Nitrides for quantum tunnelling and topological phenomena

Gallium-nitride (GaN) and its compounds have increasingly attracted attention over the last decades, particularly due to their remarkably broad spectrum of technologically relevant applications. While nitride-based compounds undoubtedly already prevail as building-blocks for visible and ultraviolet solid-state optoelectronic and high power electronic devices, plenty of room is left to these material systems for prospective functionalities in the fields of spintronics, magnetism, superconductivity, and at their cross-road. In the frame of previously granted projects, we have developed an extensive protocol of epitaxial growth, advanced (nano)characterization and theoretical analysis. This has allowed us to demonstrate that the controlled correlation and reproducibility of fabrication parameters, structural arrangement and physical/chemical properties of magnetically doped nitrides leads to striking functionalities, including e.g.: [i] ferromagnetic interaction in the absence of free carriers; [ii] engineering of magnetic complexes that extend the photonic performance of GaN-based materials to the infrared range and allow to adjust the spin state of the system; [iii] assembly of planar arrays of magnetic nanocrystals embedded in GaN with tunable magnetic (ferromagnetic to antiferromagnetic) response. The key objective of this project is to exploit the recent advancements to develop nitride-based hybrid structures that combine semiconducting, (ferro)magnetic and superconducting properties in the presence of spin-orbit coupling to: demonstrate spintronic functionalities (spin Hall and spin filtering effects); test GaN and (Ga,Mn)N based structures as potential topological insulators; and look for signatures of Majorana fermions originating from superconducting and ferromagnetic proximity effects. Substantially based on the study of the interplay between exchange interaction, spin-orbit coupling and superconducting correlations, the proposed research plan is likely to have remarkable impact specifically on the functionalities of a key material system and generally on the prospects of topological matter and topological excitations, expected to play a major role in the forthcoming developments of information processing.

The understanding and control of novel material combinations responding to the ever more challenging requirements of current and prospective functionalities are among the principal aims of nowadays research. In the frame of this project, we have refined the protocol of fabrication and advanced characterization that we had developed earlier and we have extended it to new classes of materials and structures, with the aim of identifying emerging mechanisms originating at the interface of (magnetic)semiconductors, topological insulators, 2D layered systems and superconductors. In particular, we have demonstrated: (i) the potential of non-magnetic nitride-based heterostructures (already highly relevant in the fields of photonics and high-power electronics) as hosts of spin currents; (ii) the engineering of magnetization in magnetically doped nitrides via an electric field and through a piezoelectromagnetic effect; (iii) the possibility to have crystalline magnetic topological insulators with Hall effects insensitive to disturbances and defects. Moreover, we have paved the way to the understanding and control of antiferromagnetic 2D layered structures and designed a system combining ultra-sensitive Andreev reflection and microscopy and cryogenic temperatures and high magnetic fields, which has the potential to allow the study of exotic physical objects, like, e.g. Majorana fermions stabilized at the step-edges of topological surfaces. We are confident, that our results open wide avenues for hybrid heterostructures not only as building-blocks for the next generation of non-volatile, energy-sparing devices, but also as, active elements for the detection of, for instance, elusive dark matter.