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Register a new userMagnetoresistance of 2D and 3D Electron Gas in LaAlO3/SrTiO3 Heterostructures: Influence of Magnetic Ordering, Interface Scattering and Dimensionality
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Magnetoresistance (MR) anisotropy in LaAlO3/SrTiO3 (LAO/STO) interfaces is compared between samples prepared in high oxygen partial pressure (PO2) of 10-4 mbar exhibiting quasi-two-dimensional (quasi-2D) electron gas and low PO2 of 10-6 mbar exhibiting 3D conductivity. While MR of an order of magnitude larger was observed in low PO2 samples compared to those of high PO2 samples, large MR anisotropies were observed in both cases. The MR with the out-of-plane field is always larger compared to the MR with in-plane field suggesting lower dissipation of electrons from interface versus defect scattering. The quasi-2D interfaces show a negative MR at low temperatures while the 3D interfaces show positive MR for all temperatures. Furthermore, the angle relationship of MR anisotropy for these two different cases and temperature dependence of in-plane MR are also presented. Our study demonstrates that MR can be used to distinguish the dimensionality of the charge transport and various (defect, magnetic center, and interface boundary) scattering processes in this system.
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Possible ferromagnetism induced in otherwise non-magnetic materials has been motivating intense research in complex oxide heterostructures. Here we show that a confined magnetism is realized at the interface between SrTiO3 and two insulating polar oxides, BiMnO3 and LaAlO3. By using polarization dependent x-ray absorption spectroscopy, we find that in both cases the magnetic order is stabilized by a negative exchange interaction between the electrons transferred to the interface and local magnetic moments. These local magnetic moments are associated to Ti3+ ions at the interface itself for LaAlO3/SrTiO3 and to Mn3+ ions in the overlayer for BiMnO3/SrTiO3. In LaAlO3/SrTiO3 the induced magnetic moments are quenched by annealing in oxygen, suggesting a decisive role of oxygen vacancies in the stabilization of interfacial magnetism.
The conducting interface of LaAlO$_3$/SrTiO$_3$ heterostructures has been studied by hard X-ray photoelectron spectroscopy. From the Ti~2$p$ signal and its angle-dependence we derive that the thickness of the electron gas is much smaller than the probing depth of 4 nm and that the carrier densities vary with increasing number of LaAlO$_3$ overlayers. Our results point to an electronic reconstruction in the LaAlO$_3$ overlayer as the driving mechanism for the conducting interface and corroborate the recent interpretation of the superconducting ground state as being of the Berezinskii-Kosterlitz-Thouless type.
We present low-temperature and high-field magnetotransport data on SrTiO3-LaAlO3 interfaces. The resistance shows hysteresis in magnetic field and a logarithmic relaxation as a function of time. Oscillations in the magnetoresistance are observed, showing a square root periodicity in the applied magnetic field, both in large-area unstructured samples as well as in a structured sample. An explanation in terms of a commensurability condition of edge states in a highly mobile two-dimensional electron gas between substrate step edges is suggested.
We formulate a model for magnetic and superconducting ordering at LaAlO3/SrTiO3 interfaces containing both localized magnetic moments and itinerant electrons. Though these both originate in Ti 3d orbitals, the former may be due to electrons more tightly-bound to the interface while the latter are extended over several layers. Only the latter contribute significantly to metallic conduction and superconductivity. In our model, the interplay between the two types of electrons, which is argued to be ferromagnetic, combined with strong spin-orbit coupling of the itinerant electrons, leads to magnetic ordering. Furthermore, we propose a model for interfacial superconductivity, consisting of random superconducting grains in the bulk STO driven, via coupling to the interface conduction band, towards long-ranged or quasi-long-ranged order. Most interestingly, the magnetic order and strong spin orbit coupling can lead in this manner to unconventional interfacial superconductivity, yielding a possible realization of Majorana physics.
A detailed defect energy level map was investigated for heterostructures of 26 unit cells of LaAlO3 on SrTiO3 prepared at a low oxygen partial pressure of 10-6 mbar. The origin is attributed to the presence of dominating oxygen defects in SrTiO3 substrate. Using femtosecond laser spectroscopy, the transient absorption and relaxation times for various transitions were determined. An ultrafast relaxation process of 2-3 picosecond from the conduction band to the closest defect level and a slower process of 70-92 picosecond from conduction band to intra-band defect level were observed. The results are discussed on the basis of propose defect-band diagram.
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