Gate- and magnetic field- controlled topological gap opening in the surface state of Bi 2 Se 3 by proximity to a magnetic insulator S. Mathimalar, 1, ∗ P. Rajasekhar, 1, ∗ S. Sasmal, 1, ∗ A. Bhardwaj, 1 S. Chaudhary, 1 B. Satpati, 2 and K. V. Raman 1, † 1 Tata Institute of Fundamental Research, Centre for Interdisciplinary Sciences, Hyderabad-500107, India 2 Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata, 700064, India Topological insulators are considered to be bulk insulators with exotic surface states, protected under time-reversal symmetry, which hold promise in observing many exciting condensed-matter phenomena. In this report, we show that by having a topological insulator (Bi2Se3) in proximity to a magnetic insulator (EuS), a metal-to-insulator transition in the surface state, attributed to opening of an exchange gap, is observed whose properties are tunable using bottom gate voltage and external magnetic field. Our study suggests favorable band bending at the Bi2Se3/EuS interface leading to positioning of the Fermi level near the Dirac point. Furthermore, evidence of gate- controlled enhanced interface magnetism is observed, paving way for using magnetic proximity effect in developing topological electronic devices. The process of breaking time reversal symmetry (TRS), by introducing out-of-plane magnetism, at the surface of a 3-dimensional (3-D) topological insulator (TI) [1, 2] is known to open up an exchange gap at the Dirac point of the topological surface states (TSS) [3]. Such a mechanism support several new quantum phe- nomena, such as quantum anomalous Hall effect (QAHE) [4, 5], half-integer quantum Hall effect [1, 6], topolog- ical magneto-electric effect [7, 8], and image magnetic monopoles [9]. In realizing these effects, the main chal- lenges is to successfully demonstrate the opening of gap at the surface of TI. Currently, this is made possible in magnetic TIs such as Cr and V-doped compensated TIs, where perpendicular magnetism is developed in the bulk [5, 10], showing QAHE. In comparison, inducing local magnetism at the TI surface through the short range na- ture of magnetic proximity effect (MPE) can provide a number of advantages over bulk-doping [3, 5, 11]; these include lower bulk and surface defect density due to im- proved film crystallinity, better controllability of the sur- face states (SS) from the bulk and the ability to inde- pendently control the magnetic state at each surface of TI. Conventionally, MPE can be achieved by using mag- netic insulators (MIs). However, the requirement of an out-of-plane magnetization at the TI surface makes this study difficult since most of the MIs naturally possess in- plane anisotropy. Additionally, new hybridization states [12] form at the TI/MI interface near the Fermi level (E F ) that can contaminate the topological properties of the Dirac SSs [13–15]. As a result, observing these ef- fects through transport studies can become non-trivial. In this regard, MPE studies in the 3D TI, Bismuth Se- lenide (Bi 2 Se 3 ), with an MI, Europium sulphide (EuS), have been extensively researched using magnetometry techniques [16–18]. Different research groups confirm the presence of an out-of-plane component of magnetization * equal contribution † Corresponding author: kvraman@tifrh.res.in at the Bi 2 Se 3 /EuS interface with the magnetism exist- ing at temperature much above the magnetic transition temperature (T c ) of EuS (∼ 17K). Unlike other TI/MI interfaces [13, 14, 19, 20], recent theoretical calculation of the above interface have also suggested induced mag- netism [21], localized within the first quintuple layer (QL) of Bi 2 Se 3 , with the SS retaining most of the topological character compared to a pristine Bi 2 Se 3 SS [22]. Further, an induced exchange gap of ∼ 9 meV in the surface Dirac cone is suggested [21]. Despite these reports, experimen- tal observation of the exchange gap opening at this sur- face has been elusive [23]. This is primarily due to inher- ent intrinsic doping caused by large Se vacancies leading to bulk dominated conduction with E F pinning deep in- side the conduction band (CB). In this letter, we provide evidence for favorable band bending at the Bi 2 Se 3 /EuS interface leading to E F dropping below the CB, thereby gaining access to the SSs in our transport measurements. Under these conditions, a gate and magnetic-field con- trolled metal-to-insulator transition (MIT) is observed in these SSs which we attribute to the opening of exchange gap. High quality thin films of Bi 2 Se 3 were grown on Si(100)/SiO 2 (300 nm) substrate at 250 ◦ C in our molecu- lar beam epitaxy system with a base pressure of 8 ×10 −10 mbar using co-evaporation of Bi and Se with a flux ra- tio (Bi:Se) varied between 1:15 to 1:20. The structural properties of the films were studied using Raman spec- troscopy, X-ray diffraction and Transmission electron mi- croscopy (TEM) (see Fig. S1 in Supp. Mater.). In- terestingly, cross-sectional TEM images reveal a layered growth, supporting the hexagonal structure with 0.95 nm thickness of the QL (inset of Fig.1a). For the case of transport measurements, thicker Bi 2 Se 3 films in ∼10 to 16 nm range were grown. Subsequently, a 3 nm film of EuS was deposited in a sulphur-rich environment at room temperature, without breaking vacuum, followed by a 2 nm-capping layer of Al 2 O 3 . Hall bar structures (with length, L and width, W) were patterned by mechanical etching. Figure 1a shows the temperature dependence of the longitudinal resistivity (ρ xx ) of Bi 2 Se 3 films for three arXiv:1907.12770v2 [cond-mat.mes-hall] 11 Sep 2019