Mauro Fanciulli | Spin, orbit and time-resolved spectroscopieson transition metal dichalcogenides, spin-degenerated systems and magnetic vortices

LPMS, CY Cergy Paris Université, Cergy-Pontoise, France
Université Paris-Saclay, CEA, CNRS, LIDYL, Gif-sur-Yvette, France

Light-matter interaction comprehends a number of fundamental processes at the core of most condensed
matter physics studies, where each time one measures some of the various properties (position, angular
momentum, energy, etc.) of photons, electrons and nuclei. While experimental challenging, it is of great
importance to combine different degrees of freedom, in order to have a better insight of the particular
process under examination and at the same time to exploit it as an investigation technique. In this
seminar, I will present three case studies with the common thread of combining different degrees of
freedom in spectroscopic techniques in the extreme ultraviolet (XUV) range, with the aim of better
understanding the physical processes, and to extract qualitatively new type of information.

• In photoemission, the spin polarization of the photoelectrons from spin-degenerate states and
  the emission time delay in the attosecond domain are both linked to the phase term of the
  transition matrix elements. I will present how to indirectly access the attosecond time scale from
  a measurement of the spin polarization as a function of electron binding energy [1].
  

• Alternatively, it is possible to directly access the time information in the photoemission process
  through a XUV pump – IR probe interferometer scheme. In this case, one probes the collective
  deexcitation process in the femtosecond domain. I will present a setup that allows to perform
  angle-resolved photoemission spectrscopy (ARPES) with combined spin and time resolution (ST-
  ARPES), thus giving access not only to the charge but also to the spin dynamics. I will present
  experimental results on the transition metal dichalcogenide WTe2 [2], a precursor of Weyl type-II
  semimetal topological phase.
  

• In addition to the spin angular momentum (SAM) associated to the circular polarization of a light
  wave, a photon can carry also orbital angular momentum (OAM), corresponding to a helicoidal
  wavefront of light instead of a plane wave. While SAM of light is extensively used for dichroic
  studies, this new OAM degree of freedom has been much less exploited. I will present the classical
  electromagnetic theory for the case of reflection of light carrying OAM by a non-uniform magnetic
  material, leading to a differential scattering associated to a so-called magnetic helicoidal
  dichroism (MHD) [3]. It is found that MHD gives information about the overall topology of the
  magnetic structure. I will also present the first experimental observation of MHD measured at the
  FERMI free electron laser on a permalloy magnetic vortex at the Fe 3p resonance [4]. The
  experimental results agree with the theoretical predictions, setting the ground for scan-free
  investigations of ultrafast dynamics of magnetic structures with MHD.
• 
  References:
  [1] M. Fanciulli et al., Physical Review Letters 118, 067402 (2017)
  [2] M. Fanciulli et al., Physical Review Research 2, 013261 (2020)
  [3] M. Fanciulli et al., Physical Review A 103, 013501 (2021)
  [4] M. Fanciulli et al., Physical Review Letters 128, 077401 (2022)