Author Archives: Dongwook

A little side project that can be useful for junior-level reserachers

I have always thought, it would be nice to make a series notes of essential toolkits for everyone to know in the field of magnetism/spintronics. Sure, I am talking about theroetical tools — I am quite ignorant of any experimental tools, even rudimentary ones like x-ray diffraction and Hall measurements. One of the main motivations behind this project is that unlike other established fields, in magnetism/spintronics, there is no single textbook that makes you prepare for real research projects.

For example, when I was beginning my graduate study, everyone on the condensed matter theory track was studying one of famous many-body physics books; Mahan, Coleman, Altland&Simons, Bruus&Flensberg, etc. I was not an exception. I studied some of them, and I even read all chapters of Altland&Simons and Bruus&Flensburg thorougly. Practically, Bruus&Flensburg was the most helpful, on the practical side. I understood the main ideas and detailed procedures for calculating transport coefficeints by the Kubo formalism and incorporating many-body effects (quenched disorders, electron-electron interaction, etc.). Well, Altland&Simons is an excellent textbook for an overview of research topics involving many-body physics, but it was mostly for my own intellectual pleasure (I am quite sure that the concepts and ways of thinking really helped me, but what I mean here is I do not practically use functional integral methods in my own research).

I think it was from the second and third year of my graduate study, when I started to feel what I learn from the textbooks is quite far from what I was duing in my research project. Now, looking back previous years and gaining more experience, I start to see that in the field of magnetism/spintronics, you really need the T-shape knowledge: Together with a strong core methodology (the vertical line of “T”), a diverse background (the horizontal line in “T”) is also necessary. When I observe how my fellow researchers and seniors do their research works, even if they are known for particular techniques, they also use many other techniques and knowledge.

In my case, the core methodology that I currently employ is large-scale simulation of real materials and analysis of the electronic structure. But at the same time, I often need knowledge of symmetry analysis, quantum transport theory, Berry phase physics, and spin dynamics.

So, what topics I have in mind for the little side project? I still need to think more about it, to be honest. For the moment, I think of Kubo formula (and many different versions that appear in papers), symmetry analysis of response functions (also known as the Neumann’s principle), what people mean by Berry phase and topology, simple python programming for calculating band structures, producing high-quality figures with matplotlib — I think they are what I use “everyday”.

Yes, you don’t really need a lot of techniques to do research. But I want to emphasize that what is more important is developing ideas and setteling those ideas by simple but concrete calculations. In this regard, discussion with colleagues is invaluable!

Lots of things happened in 2022

I realized that I did not post any blog article since summer. I would like to tell everyone that I am well and not dead. But the year of 2022 was the busiest moment so far in my academic career.

First, now that there is almost no Covid-related restrictions, I suddenly started to travel a lot and give talks here and there. By simply counting, I noticed that I gave 19 talks in 2022! I think it was pretty good in that I got many great opportunities to share my enthusiasm on orbitronics, and I indeed enjoyed discussing with different audiences. For seminars, I enjoyed a chance to connect with researchers during my stay, and during 45 mins I could really show how I think rather than throwing recent results. For invited talks, it was a bit frustrating because I had quite a lot of result to cover, but I also wanted to share my viewpoint and expertise. It’s alwys a challenge to spend 20 mins efficiently. But in conferences, giving invited talks is a great chance to let new people know me, and often after the talk, I had a chance to share some time for a coffee or meal in the evening. I didn’t give many contributed talks this year, but during the DPG Meeting in September, I really enjoeyd giving a 12 min talk! It is another challenge to present with a sharp focus, but it felt quite good to focus only one thing at a time.

Second, I organized SPICE Workshop on Orbitronics: from Topological Matter to Next-Level Electronics together with Tatiana G. Rappoport and Henri Jaffrès. It was a great experience to learn how to organize a scientific event. Certainly, the most important thing is to prepare a good scientific program that serves the community, where participants can learn new things, exchange expertise, and most importantly, get to know each other personally and become friends. There are other important aspects that I learned, such as diversity of participants regardless of their gender, sex, ethnic group, cultural background, and career stage (junior/senior), choice of the date that is convenient for most people, distribution of workloads between the organizers, and logistics (fortunately, this was largely covered by SPICE staffs, whom I thank sincerely). For our workshop this year, it was also quite special because it was the first international workshop on orbitronics, which just emerged in recent years.

