arXiv subjects

J. E. Prieto

Publications and source records attributed to J. E. Prieto.

29 records · Page 2Linked to original sources

arXiv2007

Forbidden island heights in stress-driven coherent Stranski-Krastanov growth

The observed height distribution of clusters obtained in strained epitaxy has been often interpreted in terms of electronic effects. We show that some aspects can be explained classically by the interplay of strain and edge energies. We find that soft materials can transform directly from monolayer into thicker islands by two-dimensional (2D) multilayer nucleation and growth. There is a critical thickness decreasing with the force constant. Thinner islands are thermodynamically forbidden, due to the insufficient stress relaxation upon clustering particularly under tensile stress. At sufficiently large misfits the barrier for 2D multilayer nucleation is significantly smaller than the barrier for subsequent single-layer nucleation. The effects are found to be quantitatively reasonable and offer a plausible explanation for the absence of thin islands and 2D growth of flattop islands usually attributed to quantum size effects.

cond-mat.mtrl-sci↗

arXiv2006

Formation and self-assembly of coherent quantum dots: some thermodynamic aspects

Quantum dots have promising properties for optoelectronic applications. They can be grown free of dislocations in highly mismatched epitaxy in the coherent Stranski-Krastanov mode. In this chapter, some thermodynamic aspects related to the wetting in the growth and self-assembly of three-dimensional (3D) coherent islands are studied using an energy minimization scheme in a 1+1-dimensional atomic model with anharmonic interactions. The conditions for equilibrium between the different phases are discussed. It is found that the thermodynamic driving force for 3D-cluster formation is the reduced adhesion of the islands to the wetting layer at their edges. In agreement with experimental observations, for values of the lattice mismatch larger than a critical misfit, a critical island size for the 2D-3D transition is found. Beyond it, monolayer islands become unstable against bilayer ones. Compressed coherent overlayers show a greater tendency to clustering than expanded ones. The transition to 3D islands takes place through a series of intermediate stable states with thicknesses discretely increasing in monolayer steps. Special emphasis is made on the analysis of the critical misfit. Additionally, the effect of neighbouring islands mediated through a deformable wetting layer is considered. The degree of wetting of the substrate by a given island depends on the size and shape distributions of the neighbouring islands. Implications for the self-assembled growth of quantum dots are discussed.

cond-mat.mtrl-sci↗

arXiv2005

Quantum-dot nucleation in strained-layer epitaxy: minimum-energy pathway in the stress-driven 2D-3D transformation

The transformation of monolayer islands into bilayer islands as a first step of the overall two-dimensional to three-dimensional (2D-3D) transformation in the coherent Stranski-Krastanov mode of growth is studied for the cases of expanded and compressed overlayers. Compressed overlayers display a nucleation-like behavior: the energy accompanying the transformation process displays a maximum at some critical number of atoms, which is small for large enough values of the misfit, and then decreases gradually down to the completion of the transformation, non-monotonically due to the atomistics of the process. On the contrary, the energy change in expanded overlayers increases up to close to the completion of the transformation and then abruptly collapses with the disappearance of the monoatomic steps to produce low-energy facets. This kind of transformation takes place only in materials with strong interatomic bonding. Softer materials under tensile stress are expected to grow predominantly with a planar morphology until misfit dislocations are introduced, or to transform into 3D islands by a different mechanism. It is concluded that the coherent Stranski-Krastanov growth in expanded overlayers is much less probable than in compressed ones for kinetic reasons.

cond-mat.mtrl-sci↗

arXiv2005

Magnetism of ordered Sm/Co(0001) surface structures

The epitaxial system Sm/Co(0001) was studied for Sm coverages up to 1 monolayer (ML) on top of ultrathin Co/W(110) epitaxial films. Two ordered phases were found for 1/3 and 1 ML Sm, respectively. The valence state of Sm was determined by means of photoemission and magnetic properties were measured by magneto-optical Kerr effect. We find that 1 ML Sm causes a strong increase of the coercivity with respect to that of the underlying 10 ML Co film. Element-specific hysteresis loops, measured by using resonant soft x-ray reflectivity, show the same magnetic behaviour for the two elements.

