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Faculty of Mathematics and Natural SciencesInstitute of Physics II Dep. of Physics

Hedayat Subgroup

Our subgroup focuses on exploring the exotic electronic and spin properties of magneto-optical materials. We are particularly interested in understanding, controlling and manipulating electrons and their spins in quantum materials with strong spin-orbit coupling on the ultra-short timescales under non-equilibrium conditions. A variety of different materials, ranging from low-dimensional materials like spin-textured 2D materials, 2D transition metal dichalcogenides to excotic quantum materials like spin liquid candidates, skyrmion and chiral magnetic materials and Weyl semimetals are examined.

For our experiments, we use state-of-the-art time-resolved optical (broadband) and magneto-optical spectroscopy techniques, time-resolved Raman spectroscopy and transient grating (and spin grating) spectroscopy.


Dr. Hamoon Hedayat

Avatar Hamoon Hedayat received his PhD in Physics (with honors) at the Politecnico di Milano, Milan (2015). His thesis focused on extracting the genuine ultrafast spin dynamics in metals and half-metals by ultrafast magneto-optic spectroscopic ellipsometry. Immediately after that, he became a Postdoctoral Researcher at the Politecnico di Milano in the same group of Prof. Claudia Dallera and Dr. Ettore Carpene together with Prof. Giulio Cerullo (Jan 2015 - Jan 2017). In 2016, he won the highly competitive Politecnico International Fellowship (PIF, edition 2016) to study the ultrafast electronic/spin dynamics in topological insulators and 2D materials employing time-resolved angle-resolved photoemission spectroscopy (TR-ARPES). After the 3 years PIF period (Jan 2017 - Jan 2020), he spent a year as a senior Research Associate at the Italian National Research Council, Institute for Photonics and Nanotechnologies, Milan. In Jan 2021, he joined the Optical Condensed Matter Science (OCMS) group of Prof. Paul van Loosdrecht at the University of Cologne as a senior researcher and subgroup leader. His research interest concerns the investigation of ultrafast microscopic interactions in various classes of strongly correlated materials using femtosecond laser pulses. He has developed several ultrafast optical setups and apparatuses to perform time-resolved optical (and magneto-optical) spectroscopy and TR-ARPES experiments.

M.Sc. Julian Wagner

Avatar Julian Wagner obtained his Bachelor of Science (2017) and Master of Science (2020) degree in physics at the University of Cologne. He held the scholarships “Deutschlandstipendium der Universität zu Köln” (2015) and “Bonn Cologne Graduate School (BCGS)” (2019). His master thesis was entitled “Transient Valley Grating Spectroscopy on Monolayer WSe2” under the supervision of Prof. Paul van Loosdrecht. Then, He started his PhD in the same group at the University of Cologne. His PhD research aims to understand and control non-equilibrium dynamics in solid-state materials using various time-resolved optical and magneto-optical spectroscopy techniques. Here, especially probing the dynamics in potential quantum spin liquid materials and their dynamical behavior after photoexcitation is the focus of his research. In addition, he is interested in optically driven phase transitions, seeking to understand the universality of the underlying dynamics. In his personal life, his passion is Ballroom dancing. He dances the Waltz, Tango, Viennese Waltz, Slow Foxtrott, Quickstep and many other dances on a competitive level.

E-Mail wagner@ph2.uni-koeln.de

Joint PhD

M.Sc. Anuja Sahasrabudhe

Anuja Sahasrabudhe is a doctoral student working on strongly correlated materials using a combination of steady-state and ultra-fast techniques like Raman scattering and magneto-optics. She grew up in Mumbai, India, where she completed her undergraduate studies. She then moved to Germany to pursue a Master's in physics from the University of Bonn. She started working with the research group of Prof Paul van Loosdrecht for her master's research work. She studied magnetic perovskites using time-resolved Raman scattering. Then she continued working with the group as a Ph.D. student. Her current work focuses on investigating the quantum spin liquid candidates with Raman scattering in high magnetic field.
As a part of her thesis work, she is jointly working with the subgroup on identifying signatures of a quantum disordered state using ultrafast magneto-optic techniques.

