Colóquio: High-End Andreev Reflection (Dr. Plamen Stamenov)

High-end Andreev Reflection

Dr. Plamen Stamenov

Point Contact Andreev Reflection (PCAR) is one of the few available methods for the determination of the Fermi level spin polarisation in metals and degenerate semiconductors. It has traditionally been applied at fixed (liquid He) temperatures, using pure niobium as the superconductor, and at essentially zero applied magnetic fields, all of which limit the amount of information that it can provide – i.e. do not allow for the extraction of the sign of the spin polarisation and make the assignment of the transport regime to ballistic or diffusive almost impossible.

Here a series of experiments is described, aimed at the expansion of this parameter space to higher magnetic fields and to higher temperatures. These require redesigned experimental setups and the use of higher performance superconductors. Demonstrations are described of the determination of the sign of the spin polarisation, at fields of more than 5 Tesla using a low-Z superconductor, as well as operations beyond 9.2 K. Doubts about the practical reliability of the PCAR technique are dispersed using systematic series of samples – the heavy rare-earths and comparisons with alternatives, such as spin-polarised field emission, photo-emission and Tedrow-Meservey tunnelling.

The specific material examples presented include 3d-metals, order-disorder transition alloys and zero-moment half-metals – Fe, FeAl and MnRuGa, alternative low-Z and high-Z superconductors – MgB₂ and NbTi, and magnetic topological insulators, such as Cr- and V-doped (Bi_1-xSb_x)₂Te₃.

Dr. Stamenov received his BSc degree from the University of Sofia  (Bulgaria) in both theoretical and experimental physics in 2002, being  lucky enough to have been selected for an individual supervised  specialisation programme.  He was privileged to have to select between  areas in high-energy physics, theoretical non-linear dynamics and  chaos, and magnetism, as directions for his PhD. Having worked on the  on the magnetic and structural properties of some manganese  perovskites (within Prof. Mikhov’s group for more than two years at  this stage) the choice was clearly magnetism.  He later ended up  completing his PhD research in 2007 under the supervision of Prof. J.  M. D. Coey at Trinity College Dublin. This research focused on metals,  semimetals and semiconductors for spin electronics applications. After  completing his PhD, he stayed on in Trinity College as a research  fellow and teaching assistant within the School of Physics and CRANN  conducting research in the field of spin-dependent transport, and  collaborating with industry in applied magnetics and microwave  technology. In 2010, he became the Ussher Lecturer in Physics and  Principal Investigator within CRANN in the area of nanomagnetism, to  change gears once again in 2012, for an SFI SIRG assistant  professorship. Stamenov groups’ research is focussed on a variety of  topics (all related to magnetism) ranging from working on magnetic  materials, magnetic nanostructures and magnetic devices, to developing  advanced characterisation techniques, based on magnetotransport and  applied superconductivity, microwave magnetics and nuclear  spectroscopy. The group is also developing relationships with a number  of Irish SMEs on RF and microwave techniques and materials, magnetic  profiling and characterisation, and magnetic structures within  accelerometers and strip-line circulators. Some of the recent projects  in which the group is involved, include the highly competitive  European Commission FET Open TRANSPIRE, focused on the development of  spin-transfer torque oscillators for the frequency region beyond 300  GHz and the Science Foundation Ireland-funded national access  thin-film deposition facility Trifolium Dubium, to be commissioned in  early 2018.