Seminars and Events
22 June 2018, 11:00 h. Salón de Actos
Quantum Transport and Topological Features in Two-dimensional Materials
Supervisor(s): Francisco Guinea, Pablo San-José
In this thesis I present three theoretical studies on novel phenomena arising in two-dimensional materials, which can be exploited to tailor their emergent topological properties. In graphene, I propose two schemes for engineering two independent topological phases of matter, both relying on its unconventional low-energy properties. On the one hand, I consider graphene deposited on a magnetic substrate as a possible detector for skyrmions, using as a working principle the anomalous Hall signal produced by these magnetic whirls. The linear semimetallic dispersion of graphene, together with its suitability for extrinsic engineering through proximity, render the detection especially effective. I then focus on a special quantum spin Hall phase with broken time-reversal symmetry that can be generated in twisted graphene bilayers in the quantum Hall state. In the regime of large twisting angles the interplay between electronic interactions and an applied electric field drives the system into the targeted phase, which can be used as a novel platform for topological superconductivity. Finally, I focus on a two-dimensional electron gas with strong spin-orbit coupling in the quantum Hall regime. The study of the topological properties of this system when it is proximitized with a superconducting strip and a Zeeman field is applied parallel to it reveals unconventional features that allow for a novel implementation of a topological $pi$ Josephson junction. This phase is relevant for implementing a universal set of protected single-qubit operations.
15 June 2018, 12:00 h. Salón de Actos
A study of complex magnetic configurations using Magnetic Force Microscopy
Eider Berganza Eguiarte
Supervisor(s): Agustina Asenjo y Miriam Jaafar
The irruption of nanomagnetism in industry has brought remarkable advances in data storage technologies. In addition, further development in this field is expected to revolutionize traditional medicine by addressing diagnosis and disease treatment from a localized approach. Applications require a deep fundamental knowledge on the magnetic behavior of nanostructures. This thesis is framed on the study of non-trivial magnetic configurations and magnetization reversal processes of different nano-objects.
The magnetic configuration and magnetization reversal process of cylindrical shaped magnets (nanowires and nanodots) have been studied using Magnetic Force Microscopy (MFM), a powerful tool to image ferromagnetic structures in the nanoscale.
Variable Field MFM measurements and in particular its advanced modes are decisive to study the pinning mechanisms in nanowires with axial magnetization, where different strategies have been explored in order to obtain a controlled motion of the domain wall.
Cobalt based cylindrical nanowires with strong magnetocrystalline anisotropy have present complex vortex-like magnetic configurations as a result of the interplay of shape and magnetocrystalline anisotropies. Multisegmented nanowires, which combine segments of different ferromagnetic structures, are of particular relevance when it comes to engineer the magnetic domain configuration of nanowires.
Finally, the detection of hedgehog skyrmions in permalloy nanodots (nanowires of short length) represents a major breakthrough due to the absence of perpendicular magnetic anisotropy or Dzyaloshinski-Moriya interaction (DMI) in the system. The behavior of nanodots of diameter below and beyond 100nm are studied and modelled under applied field. In bigger nanodots vortex configurations are present, while smaller nanodots present skyrmions.