Finally, I spent enormous amount time for my career proposals to start a junior-leader-level position in Germany. To be honest, except for the Humboldt fellowship which I wrote in the beginning of my postdoc (I failed to get one at the end), I didn’t have a chance to write a proper, rather long research proposal. Sure, everything was new and challenging. Although I still have lots of things to learn on research proposals, I find it extremely important to think from the perspective of review pannels and funding agencies. At the end, the main purpose of the proposal is not expressing myself (although it’s still important aspect), but rather convincing other people. Also, it’s an art of balance bewteen my grand ambition and limited resources and feasibility. Fortunately, I had many kind senior researchers who gave me valuable advices and willingly helped me whenever I asked for tips.

Overall, I think the year 2022 was full of new challenges and learning opportunities, which are directly related to doing actual research works. At the end, I am glad that I pushed myself to do these, getting out of my comfort zone, and I think I learned really lots of things. At the same time, I also honestly worry that I had little time for doing research. I really miss the time when I had enough time to do all the dirty works, struggle for days and weeks for technical issues, and then learn new techniques on a daily basis. So, I am actually thinking of prioritizing on my own research works and saying “no” to other unessential parts (this is a tricky to decide though) from 2023.

Young Researcher’s Workshop

In this week, I participated in SPICE Young Research Leaders Group Workshop, which was organized by Alex and Helena. It was held in Ingelheim, a charming village located not so far from Mainz and famous for many wineries in the neighborhood. As the workshop title says, only junior-level researchers could join the workshop, and the participants were mostly postdocs and junior/assistant professors. The workshop provided a unique opportunity not only to discuss recent progress in magnetism in depth, especially from the point of view of young researchers, but also to get to know each other and build a strong connection that goes beyond “collaborators”. During the workshop, we chatted on many different topics, from funny and pleasant things to more serious stuffs such as career/job and frustrating and challenging aspects in academia.

Sure, scientific contents of the workshop was undoubtly excellent, and many questions were raised in every talk. Also, listening to talks from other junior colleagues gave me nice stimulus and inspiration.

On my side, I presented the usual “orbitronics”, with emphasis on main general ideas, opportunities, impact on other fields, and future prospects. The slide that I presented can be downloaded from this link. I think my talk went pretty well although I slightly went overtime. Most questions raised were serious enough and important, for which I do not have clear answer either, but it was very good to openly discuss (instead of pretending to know…).

I hope I can participate in the nexst workshop too, if possible, but I think it’s fair for other young researchers to have a chance as well. In any case, I hope the workshop continues to occur every year.

DFT lectures

JGU Mainz campus

In the last week, just before the Easter holidays, I visited Mainz for giving lectures on the density functional theory. It was an intensive course style, rather than a broad introduction over a semester, and I dealt with lots of practical aspects. I skipped all formal stuffs such as the Hohenberg-Kohm theorem and the Kohn-Sham formulation. I think these are basics that PhD students should know already (especially if one is planning to do theory). So, I started from the discussion on the choice of basis sets (e.g. plane wave, localized, augmented plane wave, etc.) and spent most of the time on the properties of the LAPW basis, which is the implementation that I use within the FLEUR code.

My motivation was to teach that an “optimal” choice of computation parameters is not really something that can only be learned only “by practice” (still required though) but something that naturally follows by understanding properties of the basis set. If one first encounters a DFT code that implements the FLAPW method, it is quite often that one is overwhelmed by so large number of parameters that has to be set. Well, the accuracy of the method comes at a cost of sophisticated parametrization. But if one understands the propertiees of the LAPW basis and its implementation, it becomes quite obvious that only a few paramters are really essential and the rest of them naturally follow.

I think my lecture sent quite well, and people seemed to be satisfied. Well, this has been only the first part, and the second part will continue in 2 weeks. My goal is for everyone to be able to calculate the anomalous Hall effect and spin-orbit torques at the end of the lecture series 🙂

Trip to Marseille

In the previous week, I visited a colleague of mine in Marseille. Although I have been to several workshops within Germany in recent years, I just realized that this was my first busienss trip to a foreign country since 2020, which excited me a bit. As I am trying to be more responsible for my consequences on the environment, I took trains from Aachen to Marseille, instead of taking airplanes. It took roughly 10 hours. But it wasn’t that bad as I had an internet and I could work during the trip.

