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Keeping scientific curiosity

I think most peope in science would agree that as one’s career progresses, there’s less and less time to focus on actual scientific works. There can be various reasons, but in my case, recently, it’s mainly due to submitting research proposals for fundings and applying jobs in universities and research centres. Also, if there are too many ongoing collaborations, it’s difficult to focus. Hostely, in the last year, I felt not very satisfied working as a scientist becuase most of the jobs that I did or had to do were far from fullfilling my goals as a scientist. I always want to keep my scientific curiosity and have the eyes of childern — This makes the world a way more interesting and full of surprises. Sometimes having publications in prestiguous journals can be attractive, but I feel that it does not really satisfy me intrinsically. What I want is to learn new things about nature out of pure curiosity and explore pieces of findings that are new or not yet threaded to the knowledge of humanity.

Thus, from this year, or nowadays in general, I have a rule to keep at least one scientific project that keeps me alive — a topic that is almost impossible to resist thinking about. I think it doesn’t really matter whether it’s already studied by someone or not. As long as it is new to me and as long as I can learn in my own way, I believe this can inspire me in various ways and can turn into something new that is unknown to anybody. In my case, one topic that I have always been curious is the microscopic wave function of electrons; something like in between chemistry and physics. How electron’s are distributed between ions, how they form bonds, and how they move? In chemistry, we learn various “rules”, but in physics, we learn “laws”. I think my interest is what links the rules and laws. Now, emerging rules in condensed matter physics are all related to the geometry of electronic states — a.k.a Berry phase stuffs. I always wonder how the chemistry of solids and molecules are related to the Berry phase formalisms and transport effects.

I think so far in this year, I have been quite successful, not in the sense that I wrote papers or achieve something but in the sense that I was genuinely happy by keeping my curiosity. Sometimes I have a problem that I keep thinking about my scientific ideas and tend to postponing the works that I have to do. Well, at the end, I guess a happy mindset and improved mood would help to go through all the parts that grown-ups have to do in science… Please don’t argue with me that these are important. Important things are not necessarily interesting. Usually it’s the oppostite.

Seminar at Paris

On this Monday, I visited Paris to give a seminar and discuss with my colleagues. The title of my seminar was “Crucial role of long-range orbital transport in spintronic systems”, a topic that I have been working since a few years ago. But I prepared half of my talk about the pedagogical introduction of the basic concepts in orbitronics; orbital-dependent energy splittings, a.k.a. orbital quenching, orbital texture, orbital current, response, etc. I think my talk went quite well, and at least most people seem to have followed the first half on the introduction. The second half of the talk could be a bit advanced for non-experts, but I tried to emphasize that the long-range transport of orbital angular momentum comes from the charateristic features of the underlying electronic structure.

Sure, I also took an opportunity to discuss with many people at different institutions. I thank André Thiaville for arranging everything. Everone was very kind and open to share their recent intersts, and I got nicely inspired by their enthusiasm. It was paricularly nice to know that they pay attention to my theory works as well as experimental works that I published with my collaborators.

The dinner that I had with a few colleagues was very good, and it was a kind of classic French meal; Apéritif, starter and main dish with wine, and desert — very different from the German traditional dinner with cold bread. Well-fed with nice French meals and inspiration for scientific ideas, I came back nicely by a Thalys express train that connects Paris Gare du Nord and Aachen Hbf.

Left: Nice warm evening dinner (from the left: me, Henri Jaffres, Michael Viret, and Andre Thiaville). Right: In the THz spectroscopy lab of Henri Jaffres.

Life in Zürich

It’s been three weeks since I came to Zürich, as of today, and I am getting more and more used to the life here. The first thing that I got used to is I do not really care much about the bus schedule anymore. The public transportation is amazing here, and there are buses in every 5 mins for the line that I use to commute. So, what happens now is that I don’t really run or struggle to catch a particular bus (I often do that in Germany). Talking more about using public transportation, I realized that my nose reacted to new dusts that I am not used to from all other people. I sneezed from time to time on the first week upon arrival, but now it all disappeared.

The second thing, which I find interesting, is although German is the primary language in Zürich, English is much more common. I heard that even Swiss people talk in English first when they are from different regions. This is quite a big contrast compared to Germany, where I don’t really expect people to speak English when I am out of the office. I still try to use German as much as possible, and say “Grüezi!” (a Swiss dialect for hello) to people, and I like the way it is pronounced! But it’s sometimes very difficult to understand strong accent of local people speaking.

The third thing is that since people in the same group do experiments, they talk a lot about measurements during lunch time. I personally find the topic quite intersting, and I do enjoy overhearing the conversation. But I noticed that sometimes people are too kind that they change the topic to something else that I can also participate…

Next, people love cheese dishes, such as raclette! In every supermarket, I can easily find heating plates for making raclettes and the chesses that are already cut into slices. Certainly, I feel that I am in the middle of mountain cheese culture of Europe.

