An ab initio study of the magnetic properties of strontium hexaferrite.

The magnetic properties of [Formula: see text], a paradigmatic hexaferrite for permanent magnet applications, have been addressed in detail combining density functional theory including spin-orbit coupling and a Hubbard U term with Monte Carlo simulations. This multiscale approach allows to estimate the Néel temperature of the material from ab initio exchange constants, and to determine the influence of different computational conditions on the magnetic properties by direct comparison versus available experimental data. It is found that the dominant influence arises from the choice of the Hubbard U term, with a value in the 2-3 eV range as the most adequate to quantitatively reproduce the two most relevant magnetic properties of this material, namely: its large perpendicular magnetocrystalline anisotropy and its elevated Néel temperature.

Siesta: Recent developments and applications.

A review of the present status, recent enhancements, and applicability of the Siesta program is presented. Since its debut in the mid-1990s, Siesta's flexibility, efficiency, and free distribution have given advanced materials simulation capabilities to many groups worldwide. The core methodological scheme of Siesta combines finite-support pseudo-atomic orbitals as basis sets, norm-conserving pseudopotentials, and a real-space grid for the representation of charge density and potentials and the computation of their associated matrix elements. Here, we describe the more recent implementations on top of that core scheme, which include full spin-orbit interaction, non-repeated and multiple-contact ballistic electron transport, density functional theory (DFT)+U and hybrid functionals, time-dependent DFT, novel reduced-scaling solvers, density-functional perturbation theory, efficient van der Waals non-local density functionals, and enhanced molecular-dynamics options. In addition, a substantial effort has been made in enhancing interoperability and interfacing with other codes and utilities, such as wannier90 and the second-principles modeling it can be used for, an AiiDA plugin for workflow automatization, interface to Lua for steering Siesta runs, and various post-processing utilities. Siesta has also been engaged in the Electronic Structure Library effort from its inception, which has allowed the sharing of various low-level libraries, as well as data standards and support for them, particularly the PSeudopotential Markup Language definition and library for transferable pseudopotentials, and the interface to the ELectronic Structure Infrastructure library of solvers. Code sharing is made easier by the new open-source licensing model of the program. This review also presents examples of application of the capabilities of the code, as well as a view of on-going and future developments.

A first principles study of thiol-capped Au nanoparticles: structural, electronic, and magnetic properties as a function of thiol coverage.

We have studied the stability of thiolated Au38 nanoparticles (NPs) via density functional theory based calculations varying the coverage from 0 up to 32 molecules. Three different initial core arrangements were considered for the cluster, spherical, tubular, and bi-icosahedral, while thiol groups were attached to the cluster via the sulfur atom either as single molecules or forming more complex staple motifs. After molecular dynamics runs several metastable configurations are found at each coverage thus allowing to analyze the properties of the NPs in the form of ensemble averages. In particular, we address the structural and electronic properties as a function of the number of thiols. The study emphasizes the strong influence of the core structure on the stability of the NPs, and its interplay with the thiol coverage and adsorption geometries. The magnetic properties of the NPs have also been explored via spin-polarized calculations including spin-orbit coupling. No evidence for the existence of a robust intrinsic ferromagnetism is found in any of the structures.

Fully relativistic pseudopotential formalism under an atomic orbital basis: spin-orbit splittings and magnetic anisotropies.

We present an efficient implementation of the spin-orbit coupling within the density functional theory based SIESTA code (2002 J. Phys.: Condens. Matter 14 2745) using the fully relativistic and totally separable pseudopotential formalism of Hemstreet et al (1993 Phys. Rev. B 47 4238). First, we obtain the spin-orbit splittings for several systems ranging from isolated atoms to bulk metals and semiconductors as well as the Au(111) surface state. Next, and after extensive tests on the accuracy of the formalism, we also demonstrate its capability to yield reliable values for the magnetic anisotropy energy in magnetic systems. In particular, we focus on the L1(0) binary alloys and on two large molecules: Mn(6)O(2)(H -sao)(6)(O(2)CH)(2)(CH(3)OH)(4) and Co(4)(hmp)(4)(CH(3)OH)(4)Cl(4). In all cases our calculated anisotropies are in good agreement with those obtained with full-potential methods, despite the latter being, in general, computationally more demanding.

The structure of mixed H2O-OH monolayer films on Ru(0001).

Scanning tunneling microscopy (STM) and x-ray absorption spectroscopy (XAS) have been used to study the structures produced by water on Ru(0001) at temperatures above 140 K. It was found that while undissociated water layers are metastable below 140 K, heating above this temperature produces drastic transformations, whereby a fraction of the water molecules partially dissociate and form mixed H(2)O-OH structures. X-ray photoelectron spectroscopy and XAS revealed the presence of hydroxyl groups with their O-H bond essentially parallel to the surface. STM images show that the mixed H(2)O-OH structures consist of long narrow stripes aligned with the three crystallographic directions perpendicular to the close-packed atomic rows of the Ru(0001) substrate. The internal structure of the stripes is a honeycomb network of H-bonded water and hydroxyl species. We found that the metastable low temperature molecular phase can also be converted to a mixed H(2)O-OH phase through excitation by the tunneling electrons when their energy is 0.5 eV or higher above the Fermi level. Structural models based on the STM images were used for density functional theory optimizations of the stripe geometry. The optimized geometry was then utilized to calculate STM images for comparison with the experiment.

[Prognostic factors for disease progression in patients with renal cell carcinoma]. / Análisis de los factores pronósticos de progresión tumoral en el adenocarcinoma renal.

INTRODUCTION: More than 40% of patients with renal cell carcinoma present with disease progression after surgery. The objective of the current study was to identify a clinically useful set of prognostic factors that would correlate significantly with the capacity of progression. MATERIAL AND METHODS: The authors studied 252 patients with renal cell carcinoma who underwent radical nephrectomy. Followup ranged from 12-246 months (median 36 months). Several morphologic parameters of the tumors were considered. DNA content was analyzed by flow cytometry and tumor size was determined from the surgical specimen. A Cox proportional hazards regression model was used to identify significant independent prognostic factors for disease progression. RESULTS: A total of 224 out of 252 were available for suitable histograms. Of the 224 patients, 95 (42.4%) were aneuploid tumors, 106 (47.2%) were organ-confined renal cell carcinoma and 87 (39.74%) presented disease progression. At 5 and 10 years of followup, disease free survival was found to be 66.31% and 62.23%, respectively. Univariate analysis revealed that DNA ploidy, Furhman grade and stage (TNM) had a statistically significant predictive value for disease progression. Survival univariate analysis found a worse probability of survival for aneuploid tumors, grade III-IV tumors, non organ-confined tumors and conventional and undiferentiated tumors. Using multivariate survival analyses, Furhman grade, stage (TNM) and DNA ploidy were the only independent prognostic factors. So, the probability of death for aneuploid tumor was 1.7 times higher than for diploid tumors. CONCLUSIONS: Stage, DNA content and Furhman grade were the only significant independent predictors of disease progression. Tumoral size and histological type did not provide more additional information.