Studies On Electrical And Magnetic Transport Properties Of Perovskite Oxide Heterostructures

With the advances of thin film growth technology and the detection means, the termination of the layer and the growth orientation can be controlled precisely by the pulsed laser deposition (PLD) or oxide molecular beam epitaxy (OMBE) in the preparing process of perovskite-type materials. High-quality perovskite oxide heterostructures can be achieved now, and this has given great impetus to the study and the application of perovskite oxide heterostructures. Many novel and valuable interfacial properties have been found through controlling the interface at the atomic level. In addition, there are coupling and competition between the degrees of freedom in perovskite oxides, such as charge, spin, orbital, lattice, and so on. It shows a strong link between the structure, electricity and magnetism. When two different perovskite oxides building up a heterostructure, the symmetry of the perovskite oxides is damaged at the interface, and symmetry breaking is formed. Some studies have shown that the novel physical phenomena are closely tied to the symmetry breaking caused by the interface. Inversion symmetry breaking usually causes the change of the polarity in the materials, and influences the ferroelectric and piezoelectric properties. Gauge symmetry breaking usually leads to the superconductivity. Time reversal symmetry breaking usually causes the magnetism. It is because of the close connection existed between the interface structures and properties, compared with bulk materials, that more degrees of freedom are offered for people to design the microelectronics devices, and the electronics field is broadened. Even it can lead to the development of new materials in the field of energy and information technology. Especially, LaAlO3/SrTiO3 heterostructure has received much attention. Since the 2DEG with high mobility was found in the interface of LaAlO3/SrTiO3 heterostructure by Ohtomo and collaborators in 2004, it has been one of the hotspots in the research field of materials. A lot of theoretical and experimental researches have revealed that there are magnetism and superconducting in the interface of LaAlO3/SrTiO3. All these are the novel properties caused by the interface structures, because bulk LaAlO3 and SrTiO3 are both non-magnetic insulators.Though a considerable amount of theoretical and experimental researches on the interface of LaAlO3/SrTiO3 have been done, there are disagreements on the origin of the interface 2DEG, magnetism, the coexistence of magnetism and superconducting. So, it is necessary to further study. The origin of the two-dimensional electron gas which formed at oxide interface is always a hotspot in the field, and has not reached a consensus. The most commonly appreciated reason about the origin of the two-dimensional electron gas was divided into two parties, one is the intrinsic reason: reconstruction of electrons caused by the polarity discontinuity at the interface has been considered to be the dominant mechanism behind these phenomena. "Polarization catastrophe" and "reconstruction of electrons" provide a theoretical basis for this view. The other is the defect theory:oxygen vacancies at the interface or positive ions interdiffusion at the interface. Defect theory depends heavily on experimental conditions, such as oxygen partial pressure, deposition technology, and so on.The origin of the magnetism of the LaAlO3/SrTiO3 interface is another hot spot for the researchers. Some research groups think that the magnetism is the intrinsic nature of the heterostructure, that the magnetism is induced by the spin-splitting of the 2DEG near the Fermi level. Conversely, the intrinsic magnetism at the interface is very little (～0.01 μB), and can not be observed. The oxygen vacancies of the interface TiO2 layer heighten the magnetism to visible levels, that is, the magnetism is induced by the oxygen vacancies. There are disagreements with the origin of the magnetism at the LaAlO3/SrTiO3 interface, and further research is needed.There are low-temperature superconductivity and ferromagnetism in oxide heterostructures, and it challenges the traditional theory. In recent years, some research groups provide direct evidences for the coexistence of two phases in the same sample. Some researchers think that the sizable electronic correlation is the cause of the coexistence of the two mutually exclusive properties. Yet, there is still no satisfactory explanation about the novel physical phenomenon. Whether they coexist in the same region of the samples or there is a phase separation is still a matter of debate. So, further research is needed.In this thesis, the density functional theory (DFT) calculations are employed to investigate the structures, electrical and magnetic transport properties of different SrTiO3 surfaces systematically. On this basis, the stability, magnetic and electrical transport properties of the LaAlO3/SrTiO3 (110), the electrical and magnetic transport properties of BaTiO3/MgO (110) interfaces, the effect of in-plane strain on the lattice relaxation and electrical transport properties of LaAlO3/SrTiO3 interface are investigated, respectively. Through a series of researches on the electrical and magnetic transport properties of these pervoskite heterostructures, the origin of the interface 2DEG and magnetism are studied systematically, the interface effect and its mechanism of action are revealed, too. All these have deepened the understanding of the physical mechanism