I am fascinated by the potential of quantum materials, particularly altermagnets, as they are highly spin-active yet display no overall magnetic fields. As a PhD candidate, my work involves exploring these unique materials with a vision to develop computing solutions that are denser, faster, and more energy-efficient.
Altermagnetism Q&A
What are altermagnets?
Altermagets are collinear, magnetically compensated magnets that belong to a spin symmetry class (type-III), which means that atoms with opposite spins are related by rotating and/or mirrorring them. The altermagnetic type-III spin symmetry class is completely distinct from that of ferro(ferri-)magnets (type-I) and antiferromagnets (type-II). Consequently, altermagnets have alternating spin polarizations in real and wave-vector space. This combination is not available in neither ferro(ferri-)magnets nor antiferromagnets [1].
What makes altermagnets interesting?
The symmetries of altermagnets dictate that their energy-wavevector dispersions are spin-polarized and spin-split. A combination of nonrelativistic quantum mechanical exchange and crystal fields that are naturally present in the system dictates the size of this spin splitting energy. In other words, the spin splitting in altermagnets is not a relativistic correction, and its size in energy can therefore be orders of magnitude larger than splittings due to spin-orbit coupling. In turn, altermagnets are candidates for hosting many unconventional spintronics effects, such as an unconventional anomalous Hall effect [2], spin currents, and giant and tunneling magnetoresistance [3,4,5], and this without creating large stray fields like in ferromagnets or requiring large spin orbit-coupling. Moreover, several materials researched in other fields, such as magnetic semiconductors, insulators, and superconductors, also exhibit altermagnetic symmetries [6].
Is there experimental evidence of altermagnetism?
The first description of altermagnetism in RuO2 predicted that altermagnets host the anomalous (crystal) Hall effect [2]. The effect was promptly measured in RuO2 [7] and more altermagnetic candidates. Also spin current and torques have been measured in material candidates for altermagnets [8,9,10]. Such measurements indirectly point towards these systems being altermagnetic, as the effects are possible due to the specific symmetries of altermagnets. More recently, the time-reversal symmetry breaking of altermagnetic electronic band structures was observed directly [11]. Most recently, (spin-polarized) ARPES showed the band structure lifts Kramers spin degeneracy altermagnetically [12-14], which has created excitement not only in the scientific community, but also in main-stream media. A new experimental report also shows d-wave order in RuO2 [15].
[1] Šmejkal, L., Sinova, J., and Jungwirth, T., Phys. Rev. X 12, 031042 (2022).[2] Šmejkal, L., González-Hernández, R., T. Jungwirth, and J. Sinova, Sci. Adv., 6, 23, 6 (2020). arXiv:1901.00445
[3] Rafael González-Hernández, Libor Šmejkal, Karel Výborný, et al., Phys. Rev. Lett. 126, 127701 (2021).
[4] Šmejkal, L., Hellenes, A. B., González-Hernández, R., Sinova, J., and Jungwirth, T., Phys. Rev. X 12, 011028 (2022). arXiv:2103.12664.
[5] Shao, DF., Zhang, SH., Li, M. et al. Nat Commun 12, 7061 (2021). arXiv:2103.09219.
[6] Šmejkal, L., Sinova, J., and Jungwirth, T., Phys. Rev. X 12, 040501 (2022).
[7] Feng, Z., Zhou, X., et al. Nat Electron 5, 735–743 (2022). arXiv:2002.08712.
[8] Bose, A., Schreiber, N.J., Jain, R. et al. Nat Electron 5, 267–274 (2022).
[9] H. Bai, L. Han et al., Phys. Rev. Lett. 128, 197202 (2022).
[10] Karube, S., Tanaka, T., et al., Phys. Rev. Lett. 129, 137201 (2022).
[11] O. Fedchenko, J. Minar, A. Akashdeep, ..., Hellenes, A. B., et al., Sci. Adv 10, eadj4883 (2024).
[12] J. Krempaský, L. Šmejkal, ..., Hellenes, A. B., et al., Altermagnetic lifting of Kramers spin degeneracy. Nature 626, 517–522 (2024). arXiv:2308.10681.
[13] Suyoung Lee, Sangjae Lee, et al., Phys. Rev. Lett. 132, 036702 (2024). arXiv:2308.11180.[14] T. Osumi, [14] T. Osumi, S. Souma, et al., arXiv:2308.10117.
[15] Z. Lin, D. Chen, et al., arXiv:2402.04995.
Research interests
- Altermagnetism
- Giant and tunneling magnetoresistance in magnets with zero net magnetic moment
- Topological antiferromagnetic spintronics
- Noncollinear antiferromagnetism
Invited talks
Talk at the 2024 international workshop Spins, waves, and interactions in Greifswald, introducing the altermagnetism field and presenting recent results.
