Spin-Reorientation-Induced Band Gap in ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$: Optical Signatures of Weyl Nodes

Temperature- and frequency-dependent infrared spectroscopy identifies two contributions to the electronic properties of the magnetic kagome metal ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$: two-dimensional Dirac fermions and strongly correlated flat bands. The interband transitions within the linearly dispersing Dirac bands appear as a two-step feature along with a very narrow Drude component due to intraband contribution. Low-lying absorption features indicate flat bands with multiple van Hove singularities. Localized charge carriers are seen as a Drude peak shifted to finite frequencies. The spectral weight is redistributed when the spins are reoriented at low temperatures; a sharp mode appears suggesting the opening of a gap due to the spin reorientation as the sign of additional Weyl nodes in the system.

Figure 1

(a) Temperature-dependent reflectivity of ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ in a broad frequency range. The inset magnifies the low-energy reflectivity where the dip causes the peak structure in ${\sigma }_1(\omega )$. (b) Corresponding optical conductivity at different temperatures. The green circles indicate the peak position. The arrows mark the points of the van Hove singularities. The inset displays the measured dc resistivity of the sample. $\mathrm{RRR}=39$ indicates the good quality of our sample. The red circles are the dc resistivity values determined via the optical parameters matches very well with the measured resistivity.

Figure 2

(a) Decomposition of the 7 K optical conductivity. The red curve represents the fit of the overall spectrum with a Drude (green), flat band responses (blue), and two-dimensional Dirac responses (orange). The inset sketches the band structure along with the transitions. (b) Interband and intraband responses of the two-dimensional Dirac fermions (blue curve) after the flat band contributions have been subtracted from the overall fit. The two step feature and the Dirac points are highlighted. Signs of these points are visible at higher $T$ and do not change significantly with temperature; the $T=150$ and 300 K spectra are given for comparison. The inset shows the temperature-dependent SW analysis. The details of the fit procedure and the determination of the individual components and their SW can be found in [17].

Figure 3

(a) Magnetic susceptibility of ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ measured for $H\parallel ab$ plane in field-cooled and zero-field-cooled configuration. The inset shows the field dependence $\chi (B)$ normalized to 2 T value (above the saturation field) for $T=300$ and 2 K. Also shown is the slope of the low-field magnetization as a function of $T$ indicating the spin-reorientation transition. (b) Relative optical conductivity normalized to the room temperature spectrum. The spectra for $T>7\mathrm{K}$ are shifted by 0.5 each with respect to each other. The red circles mark the point where the spectra cross each other, indicating the spectral weight transfer energy. The green circles are the maximum of the low-lying absorption feature shifting in energy. Green arrows are guide to the eye following the decreasing temperature. Panel (c) shows the $T$ dependence of the red and green circles in panel (b). The solid green line represent the position of the expected localization peak extended to the lower temperatures, highlighting the clear deviation. The same scaling also given in panel (e). (d) Energy-dependent absorption feature at $T=300$, 150, and 7 K with the localization peak fit from reference [30]. (e) Scaling of the peak position with the dc conductivity suggested by [31]. Grey shaded areas in figures represent the crossover temperature, where the spin reorientation takes place.