Berlin 2024 – scientific programme
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MA: Fachverband Magnetismus
MA 9: Ultrafast Magnetization Effects II
MA 9.5: Talk
Monday, March 18, 2024, 16:15–16:30, H 2013
Non-Hermitian phase transition in optically-driven ferromagnetic semiconductors — •J. Li1, M. Matsubara2, K. Kliemt3, N. Kaya3, I. Reiser3, M. Turaev4, C. Krellner3, S. Pal5, J. Kroha4, and M. Fiebig1 — 1ETH Zurich, Switzerland — 2Tohoku Uni., Japan — 3Goethe Uni. Frankfurt, Germany — 4Bonn Uni., Germany — 5NISER Bhubaneshwar, India
Phase transition in thermodynamic equilibrium is a heavily studied topic. Recently, however, non-equilibrium phase transitions have been drawing attention. Such transitions occur typically in open quantum systems dissipatively coupled to the environment and/or driven by external fields. As a result, the Hamiltonian eigenvalues can be complex-valued, which is why these transitions are referred to as non-Hermitian phase transition (NHPT). While most of the previous works studied bosonic quantum systems, we report our observation of a NHPT in a fermionic quantum system, i.e., the ferromagnetic semiconductor EuO. EuO has a Curie temperature (TC) of 69 K. In our study, it is driven out of equilibrium by a 120 fs optical pump pulse at 1.55 eV. We probe the relaxation dynamics by measuring the pump-induced reflectivity change using another time-delayed 120 fs optical probe pulse at 1.31 eV. Below TC, we find a clear signature of bi-exponential decay of the pump-induced reflectivity change, which becomes a damped, oscillatory behavior above TC, indicating the appearance of complex eigenvalues of the optical response function. Our results reveal that the non-equilibrium situation triggers a NHPT in EuO that is marked by an exceptional point where two eigenvalues coalesce.
Keywords: Non-Hermitian phase transition; Ferromagnetic semiconductors; Non-equilibrium phase transitions