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Coexisting two-dimensional electron and hole gases highly confined at the interfaces of undoped KTaO3-sandwiching heterostructures

Two-dimensional electron gas (2DEG) in interfaces and surfaces based on perovskite SrTiO$_3$ (STO) has exhibited various interesting phenomena and is used to develop oxide electronics. Recently, KTaO$_3$ (KTO) shows great potential and is believed to host more exciting effects and phenomena toward novel devices. Here, through first-principles investigation and analysis, we find two types of coexisting 2DEG and 2D hole gas (2DHG) highly confined at the interfaces in undoped STO/KTO/BaTiO$_3$ heterostructures, when the KTO thickness $m$ reaches a crititcal value. The two interfaces are made by (SrO)$^0$/(TaO$_2$)$^+$ and (KO)$^-$/(TiO$_2$)$^0$ for the A-type, and by (TiO$_2$)$^0$/(KO)$^-$ and (TaO$_2$)$^+$/(BO)$^0$ for the B-type. The 2D electron carriers originate from Ta-$5 d_{xy}$ states at the interface including TaO$_2$ atomic layer, and the hole carriers from O-$2 p_x/p_y$ orbitals at the other interface. The electron and hole effective masses are 0.3$m_0$ and $1.06\sim 1.12 m_0$, respectively, where $m_0$ is mass of free electron, and the 2D carrier concentrations are in the order of $10 ^{13}$ cm$^{-2}$. Our analysis indicates that the interfacial 2DEG and 2DHG are simultaneously formed because of the band bending due to the polar discontinuity at the interfaces and the stress-induced polarization within the KTO layer. These could stimulate more exploration for new phenomena and novel devices.