The place where I visited was CINaM. It is located a bit far from the city center, but it’s surrounded by the beautiful nature. The institute covers wide area of “nanoscience”, which include chemists, material scientists, and physicists, etc. So, it was a bit challenge for me to give a seminar in front of people with various interests and backgrounds (but it went well, I think).

I spent effectively 4 days in Marseille, and I had great time there. Sure, scientific discussion we had were fantastic. It was so great to discuss in depth, which have totally missed during covid times. I got quite impressed that we discussed for 3 hours with 15 slides. This somehow reminded me of my PhD time. It was also a kind of small group, and we discussed quite intensively (still relaxed though) without worrying much about the time in group meetings. In fact, the atmosphere of the group was really similar to my former group!

By the way, the members of the group took me out for dinner and drinks in the evenings. I must admit that everyone was so nice to me, and I am quite happy that I had a chance to know nice people. We discussed not just science but also our lives and many other stuffs. Thanks to them, I could try some local restaurants with great food, away from tourists. Sure, I was very glad that I could try nice fresh seafood, which is hard to find in Germany.

Port area of Marseille in the night

I got covid

Finally, covid has come to me. I started to cough and have headache. For a few days in the beginnin, all the self-tests that I performed were negative, but I was quite certain that this is covid because it has been around since a few weeks ago, at the workplace and accoreding to the contact-tracing app (Corona-Warn). I got my first positive result, 3 days after I got symptoms. So, I could get a PCR test (which also showed positive).

The first thing that I did was to tell my friends because I hanged out with them a day before I had symptoms. Fortunately, nobody got it after me. Then, I decided to live and sleep in the livingroom in my flat so that my partner does not get covid because of me. Fortunately, she didn’t get covid yet, so I guess this strategy has been successful so far.

I somehow felt that I would get covid quite soon. The omiron variant is quite invasive to the immune system despite multiple vaccinations. The infection rate has been quite high in NRW Germany in the last months. Also, I have travelled Ireland recently, and hanged out in bars without wearing masks with my friends. Well, I wasn’t cautious enough as in the beginning of the lockdown. Probably I am getting tired of this constraint social life, or simply because I got bolder as I got a booster shot.

Fortunately, my symptoms have been very mild except for the first 1-2 days. I feel completely normal although I get tired easily. Today I felt much better in the morning and it’s been already 4 days, so I did test. But the result was so obvious positive yet. On this Wednesday, it becomes officially a week, when I can be free from the isolation if I get tested negative.

I just spend my days working at home. In the evening, I suddenly have a lot of time alone, so I also try to read many papers. I am particularly into old, milestone papers nowadays.

Vacation in Ireland

In the last weekend, I visited Ireland for a short vacation as covid restrictions were mostly lifted. I had great time there. To be honest, I didn’t know much about the country. Thanks to the trip, I had a chance to learn the people, their land, and their culture. At the end, I fell in love with Irish culture. Pubs in every cornerof the street, which also serve very nice Irish specialties. Inside pubs, it’s quite noisy and full of friendly people and I could also hear traditional Irish music and songs. But once I went outside of the busy urban area, I discovered wonderful nature that they have, from wide green field, where you can easily spot several cows and sheep, and breathtaking views of cliffs and coastlines. Although this trip was quite short, I will definitely visit this country again and spend more time.

Dublin city center

Trinity college and long room library

Amazing view of Moher cliff and west coast line

Galway, a cultural capital of western part of Ireland

Sure, one cannot miss their favourite sports, rugby, and great Irish beers.

Why I like giving seminar talks in small groups

In 2022, so far I have given three group seminars, roughly one per month. I think all of them went quite nice, not only for delivering the knowledge but also for exchanging ideas. What I like in particular was “friendly atmosphere” in which everyone asks questions and shares opinions freely (Probably it’s because they are in their home ground?). Many of them readily shared their preliminary data and asked me for my comments even though I am not sure if I am really helpful or not. Anyway, for this reason, a seminar talk often works as a trigger for the initiation of the collaboration.

