A System Dynamics model to predict the impact of COVID-19 in Spain

Sanz, MT.; Caselles, A.; Micó, JC.; Soler, C. (2020). A System Dynamics model to predict the impact of COVID-19 in Spain. 146-151. http://hdl.handle.net/10251/178213S14615

Magnetic interaction of Co ions near the {10\bar{1}0} ZnO surface

Co-doped ZnO is the prototypical dilute magnetic oxide showing many of the characteristics of ferromagnetism. The microscopic origin of the long range order however remains elusive, since the conventional mechanisms for the magnetic interaction, such as super-exchange and double exchange, fail either at the fundamental or at a quantitative level. Intriguingly, there is a growing evidence that defects both in point-like or extended form play a fundamental role in driving the magnetic order. Here we explore one of such possibilities by performing {\it ab initio} density functional theory calculations for the magnetic interaction of Co ions at or near a ZnO \{10$\bar{1}$0\} surface. We find that extended surface states can hybridize with the $e$-levels of Co and efficiently mediate the magnetic order, although such a mechanism is effective only for ions placed in the first few atomic planes near the surface. We also find that the magnetic anisotropy changes at the surface from an hard-axis easy-plane to an easy axis, with an associated increase of its magnitude. We then conclude that clusters with high densities of surfacial Co ions may display blocking temperatures much higher than in the bulk

Systematically improvable optimized atomic basis sets for {\it ab inito} calculations

We propose a unique scheme to construct fully optimized atomic basis sets for density-functional calculations. The shapes of the radial functions are optimized by minimizing the {\it spillage} of the wave functions between the atomic orbital calculations and the converged plane wave calculations for dimer systems. The quality of the bases can be systematically improved by increasing the size of the bases within the same framework. The scheme is easy to implement and very flexible. We have done extensive tests of this scheme for wide variety of systems. The results show that the obtained atomic basis sets are very satisfactory for both accuracy and transferability

Large-scale electronic structure theory for simulating nanostructure process

Fundamental theories and practical methods for large-scale electronic structure calculations are given, in which the computational cost is proportional to the system size. Accuracy controlling methods for microscopic freedoms are focused on two practical solver methods, Krylov-subspace method and generalized-Wannier-state method. A general theory called the 'multi-solver' scheme is also formulated, as a hybrid between different solver methods. Practical examples are carried out in several insulating and metallic systems with 10^3-10^5 atoms. All the theories provide general guiding principles of constructing an optimal calculation for simulating nanostructure processes, since a nanostructured system consists of several competitive regions, such as bulk and surface regions, and the simulation is designed to reproduce the competition with an optimal computational cost.Comment: 19 pages, 6 figures. To appear in J. Phys. Cond. Matt. A preprint PDF file in better graphics is available at http://fujimac.t.u-tokyo.ac.jp/lses/index_e.htm

Dynamics and thermalization of the nuclear spin bath in the single-molecule magnet Mn12-ac: test for the theory of spin tunneling

The description of the tunneling of a macroscopic variable in the presence of a bath of localized spins is a subject of great fundamental and practical interest, and is relevant for many solid-state qubit designs. Instead of focusing on the the "central spin" (as is most often done), here we present a detailed study of the dynamics of the nuclear spin bath in the Mn12-ac single-molecule magnet, probed by NMR experiments down to very low temperatures (T = 20 mK). We find that the longitudinal relaxation rate of the 55Mn nuclei in Mn12-ac becomes roughly T-independent below T = 0.8 K, and can be strongly suppressed with a longitudinal magnetic field. This is consistent with the nuclear relaxation being caused by quantum tunneling of the molecular spin, and we attribute the tunneling fluctuations to the minority of fast-relaxing molecules present in the sample. The transverse nuclear relaxation is also T-independent for T < 0.8 K, and can be explained qualitatively and quantitatively by the dipolar coupling between like nuclei in neighboring molecules. We also show that the isotopic substitution of 1H by 2H leads to a slower nuclear longitudinal relaxation, consistent with the decreased tunneling probability of the molecular spin. Finally, we demonstrate that, even at the lowest temperatures, the nuclear spins remain in thermal equilibrium with the lattice phonons, and we investigate the timescale for their thermal equilibration. After a review of the theory of macroscopic spin tunneling in the presence of a spin bath, we argue that most of our experimental results are consistent with that theory, but the thermalization of the nuclear spins is not.Comment: 24 pages, 18 figures. Experimental study of the spin bath dynamics in quantum nanomagnets, plus an extensive review and application of the theor

