Using state-of-the-art, aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy with atomic-scale spatial resolution, experimental evidence for an intrinsic electronic reconstruction at the LAO/STO interface is shown. Simultaneous measurements of interfacial electron density and system polarization are crucial for establishing the highly debated origin of the 2D electron gas.

We show that the growth of the heterostructure LaGaO 3 / SrTiO 3 yields the formation of a highly conductive interface. Our samples were carefully analyzed by high resolution electron microscopy, in order to assess their crystal perfection and to evaluate the abruptness of the interface. Their carrier density and sheet resistance are compared to the case of LaAlO 3 / SrTiO 3 and a superconducting transition is found. The results open the route to widening the field of polar-nonpolar interfaces, pose some phenomenological constrains to their underlying physics and highlight the chance of tailoring their properties for future applications by adopting suitable polar materials.The quasi-two-dimensional electron gas ͑q2DEG͒ recently discovered at the LaAlO 3 ͑LAO͒ / SrTiO 3 ͑STO͒ interface 1 is presently envisaged as an ideal system for the realization of nanoscale oxide devices. 2 The electronic reconstruction model attributes the origin of the q2DEG to an electronic relaxation mechanism occurring at the interface between the ͑nominally͒ non-polar ͑001͒ STO substrate and the polar ͑001͒ LAO film. The wide band gap of LAO is considered as crucial in this approach, because it determines the capability of the polar film to transfer charges over the band gap of STO. Ideally, half an electron per areal unit cell ͑Ϸ3.3ϫ 10 14 cm −2 ͒ is expected to be transferred at the TiO 2 -LaO interface, partially filling the 3d Ti levels of the STO conduction band ͑CB͒. Alternatively, a possible active role of oxygen vacancies in STO near the interface was envisaged. 3 Actually, the transport properties of the heterostructure are affected both by oxygen pressure during growth 4,5 and by the application of an oxygen postanneal. 5 Finally, it was argued that a substantial La substitution for Sr during sample growth might drive the insulating surface of STO into a conductor. [6][7][8] Obviously, also LAO poses material issues. 9 In this context, we started the search of novel heterostructures based on a different overlayer. On this basis, we identified as a first test material LaGaO 3 ͑LGO͒, a polar, wide band gap, pseudocubic perovskite.Films of LAO and LGO were deposited on nominally TiO 2 terminated STO substrates, chemically treated in deionized water and buffered-HF. 10,11 The growth was performed by reflection high energy electron diffraction ͑RHEED͒ as-sisted pulsed laser deposition ͑KrF excimer laser, 248 nm͒ with a typical fluence of Ϸ1.5-2.5 J cm −2 at the target, a substrate temperature of 800°C and different oxygen pressures within the 10 −2 -10 −4 mbar range. 12 LAO films presented regular RHEED oscillations typical of layer-by-layer growth and a final pattern reminiscent of a single crystal surface, whereas LGO films showed damped and less regular oscillations, and a streaky 2D pattern at the end of the growth ͑Fig. 1͒.The atomic and electronic structures of LAO/STO and LGO/STO interfaces were investigated by high-resolution scanning transmission electron microscopy ͑STEM͒ and electron energy loss spectroscopy ͑EELS͒ measur...

The so-called "polar catastrophe," a sudden electronic reconstruction taking place to compensate for the interfacial ionic polar discontinuity, is currently considered as a likely factor to explain the surprising conductivity of the interface between the insulators LaAlO 3 and SrTiO 3 . We applied optical second harmonic generation, a technique that a priori can detect both mobile and localized interfacial electrons, to investigating the electronic polar reconstructions taking place at the interface. As the LaAlO 3 film thickness is increased, we identify two abrupt electronic rearrangements: the first takes place at a thickness of 3 unit cells, in the insulating state; the second occurs at a thickness of 4-6 unit cells, i.e., just above the threshold for which the samples become conducting. Two possible physical scenarios behind these observations are proposed. The first is based on an electronic transfer into localized electronic states at the interface that acts as a precursor of the conductivity onset. In the second scenario, the signal variations are attributed to the strong ionic relaxations taking place in the LaAlO 3 layer.

A major challenge for future spintronics is to develop suitable spin transport channels with long spin lifetime and propagation length. Graphene can meet these requirements, even at room temperature. On the other side, taking advantage of the fast motion of chiral textures, that is, Néel-type domain walls and magnetic skyrmions, can satisfy the demands for high-density data storage, low power consumption, and high processing speed. We have engineered epitaxial structures where an epitaxial ferromagnetic Co layer is sandwiched between an epitaxial Pt(111) buffer grown in turn onto MgO(111) substrates and a graphene layer. We provide evidence of a graphene-induced enhancement of the perpendicular magnetic anisotropy up to 4 nm thick Co films and of the existence of chiral left-handed Néel-type domain walls stabilized by the effective Dzyaloshinskii-Moriya interaction (DMI) in the stack. The experiments show evidence of a sizable DMI at the gr/Co interface, which is described in terms of a conduction electron mediated Rashba-DMI mechanism and points opposite to the spin orbit coupling-induced DMI at the Co/Pt interface. In addition, the presence of graphene results in (i) a surfactant action for the Co growth, producing an intercalated, flat, highly perfect face-centered cubic film, pseudomorphic with Pt and (ii) an efficient protection from oxidation. The magnetic chiral texture is stable at room temperature and grown on insulating substrate. Our findings open new routes to control chiral spin structures using interfacial engineering in graphene-based systems for future spin-orbitronics devices fully integrated on oxide substrates.

A site-dependent charge transfer to 7,7′,8,8′-tetracyanoquinodimethane (TCNQ) adsorbed on a single layer of periodically rippled graphene grown epitaxially on Ru(0001), identified by X-ray photoemission techniques, can be spatially resolved using Scanning Tunneling Microscopy, which can also detect the formation of magnetic moments. The molecules adsorbed on the lower part of the ripples are charged with electrons donated from the doped graphene overlayer and develop a magnetic moment, while those at the upper part of the ripples are neutral. On the other hand, TCNQ adsorbed on graphene on Ir(111) shows negligible charge transfer and no magnetic moment. These observations explain the spatially dependent longrange magnetic order observed recently for TCNQ on gr/Ru(0001).