E-Mail sahasrabudhe@ph2.uni-koeln.de

M.Sc. Omar Abdul-Aziz

Avatar Omar Abdul-Aziz obtained his Bachelor degree in General Physics (with honors) at the Lebanese University in 2019. Then, he joined the Bonn-Cologne graduate School (BCGS) program, here in Germany, to pursue his Master’s degree at the University of Bonn. During his master studies, he got interested in the field of ultrafast spectroscopy of semiconducting materials. Then, he joined the research group of Prof. Paul van Loosdrecht at the University of Cologne in 2021 for his master’s thesis work, and started working on understanding of the charge carrier dynamics and molecular arrangement of Squaraine pi-conjugated systems using ultrafast transient absorption spectroscopy technique. During his bachelor and master studies, he held the scholarships “Walid Joumblatt Foundation for University Studies” (2017) and “German Academic Exchange Service DAAD” (2021). In June 2022, after completing his master’s degree, he started his research assistant position in the subgroup with the main focus on the investigating of the phonon response in the out-of-equilibrium state and carrier population dynamics of photoexcited “PdSe2”, a 2D transition metal dichalcogenides, using time-resolved Raman spectroscopy

E-Mail abdulaziz@ph2.uni-koeln.de

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Time-resolved Magneto-optical Spectroscopy

Here we use ultrashort pump and probe pulses to investigate the electronic and spin dynamics of magneto optical materials at different temperatures and under magnetic field. In particular, we study the photo-induced and photocontrolled phase transitions in magneto optical quantum materials.

Laboratory equipment:

  • Regenerative amplified Yb:KGW laser (Pharos):
    • power 15 W,
    wavelength 1028 nm, 800 nm,
    Repetition rate 1 - 200 kHz,
    pulse duration 50 femtoseconds.

  • Cryomagnetic:
    • Temperature: down to 2.5 K,
    Magnetic field up to 8 T.

  • Broadband Spectroscopy (OPA):
    • Probe is white light (approx. 500 - 2000 nm),
    Pump can be tuned from 400nm to 800nm.
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Time-resolved Raman spectroscopy

Using TR-Raman spectroscopy we investigate the time evolution of quasiparticles and collective excitations in quantum materials. The energy resolution is 7 cm-1 for picosecond setting and 50 cm-1 for femtosecond.

Laboratory equipment:

  • Laser source:
    • Regenerative amplified Yb:KGW laser (Pharos), 1024 nm 100 kHz, 11W, 150 ps.

  • Pump probe:
    • wavelengths 300 nm to 2500 nm.

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Transient grating and transient spin grating spectroscopy

We use transient grating (degenerate four-wave mixing process) to study the diffusive transport dynamics in quantum materials at temperatures ranging from 295K to 3K.

Laboratory equipment:

  • Laser source:
    • Ti:sapphire laser Coherent (MIRA 900-F), wavelength 747 nm (1.66 eV), 150 fs laser pulses.

For more information see: "Excitonic Transport and Intervalley Scattering Dynamics in Large-Size Exfoliated MoSe2 Monolayer Investigated by Heterodyned Transient Grating Spectroscopy,” Henning Kuhn, Julian Wagner, Shuangping Han, Robin Bernhardt, Yan Gao, Liantuan Xiao, Jingyi Zhu, Paul H. M. van Loosdrecht, Laser and Photonics Reviews, 14, 2000029 (2020)

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  • Julian Wagner, Anuja Sahasrabudhe, Rolf B. Versteeg, Lena Wysocki, Zhe Wang, Vladimir Tsurkan, Alois Loidl, Daniel I. Khomskii, Hamoon Hedayat and Paul H. M. van Loosdrecht, "Magneto-optical study of metamagnetic transitions in the antiferromagnetic phase of α-RuCl3,” npj Quantum Materials 7:28 (2022) .

  • Julian Wagner, Anuja Sahasrabudhe, Rolf B. Versteeg, Zhe Wang, Vladimir Tsurkan, Alois Loidl, Hamoon Hedayat and Paul H. M. van Loosdrecht, "Nonequilibrium dynamics of α-RuCl3 - a time-resolved magneto-optical spectroscopy study,” Faraday Discussions (2022) .

Selected Publications:

  • Julian Wagner, Anuja Sahasrabudhe, Rolf B. Versteeg, Lena Wysocki, Zhe Wang, Vladimir Tsurkan, Alois Loidl, Daniel I. Khomskii, Hamoon Hedayat and Paul H. M. van Loosdrecht, "Magneto-optical study of metamagnetic transitions in the antiferromagnetic phase of α-RuCl3,” npj Quantum Materials 7:28 (2022) .

  • Julian Wagner, Anuja Sahasrabudhe, Rolf B. Versteeg, Zhe Wang, Vladimir Tsurkan, Alois Loidl, Hamoon Hedayat and Paul H. M. van Loosdrecht, "Nonequilibrium dynamics of α-RuCl3 - a time-resolved magneto-optical spectroscopy study,” Faraday Discussions Arxiv link (2022) .

  • H Hedayat, C Sayers, A Ceraso, J van Wezel, S Clark, C Dallera, G Cerullo, E Da Como, E Carpene, "Investigation of the non-equilibrium state of strongly correlated materials by complementary ultrafast spectroscopy techniques,” New Journal of Physics , 23 033025 (2021).

