a major achievement in solid state and quantum computation.
The field of disordered TDSCF is also very promising, since these materials are the building blocks to realise superconducting micro-resonators, to be used as highly-sensitive photons detector. However, to this aim it is crucial to establish the optical properties, especially in the sub-gap regions where the detection mechanism is at play[12]. In addition, arrays of nanograins of conventional superconductors represent an ideal playground to test the mechanism of nanoscale-engineered inhomogeneity as a route to the Tc enhancement, with the aim of extending it to nanostructures of unconventional superconductors.
BIBLIOGRAPHY
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[2] C. Castellani et al. Z. Phys. B 103, 137 (1997);
[3] G. Seibold, et al., Physica C 481, 132 (2012).
[4] F. Kretzschmar, et al., Nat. Phys., DOI: 10.1038/NPHYS3634, (2016)
[5] S. Caprara, et al., Phys. Rev. B 84, 014514 (2011);
[6]D. Bucheli, et al., New J. Phys. 15, 023014 (2013);
[7]J. Biscaras, et al., Nat. Mater. 12, 542 (2013);
[8]S. Caprara, et al., Phys. Rev. B 88, 020504(R) (2013).
[9] N. Scopigno, et al., Phys. Rev. Lett. 116, 026804 (2016).
[10] S. Caprara, et al., Phys. Rev. Lett. 109, 196401 (2012).
[11] G. Seibold, et al., EPL 112, 17004 (2015).
[12] T.Cea et al, PRB 89, 174506 (2014).
[13] Y. Peng et al., Nature Mater. (2018), doi:10.1038/s41563-018-0108-3
[14] G. Singh, et al., Nature Commun. 9, 407 (2018).
[15] G. Dezi, et al., arXiv:1706.01274
[16] G. Seibold, et al., Phys. Rev. Lett. 119, 256801 (2017).
[17] I. Maccari, L. Benfatto and C. Castellani, Phys. Rev. B 96, 060508 (R) (2018).
[18] I. Roy et al., arXiv:1805.05193