Spin and orbital angular momentum degrees of freedom in time-resolved XUV spectroscopies
e-mail: kondo1215@issp.u-tokyo.ac.jp講演言語 : 英語
Light-matter interaction comprehends a number of fundamental processes at the core of most condensed matter physics studies. While experimental challenging, it is of great importance to seek the combination of different degrees of freedom, in order to have a better insight of the particular process and at the same time to exploit it as an investigation technique.
In this seminar, I will present two separate case studies with the common thread of combining spin and orbital angular momentum degrees of freedom in time-resolved spectroscopic techniques in the extreme ultraviolet (XUV) range, with the aim of extracting qualitatively new type of information.
• Combining a high harmonic generation (HHG) XUV beamline at the Attolab facility [1] with a hemispherical analyzer equipped with a VLEED spin polarimeter, we are able to perform a complete spin-, time- and angle-resolved photoemission spectroscopy (STAR-PES) experiment, giving access not only to the charge but also to the spin dynamics in condensed matter systems. We have investigated two prototypical transition metal dichalcogenides (TMDC): WSe2 and WTe2. Our results in the inversion-symmetric 2H-WSe2 bulk crystal [2] reveal efficient chiroptical control of bright excitons’ hidden spin polarization. After the optical photoexcitation however, intervalley scattering between nonequivalent K-K’ valleys leads to a decay of bright excitons’ hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitons acts as a local spin polarization reservoir, which could be used for spin injection in van der Waals heterostructures involving multilayer transition metal dichalcogenides. In the case of dichalcogenide WTe2 [3], a precursor of Weyl type-II semimetal topological phase, the comparison of STARPES measurements with relativistic one-step photoemission calculations reveal a spin accumulation above the Weyl points region, which is consistent with a spin-selective bottleneck effect due to the presence of spin-polarized cone-like electronic structure.
• The spin angular momentum (SAM) of a photon is associated to the circular polarization of electromagnetic radiation in the wave picture. Like other quantum particles, photons can carry also orbital angular momentum (OAM), which corresponds to a helicoidal wavefront of light instead of a plane wave. While the SAM of light is extensively used for dichroic studies, e.g. in X-ray magnetic circular dichroism (XMCD), the OAM degree of freedom has been much less exploited. I will present the classical electromagnetic theory for the case of scattering of light carrying OAM by a non-uniform magnetic material, an extension of the magneto-optic Kerr effect, leading to a differential signal associated to the so-called magnetic helicoidal dichroism (MHD) [4]. It is found that MHD can give information about the overall topology of the magnetic structure under the probing helicoidal beam. I will also present the first experimental observation of MHD measured at the FERMI free electron laser on a permalloy magnetic vortex with an XUV beam at the Fe 3p resonance [5]. The agreement of the experimental results with the theoretical predictions opens up two directions: on one hand, the extension of MHD to the time domain, allowing to track the ultrafast demagnetization and remagnetization dynamics and the transient modification of magnetic topology after a femtosecond infrared pulse [6]; on the other hand, the possibility to explore fundamental properties of light such as photon spin-orbit interaction [7].
References
[1] D. Bresteau et al., European Physical Journal Special Topics 1 (2023)
[2] M. Fanciulli et al., Physical Review Letters 131, 066402 (2023)
[3] M. Fanciulli et al., Physical Review Research 2, 013261 (2020)
[4] M. Fanciulli et al., Physical Review A 103, 013501 (2021)
[5] M. Fanciulli et al., Physical Review Letters 128, 077401 (2022)
[6-7] M. Fanciulli et al., submitted