Orbitronics at ultrafast timescale

In this month, finally the paper on the observation of the ballistic orbital current was published.

Tom S. Seifert, Dongwook Go, Hiroki Hayashi, Reza Rouzegar, Frank Freimuth, Kazuya Ando, Yuriy Mokrousov, Tobias Kampfrath
Time-domain observation of ballistic orbital-angular-momentum currents with giant relaxation length in tungsten
Nature Nanotechnology (2023). https://doi.org/10.1038/s41565-023-01470-8

A nicely written News & Views by M. Benjamin Jungfleisch can be found here:

M. Benjamin Jungfleisch
Observation of ultrafast ballistic orbital transport
Nature Nanotechnology (2023). https://doi.org/10.1038/s41565-023-01458-4

This work is an outcome of the collaboration among three different groups;

Tom Seifert, Reza Rouzegar, and Tobias Kampfrath (Free University of Berlin) on the THz spectroscopy measurement, Dongwook Go, Frank Freimuth, and Yuriy Mokrousov (Jülich Research Centre / Johannes Gutenberg University Mainz) on the first-principles calculation of the inverse orbital Rashba-Edelstein effect, and Hiroki Hayashi and Kazuya Ando on the fabrication of high-quality thin films of W/Ni and many other samples.

I exactly remember the first time when I discussed the idea with Tom in January 2022. It was at the Heraeus seminar on magnetism at Physikzentrum located in Bad Honnef, which was one of the first in-person workshops held when we were heading toward the end lockdowns. We discussed the possibility of measuring the inverse orbital Hall effect or Edelstein effect by the THz spectroscopy, but it wasn’t that clear at that time how to distinguish the signal from the spin and the signal from the orbital. But I think it was more that we didn’t understand what each of us can do and how much we know. So, I gave a seminar talk in the Berlin group in March 2022 to discuss more in-depth. It was also the moment when I mentioned several interesting results published from the group of Kazuya in Japan. So, right afte the seminar, I asked Kazuya if he could send some samples to Berlin. But thankfully, Kazuya readily agreed to make a series of samples of which the thickesses are suitable for the THz spectroscopy.

In the beginning, we really didn’t know what to expect except that the result must be very different from the conventional signals. Thouhg it is still the case, we didn’t know much about the properties of orbital currents. After the first round of experiments, we found exactly several interesting features that we have not seen before. Different signs, different THz dynamics, delayed signals, etc. We really had no idea why we get these features in the experiment. As the first experiment ended without a concrete conclusion but with lots of excitements, we asked Kazuya again to make a lot more samples this time for different substrataes and various thicknesses. From the second round of experiment, we finally start to see a systematic trends of signal delays as a function of the thickness of W.

But unfortunately, from this time, I started to become quite busy for writing two grants at the same time with rather tight deadlines. More importantly, I planned the wedding and honeymoon trip! I erased my email app from my phone during this time and did not carry my labtop. So, I had to excused myself from the project for many months. It was from February 2023 that finally I came back to normal work, but from March, I was planning to spend a month in the group of Pietro Gambardella in ETH Zürich. I was pretty occupied with additional schedules (mostly preparing for giving lectures), but I continued working on the THz experiment. I was able to sqeeze time and run some first-principles calculations.

Well, I have been working on this idea of inverse process already since 2022. All the fancy ideas of orbital pumping, etc. — I have all seen from my calculations though I did not really publish any work yet. This is because I haven’t felt that I “understood the physics”, even till now. Finally, I start to see physical pictures constructed from complicated numerical results, and I hope I write a paper during this year. Anyway, a small subset part of the result was already sufficient to explain the THz experiment, which shows a dramatically large orbital-to-charge conversion by the orbital Rashba states at the surface. You can see the result somewhere in the Supplemental Material.

By the way, I thank the editor and all the reviewers for all their efforts. The comments written were very useful for “refining” the core physics mechanism and our understanding, as well as improving the manuscript. All the reports were very professional, touching the critical aspects directly. The were way more than just “Good, recommend publication” or “I don’t like it, so I reject” kind of reports, which unfortunately is quite common nowadays. I have had various spectrum of experiences with reviewers now that my career progresses more into a profesional level, but this was certainly a very good experience.

I feel that this work just opened the door to a very big unexplored territory of ultrafast orbitronics. I am pretty confident that we will find a lot more interesting physics, which probably did not pay much attention. It’s time to think about what the next step will be.