  • H Hedayat, D Bugini, H Yi, C Chen, X Zhou, G Cerullo, C Dallera, E Carpene, Ultrafast Evolution of Bulk, Surface and Surface Resonance States in Photoexcited Bi2Te3,” Scientific Reports 11, 4924, arXiv (2021)

  • N. Sirica, H Hedayat, D. Bugini, M. R. Koehler, L. Li, D. S. Parker, D. G. Mandrus, C. Dallera, E. Carpene, N. Mannell, Disentangling electronic, lattice, and spin dynamics in the chiral helimagnet Cr1/3NbS2,” Physical Review B (Editors' Suggestion), 104, 174426,(2021).

  • H Hedayat, A Ceraso, G Soavi, S Akhavan, A Cadore, C Dallera, G Cerullo, A C Ferrari, E Carpene, “Non-equilibrium band broadening, gap renormalization and band inversion in black phosphorus,” 2D Materials, 8 (2), 025020 (2020).

  • Sayers C J, H Hedayat , A Ceraso, F Museur, M Cattelan, L S Hart, L S Farrar, S Dal Conte, G Cerullo, C Dallera, E Da Como, E Carpene “Coherent phonons and the interplay between charge density wave and mott phases in 1T−TaSe 2,” Physical Review B: Rapid Communications, 102, 161105,(2020).

  • H Hedayat, C J Sayers, D Bugini, C Dallera, D Wolverson, T Batten, S Karbas-si, S Friedemann, G Cerullo, J van Wezel, , S Clark, C Dallera, G Cerullo, E Da Como, E Carpene “Excitonic and lattice contributions to the charge density wave in 1T−TiSe 2 revealed by a phonon bottleneck,” Physical Review Research, 1, 023029, (2019).

  • Brambilla A, A Picone, D Giannotti, A Calloni, G Berti, H Hedayat , E Carpene, C Dallera, M Zani, G Vinai, P Torellic, M Foersterd, L Aballed, M Finazzia, L Duòa, F Ciccaccia “Magnetic properties of the CoO/Fe (001) system with a bottom-up engineered interface,” Journal of Magnetism and Magnetic Materials, 475, 54-59, (2019).

  • H Hedayat, D Bugini, H Yi, C Chen, X Zhou, G Cerullo, C Dallera, E Carpene, “Surface state dynamics of topological insulators investigated by femtosecond time-and angle-resolved photoemission spectroscopy,” Applied Sciences, 8, 694, (2018).

  • H Hedayat, P S Reddy, M V Manasa, G S Devi, J R Rao, G N Rao, “Nanostructure evolution of zinc stannate: A suitable material for liquefied petroleum gas detection,” Journal of Alloys and Compounds, 704 ,413-419 , (2017).

  • Bugini D, F Boschini, H Hedayat , H Yi, C Chen, X Zhou, C Manzoni, C Dallera, G Cerullo, E Carpene, “Ultrafast spin-polarized electron dynamics in the unoccupied topological surface state of Bi2Se3,” Journal of Physics: Condensed Matter, 29, 30LT01, (2017).

  • H Hedayat, D Bugini, H Yi, C Chen, X Zhou, G Cerullo, C Dallera, E Carpene, “Femtosecond dynamics of spin-polarized electrons in topological insulators,” IEEE Magnetics Letters ,9, 1-4 , (2017).

  • Giannotti D, H Hedayat , G Vinai, A Picone, A Calloni, G Berti, M Riva, G Bus-setti, F Boschini, P Torelli, G. Panaccione, E. Carpene, C. Dallera, M. Finazzi, A. Brambilla “Magnetic anisotropy at the buried CoO/Fe interface,” Applied Physics Letters, 109 ,232401, (2016).

  • Carpene E, H Hedayat , F Boschini, C Dallera, “Ultrafast demagnetization of metals: Collapsed exchange versus collective excitations,” Physical Review B , 91, 174414 (2015).

  • Boschini F, H Hedayat , C Piovera, C Dallera, A Gupta, E Carpene, “A flexible experimental setup for femtosecond time-resolved broad-band ellipsometry and mag-neto-optics,” Review of Scientific Instruments , 86, 013909 (2015).

  • Boschini F, H Hedayat , C Dallera, P Farinello, C Manzoni, A Magrez, H Ber-ger, G Cerullo, E Carpene, “An innovative yb-based ultrafast deep ultraviolet source for time-resolved photoemission experiments,” Review of Scientific Instruments, 85, 123903 (2014).

  • Carpene E, F Boschini, H Hedayat , C Piovera, C Dallera, E Puppin, M Mansurova, M Münzenberg, X Zhang, A Gupta “Measurement of the magneto-optical response of Fe and CrO2 epitaxial films by pump-probe spectroscopy: Evidence for spin-charge separation,” Physical Review B, 87, 174437 (2013).

Full Publications:

ORCID logo ORCID: Hamoon Hedayat