Impurity states in the magnetic topological insulator $\mathrm{V}:{(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$

The ferromagnetic topological insulator $\mathrm{V}:{(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ has been recently reported as a quantum anomalous Hall (QAH) system. Yet the microscopic origins of the QAH effect and the ferromagnetism remain unclear. One key aspect is the contribution of the V atoms to the electronic structure. Here the valence band of $\mathrm{V}:{(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ thin films was probed in an element-specific way by resonant photoemission spectroscopy. The signature of the V $3d$ impurity band was extracted and exhibits a high density of states near the Fermi level, in agreement with spin-polarized first-principles calculations. Our results indicate the occurrence of a ferromagnetic superexchange interaction mediated by the observed impurity band, contributing to the ferromagnetism in this system.

Figure 1

Representative spectra of (a) the V $2p$ core levels, measured at $h\nu =750$ eV, and (b) the V $L_{2,3}$ edges from a ${\mathrm{V}}_{0.2}{\left({\mathrm{Bi}}_{0.21}{\mathrm{Sb}}_{0.79}\right)}_{1.8}{\mathrm{Te}}_3$ (4 at.%) film. Satellite structures (labeled “sat.”) characterize the V $2p$ and $L$-edge lineshapes in this system. The red lines mark the position of the V $2p$ core levels. The $L_3$-edge maximum is found 1.9 eV above the $2p_{3/2}$ peak. The labels ON and OFF mark the photon energies used in the resPES measurements.

Figure 2

(a) Resonant photoemission spectra of the valence band of ${\mathrm{V}}_{0.2}{\left({\mathrm{Bi}}_{0.21}{\mathrm{Sb}}_{0.79}\right)}_{1.8}{\mathrm{Te}}_3$ at the $L_3$ absorption edge ($2p\to 3d$ transition). The red and black curves were measured in on- ($h{\nu }_{\mathrm{ON}}=515.5$ eV) and off-resonant ($h{\nu }_{\mathrm{OFF}}=510$ eV) conditions, respectively. (b) Same set of measurements as in (a), now performed on an undoped ${\left({\mathrm{Bi}}_{0.21}{\mathrm{Sb}}_{0.79}\right)}_2{\mathrm{Te}}_3$ reference sample. The absence of V in the sample leads to no resonant enhancement of the valence band photoemission. The inset shows the Fermi edge calibration at room temperature.

Figure 3

V $3d$ impurity states at the valence band of ${\mathrm{V}}_{0.2}{\left({\mathrm{Bi}}_{0.21}{\mathrm{Sb}}_{0.79}\right)}_{1.8}{\mathrm{Te}}_3$. Upper panel: resPES difference spectrum $I\left(h{\nu }_{\mathrm{ON}}\right)-I\left(h{\nu }_{\mathrm{OFF}}\right)$ [from Fig. 2], representing the contribution of the V $3d$ states to the valence band. Lower panel: calculated spin-resolved DOS (shaded areas). The highlighted exchange-split bands (full lines and dashed lines) represent the spin-polarized V $3d$ impurity states (in the ferromagnetic and paramagnetic states, respectively). The peak in the resPES difference spectrum is attributed to the V spin-up band below $E_F$ due to their remarkable agreement.