## Research outline

I am currently interested in topological states of matter, i.e., topological insulators and superconductors, in particular on the following topics:

- Topological superconductors realized in low-dimensional nanostructures, e.g., nanowires or two-dimensional gases with spin-orbit coupling, and in particular on the signatures of the topological phase transitions in these systems. To date, experimental evidence of topological superconductivity has been mainly obtained by measuring the zero-bias conductance peak via tunneling spectroscopy. We proposed[2] an alternative and complementary experimental recipe to detect topological phase transitions in these systems. We have shown in fact that, for a finite-sized system with broken time-reversal symmetry, discontinuities in the Josephson current-phase relation correspond to the presence of zero-energy modes and to a change in the fermion parity of the ground state. Such discontinuities can be experimentally revealed by the characteristic temperature dependence of the current, and can be related to a finite anomalous current at zero phase in systems with broken phase-inversion symmetry;
- Topological transport in one-dimensional quantum charge pumps. These quantum charge pumps can be realized in van der Waals heterostructures, Moirè superlattices, or nanowires in the presence of a spatially modulated perturbation. These systems exhibit a distinctive fractal energy spectra, the Hofstadter butterfly, and show a topological non-trivial phase space where the topological invariant coincides with the quantized charge pumped in each pumping cycle. This quantization is analogous to the quantum Hall effect. The charge transferred in a fraction of the pumping period is instead generally not quantized. We have shown[3] that with specific symmetries in parameter space the charge transferred at well-d actions of the pumping period is quantized as integer fractions of the Chern number;
- p-wave spin-triplet superconductivity in superconductor/ferromagnet heterostructures, with a focus on the interplay between magnetism and superconductivity.

Moreover, I am also interested in the signatures of the superconducting pairing symmetries in unconventional superconductors (in particular cuprates and pnictides), which can be in principle probed by resonant inelastic x-ray scattering (RIXS).

**Research groups:**

**Publications**

[1] “Competition between intrinsic and extrinsic effects in the quenching of the superconducting state in Fe(Se,Te) thin films”, Antonio Leo, Pasquale Marra, Gaia Grimaldi, Roberta Citro, Shrikant Kawale, Emilio Bellingeri, Carlo Ferdeghini, Sandro Pace, and Angela Nigro, arXiv:1510.05662 (2015), accepted for publication in Physical Review B;

[2] “Signatures of topological phase transitions in Josephson current-phase discontinuities”, Pasquale Marra, Roberta Citro, and Alessandro Braggio, arXiv:1508.01799 (2015), currently under review at Physical Review Letters;

[3] “Fractional quantization of the topological charge pumping in a one-dimensional superlattice”, Pasquale Marra, Roberta Citro, and Carmine Ortix, Phys. Rev. B 91, 125411 (2015), arXiv:1408.4457;

[4] “Theoretical approach to resonant inelastic x-ray scattering in iron-based superconductors at the energy scale of the superconducting gap”, Pasquale Marra, Steffen Sykora, and Jeroen van den Brink, arXiv:1405.5556 (2014);

[5] “Resonant inelastic x-ray scattering as a probe of the phase and excitations of the order parameter of superconductors”, Pasquale Marra, Steffen Sykora, Krzysztof Wohlfeld, and Jeroen van den Brink, Phys. Rev. Lett. 110, 117005 (2013), arXiv:1212.0112;

[6] “Unraveling orbital correlations with magnetic resonant inelastic x-ray scattering”, Pasquale Marra, Krzysztof Wohlfeld, and Jeroen van den Brink, Phys. Rev. Lett. 109, 117401 (2012), arXiv:1205.4940;

[7] “Fingerprints of orbital physics in magnetic resonant inelastic x-ray scattering”, Pasquale Marra, AIP Conf. Proc. 1485, 297 (2012), arXiv:1302.5028;

[8] “Paraconductivity of the K-doped SrFe2 As2 superconductor”, Pasquale Marra, Angela Nigro, Zheng Li, Gen-Fu Chen, Nan-Lin Wang, Jian-Lin Luo, and Canio Noce, New J. Phys. 14, 043001 (2012), arXiv:1203.6592;

**Title**

**Projects**