Advances in growth technology of oxide materials allow single atomic layer control of heterostructures. In particular delta doping, a key materials' engineering tool in today's semiconductor technology, is now also available for oxides. Here we show that a fully electric-field-tunable spin-polarized and superconducting quasi-2D electron system (q2DES) can be artificially created by inserting a few unit cells of delta doping EuTiO3 at the interface between LaAlO3 and SrTiO3 oxides. Spin polarization emerges below the ferromagnetic transition temperature of the EuTiO3 layer (TFM = 6-8 K) and is due to the exchange interaction between the magnetic moments of Eu-4f and of Ti-3d electrons. Moreover, in a large region of the phase diagram, superconductivity sets in from a ferromagnetic normal state. The occurrence of magnetic interactions, superconductivity and spin-orbit coupling in the same q2DES makes the LaAlO3/EuTiO3/SrTiO3 system an intriguing platform for the emergence of novel quantum phases in low-dimensional materials.

A charge-density wave (CDW) state has a broken symmetry described by a complex order parameter with an amplitude and a phase. The conventional view, based on clean, weak-coupling systems, is that a finite amplitude and long-range phase coherence set in simultaneously at the CDW transition temperature Tcdw. Here we investigate, using photoemission, X-ray scattering and scanning tunnelling microscopy, the canonical CDW compound 2H-NbSe2 intercalated with Mn and Co, and show that the conventional view is untenable. We find that, either at high temperature or at large intercalation, CDW order becomes short-ranged with a well-defined amplitude, which has impacts on the electronic dispersion, giving rise to an energy gap. The phase transition at Tcdw marks the onset of long-range order with global phase coherence, leading to sharp electronic excitations. Our observations emphasize the importance of phase fluctuations in strongly coupled CDW systems and provide insights into the significance of phase incoherence in ‘pseudogap’ states.

The physics of transition metal oxide heterostructures is a subject of great interest due to the observations of new interface electronic and magnetic states. [ 1 ] The two-dimensional electron gas (2DEG) discovered at the interface between insulating LaAlO 3 (LAO) thin fi lms and bulk SrTiO 3 (STO) crystals stands as a model and has boosted the great expectations placed in oxide electronics. [ 2 , 3 ] Recently, it has been found that also the (001) SrTiO 3 surfaces, obtained by cleaving SrTiO 3 in ultra-high vacuum (UHV) [ 4 , 5 ] or by annealing in reducing conditions, [ 6 ] exhibit two-dimensional electronic states, independent of the bulk doping. From these studies it emerges that, despite possible different doping mechanisms, a similar band splitting characterizes the electronic properties of vacuum/STO and LAO/STO [ 7 ] interfaces.As matter of fact, while electron doping is believed to occur by creation of oxygen vacancies in STO surfaces, [ 5 , 6 , 8 ] in the case of LAO/STO interfaces the primary origin of the charge carriers is attributed to an electrostatic instability of the system. In particular, a scenario of electronic reconstruction has been proposed, [ 9 , 10 ] where a Zener breakdown takes place above a LAO thickness of 3 unit cells (uc), as experimentally observed. [ 11 , 12 ] According to this picture, once the electron transfer occurs, carriers spontaneously occupy bands composed by Ti-3d orbitals, starting with d xy -symmetry bands located within the fi rst adjacent SrTiO 3 layers. [ 13 ] Moreover, according to DFT calculation on LAO/STO bilayers, [ 14 ] the distribution of electrons accumulated within the fi rst STO layers gives rise to an electric fi eld, which in turn induces a rumpling of the SrO and TiO 2 interface atomic planes within the same region hosting the 2DEG.Thus, a polar instability scenario predicts for LAO/STO heterostructures structural and electronic modifi cations of the fi rst SrTiO 3 unit cells occurring simultaneously with the appearance of a 2D-metal. However, not all the experimental results are fully consistent with this picture. For example, second harmonic generation experiments suggested that polar distortions in STO appear below the LAO threshold thickness of 4-uc, i.e., before the realization of 2D electron system. [ 15 ] More recently, a rumpling of the STO interface layers was observed also in p-type LAO/STO insulating interfaces, as shown by grazing incidence X-ray diffraction (GIXD). [ 16 ] To clarify these issues, we have used a combination of advanced X-ray synchrotron-based spectroscopic and structural measurements to study the evolution of the electronic properties and ionic positions at LAO/STO interfaces as a function of the LAO thickness ( Figure 1 ). We fi nd that an orbital reconstruction occurs also in insulating 2-uc LAO/STO, before the appearance of the metallic state. Partial or complete inversion of the 3d-t 2g energy levels in insulating samples is correlated to partial or complete coverage of the STO surface by an ordered polar layer....

Reduction of strongly oxidized carbon black by hydrazine hydrate yields water-insoluble graphene-like sheets that undergo to self-assembling in thin film on surfaces after drying. The height of a drop-casted graphene-like film was determined by atomic force microscopy (AFM) to be around 20 nm, corresponding to approximately 25 graphene-like layers. The oxidized carbon black and the corresponding reduced form were carefully characterized.