Study Of Noval Properties In Correlated Oxide Thin Films And Ionic Liquid Gating

With the rapid development of the electronic information,conventional semiconductor industry is facing two challenges.One is the quantum effects due to the size of transistors become smaller and smaller.The development of semiconductor can not feloow the Moore's Law.The switch from bipolar to complementary metal oxide semiconductor transistors lowered microchips' heat output-but increasing device densities have quickly driven it up again,which is the other challenge.However,correlated oxides give an opportunity to the electronics.Thanks to the correlated oxides,we could develop a new generation of devices based on new physical principles and new materials systems.Transition metal oxides are characterized by strong correlations that give rise to many novel physical phenomena,such as ferromagnetism,ferroelectric,superconductivity,metal-to-insulator transistion and other charge-and spin ordered states.Such phenomena arise from the complex interplay between the charge,spin,orbital,and lattice degrees of freedom that are inherent in correlated oxides simultaneously.Since the discovery of colossal magnetoresistance in manganite,of one the most famous correlated oxides,the properties of this material has attracted much interest.Recent years have witnessed a tremendous growth of research in the field of correlated oxide interfaces since the high mobility two-dimensional electron gas(2DEG)was observed at the interface.In this thesis,high-quality epitaxial manganite thin films and oxides two-dimensional electron gases were grown by pulsed laser deposition.We have observed some new physical properties in manganites and explored the new device application of the oxide two-dimensional electron gas.Specifically,the research of this thesis falls into two parts based on different materials sysytems:(1)New physical properties in manganitesa.Weak localization effect in epitacial La0.7Sr0.3MnO3 thin filmsQuantum interference effects(QIEs)dominate the appearance of low-temperature resistivity minimum in colossal magnetoresistance manganites.The T1/2 dependent resistivity under high magnetic field has been evidenced as electron-electron(e-e)interaction.However,the evidence of the other source of QIEs,weak localization(WL),still remains insufficient in manganites.Here we report on the direct experimental evidence of WL in QIEs observed in the single-crystal La0.7Sr0.3MnO3(LSMO)ultrathin films deposited by laser molecular beam epitaxy.The sharp cusps around zero magnetic field in magnetoresistance measurements is unambiguously observed,which corresponds to the WL effect.This convincingly leads to the solid conclusion that the resistivity minima at low temperatures in single-crystal manganites are attributed to both the e-e interaction and the WL effect.Moreover,the temperature-dependent phase-coherence length corroborates the WL effect of LSMO ultrathin films is within a two-dimensional localization theory.b.Emergent ferromagnetism in LaMn03 heterostructuresWe also performed the research in magnetic properties.Novel ferromagnetism was observed in otherwise antiferromagneic LaMnO3.The stoichiometric bulk-state LaMnO3(LMO)is an A-type antiferromagnetic insulator with the orthorhombic perovskite crystal structure.However,it turns to be a ferromagnetic insulator when formed as thin films.The origin of such emergent ferromagnetic state has been vigorously investigated but the underlying mechanisms remains controversial,such as polar-distinuty induced electronic reconstruction,strain effect,oxygen octahedron distortion.By combing spatially resolved electron energy-loss spectroscopy,element-specific X-ray absorption spectroscopy and X-ray magnetic circular dichroism,we find that the mixed valence state of Mn ions(Mn4+,Mn3+ and Mn2+)and attribute the ferromagnetism to the Mn3+-O-Mn4+ double exchange mechanism.In addition,the magnetic dead-layer effect,is found to be associated with an accumulation of Mn2+induced by electronic reconstruction in the proximity of the interface.These findings provide a hitherto-unexplored multivalence state of Mn on the emergent magnetism in undoped manganite epitaxial thin films,such as LaMnO3 and BiMnO3,and shed new light on all-oxide spintronic devices.(2)New devices of oxide two-dimensional electron gasa.Fabrication of the high-mobility two-dimensional electron gas devicesThe two-dimensional electron gas(2DEG)at the non-isostructural interface between spinel ?-Al2O3 and perovskite SrTiO3 is featured by a record electron mobility among complex oxide interfaces in addition to a high carrier density up to the order of 1015 cm-2.We patterned the 2DEG at the ?-Al2O3/SrTiO3 interface using a hard mask of LaMnO3.Remarkably,a high electron mobility of approximately 3,600 cm2V-1s-1 was obtained at low temperatures for the patterned 2DEG at a carrier density of?7×1012 cm-2,much lower than that of the unpatterned sample(?1015 cm-2),which exhibits clear Shubnikov-de Hass quantum oscillations.The patterned high-mobility 2DEG at the ?-Al2O3/SrTiO3 interface paves the way for the design and application of spinel/perovskite interfaces for high-mobility all-oxide electronic devices.b.Giant tenability of the two-dimensional electron gas by ionic liquid gating2DEGs formed at the interface between two oxide insulators provide a rich platform for the next generation of electronic devices.However,their high carrier density makes it rather challenging to control the interface properties under a low electric field through a dielectric solid insulator,that is,in the configuration of conventional field-effect transistors.To surpass this long-standing limit,we used ionic liquids as the dielectric layer for electrostatic gating of oxide interfaces in an electric double layer transistor configuration.By modulating the carrier density thus the band filling with ionic-liquidgating,the system experiences a Lifshitz transition at a critical carrier density of 3.0 × 1013 cm-2,where a remarkably strong enhancement of Rashba spin-orbit interaction and an emergence of Kondo effect at low temperatures are observed.Moreover,as the carrier concentration depletes with decreasing gating voltage,the electron mobility is enhanced by nearly 7 times in magnitude(from-500 cm2V-1s-1 to?3400 cm2V-1s-1),leading to the observation of clear quantum oscillations.The great tunability ionic gating not only shows promise for design of oxide devices with on-demand properties but also sheds new light on the electronic structure of 2DEG at the nonisostructural spinel/perovskite interface.