behind the novel phenomena, which occur in the interfaces of pervoskite heterostructures. Main achievements are listed as follows:(1)The electrical and magnetic transport properties of different SrTiO3 surfaces.SrTiO3 (001) surfaces:Stability and surface relaxation of TiO2-and SrO-terminated SrTiO3 (001) surfaces are investigated, firstly. The calculated results show that the surface energy of SrO-terminated SrTiO3 (001) surface is close to that of the TiO2-terminated SrTiO3 (001) surface, and the TiO2-terminated SrTiO3 (001) surface is more stable. There are different levels of surface folding in the relaxed SrTiO3 (001) surfaces. Electronic structures show that the two kinds of surfaces have good insulation.SrTiO3 (111) surfaces:Ti-and SrO3-terminated SrTiO3(111) surfaces are investigated, respectively. The results show that the surface energy of Ti-terminated SrTiO3 (111) is smaller than that of SrO3-terminated SrTiO3 (111) surface energy. On this basis, electronic structures of the Ti-terminated SrTiO3 (111) surface are investigated, and the band structure shows that a magnetic two-dimensional electron gas (2DEG) is formed in the surface. The origin of 2DEG is the t2g (dXy/xz/yz) electrons of Ti atoms at the surface. Near the Fermi level, the t2g electrons of Ti atoms at the surface have obvious spin-splitting, which leads to a net magnetic moment more than 0.55 μB per 2D unit cell on the surface. At last, the DFT+SOC are used to investigate the magnetic anisotropy of the 2DEG, and it is found that the magnetic dipoles lie in the plane of the Ti-terminated SrTiO3 (111) surface, which is perpendicular to the [111] direction.SrTiO3 (110) surface:SrTiO-and O2-terminated SrTiO3 (110) surface are investigated, respectively. The results show that the surface energy of the O2-terminated SrTiO3(110) surface is smaller than that of the SrTiO-terminated SrTiO3 (110) surfaces. In the surface, the obvious distortion of the Ti-O octahedron occurs. A highly confined conductivity and magnetism are formed in the surfaces. For the SrTiO-terminated SrTiO3 (110) surface, the partly occupied Ti 3d electrons are confined in the surface and they are the origin of the 2DEG. Near the Fermi level, the surface Ti 3d electrons have obvious spin splitting, leading to a net magnetic moment more than 0.42 μB per 2D unit cell on the surface. For the O2-terminated SrTiO3 (110) surface, the partially filled electronic states near the Fermi energy level are mainly the surface O 2p states, and they are the origin of the holes in the surface. Each surface O atom has the biggest magnetic moment about 0.32uB. The magnetic moment decreases rapidly with the increase of the distance from the surface, and thus the surface O atoms make important contributions to the conductivity and magnetism of the system.(2) The stability, electrical and magnetic transport properties of the LaAlO3/SrTiO3(110).The structures of SrTiO3 and LaAlO3 (110) surfaces are calculated, firstly. Then, the cleavage energies E(Cl), relaxation energies E(rel), and surface energies E(surf) of different terminations of SrTiO3 (110) and LaAlO3 (110) surfaces are calculated. From the results of the calculation, it is found that the O2-terminated surface is more stable for whether SrTiO3 (110) surface or LaA103 (110) surface. On this basis, LaAlO3/SrTiO3 (110) interface is investigated. Two kinds of interfaces were proposed: abrupt interface and buckled interface. By comparing interface energy, it is obtained that the buckled interface is more stable than the abrupt interface. And this is consistent with experimental observations. At the interface of LaAlO3/SrTiO3 (110) heterostructure, the Ti-O octahedral distortions cause the interface Ti t2g orbitals spin-splitting, and the partly filled two-fold degenerate dxz/dyz orbitals are the origin of two-dimensional electron gas (2DEG) and result in 0.35 uB on the interface Ti atom.(3) The electrical and magnetic transport properties of BaTiO3/MgO (110) interfaces.Both n-type and p-type interfaces have shown the two-dimensional metallic behavior. For the n-type interface, the interface Ti 3d electrons are the origin of the metallic and magnetic properties. Varying the thickness of BaTiO3 may induce an insulator-metal transition, and the critical thickness is 4 unit cells. For the p-type interface, holes preferentially occupy the interface O 2py state, resulting in a conducting interface. The unbalance of the spin splitting of the O 2p states in interface MgO layer leads to a magnetic moment of about 0.25 μB per O atom at the interface.(4) The effect of in-plane strain on the electrical transport property of LaAlO3/SrTi03 interface.It is obtained that the in-plane strain influences the lattice relaxation obviously and regularly:the relative relaxation of Ti-O decreases with the increasing of the in-plane lattice constant, while the relative relaxation of La-0 increases with the increasing of the in-plane lattice constant. The distortion of interfacial Ti-O octahedron and the roughness of the interface are enhanced with the increasing of the in-plane compressive strain. The Ti 3dxy orbital at the interface is partly filled and it is the origin of the interfacial two-dimensional electron gas (2DEG). The in-plane strain can effectively modulate the carrier concentration and consequently the conductivity of the 2DEG formed at the interface. In-plane strain may reduce the carrier concentration significantly and induce a metal-insulator transition when the in-plane compressive (tensile) strain is added up to 4.98%(5.29%).