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Nat Mater ; 18(11): 1194-1200, 2019 11.
Artículo en Inglés | MEDLINE | ID: mdl-31527810


A superconducting spin valve consists of a thin-film superconductor between two ferromagnetic layers. A change of magnetization alignment shifts the superconducting transition temperature (ΔΤc) due to an interplay between the magnetic exchange energy and the superconducting condensate. The magnitude of ΔΤc scales inversely with the superconductor thickness (dS) and is zero when dS exceeds the superconducting coherence length (ξ). Here, we report a superconducting spin-valve effect involving a different underlying mechanism in which magnetization alignment and ΔΤc are determined by nodal quasiparticle excitation states on the Fermi surface of the d-wave superconductor YBa2Cu3O7-δ sandwiched between insulating layers of ferromagnetic Pr0.8Ca0.2MnO3. We observe ΔΤc values that approach 2 K with the sign of ΔΤc oscillating with dS over a length scale exceeding 100ξ and, for particular values of dS, the superconducting state reinforces an antiparallel magnetization alignment. These results pave the way to all-oxide superconducting memory in which superconductivity modulates the magnetic state.

Sci Rep ; 7(1): 12061, 2017 09 21.
Artículo en Inglés | MEDLINE | ID: mdl-28935891


We present a comprehensive study of the crystal structure of the thin-film, ferromagnetic topological insulator (Bi, Sb)2-x V x Te3. The dissipationless quantum anomalous Hall edge states it manifests are of particular interest for spintronics, as a natural spin filter or pure spin source, and as qubits for topological quantum computing. For ranges typically used in experiments, we investigate the effect of doping, substrate choice and film thickness on the (Bi, Sb)2Te3 unit cell using high-resolution X-ray diffractometry. Scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements provide local structural and interfacial information. We find that the unit cell is unaffected in-plane by vanadium doping changes, and remains unchanged over a thickness range of 4-10 quintuple layers (1 QL ≈ 1 nm). The in-plane lattice parameter (a) also remains the same in films grown on different substrate materials. However, out-of-plane the c-axis increases with the doping level and thicknesses >10 QL, and is potentially reduced in films grown on Si (1 1 1).

Nanoscale ; 9(14): 4700-4706, 2017 Apr 06.
Artículo en Inglés | MEDLINE | ID: mdl-28345699


Operational stability is the main issue hindering the commercialisation of perovskite solar cells. Here, a long term light soaking test was performed on large area hybrid halide perovskite solar cells to investigate the morphological and chemical changes associated with the degradation of photovoltaic performance occurring within the devices. Using Scanning Transmission Electron Microscopy (STEM) in conjunction with EDX analysis on device cross sections, we observe the formation of gold clusters in the perovskite active layer as well as in the TiO2 mesoporous layer, and a severe degradation of the perovskite due to iodine migration into the hole transporter. All these phenomena are associated with a drastic drop of all the photovoltaic parameters. The use of advanced electron microscopy techniques and data processing provides new insights on the degradation pathways, directly correlating the nanoscale structure and chemistry to the macroscopic properties of hybrid perovskite devices.

Nat Commun ; 6: 8053, 2015 Sep 02.
Artículo en Inglés | MEDLINE | ID: mdl-26329811


The theory of superconductivity developed by Bardeen, Cooper and Schrieffer (BCS) explains the stabilization of electron pairs into a spin-singlet, even frequency, state by the formation of an energy gap within which the density of states is zero. At a superconductor interface with an inhomogeneous ferromagnet, a gapless odd frequency superconducting state is predicted, in which the Cooper pairs are in a spin-triplet state. Although indirect evidence for such a state has been obtained, the gap structure and pairing symmetry have not so far been determined. Here we report scanning tunnelling spectroscopy of Nb superconducting films proximity coupled to epitaxial Ho. These measurements reveal pronounced changes to the Nb subgap superconducting density of states on driving the Ho through a metamagnetic transition from a helical antiferromagnetic to a homogeneous ferromagnetic state for which a BCS-like gap is recovered. The results prove odd frequency spin-triplet superconductivity at superconductor/inhomogeneous magnet interfaces.

Nanotechnology ; 23(47): 475101, 2012 Nov 30.
Artículo en Inglés | MEDLINE | ID: mdl-23111156


Nanostructured cubic zirconia is a strategic material for biomedical applications since it combines superior structural and optical properties with a nanoscale morphology able to control cell adhesion and proliferation. We produced nanostructured cubic zirconia thin films at room temperature by supersonic cluster beam deposition of nanoparticles produced in the gas phase. Precise control of film roughness at the nanoscale is obtained by operating in a ballistic deposition regime. This allows one to study the influence of nanoroughness on cell adhesion, while keeping the surface chemistry constant. We evaluated cell adhesion on nanostructured zirconia with an osteoblast-like cell line using confocal laser scanning microscopy for detailed morphological and cytoskeleton studies. We demonstrated that the organization of cytoskeleton and focal adhesion formation can be controlled by varying the evolution of surface nanoroughness.

Materiales Biocompatibles/química , Adhesión Celular , Nanoestructuras/química , Nanoestructuras/ultraestructura , Osteoblastos/citología , Circonio/química , Línea Celular Tumoral , Proliferación Celular , Supervivencia Celular , Humanos , Propiedades de Superficie
Nano Lett ; 10(7): 2562-7, 2010 Jul 14.
Artículo en Inglés | MEDLINE | ID: mdl-20565087


Hierarchical or one-dimensional architectures are among the most exciting developments in material science these recent years. We present a nanostructured TiO(2) assembly combining these two concepts and resembling a forest composed of individual, high aspect-ratio, treelike nanostructures. We propose to use these structures for the photoanode in dye-sensitized solar cells, and we achieved 4.9% conversion efficiency in combination with C101 dye. We demonstrate this morphology beneficial to hamper the electron recombination and also mass transport control in the mesopores when solvent-free ionic liquid electrolyte is used.