Main Content

2024

• Inhalt ausklappen Inhalt einklappen M. Merboldt, M. Schüler, D. Schmitt, J. P. Bange, W. Bennecke, K. Gadge, K. Pierz, H. W. Schuhmacher, D. Momeni, D. Steil, S. R. Manmana, M. Sentef, M. Reutzel, and S. Mathias, Observation of Floquet states in graphene, Nature Physics (in-press 2025), arXiv:2404.12791 (2024).

  Recent advances in the field of condensed-matter physics have unlocked the potential to realize and control emergent material phases that do not exist in thermal equilibrium. One of the most promising concepts in this regard is Floquet engineering, the coherent dressing of matter via time-periodic perturbations. However, the broad applicability of Floquet engineering to quantum materials is still unclear. For the paradigmatic case of monolayer graphene, the theoretically predicted Floquet-induced effects, despite a seminal report of the light-induced anomalous Hall effect, have been put into question. Here, we overcome this problem by using electronic structure measurements to provide direct experimental evidence of Floquet engineering in graphene. We report light-matter-dressed Dirac bands by measuring the contribution of Floquet sidebands, Volkov sidebands, and their quantum path interference to graphene's photoemission spectral function. Our results finally demonstrate that Floquet engineering in graphene is possible, paving the way for the experimental realization of the many theoretical proposals on Floquet-engineered band structures and topological phases. https://arxiv.org/abs/2404.12791

• Inhalt ausklappen Inhalt einklappen J. P. Bange, D. Schmitt, W. Bennecke, G. Meneghini, A. AlMutairi, K. Watanabe, T. Taniguchi, S. Steil, D. Steil, R. T. Weitz, G. S. M. Jansen, S. Hofmann, S. Brem, E. Malic, M. Reutzel, and S. Mathias, Probing electron-hole Coulomb correlations in the exciton landscape of a twisted semiconductor heterostructure, Science Advances 10, eadi1323 (2024).

  In two-dimensional semiconductors, cooperative and correlated interactions determine the material’s excitonic properties and can even lead to the creation of correlated states of matter. Here, we study the fundamental two-particle correlated exciton state formed by the Coulomb interaction between single-particle holes and electrons. We find that the ultrafast transfer of an exciton’s hole across a type II band-aligned semiconductor heterostructure leads to an unexpected sub-200-femtosecond upshift of the single-particle energy of the electron being photoemitted from the two-particle exciton state. While energy relaxation usually leads to an energetic downshift of the spectroscopic signature, we show that this upshift is a clear fingerprint of the correlated interaction of the electron and hole parts of the exciton. In this way, time-resolved photoelectron spectroscopy is straightforwardly established as a powerful method to access electron-hole correlations and cooperative behavior in quantum materials. Our work highlights this capability and motivates the future study of optically inaccessible correlated excitonic and electronic states of matter.

  DOI: 10.1126/sciadv.adi1323

• Inhalt ausklappen Inhalt einklappen W. Bennecke, A. Windischbacher, D. Schmitt, J. P. Bange, R. Hemm, C. S. Kern, G. DAvino, X. Blase, D. Steil, S. Steil, M. Aeschlimann, B. Stadtmüller, M. Reutzel, P. Puschnig, G. S. M. Jansen, and S. Mathias, Disentangling the multiorbital contributions of excitons by photoemission exciton tomography, Nature Communications 15 (1), 1804 (2024).

  Excitons are realizations of a correlated many-particle wave function, specifically consisting of electrons and holes in an entangled state. Excitons occur widely in semiconductors and are dominant excitations in semiconducting organic and low-dimensional quantum materials. To efficiently harness the strong optical response and high tuneability of excitons in optoelectronics and in energy-transformation processes, access to the full wavefunction of the entangled state is critical, but has so far not been feasible. Here, we show how time-resolved photoemission momentum microscopy can be used to gain access to the entangled wavefunction and to unravel the exciton’s multiorbital electron and hole contributions. For the prototypical organic semiconductor buckminsterfullerene (C60), we exemplify the capabilities of exciton tomography and achieve unprecedented access to key properties of the entangled exciton state including localization, charge-transfer character, and ultrafast exciton formation and relaxation dynamics.

