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Register a new userRaman and fluorescence contributions to resonant inelastic soft x-ray scattering on LaAlO$_3$/SrTiO$_3$ heterostructures
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We present a detailed study of the Ti 3$d$ carriers at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures by high-resolution resonant inelastic soft x-ray scattering (RIXS), with special focus on the roles of overlayer thickness and oxygen vacancies. Our measurements show the existence of interfacial Ti 3$d$ electrons already below the critical thickness for conductivity and an increase of the total interface charge up to a LaAlO$_3$ overlayer thickness of 6 unit cells before it levels out. By comparing stoichiometric and oxygen deficient samples we observe strong Ti 3$d$ charge carrier doping by oxygen vacancies. The RIXS data combined with photoelectron spectroscopy and transport measurements indicate the simultaneous presence of localized and itinerant charge carriers. However, it is demonstrated that the relative amount of localized and itinerant Ti $3d$ electrons in the ground state cannot be deduced from the relative intensities of the Raman and fluorescence peaks in excitation energy dependent RIXS measurements, in contrast to previous interpretations. Rather, we attribute the observation of either the Raman or the fluorescence signal to the spatial extension of the intermediate state reached in the RIXS excitation process.
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We investigated the electronic structure of the SrTiO$_3$/LaAlO$_3$ superlattice (SL) by resonant soft x-ray scattering. The (003) peak, which is forbidden for our ideal SL structure, was observed at all photon energies, indicating reconstruction at the interface. From the peak position analyses taking into account the effects of refraction, we obtained evidence for electronic reconstruction of Ti 3d and O $2p$ states at the interface. From reflectivity analyses, we concluded that the AlO$_2$/LaO/TiO$_2$/SrO and the TiO$_2$/SrO/AlO$_2$/LaO interfaces are quite different, leading to highly asymmetric properties.
Effects of X-ray irradiation on the electronic structure of LaAlO$_3$/SrTiO$_3$ (LAO/STO) samples, grown at low oxygen pressure and post-annealed ex-situ till recovery of their stoichiometry, were investigated by soft-X-ray ARPES. The irradiation at low sample temperature below ~100K creates oxygen vacancies (VOs) injecting Ti t2g-electrons into the interfacial mobile electron system (MES). At this temperature the oxygen out-diffusion is suppressed, and the VOs are expected to appear mostly in the top STO layer. However, we observe a pronounced three-dimensional (3D) character of the X-ray generated MES in our samples, indicating its large extension into the STO depth, which contrasts to the purely two-dimensional (2D) character of the MES in standard stoichiometric LAO/STO samples. Based on self-interaction-corrected DFT calculations of the MES induced by VOs at the interface and in STO bulk, we discuss possible mechanisms of this puzzling three-dimensionality. They may involve VOs remnant in the deeper STO layers, photoconductivity-induced metallic states as well as more exotic mechanisms such as X-ray induced formation of Frenkel pairs.
In this paper we study LaAlO$_3$/Eu$_{1-x}$La$_x$TiO$_3$/SrTiO$_3$ structures with nominally x = 0, 0.1 and different thicknesses of the Eu$_{1-x}$La$_x$TiO$_3$ layer. We observe that both systems have many properties similar to previously studied LaAlO$_3$/EuTiO$_3$/SrTiO$_3$ and other oxide interfaces, such as the formation of a 2D electron liquid for 1 or 2 unit cells of Eu$_{1-x}$La$_x$TiO$_3$; a metal-insulator transition driven by the thickness increase of Eu$_{1-x}$La$_x$TiO$_3$ layer; the presence of an Anomalous Hall effect (AHE) when driving the systems above the Lifshitz point with a backgate voltage; and a minimum in the temperature dependence of the sheet resistance below the Lifshitz point in the one-band regime, which becomes more pronounced with increasing gate voltage. However, and notwithstanding the likely presence of magnetism in the system, we do not attribute that minimum to the Kondo effect, but rather to the properties of SrTiO$_3$ crystal and the inevitable effects of charge trapping when using back gates.
Ionic crystals terminated at oppositely charged polar surfaces are inherently unstable and expected to undergo surface reconstructions to maintain electrostatic stability. Essentially, an electric field that arises between oppositely charged atomic planes gives rise to a built-in potential that diverges with thickness. In ultra thin film form however the polar crystals are expected to remain stable without necessitating surface reconstructions, yet the built-in potential has eluded observation. Here we present evidence of a built-in potential across polar lao ~thin films grown on sto ~substrates, a system well known for the electron gas that forms at the interface. By performing electron tunneling measurements between the electron gas and a metallic gate on lao ~we measure a built-in electric field across lao ~of 93 meV/AA. Additionally, capacitance measurements reveal the presence of an induced dipole moment near the interface in sto, illuminating a unique property of sto ~substrates. We forsee use of the ionic built-in potential as an additional tuning parameter in both existing and novel device architectures, especially as atomic control of oxide interfaces gains widespread momentum.
We report a detailed analysis of magneto-transport properties of top- and back-gated LaAlO$_3$/SrTiO$_3$ heterostructures. Efficient modulation in magneto-resistance, carrier density, and mobility of the two-dimensional electron liquid present at the interface is achieved by sweeping top and back gate voltages. Analyzing those changes with respect to the carrier density tuning, we observe that the back gate strongly modifies the electron mobility while the top gate mainly varies the carrier density. The evolution of the spin-orbit interaction is also followed as a function of top and back gating.
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