COMMUNICATION 1707428 (1 of 9) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de Large Enhancement of 2D Electron Gases Mobility Induced by Interfacial Localized Electron Screening Effect Xiao Chi, Zhen Huang, Teguh C. Asmara, Kun Han, Xinmao Yin, Xiaojiang Yu, Caozheng Diao, Ming Yang, Daniel Schmidt, Ping Yang, Paolo E. Trevisanutto, T. J. Whitcher, T. Venkatesan, Mark B. H. Breese, Ariando,* and Andrivo Rusydi* Dr. X. Chi, Dr. T. C. Asmara, Dr. X. Yin, Dr. X. J. Yu, Dr. C. Diao, Dr. D. Schmidt, Dr. P. Yang, Dr. P. E. Trevisanutto, Dr. T. J. Whitcher, Prof. M. B. H. Breese, Prof. A. Rusydi Singapore Synchrotron Light Source National University of Singapore 5 Research Link, Singapore 117603, Singapore E-mail: phyandri@nus.edu.sg Dr. X. Chi, Dr. Z. Huang, Dr. T. C. Asmara, Dr. K. Han, Dr. X. Yin, Prof. T. Venkatesan, Prof. M. B. H. Breese, Prof. Ariando, Prof. A. Rusydi Department of Physics National University of Singapore Singapore 117542, Singapore E-mail: ariando@nus.edu.sg Dr. Z. Huang, Dr. T. C. Asmara, Dr. K. Han, Prof. T. Venkatesan, Prof. Ariando, Prof. A. Rusydi NUSSNI-NanoCore National University of Singapore Singapore 117576, Singapore Dr. M. Yang Centre for Advanced 2D Materials and Graphene Research Centre National University of Singapore Singapore 117546, Singapore Dr. M. Yang Institute of Materials Research and Engineering A*-STAR, 2 Fusionopolis Way, Singapore 138634, Singapore Prof. T. Venkatesan, Prof. Ariando, Prof. A. Rusydi National University of Singapore Graduate School for Integrative Sciences and Engineering (NGS) 28 Medical Drive, Singapore 117456, Singapore Prof. T. Venkatesan Department of Electrical and Computer Engineering National University of Singapore Singapore 117583, Singapore DOI: 10.1002/adma.201707428 origin of the 2DEGs is widely discussed with polar divergence, [2,8] as well as oxygen vacancies, [4,11,16,17] cation defects, [13] and chemical stoichiometry. [18] Unlike the conventional 2DEGat the semiconductor interface of which the typical carrier den- sity is only 10 10 –10 11 electrons cm -2 , the 2DEGs at the STO-based interface usually showed a much higher charge density and multifunctionalities that might arise from strongly correlated electrons. [7,16,17] Taking the LAO/STO interface for an example, the polar divergence in LAO is nicely com- pensated through a 0.5e - per unit-cell area (≈3.3 × 10 14 electrons cm -2 ) [16,19] charge transfer from LAO into the interfacial LAO/ STO layer yielding insulator–metal transi- tion. Interestingly, not all those charges are mobile—transport measurements reveal that only 10% of the total charges (≈2–3 × 10 13 electrons cm -2 ) are mobile, [3,11] suggesting that the rest 90% charges (≈3 × 10 14 electrons cm -2 ) may be localized. This coexistence of high-density localized charges and mobile elec- trons has been shown by recent experimental results [16] and theoretical calculations. [20,21] However, due to the difficulty and limited information in measurement techniques, important information of those localized charges are still missing: what is The interactions between delocalized and localized charges play important roles in correlated electron systems. Here, using a combination of transport measurements, spectroscopic ellipsometry (SE), and X-ray absorption spectro- scopy (XAS) supported by theoretical calculations, we reveal the important role of interfacial localized charges and their screening effects in determining the mobility of (La 0.3 Sr 0.7 )(Al 0.65 Ta 0.35 )O 3 /SrTiO 3 (LSAT/SrTiO 3 ) interfaces. When the LSAT layer thickness reaches the critical value of 5 uc, the insulating inter- face abruptly becomes conducting, accompanied by the appearance of a new midgap state. This midgap state emerges at ≈1 eV below the Ti t 2g band and shows a strong character of Ti 3d xy – O 2p hybridization. Increasing the LSAT layer from 5 to 18 uc, the number of localized charges increases, resulting in an enhanced screening effect and higher mobile electron mobility. This observation contradicts the traditional semiconductor interface where the localized charges always suppress the carrier mobility. These results demonstrate a new strategy to probe localized charges and mobile electrons in correlated electronic systems and highlight the important role of screening effects from localized charges in improving the mobile electron mobility at complex oxide interfaces. Electron Mobility Since the first discovery of the 2D electron gas (2DEG) at the semiconductor interface of GaAs/AlGaAs, [1] much effort has been made to search for the novel 2D electronic system at var- ious interfaces. [2–15] One example is the family of SrTiO 3 (STO)- based interfaces, such as LaAlO 3 /STO (LAO/STO), [2–8] where the Adv. Mater. 2018, 1707428