Seminars 2021

A Majorana fermion is a special type of fundamental particle which is its own antiparticle. The possible realization of these exotic Majorana fermions as quasiparticle excitations in condensed matter physics has created much excitement. Most recent studies have focused on Majorana bound states which can serve as topological qubits. More generally, akin to elementary particles, Majorana fermions can propagate and display linear dispersion. These excitations have not yet been directly observed, and can also be used for quantum information processing. This talk is focused on our recent work in realizing dispersing Majorana modes. I will describe the conditions under which such states can be realized in condensed matter systems and what their signatures are. Finally, I will describe our scanning tunneling experiments of domain walls in the superconductor FeSe0.45Te0.55, which might potentially be first realization of dispersing Majorana states in 1D.

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Van der Waals materials provide an ideal platform to explore superconductivity in the presence of strong electronic correlations, which are detrimental of the conventional phonon-mediated Cooper pairing in the BCSEliashberg theory and, simultaneously, promote magnetic fluctuations. Despite recent progress in understanding superconductivity in layered materials, the glue pairing mechanism remains largely unexplored in the singlelayer limit, where electron- electron interactions are dramatically enhanced. In this talk, I will present experimental evidence of unconventional Cooper pairing mediated by magnetic excitations in monolayers of Se -based transition metal superconductors (NbSe2 and TaSe2), two model strongly correlated 2D materials. Our high-resolution spectroscopic measurements (STS) reveal a characteristic spin resonance excitation in the density of states that emerges from the quasiparticle coupling to a collective bosonic mode. This resonance gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values, which sets an unambiguous link to the superconducting state. Furthermore, we find clear anticorrelation between the energy of the spin resonance and the local superconducting gap, which invokes pairing of electronic origin associated with spin fluctuations. 2D TMD materials will reduce the enormous complexity associated with the investigation of unconventional superconductivity, and will rapidly allow us to expand our current limited knowledge of non-phononic Cooper pairing. They offer unprecedented simplicity for modelling as compared to the most studied bulky unconventional superconductors, i.e., cuprates, Fe-pnictides and heavy-fermion compounds. In two dimensions, TMD superconductors are even simpler to model than twisted bilayer graphene, where superconductivity is intrinsically linked to specific magic angles. From the experimental point of view, our work opens the tantalizing possibility to explore unconventional superconductivity in simple, scalable and widely accessible 2D materials.

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