  DOI: 10.1038/s41467-024-45973-x

• Inhalt ausklappen Inhalt einklappen M. Reutzel, G. S. M. Jansen, and S. Mathias, Probing excitons with time-resolved momentum microscopy, Advances in Physics: X 9 (1), 2378722 (2024).

  Excitons – two-particle correlated electron-hole pairs – are the dominant low-energy optical excitation in the broad class of semiconductor materials, which range from classical silicon to perovskites, and from two-dimensional to organic materials. The study of excitons has been brought on a new level of detail by the application of photoemission momentum microscopy – a technique that has dramatically extended the capabilities of time- and angle resolved photoemission spectroscopy. Here, we review how the photoelectron detection scheme enables direct access to the energy landscape of bright and dark excitons, and, more generally, to the momentum-coordinate of the exciton wavefunction. Focusing on two-dimensional materials and organic semiconductors, we first discuss the typical photoemission fingerprint of excitons in momentum microscopy and highlight that it is possible to obtain information not only on the electron- but also hole-component. Second, we focus on the recent application of photoemission orbital tomography to such excitons, and discuss how this provides a unique access to the real-space properties of the exciton wavefunction. We detail how studies performed on two-dimensional transition metal dichalcogenides and organic semiconductors lead to very similar conclusions, and, in this manner, highlight the strength of momentum microscopy for the study of optical excitations in semiconductors.

  DOI: 10.1080/23746149.2024.2378722

• Inhalt ausklappen Inhalt einklappen H. Probst, C. Möller, M. Schumacher, T. Brede, J. K. Dewhurst, M. Reutzel, D. Steil, S. Sharma, G. S. M. Jansen, S. Mathias, Unraveling femtosecond spin and charge dynamics with extreme ultraviolet transverse MOKE spectroscopy, Physical Review Research 6 (1), 013107 (2024). 

  The magneto-optical Kerr effect (MOKE) in the extreme ultraviolet (EUV) regime has helped to elucidate some of the key processes that lead to the manipulation of magnetism on ultrafast timescales. However, as we show in this paper, the recently introduced spectrally resolved analysis of such data can lead to surprising experimental observations, which might cause misinterpretations. Therefore, an extended analysis of the EUV magneto-optics is necessary. Via experimental determination of the dielectric tensor, we find here that the nonequilibrium excitation in an ultrafast magnetization experiment can cause a rotation of the off-diagonal element of the dielectric tensor in the complex plane. In direct consequence, the commonly analyzed magneto-optic asymmetry may show time-dependent behavior that is not directly connected to the magnetic properties of the sample. We showcase such critical observations for the case of ultrafast magnetization dynamics in Ni, and give guidelines for the future analysis of spectrally resolved magneto-optical data and its comparison with theory.

  DOI: 10.1103/PhysRevResearch.6.013107

• Inhalt ausklappen Inhalt einklappen C. Möller, H. Probst, G. S. M. Jansen, M. Schumacher, M. Brede, J. K. Dewhurst, M. Reutzel, D. Steil, S. Sharma, S. Mathias, Verification of ultrafast spin transfer effects in FeNi alloys, Communications Physics 7 (1), 74 (2024).

  The optical intersite spin transfer (OISTR) effect was recently verified in Fe50Ni50 using extreme ultraviolet magneto-optical Kerr measurements. However, one of the main experimental signatures analyzed in this work, namely a magnetic moment increase at a specific energy in Ni, was subsequently found also in pure Ni, where no transfer from one element to another is possible. Hence, it is a much-discussed issue whether OISTR in FeNi alloys is real and whether it can be verified experimentally or not. Here, we present a comparative study of spin transfer in Fe50Ni50, Fe19Ni81 and pure Ni. We conclusively show that an increase in the magneto-optical signal is indeed insufficient to verify OISTR. However, we also show how an extended data analysis overcomes this problem and allows to unambiguously identify spin transfer effects. Concomitantly, our work solves the long-standing riddle about the origin of delayed demagnetization behavior of Ni in FeNi alloys.

  DOI: 10.1038/s42005-024-01555-3