cond-mat.mtrl-sci↗

arXiv2005

X-ray magneto-optics of lanthanide materials: principles and applications

Lanthanide metals are a particular class of magnetic materials in which the magnetic moments are carried mainly by the localized electrons of the 4f shell. They are frequently found in technically relevant systems, to achieve, e.g., high magnetic anisotropy. Magneto-optical methods in the x-ray range are well suited to study complex magnetic materials in an element-specific way. In this work, we report on recent progress on the quantitative determination of magneto-optical constants of several lanthanides in the soft x-ray region and we show some examples of applications of magneto-optics to hard-magnetic interfaces and exchange-coupled layered structures containing lanthanide elements.

cond-mat.mtrl-sci↗

arXiv2004

Rashba Effect at Magnetic Metal Surfaces

We give experimental and theoretical evidence of the Rashba effect at the magnetic rare-earth metal surface Gd(0001). The Rashba effect is substantially enhanced and the Rashba parameter changes its sign when a metal-oxide surface layer is formed. The experimental observations are quantitatively described by ab initio calculations that give a detailed account of the near-surface charge density gradients causing the Rashba effect. Since the sign of the Rashba splitting depends on the magnetization direction, the findings open up new opportunities for the study of surface and interface magnetism.

cond-mat.mtrl-sci↗

arXiv2004

Self-assembly of quantum dots: effect of neighbor islands on the wetting in coherent Stranski-Krastanov growth

The wetting of the homogeneously strained wetting layer by dislocation-free three-dimensional islands belonging to an array has been studied. The array has been simulated as a chain of islands in 1+1 dimensions. It is found that the wetting depends on the density of the array, the size distribution and the shape of the neighbor islands. Implications for the self-assembly of quantum dots grown in the coherent Stranski-Krastanov mode are discussed.

cond-mat.mtrl-sci↗

arXiv2003

Magneto-optics of Gd and Tb in the soft x-ray resonance regions

We present x-ray absorption spectra around the 3d-4f and 4d-4f excitation thresholds of in-plane magnetized Gd and Tb films measured by total electron yield using circularly polarized synchrotron radiation. By matching the experimental spectra to tabulated absorption data far below and above the thresholds, the imaginary parts of the complex refractive index are determined quantitatively. The associated real parts for circularly polarized light propagating nearly parallel or antiparallel to the magnetization direction are obtained through the Kramers-Kronig relations. The derived magnetooptical parameters are used to calculate soft x-ray reflectivity spectra of a magnetized Gd film at the 3d-4f threshold, which are found to compare well with our experimental spectra.

cond-mat.mtrl-sci↗

arXiv2003

Some thermodynamic aspects of self-assembly of arrays of quantum dots

We have studied the relative adhesion (the wetting) of dislocation-free three-dimensional islands belonging to an array of islands to the wetting layer in Stranski-Krastanov growth mode. The array has been simulated as a chain of islands in 1+1 dimensions placed on top of a wetting layer. In addition to the critical size of the two-dimensional islands for the 2D-3D transformation to occur, we find that the wetting depends strongly on the density of the array, the size distribution and the shape of the islands.

cond-mat.mtrl-sci↗

arXiv2002

Prediction of huge X-ray Faraday rotation at the Gd N_4,5 threshold

X-ray absorption spectra in a wide energy range around the 4d-4f excitation threshold of Gd were recorded by total electron yield from in-plane magnetized Gd metal films. Matching the experimental spectra to tabulated absorption data reveals unprecedented short light absorption lengths down to 3 nm. The associated real parts of the refractive index for circularly polarized light propagating parallel or antiparallel to the Gd magnetization, determined through the Kramers-Kronig transformation, correspond to a magneto-optical Faraday rotation of 0.7 degrees per atomic layer. This finding shall allow the study of magnetic structure and magnetization dynamics of lanthanide elements in nanosize systems and dilute alloys.

cond-mat.mtrl-sci↗

arXiv2002

Thermodynamic driving force of formation of coherent three-dimensional islands in Stranski-Krastanov growth

The formation of coherent three-dimensional islands in highly mismatched epitaxy is discussed in terms of the traditional concept of wetting. It is shown that the wetting layer and the 3D islands represent different phases which cannot be in equilibrium with each other, and the transfer of matter from the stable wetting layer to the 3D islands is thermodynamically unfavored. The experimentally observed critical misfit for coherent 3D islanding to occur and the coexistence of pyramids with discrete heights of two, three, four... monolayers can be explained assuming that the thermodynamic driving force of formation of coherent 3D islands on the surface of the wetting layer of the same material is the reduced average adhesion of the islands to that layer and that the islands height is a discrete variable.

cond-mat.mtrl-sci↗