There are many electronic and magnetic properties exhibited by complex oxides. Electronic phase separation (EPS) is one of those, the presence of which can be linked to exotic behaviours, such as ...colossal magnetoresistance, metal-insulator transition and high-temperature superconductivity. A variety of new and unusual electronic phases at the interfaces between complex oxides, in particular between two non-magnetic insulators LaAlO(3) and SrTiO(3), have stimulated the oxide community. However, no EPS has been observed in this system despite a theoretical prediction. Here, we report an EPS state at the LaAlO(3)/SrTiO(3) interface, where the interface charges are separated into regions of a quasi-two-dimensional electron gas, a ferromagnetic phase, which persists above room temperature, and a (superconductor like) diamagnetic/paramagnetic phase below 60 K. The EPS is due to the selective occupancy (in the form of 2D-nanoscopic metallic droplets) of interface sub-bands of the nearly degenerate Ti orbital in the SrTiO(3). The observation of this EPS demonstrates the electronic and magnetic phenomena that can emerge at the interface between complex oxides mediated by the Ti orbital.
The relative importance of atomic defects and electron transfer in explaining conductivity at the crystalline LaAlO3/SrTiO3 interface has been a topic of debate. Metallic interfaces with similar ...electronic properties produced by amorphous oxide overlayers on SrTiO3 Y. Chen et al., Nano Lett. 11, 3774 (2011); S. W. Lee et al., Nano Lett. 12, 4775 (2012) have called in question the original polarization catastrophe model N. Nakagawa et al., Nature Mater. 5, 204 (2006). We resolve the issue by a comprehensive comparison of (100)-oriented SrTiO3 substrates with crystalline and amorphous overlayers of LaAlO3 of different thicknesses prepared under different oxygen pressures. For both types of overlayers, there is a critical thickness for the appearance of conductivity, but its value is always 4 unit cells (around 1.6 nm) for the oxygen-annealed crystalline case, whereas in the amorphous case, the critical thickness could be varied in the range 0.5 to 6 nm according to the deposition conditions. Subsequent ion milling of the overlayer restores the insulating state for the oxygen-annealed crystalline heterostructures but not for the amorphous ones. Oxygen post-annealing removes the oxygen vacancies, and the interfaces become insulating in the amorphous case. However, the interfaces with a crystalline overlayer remain conducting with reduced carrier density. These results demonstrate that oxygen vacancies are the dominant source of mobile carriers when the LaAlO3 overlayer is amorphous, while both oxygen vacancies and polarization catastrophe contribute to the interface conductivity in unannealed crystalline LaAlO3/SrTiO3 heterostructures, and the polarization catastrophe alone accounts for the conductivity in oxygen-annealed crystalline LaAlO3/SrTiO3 heterostructures. Furthermore, we find that the crystallinity of the LaAlO3 layer is crucial for the polarization catastrophe mechanism in the case of crystalline LaAlO3 overlayers.
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Magnetic interactions in solids are normally mediated by short-range exchange or weak dipole fields. Here we report a magnetic interaction that can propagate over long distances (∼10 nm) across a ...polar insulating oxide spacer. Evidence includes oscillations of magnetization, coercivity and field-cooled loop shift with the thickness of LaAlO3 in La0.67Sr0.33MnO3/LaAlO3/SrTiO3 heterostructures. Similar modifications of the hysteresis loop appear when two coupled films of La0.67Sr0.33MnO3 are separated by LaAlO3, or another polar insulator, but they are absent when the oxide spacer layer is nonpolar. The loop shift is attributed to strong spin-orbit coupling and Dzyaloshinskii-Moriya interaction at the interfaces. There is evidence from inelastic light scattering that the polar spacer mediates long-range transmission of orbital magnetization. This coupling mechanism is expected to apply for any conducting ferromagnetic oxide with mixed valence; in view of electron hopping frequency involved, it raises the prospect of terahertz tunability of magnetic coupling.
The observation of a high-mobility two-dimensional electron gas between two insulating complex oxides, especially LaAlO₃/SrTiO₃, has enhanced the potential of oxides for electronics. The occurrence ...of this conductivity is believed to be driven by polarization discontinuity, leading to an electronic reconstruction. In this scenario, the crystal orientation has an important role and no conductivity would be expected, for example, for the interface between LaAlO₃ and (110)-oriented SrTiO₃, which should not have a polarization discontinuity. Here we report the observation of unexpected conductivity at the LaAlO₃/SrTiO₃ interface prepared on (110)-oriented SrTiO₃, with a LaAlO₃-layer thickness-dependent metal-insulator transition. Density functional theory calculation reveals that electronic reconstruction, and thus conductivity, is still possible at this (110) interface by considering the energetically favourable (110) interface structure, that is, buckled TiO₂/LaO, in which the polarization discontinuity is still present. The conductivity was further found to be strongly anisotropic along the different crystallographic directions with potential for anisotropic superconductivity and magnetism, leading to possible new physics and applications.
