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Register a new userInfrared study of the quasi-two-dimensional electron system at the interface between SrTiO$_{3}$ and crystalline or amorphous LaAlO$_3$
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We have used grazing-angle infrared spectroscopy to detect the Berreman effect (BE) in the quasi-two-dimensional electron system (q-2DES) which forms spontaneously at the interface between SrTiO$_{3}$ (STO) and a thin film of LaAlO$_3$ (LAO). From the BE, which allows one to study longitudinal optical excitations in ultrathin films like the q-2DES, we have extracted at different temperatures its thickness, the charge density and mobility of the carriers under crystalline LAO (sample A), and the charge density under amorphous LAO (sample B). This quantity turns out to be higher than in sample A, but a comparison with Hall measurements shows that under amorphous LAO the charges are partly localized at low $T$ with a low activation energy (about 190 K in $k_B$ units), and are thermally activated according to a model for large polarons. The thickness of the q-2DES extracted from our spectra turns out to be 4 $pm 1$ nm for crystalline LAO, 7 $pm 2$ nm for amorphous LAO.
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We have investigated the illumination effect on the magnetotransport properties of a two-dimensional electron system at the LaAlO$_3$/SrTiO$_3$ interface. The illumination significantly reduces the zero-field sheet resistance, eliminates the Kondo effect at low-temperature, and switches the negative magnetoresistance into the positive one. A large increase in the density of high-mobility carriers after illumination leads to quantum oscillations in the magnetoresistance originating from the Landau quantization. The carrier density ($sim 2 times 10^{12}$ cm$^{-2}$) and effective mass ($sim 1.7 ~m_e$) estimated from the oscillations suggest that the high-mobility electrons occupy the d$_{xz/yz}$ subbands of Ti:t$_{2g}$ orbital extending deep within the conducting sheet of SrTiO$_3$. Our results demonstrate that the illumination which induces additional carriers at the interface can pave the way to control the Kondo-like scattering and study the quantum transport in the complex oxide heterostructures.
We report the angular dependence of magnetoresistance in two-dimensional electron gas at LaAlO$_3$/SrTiO$_3$ interface. We find that this interfacial magnetoresistance exhibits a similar angular dependence to the spin Hall magnetoresistance observed in ferromagnet/heavy metal bilayers, which has been so far discussed in the framework of bulk spin Hall effect of heavy metal layer. The observed magnetoresistance is in qualitative agreement with theoretical model calculation including both Rashba spin-orbit coupling and exchange interaction. Our result suggests that magnetic interfaces subject to spin-orbit coupling can generate a nonnegligible contribution to the spin Hall magnetoresistance and the interfacial spin-orbit coupling effect is therefore key to the understanding of various spin-orbit-coupling-related phenomena in magnetic/non-magnetic bilayers.
We create a two-dimensional electron system (2DES) at the interface between EuO, a ferromagnetic insulator, and SrTiO3, a transparent non-magnetic insulator considered the bedrock of oxide-based electronics. This is achieved by a controlled in-situ redox reaction between pure metallic Eu deposited at room temperature on the surface of SrTiO3, an innovative bottom-up approach that can be easily generalized to other functional oxides and scaled to applications. Additionally, we find that the resulting EuO capping layer can be tuned from paramagnetic to ferromagnetic, depending on the layer thickness. These results demonstrate that the simple, novel technique of creating 2DESs in oxides by deposition of elementary reducing agents [T. C. Rodel et al., Adv. Mater. 28, 1976 (2016)] can be extended to simultaneously produce an active, e.g. magnetic, capping layer enabling the realization and control of additional functionalities in such oxide-based 2DESs.
By combined top- and backgating, we explore the correlation of superconductivity with band filling and electron confinement at the LaAlO$_3$-SrTiO$_3$ interface. We find that the top- and backgate voltages have distinctly different effects on the superconducting critical temperature, implying that the confining potential well has a profound effect on superconductivity. We investigate the origin of this behavior by comparing the gate-dependence of $T_c$ to the corresponding evolution of the band filling with gate voltage. For several backgate voltages, we observe maximum $T_c$ to consistently coincide with a kink in tuning the band filling for high topgate voltage. Self-consistent Schrodinger-Poisson calculations relate this kink to a Lifshitz transition of the second $d_{xy}$ subband. These results establish a major role for confinement-induced subbands in the phase diagram of SrTiO$_3$ surface states, and establish gating as a means to control the relative energy of these states.
We have studied the electronic properties of the 2D electron liquid present at the LaAlO$_3$/SrTiO$_3$ interface in series of samples prepared at different growth temperatures. We observe that interfaces fabricated at 650{deg}C exhibit the highest low temperature mobility ($approx 10000 textrm{ cm}^2/textrm{Vs}$) and the lowest sheet carrier density ($approx 5times 10^{12} textrm{ cm}^{-2}$). These samples show metallic behavior and Shubnikov-de Haas oscillations in their magnetoresistance. Samples grown at higher temperatures (800-900{deg}C) display carrier densities in the range of $approx 2-5 times 10^{13} textrm{ cm}^{-2}$ and mobilities of $approx 1000 textrm{ cm}^2/textrm{Vs}$ at 4K. Reducing their carrier density by field effect to $8times 10^{12} textrm{ cm}^{-2}$ lowers their mobilites to $approx 50 textrm{ cm}^2/textrm{Vs}$ bringing the conductance to the weak-localization regime.
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