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Register a new userCorrelation between Superconductivity, Band Filling and Electron Confinement at the LaAlO$_{3}$-SrTiO$_{3}$ Interface
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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.
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The interface superconductivity in LaAlO$_{3}$-SrTiO$_{3}$ heterostructures reveals a non-monotonic behavior of the critical temperature as a function of the two-dimensional density of charge carriers. We develop a theoretical description of interface superconductivity in strongly polar heterostructures, based on the dielectric function formalism. The density dependence of the critical temperature is calculated accounting for all phonon branches including different types of optical (interface and half-space) and acoustic phonons. The LO- and acoustic-phonon-mediated electron-electron interaction is shown to be the dominating mechanism governing the superconducting phase transition in the heterostructure.
Using a combination of vertical transport measurements across and lateral transport measurements along the LaAlO$_{3}$/SrTiO$_{3}$ heterointerface, we demonstrate that significant potential barrier lowering and band bending are the cause of interfacial metallicity. Barrier lowering and enhanced band bending extends over 2.5 nm into LaAlO$_{3}$ as well as SrTiO$_{3}$. We explain origins of high-temperature carrier saturation, lower carrier concentration, and higher mobility in the sample with the thinnest LaAlO$_{3}$ film on a SrTiO$_{3}$ substrate. Lateral transport results suggest that parasitic interface scattering centers limit the low-temperature lateral electron mobility of the metallic channel.
Novel physical phenomena arising at the interface of complex oxide heterostructures offer exciting opportunities for the development of future electronic devices. Using the prototypical LaAlO$_3$/SrTiO$_3$ interface as a model system, we employ a single-step lithographic process to realize gate tunable Josephson junctions through a combination of lateral confinement and local side gating. The action of the side gates is found to be comparable to that of a local back gate, constituting a robust and efficient way to control the properties of the interface at the nanoscale. We demonstrate that the side gates enable reliable tuning of both the normal-state resistance and the critical (Josephson) current of the constrictions. The conductance and Josephson current show mesoscopic fluctuations as a function of the applied side gate voltage, and the analysis of their amplitude enables the extraction of the phase coherence and thermal lengths. Finally, we realize a superconducting quantum interference device in which the critical currents of each of the constriction-type Josephson junctions can be controlled independently via the side gates.
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.
A magnetic field parallel to an electrical current does not produce a Lorentz force on the charge carriers. Therefore, orbital longitudinal magnetoresistance is unexpected. Here we report on the observation of a large and non saturating magnetoresistance in lightly doped SrTiO$_{3-x}$ independent of the relative orientation of current and magnetic field. We show that this quasi-isotropic magnetoresistance can be explained if the carrier mobility along all orientations smoothly decreases with magnetic field. This anomalous regime is restricted to low concentrations when the dipolar correlation length is longer than the distance between carriers. We identify cyclotron motion of electrons in a potential landscape tailored by polar domains as the cradle of quasi-isotropic orbital magnetoresistance. The result emerges as a challenge to theory and may be a generic feature of lightly-doped quantum paralectric materials.
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