R. Miranda

Atomic aspects in the epitaxial growth of metallic superlattices and nanostructures

The properties of materials (mechanical, electronic, magnetic, etc) derive ultimately from the identity and spatial arrangement of their constituents. Nowadays, with the dimensions of technological devices and nanostructures reaching a few atomic constants, descriptions in terms of macroscopic concepts appear to be frequently inadequate and must give way to atomistic formulations based on elementary processes. Focusing on metallic materials, and more specifically on low-dimensional systems such as ultrathin films, superlattices or nanostructures, this paper reviews the atomic scale phenomena responsible for the most common types of defects (interfacial alloying, etching and roughness, formation of dislocations and pinholes, film discontinuities and twinning). It is shown that many of these features are related to the different mechanisms of strain relaxation in heteroepitaxial systems as well as to specific characteristics of atomic diffusion, such as the presence of Ehrlich–Schwoebel barriers hindering step crossings. Some special growth techniques (use of surfactants and codeposition) are also presented together with experimental examples demonstrating their usefulness to overcome the elements natural limitations and produce accurately controlled, custom-designed epitaxial samples. Finally, a brief overview is given of different phenomena that can be exploited to produce self-assembled or self-organized structures.

Enantiospecific Spin Polarization of Electrons Photoemitted Through Layers of Homochiral Organic Molecules

Electrons photoemitted through layers of purely organic chiral molecules become strongly spin-polarized even at room temperature and for double-monolayer thicknesses. The substitution of one enantiomer for its mirror image does not revert the sign of the spin polarization, rather its direction in space. These findings might lead to the obtention of highly efficient spin filters for spintronic applications.

Nonstochastic Behavior of Atomic Surface Diffusion on Cu(111) down to Low Temperatures

Atomic diffusion is usually understood as a succession of random, independent displacements of an adatom over the surfaces potential energy landscape. Nevertheless, an analysis of molecular dynamics simulations of self-diffusion on Cu(111) demonstrates the existence of different types of correlations in the atomic jumps at all temperatures. Thus, the atomic displacements cannot be correctly described in terms of a random walk model. This fact has a profound impact on the determination and interpretation of diffusion coefficients and activation barriers.

Large-Area Heterostructures from Graphene and Encapsulated Colloidal Quantum Dots via the Langmuir–Blodgett Method

This work explores the assembly of large-area heterostructures comprised of a film of silica-encapsulated, semiconducting colloidal quantum dots, deposited via the Langmuir-Blodgett method, sandwiched between two graphene sheets. The luminescent, electrically insulating film served as a dielectric, with the top graphene sheet patterned into an electrode and successfully used as a top gate for an underlying graphene field-effect transistor. This heterostructure paves the way for developing novel hybrid optoelectronic devices through the integration of 2D and 0D materials.

Chiral asymmetry driven by unidirectional magnetic anisotropy in spin-orbitronic systems

Spin-Orbit (SO) effects of a ferromagnetic (FM) layer can be artificially modified by interfacial exchange coupling with an antiferromagnet (AFM).

Chiral asymmetry driven by unidirectional magnetic anisotropy in Spin-Orbitronic systems

Spin-Orbit (SO) effects of a ferromagnetic (FM) layer can be artificially modified by interfacial exchange coupling with an anti-ferro magnet (AFM). Non-symmetric magnetization reversals as well as asymmetric transport behaviors are distinctive signatures of the symmetry-breaking induced by such interfacial coupling. We present a complete picture of the symmetry of the SO effects by studying the magneto-transport properties of single FM film and FM/AFM systems (exchanged-biased bilayer and spin-valve structures) with specific in-plane magnetic anisotropy. Single FM films with a well-defined (two-fold) uniaxial magnetic anisotropy display symmetric magnetization reversals and magneto-resistance responses for any value and direction of the applied magnetic field. On the contrary, in the exchange-biased structures, the exchange interaction at the interface between the FM and AFM layers is responsible of chiral asymmetries in magnetization reversal pathways as well as in the magneto-resistance behaviors. Such asymmetries are directly related to the additional unidirectional (one-fold) magnetic anisotropy imposed by the AFM. In particular, chiral reversals and MR responses are found around the magnetization hard-axis direction. This has been shown in FM/AFM bilayer and spin-valve (where the MR outputs are related to different transport phenomena, i.e. anisotropic magneto-resistance and giant magneto-resistance respectively), hence indicating that the chiral asymmetries are intrinsic of systems with unidirectional anisotropy.