Alternatively, a few studies suggested that the EGs can be generated without polar oxides (still on T-STO, when STO(001) was used) 23, 24. However, the properties of EGs involving polar oxides can be dramatically altered by polar adsorbates 19, 22, and thus suffer from ambient water 19. In fact, nowadays interface engineering with polar oxides has become a very popular designing concept in controlling strongly correlated electrons in transition metal oxides. Following a similar idea, very recently a hole gas was achieved in a sandwiched STO/LaAlO 3/STO heterostructure 20, 21. Inspired by this EG, more EGs have been formed between STO and a few different polar oxides 14, 15, 16, 17, 18, 19. This extreme asymmetry originates from the polar arrangement of atomic layers in LaAlO 3, and can be understood in an elegant polar discontinuity and electronic reconstruction picture 13. One most remarkable result in previous studies is that the EG can only be formed on TiO 2-terminated STO (T-STO), and the heterointerface with SrO-terminated STO (S-STO) is highly insulating 1, 13. Applications in field-effect transistors (FETs) 9, 10, 11 and integrated circuit devices 12 have also been demonstrated. Many intriguing properties that are not present in conventional semiconductor heterostructures, such as interface magnetism 3, 4, interface superconductivity 5, 6, and even the coexistence of magnetism and superconductivity at the interface 7, 8, have been observed. Since the discovery of the high-mobility electron gas (EG) at the LaAlO 3/STO heterointerface 1, much effort has been undertaken to explore and understand the emergent phenomena at this heterointerface 2. SrTiO 3 (STO) is the workhorse oxide semiconductors.
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