Topological Magnetism Group

Abstract:

International audienceFriction plays an essential role in most physical processes that we experience in our everyday life. Examples range from our ability to walk or swim, to setting boundaries of speed and fuel efficiency of moving vehicles. In magnetic systems, the displacement of chiral domain walls (DW) and skyrmions (SK) by Spin Orbit Torques (SOT), is also prone to friction. Chiral damping (α c), the dissipative counterpart of the Dzyaloshinskii Moriya Interaction (DMI), plays a central role in these dynamics. Despite experimental observation, and numerous theoretical studies confirming its existence, the influence of chiral damping on DW and SK dynamics has remained elusive due to the difficulty of discriminating from DMI. Here we unveil the effect that α c has on the flow motion of DWs and SKs driven by current and magnetic field. We use a static in-plane field to lift the chiral degeneracy. As the in-plane field is increased, the chiral asymmetry changes sign. When considered separately, neither DMI nor α c can explain the sign reversal of the asymmetry, which we prove to be the result of their competing effects. Finally, numerical modelling unveils the non-linear nature of chiral dissipation and its critical role for the stabilization of moving SKs

Abstract:

The generation and control of terahertz pulses is vital for realizing the potential of terahertz radiation in several sectors, including 6G communication, security and imaging. In this work, we present the topological Dirac semimetal cadmium arsenide as a novel helicity-dependent terahertz source. We show both broadband (single-cycle) and narrowband (multi-cycle) terahertz pulses upon near-infrared photoexcitation at oblique incidence. By varying the incident angle of the photoexcitation pulse, control of the emission frequency can also be achieved, providing a candidate for a tuneable narrowband terahertz source.

Abstract:

Abstract:

The low-temperature electronic structure of the van der Waals ferromagnet CrSiTe3 has been investigated. This ferromagnetic semiconductor has a magnetic bulk transition temperature of 33 K, which can reach up to 80 K in single- and few-layer flakes. X-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements, carried out at the Cr L_2,3 and Te M_b edges on in vacuo cleaved single crystals, give strong evidence for hybridization-mediated super-exchange between the Cr atoms. The observed chemical shift in the XAS, as well as the comparison of the XMCD with the calculated Cr L_2,3 multiplet spectra, confirm a strongly covalent bond between the Cr 3d(e_g) and Te 5p states. Application of the XMCD sum rules gives a non-vanishing orbital moment, supporting a partial occupation of the e_g states, apart from the t_2g. Also, the presence of a non-zero XMCD signal at the Te Mb edge confirms a Te 5p spin polarization due to mixing with the Cr e_g bonding states. The results strongly suggest that superexchange, instead of the previously suggested single ion anisotropy, is responsible for the low-temperature ferromagnetic ordering of 2D materials such as CrSiTe₃ and CrGeTe₃. This demonstrates the interplay between electron correlation and ferromagnetism in insulating two-dimensional materials.