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Polar discontinuities occurring at interfaces between two different materials constitute both a challenge and an opportunity in the study and application of a variety of devices. In order to cure the large electric field occurring in such structures, a reconfiguration of the charge landscape sets in at the interface via chemical modifications, adsorbates or charge transfer. In the latter case, one may expect a local electronic doping of one material: one sparkling example is the two-dimensional electron liquid (2DEL) appearing in SrTiO$_3$ once covered by a polar LaAlO$_3$ layer. Here we show that tuning the formal polarisation of a (La,Al)$_{1-x}$(Sr,Ti)$_x$O$_3$ (LASTO:$x$) overlayer through chemical composition modifies the quantum confinement of the 2DEL in SrTiO$_3$ and its electronic band structure. The analysis of the behaviour in magnetic field of superconducting field-effect devices reveals, in agreement with $ab\ initio$ calculations and self-consistent Poisson-Schr\"odinger modelling, that quantum confinement and energy splitting between electronic bands of different symmetries strongly depend on interface charge densities. These results not only strongly support the polar discontinuity mechanisms with a full charge transfer to explain the origin of the 2DEL at the celebrated LaAlO$_3$/SrTiO$_3$ interface, but also demonstrate an effective tool for tailoring the electronic structure at oxide interfaces., Comment: 18 pages, 4 figures, 1 ancillary file (Supporting Information)

For more than 30 years, the remarkable superconducting properties of REBa$_\textrm{2}$Cu$_\textrm{3}$O$_\textrm{7-x}$ (RE = rare earth) compounds have triggered research studies across the world. Accordingly, significant progresses have been made both from a basic understanding and a fabrication processes perspective. Yet, today, the major technological bottleneck towards the spread of their practical uses remains the exponential decay of their critical current with grain misorientation in polycrystalline samples. In this work, we used an ionic liquid to apply extremely high transverse electric fields to YBa$_\textrm{2}$Cu$_\textrm{3}$O$_\textrm{7-x}$ thin films containing a single well-defined low-angle grain boundary. Our study shows that this technique is very effective to tune the IV characteristics of these weak-links. In-magnetic field measurements allow us to discuss the type of the vortices present at the grain boundary and to unveil a large variation of the local depairing current density with gating. Comparing our results with the ones obtained on chemically-doped grain boundaries, we discuss routes to evaluate the role of local strain in the loss of transparency at cuprates low-angle grain boundaries. In short, this study offers a new opportunity to discuss scenarios leading to the reduced transport capabilities of grain boundaries in cuprates.

Superconductivity develops in bulk doped SrTiO$_3$ and at the LaAlO$_3$/SrTiO$_3$ interface with a dome-shaped density dependence of the critical temperature $T_c$, despite different dimensionalities and geometries. We propose that the $T_c$ dome of LaAlO$_3$/SrTiO$_3$ is a shape resonance due to quantum confinement of superconducting bulk SrTiO$_3$. We substantiate this interpretation by comparing the exact solutions of a three-dimensional and quasi-two-dimensional two-band BCS gap equation. This comparison highlights the role of heavy bands for $T_c$ in both geometries. For bulk SrTiO$_3$, we extract the density dependence of the pairing interaction from the fit to experimental data. We apply quantum confinement in a square potential well of finite depth and calculate $T_c$ in the confined configuration. We compare the calculated $T_c$ to transport experiments and provide an explanation as to why the optimal $T_c$'s are so close to each other in two-dimensional interfaces and the three-dimensional bulk material., Comment: 6 pages, 5 figures

In this paper, we present a detailed investigation of the self-field transport properties of an ionic liquid gated ultra-thin YBa$_2$Cu$_3$O$_{7-x}$ film. From the high temperature dynamic of the resistivity ($> 220 \textrm{ K}$) different scenarios pertaining to the interaction between the liquid and the thin film are proposed. From the low temperature evolution of $J_{\textrm{c}}$ and $T_{\textrm{c}}$ a comparison between the behavior of our system and the standard properties of YBCO is drawn.

We describe the transport properties of mesoscopic devices based on the two dimensional electron gas (2DEG) present at the LaAlO$_3$/SrTiO$_3$ interface. Bridges with lateral dimensions down to 500~nm were realized using electron beam lithography. Their detailed characterization shows that processing and confinement do not alter the transport parameters of the 2DEG. The devices exhibit superconducting behavior tunable by electric field effect. In the normal state, we measured universal conductance fluctuations, signature of phase-coherent transport in small structures. The achievement of reliable lateral confinement of the 2DEG opens the way to the realization of quantum electronic devices at the LaAlO$_3$/SrTiO$_3$ interface., Comment: 10 pages, 4 figures

We report measurements of the normal state in-plane magnetoconductance in gated LaAlO$_3$-SrTiO$_3$ samples. As the orientation of the magnetic field changes within the plane of the interface, the signal displays periodic oscillations with respect to the angle between the field and the direction of the current. We show that in the underdoped to optimally doped range, a Fermi surface reconstruction takes place due to the Rashba spin-orbit term and that the oscillations are due to a magnetic field induced opening and closing of a gap at the $\Gamma$ point for those Ti out-of-plane orbitals having their band minimum close to the Fermi energy of the system., Comment: 5 pages, 4 figures