Combustibles forestales superficiales y riesgo de incendio en tres estadios de sucesión de bosques en el Parque Nacional Palo Verde, Costa Rica.

Proyecto de Graduación (Licenciatura en Ingeniería Forestal) Instituto Tecnológico de Costa Rica, Escuela de Ingeniería Forestal, 2014.In this study, forest fuel of the standing trees and aboveground mulch was quantified. Also, the floristic composition was characterized in three successional stages of treatment, old or primary forest (38.36 ha), more than 30 years (22.75 ha) and 10-25 years (29.88 ha). The sampling design for temporary plots was a simple random and stratified, with seven replicates per treatment (15 x 50 m), with sub plots 1m2 for quantification of surface soil material. Topsoil samples were taken to the laboratory of the Center for Integration Research Forest Industry (CIIBI) at the Instituto Tecnológico de Costa Rica. The consumption tests were performed. The hazard index was determined from surface fuels. A total of 567 individuals, 57 species and 28 families (five families represented by only one individual) was identified. Only 10 species were present in the three successional stages. Most species are found in primary forest (37), while the > 30 years forest had the highest number of families (20). The primary forest has the highest amount of forestry fuel with a value of 254.67 Mg ha-1. The mature secondary forest (> 30 years) was the second highest with 196.27 Mg ha-1. The lowest values (71.31 Mg ha-1) correspond to the 10-25 years forest. The fuels accumulated in the topsoil was higher for primary forest (14.06 Mg ha-1), followed by the largest forest to 30 years (11.87 Mg ha-1), and finally the forest 10-25 years (7.75 Mg ha-1). It was found that the amount of fuel tends to be higher in the older sites. The average value obtained of IPCS was 754.32, according to the classification criteria described by Galiana (2009) and modified by Parra (2013), the three stages of succession are placed in the medium-low risk category.Instituto Tecnológico de Costa Rica, Escuela de Ingeniería Forestal

The split-operator technique for the study of spinorial wavepacket dynamics

The split-operator technique for wave packet propagation in quantum systems is expanded here to the case of propagating wave functions describing Schr\"odinger particles, namely, charge carriers in semiconductor nanostructures within the effective mass approximation, in the presence of Zeeman effect, as well as of Rashba and Dresselhaus spin-orbit interactions. We also demonstrate that simple modifications to the expanded technique allow us to calculate the time evolution of wave packets describing Dirac particles, which are relevant for the study of transport properties in graphene.Comment: 19 pages, 4 figure

Theoretical investigation of electron-hole complexes in anisotropic two-dimensional materials

Trions and biexcitons in anisotropic two-dimensional materials are investigated within an effective mass theory. Explicit results are obtained for phosphorene and arsenene, materials that share features such as a direct quasi-particle gap and anisotropic conduction and valence bands. Trions are predicted to have remarkably high binding energies and an elongated electron-hole structure with a preference for alignment along the armchair direction, where the effective masses are lower. We find that biexciton binding energies are also notably large, especially for monolayer phosphorene, where they are found to be twice as large as those for typical monolayer transition metal dichalcogenides.Comment: 3 figures, 5 pages + Supplementary Material, accepted for publication in Phys. Rev.

Anisotropic exciton Stark shift in black phosphorus

We calculate the excitonic spectrum of few-layer black phosphorus by direct diagonalization of the effective mass Hamiltonian in the presence of an applied in-plane electric field. The strong attractive interaction between electrons and holes in this system allows one to investigate the Stark effect up to very high ionizing fields, including also the excited states. Our results show that the band anisotropy in black phosphorus becomes evident in the direction dependent field induced polarizability of the exciton

Wave packet dynamics and valley filter in strained graphene

The time evolution of a wavepacket in strained graphene is studied within the tight-binding model and continuum model. The effect of an external magnetic field, as well as a strain-induced pseudo-magnetic field, on the wave packet trajectories and zitterbewegung are analyzed. Combining the effects of strain with those of an external magnetic field produces an effective magnetic field which is large in one of the Dirac cones, but can be practically zero in the other. We construct an efficient valley filter, where for a propagating incoming wave packet consisting of momenta around the K and K' Dirac points, the outgoing wave packet exhibits momenta in only one of these Dirac points, while the components of the packet that belong to the other Dirac point are reflected due to the Lorentz force. We also found that the zitterbewegung is permanent in time in the presence of either external or strain-induced magnetic fields, but when both the external and strain-induced magnetic fields are present, the zitterbewegung is transient in one of the Dirac cones, whereas in the other cone the wave packet exhibits permanent spatial oscillations.Comment: 13 pages, 10 figure

Electric and magnetic fields effects on the excitonic properties of elliptic core-multishell quantum wires

The effect of eccentricity distortions of core-multishell quantum wires on their electron, hole and exciton states is theoretically investigated. Within the effective mass approximation, the Schrodinger equation is numerically solved for electrons and holes in systems with single and double radial heterostructures, and the exciton binding energy is calculated by means of a variational approach. We show that the energy spectrum of a core-multishell heterostructure with eccentricity distortions, as well as its magnetic field dependence, are very sensitive to the direction of an externally applied electric field, an effect that can be used to identify the eccentricity of the system. For a double heterostructure, the eccentricities of the inner and outer shells play an important role on the excitonic binding energy, specially in the presence of external magnetic fields, and lead to drastic modifications in the oscillator strength.Comment: 17 pages, 10 figure