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Magnetic Cr doping of Bi2Se3: Evidence for divalent Cr from x-ray spectroscopy

DOI: 10.1103/PhysRevB.90.134402 DOI Help

Authors: Ad. I. Figueroa-garcia (Diamond Light Source) , G. Van Der Laan (Diamond Light Source) , L. J. Collins-mcintyre (Department of Physics, University of Oxford, Oxford, UK) , S.-l. Zhang (Department of Physics, University of Oxford, Oxford, UK) , A. A. Baker (Department of Physics, Clarendon Laboratory, University of Oxford, Oxford, UK) , S. E. Harrison (Department of Physics, Clarendon Laboratory, University of Oxford, Oxford, UK) , P. Schönherr (Department of Physics, Clarendon Laboratory, University of Oxford, Oxford, UK) , G. Cibin (Diamond Light Source) , T. Hesjedal (Department of Physics, University of Oxford, Oxford, UK)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Physical Review B , VOL 90 (13)

State: Published (Approved)
Published: October 2014
Diamond Proposal Number(s): 9178 , 9234

Abstract: 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 delta d = 0.36 Angstrom , 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 muB/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.

Subject Areas: Physics, Materials, Energy


Instruments: B18-Core EXAFS , I10-Beamline for Advanced Dichroism