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.

Research interests

• Altermagnetism
• P-wave and odd-parity magnets
• Giant and tunneling magnetoresistance in altermagnets

Invited talks

• “Unconventional p-wave magnets”

ICM 2024, Focus Session on Altermagnetism (major magnetism conference)

• “From altermagnets to p-wave magnets: electronic structure and spintronics applications”

Spins, Waves and Interactions 2024, Greifswald (spintronics workshop)

• “From spin symmetries to magnetoresistance effects in altermagnets”

Spintronics and Quantum transformation 2023, Jülich (EU-Japan workshop)

• “Altermagnetic crystals and spin-polarized transport”

Talk in seminar series "Nanophysics and Nanotechnology" organized by Prof. Roland Wiesendanger, University of Hamburg, 2023

• “Giant magnetoresistance effects in altermagnets”

Seminar in group of Prof. Stefano Sanvito, Trinity College Dublin, 2023

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 [1]. 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 [3-5], 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 the altermagnetic electronic band structure in the entire Brillouin zone predicted that altermagnets host time-reversal symmetry broken energy bands and the anomalous (crystal) Hall effect [2]. This effect was measured in several altermagnetic candidates [7-9]. Also, spin currents have been indirectly measured via the torque an altermagnetic candidate exerts on a ferromagnet [10-12]. Such measurements suggests that these systems can be altermagnetic, as the effects are possible due to the specific symmetries of altermagnets. Moreover, time-reversal symmetry breaking of the electronic band structure was observed directly in a material candidate [13]. Most recently, (spin-polarized) ARPES showed the band structure lifts Kramers spin degeneracy altermagnetically [14-19], which is considered direct evidence of altermagnetism. This has created excitement also beyond the scientific community.

[1] Šmejkal, L., Sinova, J., and Jungwirth, T., Phys. Rev. X 12, 031042 (2022). arXiv:2105.05820.
[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] Gonzalez Betancourt, R. D., et al. Phys. Rev. Lett. 130, 036702 (2023). arXiv:2112.06805.
[9] Reichlova, H., ..., Hellenes, A. B., et al. Nat Commun 15, 4961 (2024). arXiv:2012.15651.
[10] Bose, A., Schreiber, N.J., Jain, R. et al. Nat Electron 5, 267–274 (2022).
[11] H. Bai, L. Han et al., Phys. Rev. Lett. 128, 197202 (2022).
[12] Karube, S., Tanaka, T., et al., Phys. Rev. Lett. 129, 137201 (2022).
[13] O. Fedchenko, J. Minar, A. Akashdeep, ..., Hellenes, A. B., et al., Sci. Adv 10, eadj4883 (2024).
[14] J. Krempaský, L. Šmejkal, ..., Hellenes, A. B., et al., Altermagnetic lifting of Kramers spin degeneracy. Nature 626, 517–522 (2024). arXiv:2308.10681.
[15] Suyoung Lee, Sangjae Lee, et al., Phys. Rev. Lett. 132, 036702 (2024). arXiv:2308.11180.
[16] T. Osumi, [16] T. Osumi, S. Souma, et al., arXiv:2308.10117.
[17] Reimers, …, Hellenes, A. B., et al. Nat. Commun. 15, 2116 (2024), arXiv:2310.17280.
[18] Ding et al., Phys. Rev. Lett. 133, 206401 (2024), arXiv:2405.12687.
[19] Zeng et al., Adv. Sci. 11, 43, 2406529 (2024), arXiv:2405.12679.

News

Science Magazine recognized altermagnetism as one of the top 10 science breakthroughs of 2024, Dec. 12, 2024.

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 "Applied Artificial Intelligence and Innovation" of Dipl.-Math. Jürgen Halt, for business students at the master level at the University of Applied Sciences Ansbach/Germany, June 2023. "How Computing Hardware Meets the Demands of Today's World"

Gave talk in focus session "Altermagnetism: Transport, Optics, Excitations" at the DPG meeting, March 2023.

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 with them.

Gave talk in course "Applied Artificial Intelligence and Innovation" of Dipl.-Math. Jürgen Halt, for business students at the master level at the University of Applied Sciences Ansbach/Germany, June 2022. "Antiferromagnetic spintronics"

Selected Publications

arXiv:2309.01607

P-wave magnets. Anna Birk Hellenes, Tomáš Jungwirth, Jairo Sinova, Libor Šmejkal.

Nature

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

Nature Communications

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.

Science Advances

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).

Physical Review X

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).

Physical Review Letters

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 Semester, Nagoya University | 2016