I enjoy giving a talk because arranging various concepts and topics in a coherent manner gives me a fresh look on the subject that I thought I am quite familiar with. Also, each research group has different interest and strength. This means that I tend to look at the topic from their point of view, and I often discover new perspectives.

But preparation of a seminar takes at least a half to one day of working hour, and a new collaboration project starts. Well, I think I am quite swamped by too many projects, but at the same time, I am also always excited to learn new things by working with other groups. At least, I am trying to secure my private time for my partner and hobby…

I still enjoy dirty works in science

Nowadays I spend more time in supervising someone than in my own project, and I am also involved in many other collaboration projects. A good thing is that as I discuss on various topics in spin-orbitronics, this really helps me to have a broader and comprehensive perspective on the field. However, I cannot spend too much time in every projects, and I rather try to understand the gist of ideas and motivations. As human brains are not developed for multi-tasking, situation of having multiple projects simultaneously make me exhausted. Importantly, it seems to take away a “fun” part of doing science. In my experience, even for a seemingly boring project, at the end I find a lot of interesting aspects as I start to understand more things that are not so obvious from the surface. Often we get a result that goes against our intuitiion and we are surprised by that. After spending some time, there is a moment when everything perfectly makes sense.

In this week, I had a chance to work on a tight-binding model and symmetry reduction of the Hamiltonian. In the beginning, this has been done by my master student, but he found that the final result does not make any sense: We have seen a net orbital magnetic moment even though the system posesses the time-reversal symmetry. Quite often, this signals that there must be a rather dumb mistake. Since it’s quite difficult to find a mistake from one’s own notes, I decided to work on the problem independently from the master student. To be honest, I was a bit impatient in the beginning as I had many other works which have priority. But once I started to work on it, I forgot about it and started to enjoy doing science. Especially, now that I knew most of the technical details, we could have nice discussion on the same page (so far, quite often I only checked the final result and see whether it makes sense in terms of symmetry or microscopics, etc.). At the end, we end up having hours of stimulating discussion, although I left a dirty part of the calculation to the student.

This little moment in this week started to make me realize that doing science is not only about making general statements. Every scientific arguments should be based on solid data, which requires understanding of technical side of the work as well. Details and technical aspects really matter in many cases, if you want to really understand and “feel” a physical mechanism that we study. Sometimes it’s frustrating, but I do enjoy dirty works in science, still!

Recording a conference presentation

In the last week, I recorded my presentation for the upcoming Joint MMM-Intermag conference. I was invited to give a talk in the symposium Frontiers of Orbital Physics: Statics, Dynamics, and Transport of Orbital Angular Momentum, and my presentation is supposed to cover theoretical aspects of the orbital current, which is my main research expertise.

In the beginning of virtual conferences due to travel restriction with the surge of COVID, I clearly remember that I struggled for more than 5 hours to record a 30 min presentation. There were many reasons.

First, most importantly, it felt very strange to talk in front of my camera with no audience. This is something I am not completely used yet. Even in live online seminars, I do not see people’s faces anymore as the slide show starts unless I have a second monitor. I think I just got used to it by time although I’m not sure if my presentation improved or not. As usual, in every presentation, I try to apply the two simple rules: (1) Connect with audience first before I strat my story, (2) Be myself such that I truely enjoy developing scientific thoguths. Well, at the end, we do what we can.

The second difficulty was more about technical side. I wasn’t used to using any software for the screen recording. In the beginning of the pandemic, I also somehow noticed that conference organizers were also a bit lost and were not sure which option is the best. After 2 years, most conferences now provide a very clear guideline and tips for recording. For me, I do not use any fancy software. I simply use OBS, which is a free and open-source software. Importantly, it supports all different kinds of platforms, including linux. Then, I use QuicktimePlayer to trim the video.

Even after I successfully record my presentation, what I am not sure is how people would react to my presentation. Some conferences provide a comment section to make it more interactive, but it didn’t seem to work quite well. Most people, including myself, seem to be less active. In offline conferences, one can easily talk to the speaker after the presentation even if one misses a chance to ask a question at the end of the talk. In many previous conerences, I found it super nice to have a long conversation with colleagues in the coridoor after the session.

It’s not easy. Recording a presentation takes more time than preparing for a live presentation, and it is much less effective when it comes to exchanging knowledges and having networks. But we do what we can for now until we overcome the difficult time.