Finally, beautiful mountains are nearby. During the last week, I took a chance to go to Rigi mountain near Luzern. It was amazing to see mountains covered with snow and shiny like mirrors.

Visit of ETH Zürich

Since the beginning of this month, I have been in Zürich for visiting the group of Prof. Pietro Gambardella at ETH Zürich, and I am going to stay until the end of the month. The main purpose of the visit is to have in-depth discussion on the orbital transport effects in magnetic heterostructues and interfaces. For this purpose, I plan to give a serious of lectures that introduce basic concepts in this topic of research.

I already gave the first lecture on the last Friday. I covered how the atomic orbital information is described as an internal degree of freedom of Bloch states, how the crystal field potential splits the bands of Bloch states with differnet orbital characters in a specific way depending on the crystal symmetry, and what are often misunderstood regarding the orbital quenching. I also introduced minimal models for describing the orbital physics in the bulk and at the surface/interface, for which we discussed the properties of the eigenstates and eigenvalues in detail.

The lecture took more than 2 hours with lots of questions and discussions. My intention was to give a message that we need to start from fundamental principles rather than relying on our intuition which is quite bad in the orbital physics; the orbital physics has been largely overlooked because most people simply took orbital quenching from the intuition that the crystal field potential merely suppresses the orbital motion of electrons. I think my approach was quite effective, seeing the level of questions, and I hope I triggered inspirations.

The second lecture is going to be in this week, and I plan to explain the physics of orbital Hall effect. The last lecture, which is going to take place a week after, will be about how the nonequilibrium orbital angular momentum interacts with local magnetic moments.

Apart from science, I also enjoy my stay in Zürich. I find it nice to get to know new people, including those whom I have worked with in the last years, and hang out with them after work and out of the office. On the weekend, I enjoy walking along the walking trail right behind my apartment. The trail leads to the top of the hill, and from there I can see a beautiful view of the city and the lake. On this weekend, there was snow in Zürich, and it was so nice to walk under the snow. If it doesn’t snow or rain in the next weekend, I will try to go to another place in Switzerland such as Luzern.

Every week is very precious because I stay only one month. Unfortunately, I have been very busy with other works that I could not discuss as much with other colleagues as I wished. As my other works and responsibilities are getting cleared, I hope I can spend more time to discuss with others.

Visit of the Institute of PHysics of the Czech Acdemy of Science

In the previous week, I visitied the Institute of Physics of Czech Academy of Science, thanks to the invitation by Helena (Reichlova). She showed great interest since the last SPICE junior workshop in Ingelheim, but unfotunately I did not have chance to explain big ideas and basic aspects. My talk in that workshop was rather technical, as I was presenting the most recent result.

So the main purpose of this visit was to (try to) give a pedagogical lecture, instead of presenting what’s at the upfront of my research. Regardless of the outcome of my presentation, I enjoyed preparing the talk so much this time because I could feel that my work is now standing on a rather solid ground, and I could construct formal physical arguments and write down equations which exactly correspond to physical situations. This was a big difference compared to so-called “state-of-the-art”, which kind of seems not wrong and probably right. I always feel like I am swimming in an open ocean without much navigation…

Talking more about fundamentals, I personally believe that we don’t need to fully understand what other people are doing “now” although we have to “know” them at some point before writing papers. What is more important is to figure out the motivation of the research at the very beginning and understand fundamentals in the deepest level possible. For me, at least, that’s how I get the most creative ideas, and I don’t really get much motivation from Nature papers…

Anyway, hopefully most people seem to have understood my talk qutie well, which I could see from the level of questions. What I particuarly liked was a relaxed atmosphere of the seminar. People raised questions freely during the seminar, and I was not pressured to finish all the slides. I just focused on explaining physics to the eyes of the audience (I tried). Sure, there were things that I could not discuss much, but usually these are topics that can be discussed later.

On the second day, they took me to experimental labs. To me, everything looked amazing and exciting, but I tried not to touch anything 😂 Also, it was surprising that there is still a PC with Windows95, which was wired to their MBE machine.

Most of the time, I spent time with Helena and Jakub (Železný). It was funny to realized that I talked to Jakub about visiting Prague in 2018, but I could not make it (due to busy life after moving to Germany and Covid…). Anyway, we not only discussed physics but also talked about many other things, about life, career, academia, science community… Overall, it was very pleasant time in a friendly environemnt. They also took me to a nice local restaurnt for dinner, where I tasted quite nice Goulash and a local beer. It was pity that I had a slight stomach pain (probably eaten something wrong before the trip…) and could not fully appreciate the taste of food and beer.

In the airport on the way back, as usual, I had quite a lot of thoughts and ideas as I was sitting while waiting for the plane. Although I wanted to write down some equations, as soon as I was on board, I immediately fell asleep. When I opened my eyes, the plane was already flying above the Rhein river and was preparing for landing. I landed near 10pm, and I could finally come back to my home by midnight.

