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Marcel Hoek

At the interface between electron & hole-doped cuprates

Promotion date: August 28.

Promotor: Prof. dr. ir. Hans Hilgenkamp

The combination of high temperature superconductors with different carrier types is described, specifically the interaction between electrons and holes in the cuprates Nd2-xCexCuO4 (NCCO, electron-doped) and La2-xSrxCuO4 (LSCO, hole-doped). The idea of combining oppositely doped cuprates has led to theoretical predictions ranging from unusual Josephson effects to exciton formation and Bose-Einstein condensation.

The materials are grown as thin films by pulsed laser deposition (PLD). The requirements for successful combination of LSCO and NCCO are studied, and in the process a technique has been developed to greatly improve the quality and properties of the NCCO. The materials in the (a,b)-plane through ramp-edge junctions and in the c-axis direction through bilayers, were combined. For both types of junctions an insulating barrier at the interface was found.

For the ramp-edge junctions, low temperature transport measurements were used and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), to show that this barrier is caused by a combination of electronic depletion in the NCCO and strain in the LSCO. Furthermore, a tilting of the LSCO lattice is found on the ramp of the ramp-edge junctions, using both HAADF-STEM and nano-focus X-ray diffraction (nXRD). The lattice tilt is explained by strain accommodation and can be described using a geometrical model. It is shown that the NCCO/LSCO contact is crystalline and that ramp-edge structures can potentially be used to create artificial domain structures.

Was your work fundamental in nature?

Yes, very much so. Exploring the semiconductor-like properties of the high temperature superconductors in devices may help theorists and experimentalists to find further insight into the fundamentals of materials science. High temperature superconductors are widely used in experiments and even in some applications, but the fundamental mechanism behind the superconductivity in these materials is still not understood. This makes them interesting materials to study.

In our experiments we have explored the interface between two high temperature superconductors with opposite charge carrier type in the hope of learning more about the underlying physics.

What can Mesa+ and, more specifically, the Interfaces & Correlates Electrons (ICE) Group, contribute to this worldwide quest?

Here in Twente we are leading in building devices needed for these scientific research ends. Traditionally pulsed laser deposition is a distinctive technique in Twente. On top of that, we have a long tradition in device fabrication, which was very useful for the ramp-edge junctions I fabricated in my PhD project.

In the project, we have taken great care to ensure the quality of the devices we fabricated. This would not have been possible without collaborations with groups at the European Synchrotron Radiation facility (ESRF) in Grenoble, at the University of Antwerp (for transmission electron microscopy) and at the National University of Singapore (for X-ray absorption experiments). It is through the network of the people working at Mesa+ and ICE that these collaborations were possible.

Do you recall some special moments during your PhD work?

Demonstrating the tilting of the LSCO lattice was an unexpected result, never observed or published about before. Several hurdles had to be overcome here. As the measurements were performed in another lab, I first had to figure out the scanning strategy used. Once I was able to reconstruct the results and make reliable images from them, I started to understand the tilting phenomena and could propose a solid theoretical base, to explain why this tilting behavior took place.

I enjoyed working with experts from universities and research centers abroad, especially with the European Synchotron Radiation Facility (ESRF) in Grenoble. The collaboration started when our group was joined by an Italian post-doc who had beam time available at ESRF. By studying the interfaces closely, we were able to show the tilting behavior and could confirm these findings by specialized transmission electron microscopy (TEM) measurements performed in Antwerp.

In which journals did you publish your results?

One paper was published in the Journal of Superconductor Science & Technology. During my PhD I have also co-authored several papers on topological insulators, materials I worked on during my Masters in the ICE group (Interfaces and Correlated Electron Systems). These have appeared in Nature Materials, Physica Status Solidi RRL and Physical Review B. I am still working on two or three papers on my PhD work at this very moment.

What are your future plans?

I am applying for post-doc positions, preferably abroad. A position in the United States would be great. A post-doc abroad is favorable for starting an academic career and it would be a once in a life chance to do so now. I like the academic world, as I am always curious about the latest developments. The conferences I attended all were highlights in my PhD period.

What in your opinion is important for Mesa+ to stay successful in future?

Mesa+ is a renowned institution all over the world. The fabrication facilities at Mesa+ are of world-class level, especially laser deposition. The collaboration with universities and colleagues abroad is extensive, making the expertise and impact of Mesa+ even broader.

The ICE Group shares a lot of lab facilities with the Inorganic Materials Science Group. I value the initiatives of Mesa+, to facilitate the collaboration and expertise sharing of the groups involved in thin film physics. Next to ICE and IMS, also Physics of Interfaces and Nanomaterials (PIN) and NanoElectronics were involved. It is interesting to hear of the work of colleagues from other groups and to share expertise on using shared cleanroom facilities. This is already taking place in the nanomaterials colloquia, which I found very interesting.