B. Gobaut

Reversible Photoswitching of a Spin-Crossover Molecular Complex in the Solid State at Room Temperature

Spin-crossover metal complexes are highly promising magnetic molecular switches for prospective molecule-based devices. The spin-crossover molecular photoswitches developed so far operate either at very low temperatures or in the liquid phase, which hinders practical applications. Herein, we present a molecular spin-crossover iron(II) complex that can be switched between paramagnetic high-spin and diamagnetic low-spin states with light at room temperature in the solid state. The reversible photoswitching is induced by alternating irradiation with ultraviolet and visible light and proceeds at the molecular level.

Direct growth of InAsP/InP quantum well heterostructures on Si using crystalline SrTiO3/Si templates

Integrating III-V semiconductors on Si is one of the major challenges of epitaxial growth and presents important applicative interest. We describe here an approach based on the use of crystalline SrTiO3 (STO)/Si templates. The structural and optical properties of InAsP/InP quantum well heterostructures grown directly on Si and on STO/Si templates are compared. Using STO/Si templates strongly improves the structural properties of the III-V heterostructure, and allows observing room-temperature photoluminescence from the quantum well.

New strategy for magnetic gas sensing

A new-concept approach to room temperature magnetic gas sensing has been developed, based on newly designed Co/ZnO hybrid nanostructures. The sensor prototype has been demonstrated to be sensitive, reversible, fast and scalable. In this work, the role of the Co/ZnO surface and interface in the gas-sensing mechanism has been clarified. In order to attain a comprehensive understanding of the physics governing the remarkable properties of the Co/ZnO stack, an extensive electronic and structural investigation has been carried out. The reaction at room temperature with target gases involves the surface and the lateral faces of the ZnO nanorods, with the formation of structural defects and vacancies. In particular, it has been discovered that the stress enhancement as well as the change in the polarizability of the ZnO nanorods are transduced by Co in a change of its magnetization. The interplay between these phenomena may provide versatile approaches to tune the intrinsic electronic, magnetic and optical properties of the hybrid nanostructure.

Quantifying the critical thickness of electron hybridization in spintronics materials

In the rapidly growing field of spintronics, simultaneous control of electronic and magnetic properties is essential, and the perspective of building novel phases is directly linked to the control of tuning parameters, for example, thickness and doping. Looking at the relevant effects in interface-driven spintronics, the reduced symmetry at a surface and interface corresponds to a severe modification of the overlap of electron orbitals, that is, to a change of electron hybridization. Here we report a chemically and magnetically sensitive depth-dependent analysis of two paradigmatic systems, namely La1−xSrxMnO3 and (Ga,Mn)As. Supported by cluster calculations, we find a crossover between surface and bulk in the electron hybridization/correlation and we identify a spectroscopic fingerprint of bulk metallic character and ferromagnetism versus depth. The critical thickness and the gradient of hybridization are measured, setting an intrinsic limit of 3 and 10 unit cells from the surface, respectively, for (Ga,Mn)As and La1−xSrxMnO3, for fully restoring bulk properties.

Structural and electronic properties of Bi2Se3 topological insulator thin films grown by pulsed laser deposition

We report on epitaxial growth of Bi2Se3 topological insulator thin films by Pulsed Laser Deposition (PLD). X-ray diffraction investigation confirms that Bi2Se3 with a single (001)-orientation can be obtained on several substrates in a narrow (i.e., 20 °C) range of deposition temperatures and at high deposition pressure (i.e., 0.1 mbar). However, only films grown on (001)-Al2O3 substrates show an almost-unique in-plane orientation. In-situ spin-resolved angular resolved photoemission spectroscopy experiments, performed at the NFFA-APE facility of IOM-CNR and Elettra (Trieste), show a single Dirac cone with the Dirac point at E B ∼ 0.38 eV located in the center of the Brillouin zone and the spin polarization of the topological surface states. These results demonstrate that the topological surface state can be obtained in PLD-grown Bi2Se3 thin films.