Back to normal working routine

I have been on a quite long vacation recently and had wonderful time. It was the first time since I started my PhD study that I haven’t really studied or worked more than 2 months. Sure, I was thinking about physics and science, but my thoughts were more from very childish curiosity, like “how does the electronic wave function change — in its amplitude and phase — such that various (charge, spin, orbital) current flows?”, “What is happening (quantum mechanically) at the electrode in the transport geometry?” (I was thinking how the “quantum” measurement takes places in macroscopic scale), etc. I also got a few nice ideas on my own resarch projects, and I am testing them one by one nowadays.

The first week since I am back to work was kind of like the beginning of every semester when I was studying in schools and university. It felt so ackward to write equations (or just writing itself), and my way of describing physics seems rusty. But my brain was certainly much more malleable, and all the strains from too much work seem to have disappeared. But my productivity was extremely low 😂

Now I am almost back to normal working routine. Every evening, I write things to do tomorrow, and as soon as I wake up (feeling quite sleepy for sure, but I try to kick-start everyday…), I just eat something, hopefully healthy stuffs, and drive to the research centrer. I try to do creative jobs in the morning time and do the rest of the jobs (usually things that I should do – not necessarily boring nor unintersting). Anyway, feels good to be back!

“Inverse” orbital torque and “spin-to-orbital” conversion

Another recent experimental paper in which I got coauthored in 2023 is one from the group of Antonio Azevedo in Federal University of Pernambuco:

  • E. Santos, J. E. Abrão, Dongwook Go, L. K. de Assis, Y. Mokrousov, J.B.S. Mendes, A. Azevedo
    Inverse Orbital Torque via Spin-Orbital Interwined States
    Physical Review Applied 19, 014069 (2023) *Editor’s Suggestion

As far as I know, the experiment began soon after our previous work on orbital-to-spin conversion in CuOx/Pt/TmIG (experiemnt performed in Kläui-Lab in Mainz)

  • Shilei Ding, Andrew Ross, Dongwook Go, Zengyao Ren, Frank Freimuth, Sven Becker, Fabian Kammerbauer, Jinbo Yang, Gerhard Jakob, Yuriy Mokrousov, Mathias Kläui
    Harnessing non-local orbital-to-spin conversion of interfacial orbital currents for efficient spin-orbit torques
    Physical Review Letters 125, 177201 (2020)

and they asked the question on how the reciprocal process would take place. By injecting spin currents to Pt/CuOx interface (by spin pumping and spin Seebeck effect from YIG), they confirmed that the reciprocal process also exists, and qualitative features are very much alike. The experiment clearly showed that spin-to-orbital conversion occurs as much as orbital-to-spin conversion occurs. Even though this is somewhat expected in theory – the reciprocity is always guaranteed to be present in near-equilibrium transport phenomena, I was very glad to see this result and reassured, which “closed the second half of the loop” in our conceptual understanding.

> 50 nm orbital relaxation length revealed

Some time ago, I submitted a theory paper that predicts exceptionally long orbital dephasing in ferromagnets:

  • Dongwook Go, Daegeun Jo, Kyoung-Whan Kim, Soogil Lee, Min-Gu Kang, Byong-Guk Park, Stefan Blügel, Hyun-Woo Lee, Yuriy Mokrousov
    Long-Range Orbital Magnetoelectric Torque in Ferromagnets

(Yes, it’s still “Under Review” because the process took time, and I myself also took a lot of time due to many other exciting stuffs on the way.)

Probably soon after I uploaded the preprint (a few months?), Hiroki Hayashi in Kazuya Ando’s group in Keio University has found “striking” result that the orbital relaxation/dephasing lengths seem to go beyond tens of nanometres. The data were nowhere comparable to other similar-type experiments on spin-orbit torques, and I got totally astonished. Although I made the theroetical prediction myself, I could not believe that the length scale of orbital relaxation/dephasing can be this large! This is not a regime that I could even test in numerical simulations. Soon, I became more confident that this signals a previously unnoticed physics, and the underlying mechanism may involve interactions whose energy scale is very robust ~ 1 eV. This is one of the main features of many orbitronic effects driven by the interaction of electronic wave functions with the crystal potential.

Since then, we exchanged several emails, together with Daegeun Jo in Hyun-Woo Lee’s group in POSTECH, and we began to collaborate seriously. Now, all the results from our long discussions are summarized in this paper:

  • Hiroki Hayashi, Daegeun Jo, Dongwook Go, Yuriy Mokrousov, Hyun-Woo Lee, Kazuya Ando
    Observation of long-range orbital transport and giant orbital torque
    Communications Physics 6, 32 (2023)

I especially like this work because publishing this kind of new experimental work requires braveness and confidence in measurements because the data are so different from “conventional” types.

I hope this paper inspires you, and we will be happy to know your opinions (email, skype, conference, in our/your group, etc.)!

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!