COMMUNICATION 1703062 (1 of 6) © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de Enhancing the Quantum Anomalous Hall Effect by Magnetic Codoping in a Topological Insulator Yunbo Ou, Chang Liu, Gaoyuan Jiang, Yang Feng, Dongyang Zhao, Weixiong Wu, Xiao-Xiao Wang, Wei Li, Canli Song, Li-Li Wang, Wenbo Wang, Weida Wu, Yayu Wang,* Ke He,* Xu-Cun Ma, and Qi-Kun Xue Dr. Y. Ou, C. Liu, G. Jiang, Y. Feng, D. Zhao, W. Wu, X.-X. Wang, Prof. W. Li, Prof. C. Song, Prof. L.-L. Wang, Prof. Y. Wang, Prof. K. He, Prof. X.-C. Ma, Prof. Q.-K. Xue State Key Laboratory of Low Dimensional Quantum Physics Department of Physics Tsinghua University Beijing 100084, China E-mail: yayuwang@tsinghua.edu.cn; kehe@tsinghua.edu.cn Dr. Y. Ou, C. Liu, G. Jiang, Y. Feng, D. Zhao, W. Wu, X.-X. Wang, Prof. W. Li, Prof. C. Song, Prof. L.-L. Wang, Prof. Y. Wang, Prof. K. He, Prof. X.-C. Ma, Prof. Q.-K. Xue Collaborative Innovation Center of Quantum Matter Beijing 100084, P. R. China W. Wang, Prof. W. Wu Department of Physics and Astronomy School of Arts and Sciences Rutgers University Piscataway, NJ 08854, USA The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201703062. DOI: 10.1002/adma.201703062 and topologically protected gapless sur- face states with Dirac-like linear band dispersion. [17,18] Introducing ferromag- netism into a 3D TI can open a gap at the Dirac surface states, which, in a film of appropriate thickness, will lead to the QAH effect. [4] Up to now the QAH effect has only been observed in Cr- [10–13] or V-doped [14–16] (Bi,Sb) 2 Te 3 TI films. The two magnetically doped TI materials exhibit distinct magnetic and QAH behaviors, but a temperature <100 mK is always required to achieve perfect quantization in either case. [10–16] The ultralow temperature needed is still the biggest challenge and puzzle for the studies on the QAH effect. Recently Mogi et al. reported the observa- tion of QAH effect at higher temperature in modulation-Cr-doped (Bi,Sb) 2 Te 3 films which however involve a highly complex heterostructure diffi- cult to fabricate and investigate. [19] Several recent experiments with different techniques on mag- netically doped (Bi,Sb) 2 Te 3 TI films revealed evidences for very inhomogeneous ferromagnetism which is suspected of contrib- uting to the unexpected low temperature of the QAH effect. [16,20–22] Alloying is a common and effective way to refine the perfor- mance of ferromagnetic metals, and, similarly, codoping of dilute magnetic semiconductors (DMSs) can promote ferromagnetic coupling in those systems. [23] It has recently been proposed that codoping V and I in Sb 2 Te 3 TIs may increase the quantization temperature of the QAH effect. [24] In our study, we prepared Cr and V codoped (Bi,Sb) 2 Te 3 films with MBE and found a signifi- cant increase in the temperature of the QAH effect. We report both our new approach to achieving higher temperature QAH materials and our investigations of the factors determining the temperature for the QAH effect in magnetically doped TIs. We grew a series of (Cr y V 1-y ) 0.19 (Bi x Sb 1-x ) 1.81 Te 3 films with the nominal Cr concentration y = 0, 0.05, 0.16, 0.32, 0.54, 0.75, and 1 on SrTiO 3 (111) substrates by MBE. The nominal Bi con- centration x is ≈0.4 and is modified for each sample in order to keep them nearly charge neutral. All the films have a thick- ness d = 5 quintuple-layer (QL), which is confirmed with atom force microcopy. [10] The SrTiO 3 substrate serves as a dielectric layer for an applied bottom gate voltage (V g ), which permits the chemical potential of the films to be finely tuned during transport measurements (see the Supporting Information). [25] The quantum anomalous Hall (QAH) effect, which has been realized in mag- netic topological insulators (TIs), is the key to applications of dissipationless quantum Hall edge states in electronic devices. However, investigations and utilizations of the QAH effect are limited by the ultralow temperatures needed to reach full quantization—usually below 100 mK in either Cr- or V-doped (Bi,Sb) 2 Te 3 of the two experimentally confirmed QAH materials. Here it is shown that by codoping Cr and V magnetic elements in (Bi,Sb) 2 Te 3 TI, the temperature of the QAH effect can be significantly increased such that full quantization is achieved at 300 mK, and zero-field Hall resistance of 0.97 h/e 2 is observed at 1.5 K. A systematic transport study of the codoped (Bi,Sb) 2 Te 3 films with varied Cr/V ratios reveals that magnetic codoping improves the homogeneity of ferromagnetism and modulates the surface band structure. This work demonstrates magnetic codoping to be an effective strategy for achieving high-temperature QAH effect in TIs. Topological Insulators The quantum anomalous Hall (QAH) effect results from 2D electronic band structure with topologically nontrivial prop- erty characterized by a nonzero Chern number. [1–9] After remaining as a theoretical hypothesis for over two decades, [1–6] the effect was experimentally realized in molecular beam epi- taxy (MBE)-grown thin films of magnetically doped 3D topo- logical insulators (TIs). [10–16] A 3D TI has gapped bulk bands Adv. Mater. 2017, 1703062