First-Principles Study of Substitutional Metal Impurities in Graphene: Structural, Electronic and Magnetic Properties

We present a theoretical study using density functional calculations of the structural, electronic and magnetic properties of 3d transition metal, noble metal and Zn atoms interacting with carbon monovacancies in graphene. We pay special attention to the electronic and magnetic properties of these substitutional impurities and found that they can be fully understood using a simple model based on the hybridization between the states of the metal atom, particularly the d shell, and the defect levels associated with an unreconstructed D3h carbon vacancy. We identify three different regimes associated with the occupation of different carbon-metal hybridized electronic levels: (i) bonding states are completely filled for Sc and Ti, and these impurities are non-magnetic; (ii) the non-bonding d shell is partially occupied for V, Cr and Mn and, correspondingly, these impurties present large and localized spin moments; (iii) antibonding states with increasing carbon character are progressively filled for Co, Ni, the noble metals and Zn. The spin moments of these impurities oscillate between 0 and 1 Bohr magnetons and are increasingly delocalized. The substitutional Zn suffers a Jahn-Teller-like distortion from the C3v symmetry and, as a consequence, has a zero spin moment. Fe occupies a distinct position at the border between regimes (ii) and (iii) and shows a more complex behavior: while is non-magnetic at the level of GGA calculations, its spin moment can be switched on using GGA+U calculations with moderate values of the U parameter.Comment: 13 figures, 4 tables. Submitted to Phys. Rev. B on September 26th, 200

Mechanical properties of Graphene Nanoribbons

Herein, we investigate the structural, electronic and mechanical properties of zigzag graphene nanoribbons upon the presence of stress applying Density Functional Theory within the GGA-PBE approximation. The uniaxial stress is applied along the periodic direction, allowing a unitary deformation in the range of +/- 0.02%. The mechanical properties show a linear-response within that range while the non-linear dependence is found for higher strain. The most relevant results indicate that Young's modulus is considerable higher than those determined for graphene and carbon nanotubes. The geometrical reconstruction of the C-C bonds at the edges hardness the nanostructure. Electronic structure features are not sensitive to strain in this linear elastic regime, being an additional promise for the using of carbon nanostructures in nano-electronic devices in the near future.Comment: 30 pages. J. Phys.: Condens. Matter (accepted

Band-theoretical prediction of magnetic anisotropy in uranium monochalcogenides

Magnetic anisotropy of uranium monochalcogenides, US, USe and UTe, is studied by means of fully-relativistic spin-polarized band structure calculations within the local spin-density approximation. It is found that the size of the magnetic anisotropy is fairly large (about 10 meV/unit formula), which is comparable with experiment. This strong anisotropy is discussed in view of a pseudo-gap formation, of which crucial ingredients are the exchange splitting of U 5f states and their hybridization with chalcogen p states (f-p hybridization). An anomalous trend in the anisotropy is found in the series (US>>USe<UTe) and interpreted in terms of competition between localization of the U 5f states and the f-p hybridization. It is the spin-orbit interaction on the chalcogen p states that plays an essential role in enlarging the strength of the f-p hybridization in UTe, leading to an anomalous systematic trend in the magnetic anisotropy.Comment: 4 pages, 5 figure

Simulação do crescimento e desenvolvimento do trigo irrigado utilizando o modelo CERES-Wheat na região de Campinas - SP.

Este trabalho tem como objetivos apresentar de forma sucinta o funcionamento do modelo CERES-Wheat inserido na plataforma DSSAT 3.5, assim como apresentar os resultados obtidos das simulações realizadas com o modelo e os observados em experimento de campo, em especial a sua capacidade de detectar os efeitos da aplicação de N sob a fenologia e produtividade de grãos do trigo sob irrigação.bitstream/CNPTIA/9961/1/circtec2.pdfAcesso em: 28 maio 2008

Chirality in Bare and Passivated Gold Nanoclusters

Chiral structures have been found as the lowest-energy isomers of bare (Au$_{28}$ and Au$_{55}) and thiol-passivated (Au$_{28}(SCH$_{3})$_{16}$and Au$_{38}(SCH_{3})_{24}) gold nanoclusters. The degree of chirality existing in the chiral clusters was calculated using the Hausdorff chirality measure. We found that the index of chirality is higher in the passivated clusters and decreases with the cluster size. These results are consistent with the observed chiroptical activity recently reported for glutahione-passivated gold nanoclusters, and provide theoretical support for the existence of chirality in these novel compounds.Comment: 5 pages, 1 figure. Submitted to PR