Unraveling the magnetic properties of BiFe0.5Cr0.5O3 thin films

We investigate the structural, chemical, and magnetic properties on BiFe0.5Cr0.5O3 (BFCO) thin films grown on (001) (110) and (111) oriented SrTiO3 (STO) substrates by x-ray magnetic circular dichroism and x-ray diffraction. We show how highly pure BFCO films, differently from the theoretically expected ferrimagnetic behavior, present a very weak dichroic signal at Cr and Fe edges, with both moments aligned with the external field. Chemically sensitive hysteresis loops show no hysteretic behavior and no saturation up to 6.8 T. The linear responses are induced by the tilting of the Cr and Fe moments along the applied magnetic field.

Interface accommodation mechanism for weakly interacting epitaxial systems

We report here an interface accommodation mechanism observed by using in situ grazing incidence X-ray diffraction in the very early stages of Ge epitaxial growth on SrTiO3. This mechanism leads to interface-localized misfit accommodation and involves two regimes: very early dislocation emergence followed by a damped collective oscillatory lattice parameter evolution. We show that this behavior is compatible with the simplest nonlinear Frenkel-Kontorova model assuming the weak elastic-chain/substrate interaction.

X-ray absorption spectroscopy study of annealing process on Sr1–xLaxCuO2 electron-doped cuprate thin films

The superconducting properties of Sr1–xLaxCuO2 thin films are strongly affected by sample preparation procedures, including the annealing step, which are not always well controlled. We have studied the evolution of Cu L2,3 and O K edge x-ray absorption spectra (XAS) of Sr1–xLaxCuO2 thin films as a function of reducing annealing, both qualitatively and quantitatively. By using linearly polarized radiation, we are able to identify the signatures of the presence of apical oxygen in the as-grown sample and its gradual removal as a function of duration of 350 °C Ar annealing performed on the same sample. Even though the as-grown sample appears to be hole doped, we cannot identify the signature of the Zhang-Rice singlet in the O K XAS, and it is extremely unlikely that the interstitial excess oxygen can give rise to a superconducting or even a metallic ground state. XAS and x-ray linear dichroism analyses are, therefore, shown to be valuable tools to improving the control over the annealing process of electron ...

Linking Electronic Transport through a Spin Crossover Thin Film to the Molecular Spin State Using X-ray Absorption Spectroscopy Operando Techniques

One promising route toward encoding information is to utilize the two stable electronic states of a spin crossover molecule. Although this property is clearly manifested in transport across single molecule junctions, evidence linking charge transport across a solid-state device to the molecular films spin state has thus far remained indirect. To establish this link, we deploy materials-centric and device-centric operando experiments involving X-ray absorption spectroscopy. We find a correlation between the temperature dependencies of the junction resistance and the Fe spin state within the devices [Fe(H2B(pz)2)2(NH2-phen)] molecular film. We also factually observe that the Fe molecular site mediates charge transport. Our dual operando studies reveal that transport involves a subset of molecules within an electronically heterogeneous spin crossover film. Our work confers an insight that substantially improves the state-of-the-art regarding spin crossover-based devices, thanks to a methodology that can benefit device studies of other next-generation molecular compounds.

Role of Oxygen Deposition Pressure in the Formation of Ti Defect States in TiO2(001) Anatase Thin Films

We report the study of anatase TiO2(001)-oriented thin films grown by pulsed laser deposition on LaAlO3(001). A combination of in situ and ex situ methods has been used to address both the origin of the Ti3+-localized states and their relationship with the structural and electronic properties on the surface and the subsurface. Localized in-gap states are analyzed using resonant X-ray photoelectron spectroscopy and are related to the Ti3+ electronic configuration, homogeneously distributed over the entire film thickness. We find that an increase in the oxygen pressure corresponds to an increase in Ti3+ only in a well-defined range of deposition pressure; outside this range, Ti3+ and the strength of the in-gap states are reduced.