Electronic Transport Properties Of Electron Gas At The Interface Of Oxide Film/Strontium Titanate Single Crystal Heterojunction

In 2004,researchers discovered a high mobility two-dimensional electron gas at the interface of La Al O₃/Sr Ti O₃（LAO/STO）heterojunction,which attracted widespread attention from condensed matter physicists and materials scientists.As the research progressed,researchers discovered many novel physical phenomena at the interface of the STO single crystal-based heterostructures,such as superconductivity,ferromagnetism,high mobility,spin polarization,spin-orbit coupling,Shubnikov-de Haas（Sd H）oscillations,spin-charge conversion,and huge tunability of physical properties under external stimulation.Considering the strong correlation interactions at the oxide heterostructure interface,the interface electron gas may exhibit transport behaviors related to magnetism.Through experimental studies,researchers confirmed the existence of ferromagnetism at the interface of the LAO/STO system and extended the two-dimensional electron gas at the interface to different oxide heterostructures.However,the characteristic physical phenomena of the interfacial electron spin polarization,such as low-field negative magnetoresistance,butterfly-shaped magnetoresistance,and anomalous Hall effect,are rarely reported.Finding new oxide interface electron systems and proving the spin polarization of the interfacial electron gas through electronic transport and magnetic measurements is still a challenge.This thesis chose different oxide thin films as the covering layer,using STO single crystals as the substrates,and successfully obtained a high mobility electron gas at the heterostructure interface,where the electron gas may be spin-polarized,providing useful references for the development of oxide interface spintronics.The main research content and conclusions of this thesis include:1.Using pulsed laser deposition,high-quality Sr₂Cr Mo O₆（SCMO）thin films were grown on STO（110）single-crystal substrates.The interface of the SCMO/STO heterostructure exhibits metallic conductivity.The temperature dependence of the low-temperature resistivity in the zero-field regime shows T² dependence,indicating the presence of Fermi liquid behaviors.In the temperature range of 0.1-3.8 K,when the direction of the magnetic field is perpendicular to the film plane,the SCMO/STO heterostructure exhibits quantum oscillations（Sd H）in strong a magnetic field regime.Interestingly,when the direction of an external magnetic field is parallel to the[1-10]crystallographic direction（i.e.,B//electron gas plane）,the interfacial electronic transport also exhibits strong Sd H oscillations,implying that the thickness of the interface electron gas layer is large enough to accommodate electron cyclotron motion.The angle-dependent Sd H oscillation measurements show that the strongest oscillations occur atθ=0°,90°,180°,and 270°positions,which are all equivalent to the[110]crystallographic direction,thus confirming that the interfacial states responsible for the Sd H oscillations exist in a two-dimensional Fermi surface and their normal direction follows the 12 equivalent[110]directions,thus the Fermi surface has quasi-3D characteristics,which is completely different from the Fermi surfaces of conducting STO single crystals and LAO/STO heterostructures.The SCMO/STO heterostructure exhibits negative magnetoresistance in the low-temperature regime（1.8-5 K）,implying that the interfacial electron gas is probably spin polarized.Neutron diffraction and magnetic measurements show that the SCMO thin film has ferrimagnetism.2.Using pulsed laser deposition,high-quality Eu Ti O₃（ETO）epitaxial films were grown on STO（001）and（110）single-crystal substrates.The electronic transport of the ETO/STO（001）and ETO/STO（110）heterostructure interfaces exhibits metallic conductivity behavior,and the Hall effect measurements show that the interface has high mobility(1.4×10⁴ cm²V^-1s^-1)electrons while at the same time,the Sd H oscillation is observed at low temperatures（between 1.8 and 3 K）in a strong magnetic field.At low temperatures,the variation of the magnetoresistance of the ETO/STO（110）heterostructure with the magnetic field shows a butterfly-shaped negative magnetoresistance versus the magnetic field curve at low fields（<500 Oe）,with a negative magnetoresistance up to 58%at 1.8 K,which indicates that the interface has formed a high-mobility spin-polarized electron gas.The butterfly-shaped magnetoresistance comes from the ferromagnetic interface,which originates from two factors:（1）the oxygen vacancies in the interface region of STO substrate,which induces Ti³⁺ions with magnetic moments;（2）the RKKY interaction between the interfacial Eu²⁺-4f spin and the itinerant electrons.The angle-dependent Sd H oscillation measurements shows that the electron gas at the interface of the ETO/STO（110）heterostructures has a quasi-2D Fermi surface,and the interfacial in-plane and out-of-plane magnetoresistance of the heterostructure is anisotropy.Low-temperature electronic transport of the interface of the ETO/STO（110）heterostructure exhibits the Kondo effect.The magnetic measurement shows that the ETO/STO（110）heterostructure is ferromagnetic,which comes from the ETO film itself and the oxygen-deficient interfacial layer.3.Using molecular beam epitaxy,metal Cr thin films were deposited on Sr Ti O₃（111）single-crystal substrates under high vacuum(5×10^-10 mbar)and high temperature（700°C）.In this process,Cr atoms captured surface oxygen atoms on the surface region of STO substrate to form a Cr₂O₃ thin-film layer with a thickness of about 4-5 nm.The Cr₂O₃ thin-film layer blocks the metal Cr from further absorbing oxygen atoms from the STO surface,ultimately forming heterostructures of Cr₂O₃/Sr Ti O₃ or Cr/Cr₂O₃/Sr Ti O₃.The X-ray photoelectron spectroscopy was measured for the heterostructure with a film thickness of 5 nm and a deposition time of 5 min,and it was found that the Cr ions in the Cr₂O₃ thin film were mainly in the trivalent state,and hexavalent Cr ions.A high mobility electron gas was observed at the Cr₂O₃/Sr Ti O₃interface of the Cr/Cr₂O₃/Sr Ti O₃ heterostructures deposited for 90 min,and the Sd H oscillation measurements showed that the shape of Fermi surface was an ellipsoidal profile,rather than a two-dimensional profile,indicating that the Cr metal can effectively creat a certain depth electron gas layer within the surface layer of STO substrate.The Cr₂O₃/STO heterostructure has weak ferromagnetism,and it is inferred that the ferromagnetism originates from the magnetic Ti³⁺ions which were induced by interfacial oxygen vacancies.