Talk at ICM 2024, Altermagnetism focus session. "Exchange spin-orbit coupling and unconventional p-wave magnetism"
Seminar in the group of Prof. Dr. Roland Wiesendanger, University of Hamburg, November 2023. "Altermagnetic crystals and spin polarized transport"
Talk at the EU-Japan workshop Spintronics and Quantum Transformation, Jülich Forschungszentrum, Peter Grünberg Institute, August 2023. "From spin symmetries to magnetoresistance effects in altermagnets"
Seminar in the group of Prof. Dr. Stefano Sanvito, Trinity College Dublin, May 2023. "Giant magnetoresistance effects in altermagnets"
News
Science Magazine article wrote about the evolution of the altermagnetism field and the newest spectroscopy experiments on Feb. 6, 2024.
The Economist wrote about altermagnetism and spectroscopic evidence on Jan. 24, 2024.
Organizing the PhD focus session at the 2024 German physics society (DPG) meeting in Berlin on the topic "Altermagnets: foundations and experimental evidence". The program is available here.
Gave talk in course "
Gave talk in focus session "Altermagnetism: Transport, Optics, Excitations" at the DPG meeting, March 2023.
Received poster prize at the European school of magnetism 2022 together with Elena Stetco, who herself won a price for writing the winning nomination. The topic of the poster was altermagnetism and giant magnetoresistance effects using them.
Gave talk in course "
Publications
Unconventional p-wave magnets. Anna Birk Hellenes, Tomáš Jungwirth, Jairo Sinova, Libor Šmejkal.
Altermagnetic lifting of Kramers spin degeneracy. J. Krempaský, L. Šmejkal, S.W. D'Souza, M. Hajlaoui, G. Springholz, K. Uhlířová, F. Alarab, P.C. Constantinou, V. Strokov, D. Usanov, W.R. Pudelko, R. González-Hernández, A. Birk Hellenes, Z. Jansa, H. Reichlová, Z. Šobáň, R. D. Gonzalez Betancourt, P. Wadley, J. Sinova, D. Kriegner, J. Minár, J.H. Dil, T. Jungwirth. arXiv:2308.10681
Direct observation of altermagnetic band splitting in CrSb thin films. Sonka Reimers, Lukas Odenbreit, Libor Smejkal, Vladimir N. Strocov, Procopios Constantinou, Anna Birk Hellenes, Rodrigo Jaeschke Ubiergo, Warlley H. Campos, Venkata K. Bharadwaj, Atasi Chakraborty, Thiboud Denneulin, Wen Shi, Rafal E. Dunin-Borkowski, Suvadip Das, Mathias Kläui, Jairo Sinova, Martin Jourdan.
Observation of time-reversal symmetry breaking in the band structure of altermagnetic RuO2. Olena Fedchenko, Jan Minár, Akashdeep Akashdeep, Sunil Wilfred D’Souza, Dmitry Vasilyev, Olena Tkach, Lukas Odenbreit, Quynh Nguyen, Dmytro Kutnyakhov, Nils Wind, Lukas Wenthaus, Markus Scholz, Kai Rossnagel, Moritz Hoesch, Martin Aeschlimann, Benjamin Stadtmüller, Mathias Kläui, Gerd Schönhense, Tomas Jungwirth, Anna Birk Hellenes, Gerhard Jakob, Libor Šmejkal, Jairo Sinova, Hans-Joachim Elmers (2024).
Macroscopic time reversal symmetry breaking by staggered spin-momentum interaction. Helena Reichlova, Rafael Lopes Seeger, Rafael González-Hernández, Ismaila Kounta, Richard Schlitz, Dominik Kriegner, Philipp Ritzinger, Michaela Lammel, Miina Leiviskä, Anna Birk Hellenes, Vaclav Petřiček, Petr Dolězal, Lukas Horak, Eva Schmoranzerova, Antonín Badura, Sylvain Bertaina, Andy Thomas, Vincent Baltz, Lisa Michez, Jairo Sinova, Sebastian T. B. Goennenwein, Tomáš Jungwirth, and Libor Šmejkal, submitted.
Giant and Tunneling Magnetoresistance in Unconventional Collinear Antiferromagnets with Nonrelativistic Spin-Momentum Coupling. Libor Šmejkal, Anna Birk Hellenes, Rafael Gonzáles-Hernández, Jairo Sinova, Tomáš Jungwirth (2022).
Nonlocal conductance spectroscopy of Andreev bound states: Symmetry relations and BCS charges. Jeroen Danon, Anna Birk Hellenes, Esben Bork Hanse, Lucas Casparis, Andrew P. Higginbotham, and Karsten Flensberg (2020).
Education
Ph.D. Computational Condensed Matter Physics, Johannes Gutenberg Univeristy | 2020-Now
M.Sc. Quantum Physics, Niels Bohr Institute, University of Copenhagen | 2017-2019
B.Sc. Physics, University of Oslo | 2014-2017
Academic Exchange, Nagoya University | 2016