| Summary: | Ferromagnetically doped topological insulators with broken time-reversal symmetry are a prerequisite for observing the quantum anomalous Hall effect. Cr-doped (Bi,Sb)2(Se,Te)3 is the most successful materials system so far, as it combines ferromagnetic ordering with acceptable levels of additional bulk doping. Here, we report a study of the local electronic structure of Cr dopants in epitaxially grown Bi2Se3 thin films. Contrary to the established view that the Cr dopant is trivalent because it substitutionally replaces Bi3+, we find instead that Cr is divalent. This is evidenced by the energy positions of the Cr K and L2,3 absorption edges relative to reference samples. The extended x-ray absorption fine structure at the K edge shows that the Cr dopants substitute on octahedral sites with the surrounding Se ions contracted by Δd=-0.36 Å, in agreement with recent band structure calculations. Comparison of the Cr L2,3 x-ray magnetic circular dichroism at T=5 K with multiplet calculations gives a spin moment of 3.64 μB/Crbulk, which is close to the saturation moment for Cr2+ d4. The reduced Cr oxidation state in doped Bi2Se3 is ascribed to the formation of a covalent bond between Cr d(eg) and Se p orbitals, which is favored by the contraction of the Cr-Se distances.
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