Ultrafast photoelectron spectroscopy of correlated materials
Prof. Martin Weinelt
Freie Universitaet Berlin, Halle-Berlin-Regensburg Cluster of Excellence CCE
16:30 - 17:30 Tuesday 07 October 2025 TUG HS P2 We use time- and angle-resolved photoemission spectroscopy to follow the signature of quasiparticle dynamics in the electronic structure and reveal fundamental couplings of electron, spin, and phonon subsystems in correlated materials.
In the prototype 3d ferromagnet iron probing exchange splitting and magnetic linear dichroism (MLD) allows us to distinguish between longitudinal and transverse spin excitations [1]. Comparing spin-split partner bands at the Fermi surface shows that the exchange splitting remains constant upon optical excitation. In contrast, the different MLD response of spin-orbit-split valence bands reveals transverse spin dynamics. Non-equilibrium magnon scattering drives angular momentum transfer between spin and phonon subsystems and the collapse of spin-orbit hybridization gaps.
Optically induced intersite spin transfer (OISTR) promises manipulation of spin systems within the ultimate time limit of laser excitation. Following its prediction, signatures of OISTR have been observed in magnetic alloys and multilayers [2]. We show that ultrafast spin transfer occurs even in ferromagnetic gadolinium metal, where charge transfer between localized surface and extended valence-band states leads to a decrease of the surface spin polarization. This synchronously alters the exchange splitting of the valence-bands. Changing the temperature-dependent spin mixing shows a promising route to ultrafast control of the magnetization, widening the applicability of OISTR [3].
Capturing both the timescales of electronic and lattice response in optically excited charge-density wave materials, allows for unveiling the complex interplay between electron correlation and atomic displacements. Upon optical excitation we observe a periodic modulation of the charge-density wave gap in TiSe2. The modulation frequency of the CDW gap depends on the photocarrier density and reveals a progressive instantaneous change of the potential energy surface as consequence of carrier screening. Our results provide insights into excitation and coupling of coherent lattice vibrations and electronic states and give a unified view of the distinctly different dynamics of TiSe2 in the low and high excitation regime [4].
[1] B. Andres, M. Weinelt et al., Strong momentum-dependent electron–magnon renormalization of a surface resonance on iron. Appl. Phys. Lett. 120, 202404 (2022).
[2] F. Siegrist,..., M. Schultze, Light-wave dynamic control of magnetism. Nature 571, 240 (2019).
[3] K. Bobowski,..., M. Weinelt, Ultrafast spin transfer and its impact on the electronic structure. Sci. Adv. 10, eadn4613 (2024).
[4] M. Burian et al., Structural involvement in the melting of the charge density wave in 1T-TiSe2. Phys. Rev. Research 3, 013128 (2021).
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