My stay at UNIST is coming to an end. Since arriving here nearly 3 months ago, I observed the end of a presidency, the start of a beautiful spring and the continuation of missile threats. Now, the latter is not much of a topic in Korea and is certainly amplified in western media. But culminating in the impeachment process, it is nice to see a lively democratic
movement in this country and the cherry blossom in April is definitely something you have to see for yourself.
I was warmly welcomed by Prof. Kwang-Soo Kim and his group on day 1 at the Center for Superfunctional Materials. Over the course of two seminars I was able to present and explain the project and goals, which initiated interesting discussions. UNIST, established less than 10 years ago, is a very modern university aiming to attract top researches from all over the world – and as such the Campus is very international. Situated in a charming hilly landscape, the bus trip to the center of Ulsan takes almost one hour. But UNIST is a city by its own – with 5 convenience stores, over 4 cafeterias, coffee shops, stationary store, restaurants… and the list goes on. It is designed to focus.
However, you have to get out to see and feel the Korean culture. High-speed trains and super-punctual express buses are a treat. Ancient capital city Gyeongju, charming Daegu or the vibrant Busan are easy to reach and a must-see visit. Even Seoul is only a 2.5h KTX ride away. Indulge in Korean food and culture. Don’t miss San-nakji (산낙지), daege (대게) or one of the many BBQ places for Dak-galbi (닭갈비) or Samgyeopsal (삼겹살), together with a bottle of Soju (소주) or Makgeolli (막걸리).
I will leave you with some photos and get back to work: After discussing various mesoscopic silicon systems and their importance on future directions of nanoelectronics, we focussed on time-dependent transport calculation. In particular, electron transport in silicon dangling bond circuits. Consider a slab of pure Silicon, with its entire surface passivated by Hydrogen atoms. After removing single Hydrogens (e.g. by Scanning tunnelling microscope techniques), a dangling bond state appears on the (now depassivated) Silicon atom. This electronic state is localised, but can hybridise with neighbouring dangling bond states to extended states – and ultimately: wires. Those silicon dangling bond wires are 1D electronic states on the silicon surface, with an energy right inside the band gap of bulk silicon. Therefor, electron transport selectively through those DB states is possible. Thus, those channels can be used to construct arbitrary circuits: from loops, to logic gates, single-molecule contacts or more. Although many groups started working on this subject, transport properties of those systems are not yet well understood, especially in the time-dependent domain. Voltage pulses or AC fields demand for more sophisticated approaches, such as an extension of DFT to TD-DFT and the time-dependent Keldysh formalism TD-NEGF. Using the latter, we solve the problem by recasting the problem in terms of Green functions, where lot of work goes into adapting this to efficient numerical algorithms, such as the Auxiliary Mode Expansion by A. Croy. After implementation, this algorithm allows us to study the electric response to arbitrary time-dependent fields in real-sized DB systems. This is especially noteworthy, because many of the time-dependent studies are on simplistic toy models. However, multiple approaches exist for time-dependent electron transport. The group of Prof. Kwang-Soo Kim, together with Prof. Geunsik Lee, is specialised in a complementary method, called Dynamical Mean Field Theory, which can be combined with the NEGF approach. This theory allows to study strongly correlated materials, where DFT breaks down. Altogether, we developed a strong expertise on time-dependent electron transport with application beyond toy models, where both project partners complemented each other and pushed the research forward. PhD student Abdoreza Ershadiniai from Prof. Kim’s group will join us in Dresden later this year, further extending our collaboration. I also reached out personally to the other collaborators at UNIST, namely the group of Prof. Hyeon-Suk Shin and Prof. Rodney Ruoff of the Chemistry Department, who are delighted to be part of the project and look forward to the Dresden visits and German visitors.
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