We show here a new phenomenon in La0.5Sr0.5TiO3/SrTiO3 (LSTO/STO) heterostructures; that is a coexistence of three-dimensional electron liquid (3DEL) and 2D electron gas (2DEG), separated by an ...intervening insulating LSTO layer. The two types of carriers were revealed through multi-channel analysis of the evolution of nonlinear Hall effect as a function of film thickness, temperature and back gate voltage. We demonstrate that the 3D electron originates from La doping in LSTO film and the 2D electron at the surface of STO is due to the polar field in the intervening insulating layer. As the film thickness is reduced below a critical thickness of 6 unit cells (uc), an abrupt metal-to-insulator transition (MIT) occurs without an intermediate semiconducting state. The properties of the LSTO layer grown on different substrates suggest that the insulating phase of the intervening layer is a result of interface strain induced by the lattice mismatch between the film and substrate. Further, by fitting the magnetoresistance (MR) curves, the 6 unit cell thick LSTO is shown to exhibit spin-orbital coupling. These observations point to new functionalities, in addition to magnetism and superconductivity in STO-based systems, which could be exploited in a multifunctional context.
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The surface termination of (100)-oriented LaAlO3 (LAO) single crystals was examined by atomic force microscopy and optimized to produce a single-terminated atomically flat surface by annealing. Then ...the atomically flat STO film was achieved on a single-terminated LAO substrate, which is expected to be similar to the n-type interface of two-dimensional electron gas (2DEG), i.e., (LaO)-(TiO2). Particularly, that can serve as a mirror structure for the typical 2DEG heterostructure to further clarify the origin of 2DEG. This newly developed interface was determined to be highly insulating. Additionally, this study demonstrates an approach to achieve atomically flat film growth based on LAO substrates.
The interface between the polar LaAlO sub(3) and nonpolar SrTiO sub(3) layers has been shown to exhibit various electronic and magnetic phases such as two-dimensional electron gas (2DEG), ...superconductivity, magnetism, and electronic phase separation. These rich phases are expected due to the strong interplay between charge, spin, and orbital degree of freedom at the interface between these complex oxides, leading to the electronic reconstruction in this system. However, until now all of these new properties have been studied extensively based on the interfaces which involve a polar LaAlO sub(3) layer. To investigate the role of the ABO sub(3) polar layer, here we study various combinations of polar/nonpolar oxide (NdAlO sub(3)/SrTiO sub(3), PrAlO sub(3)/SrTiO sub(3), and NdGaO sub(3)/SrTiO sub(3)) interfaces which are similar in nature to the LaAlO sub(3)/SrTiO sub(3) interface. Our results show that all of these new interfaces can also produce 2DEG at their interfaces, supporting the idea that the electronic reconstruction could be the driving mechanism for the creation of the 2DEG at these oxide interfaces. Furthermore, the electrical properties of these interfaces are shown to be governed by the interface strain and the type of cations in the polar overlayers. Our observations may provide an approach to further tune the properties of the 2DEG at the selected polar/nonpolar oxide interfaces.
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The two-dimensional electron gas (2DEG) formed at the perovskite oxides heterostructures is of great interest because of its potential applications in oxides electronics and nanoscale multifunctional ...devices. A canonical example is the 2DEG at the interface between a polar oxide LaAlO3 (LAO) and non-polar SrTiO3 (STO). Here, the LAO polar oxide can be regarded as the modulating or doping layer and is expected to define the electronic properties of 2DEG at the LAO/STO interface. However, to practically implement the 2DEG in electronics and device design, desired properties such as tunable 2D carrier density are necessary. Here, we report the tuning of conductivity threshold, carrier density and electronic properties of 2DEG in LAO/STO heterostructures by insertion of a La0.5Sr0.5TiO3 (LSTO) layer of varying thicknesses, and thus modulating the amount of polarization of the oxide over layers. Our experimental result shows an enhancement of carrier density up to a value of about five times higher than that observed at the LAO/STO interface. A complete thickness dependent metal-insulator phase diagram is obtained by varying the thickness of LAO and LSTO providing an estimate for the critical thickness needed for the metallic phase. The observations are discussed in terms of electronic reconstruction induced by polar oxides.
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