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Electronic correlations in the van der Waals ferromagnet Fe3GeTe2 revealed by its charge dynamics

The layered van der Waals (vdW) material Fe3GeTe2 is one of the rare and elusive ferromagnetic (FM) topological semimetals with a high Curie temperature TC ≃ 200 K and harbours an unconventional interplay between topology and magnetism, leading to a large anomalous Hall conductivity at low temperatures. The vdW materials are generally considered as front-runners of two-dimensional (2D) electronic systems with weak interlayer coupling, which facilitates exfoliation into few-layer nanosheets. The 2D quantum confinement of their electronic properties makes them of interest for potential applications in the area of electronic and spintronics devices.

We thoroughly investigated the optical response of Fe3GeTe2 as a function of temperature (T) which addresses the complete charge dynamics. We can image the T evolution of the electronic properties and extract the nature of their correlations; first by uniquely disentangling various relevant parameters and quantities, such as the scattering rate and the plasma frequency within the intraband contribution of the absorption spectrum, and then by accessing the interband excitations spanning the energy interval from the Fermi level (EF) to states deep into the electronic structure. We discover a two-fold spectral weight transfer from the mid-infrared towards lower as well as higher energy scales and developing within an energy interval of about 2 eV (Fig. 1.16).

Enlarged view: Fig. 1.16
Fig. 1.16: Real part (σ1(ω)) of the optical conductivity of Fe3GeTe2 as a function of T in the spectral range below 104 cm-1. The insets are its blow-up in the FIR-MIR and MIR-NIR spectral range at 300 and 10 K, together with the Drude-Lorentz fit and its constituent components. Reddish and bluish colours refer to 300 and 10 K, respectively. The black arrow in the main panel points out the absorption at ~800 cm-1. (b) Integrated spectral weight (SW) as a function of frequency at 10, 100 and 200 K, normalised by its value at 300 K. The inset shows the bare SW quantity at 10 and 300 K. The dotted, vertical blue line across both panels divides the investigated spectral range into two intervals, for which the reshuffling of SW at 10 K occurs towards low (left-hand side) and high (right-hand side) energy scales, respectively (as schematically indicated by the (rounded) blue arrows in panel (a)). At this energy scale ω*, SW(ω*; T)/SW(ω*; 300 K) = 1. The dotted, vertical orange and green segments in panel (b) define the corresponding ω* at 200 and 100 K, respectively.

This bears testimony to electronic correlations driven by Hund's coupling and to the presence of an incoherent–coherent crossover at low T, likely underlining the enhancement of the effective electronic dimensionality. Furthermore, the gradual increase of SW associated with the excitation involving the topological nodal-line gapped by spin–orbit coupling may account for the remarkable anomalous Hall effect (AHE) upon lowering T (Fig. 1.17). This testifies the unique interplay of FM and topology, which here depends ultimately on the environment of the Fe atom, tunable by parameters affecting the design of the Fe3Ge slabs. In a broad and generic context, this may shed new light on related 2D materials and foster ways for future manipulations of their AHE and its 2D quantum limit, in view of novel electronic functionalities.

Enlarged view: Fig. 1.17
Fig. 1.17: Temperature dependence of the squared strength (Ω12) of the harmonic oscillator (HO L1) fitting the bump (black arrow in Fig. 1.16(a) and its inset) and of the Hall conductivity σxy, reproduced from the literature. The linear fits (dashed lines) are guides to the eyes. Inset: schematic sketch of the bands gapped by the spin-orbit coupling at the K-point of BZ. Red and blue refer to different combinations of the orbital degrees of freedom within the Fe3GeTe2 bilayer structure. The pink arrow indicates the excitation represented by HO L1 in our Drude-Lorentz fit (inset in Fig. 1.16(a)).

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