Transient Charging and Discharging of Spin-polarized Electrons in a Quantum Dot

We study spin-polarized transient transport in a quantum dot coupled to two ferromagnetic leads subjected to a rectangular bias voltage pulse. Time-dependent spin-resolved currents, occupations, spin accumulation, and tunneling magnetoresistance (TMR) are calculated using both nonequilibrium Green function and master equation techniques. Both parallel and antiparallel leads' magnetization alignments are analyzed. Our main findings are: a dynamical spin accumulation that changes sign in time, a short-lived pulse of spin polarized current in the emitter lead (but not in the collector lead), and a dynamical TMR that develops negative values in the transient regime. We also observe that the intra-dot Coulomb interaction can enhance even further the negative values of the TMR.Comment: 7 pages, 6 figures. Typos corrections corresponding to the published versio

First Principles Calculations of Fe on GaAs (100)

We have calculated from first principles the electronic structure of 0.5 monolayer upto 5 monolayer thick Fe layers on top of a GaAs (100) surface. We find the Fe magnetic moment to be determined by the Fe-As distance. As segregates to the top of the Fe film, whereas Ga most likely is found within the Fe film. Moreover, we find an asymmetric in-plane contraction of our unit-cell along with an expansion perpendicular to the surface. We predict the number of Fe 3d-holes to increase with increasing Fe thickness on $p$-doped GaAs.Comment: 9 pages, 14 figures, submitted to PR

Magnetization reversal processes in epitaxial Fe/GaAs(001) films

Copyright © 1994 American Institute of PhysicsIn this article we present the results of a detailed study of the switching behavior observed in epitaxial single Fe films of thickness between 30 and 450 Å, and a wedge shaped Fe film with a thickness range of 10–60 Å grown on GaAs (001). These films have cubic and uniaxial anisotropies which change with film thickness. For the fixed thickness films the values of the anisotropy constants were accurately determined by Brillouin light scattering (BLS) measurements together with polar magneto‐optic Kerr effect (MOKE) measurements that gave the value of the magnetization. The switching behavior of these samples was observed with in‐plane MOKE magnetometry as a function of the angle between the applied field and the in‐plane crystallographic axes. Measurements of the component of magnetization perpendicular to the applied field allow a precise determination of the relative orientation of the hard and easy in‐plane anisotropy axes. This can be used to accurately determine the ratio of uniaxial to cubic anisotropy constants, when this ratio is less than one. The ratios obtained from MOKE agree well with those obtained from BLS. Minimum energy calculations predict that the reversal process should proceed by a continuous rotation of the magnetization vector with either one or two irreversible jumps, depending on the applied field orientation and the nature of the anisotropy of the film. The calculations provide a good qualitative description of the observed reversal process, although the magnetic microstructure influences the exact values of the switching fields

Continuous evolution of the in-plane magnetic anisotropies with thickness in epitaxial Fe films

Copyright © 1996 American Institute of Physics.We have studied the evolution of the magnetic in‐plane anisotropy in epitaxial Fe/GaAs films of both (001) and (110) orientation as a function of the Fe layer thickness using the longitudinal magneto‐optic Kerr effect and Brillouin light scattering. Magnetization curves which are recorded in situ during film growth reveal a continuous change of the net anisotropy axes with increasing film thickness. This behavior can be understood to arise from the combination of a uniaxial and a cubic in‐plane magnetic anisotropy which are both thickness dependent. Structural analysis of the substrate and Fe film surfaces provides insight into the contribution of atomic steps at the interfaces to the magnetic anisotropy. Changing the degree of crystalline order at the Fe–GaAs interface allows us to conclude that the magnetic anisotropies are determined by atomic scale order

Nonlinear optical response of α -terpinolene and β -phellandrene chromophores: An octupolar gas-to-solvent enhancement able to retain light conduction

The search for new materials with improved nonlinear optical (NLO) response is a field of growing interest in materials science. Typically, the dipole ΦJ=1 contributions exceed the octupolar ΦJ=3 ones and dominate the optical behavior. However, the latter is essential for NLO device engineering. Under this scenario, this work investigates the electronic-optical properties of the α-terpinolene and β-phellandrene molecules within the Hyper-Rayleigh scattering formalism (HRS). It includes solvent contributions using a sequential Monte Carlo/Quantum Mechanics procedure. According to Density Functional Theory analysis, molecular solvatochromism acts differently for the two chromophores. While α-terpinolene undergoes a hypsochromic effect, the β-phellandrene molecule shows moderate bathochromism, both with a strong absorption band in the ultraviolet region (λmax<240nm), making them attractive for potential UV filters. Regarding the NLO response, both compounds present similar values for the first frequency-dependent hyperpolarizability (βHRS) with values that vary from 62.46 to 138.73 au in aqueous environment, superating urea (β=42.82 au), a standard optical material. Furthermore, while one of these chromophores is best described by dipole contributions (ΦJ=1≈68%), the other is dominated by the octupolar term (ΦJ=3≈60%) even when the solvent moderates it. These characteristics allow the building of optical switches without losing the strength of the NLO response. In addition, the polarization of the solute due to the solvent conveniently reduces the refractive index (n), providing light conduction applications. Therefore, these chromophores can be used to promote a decoupling between dipolar and octupolar contributions in NLO.Fil: dos Santos, Neidy S. S.. Universidade Federal do Pará; BrasilFil: Fonseca, Sávio. Universidade Federal do Pará; BrasilFil: Moura, André. Universidade Federal do Pará; BrasilFil: da Cunha, Antonio R.. Universidade Federal do Maranhao; BrasilFil: Provasi, Patricio Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnológica; ArgentinaFil: Andrade Filho, Tarciso. Universidade Federal do Pará; BrasilFil: Gester, Rodrigo. Universidade de Sao Paulo; Brasi

Evaluation of a Desktop 3D Printed Rigid Refractive-Indexed-Matched Flow Phantom for PIV Measurements on Cerebral Aneurysms

Purpose Fabrication of a suitable flow model or phantom is critical to the study of biomedical fluid dynamics using optical flow visualization and measurement methods. The main difficulties arise from the optical properties of the model material, accuracy of the geometry and ease of fabrication. Methods Conventionally an investment casting method has been used, but recently advancements in additive manufacturing techniques such as 3D printing have allowed the flow model to be printed directly with minimal post-processing steps. This study presents results of an investigation into the feasibility of fabrication of such models suitable for particle image velocimetry (PIV) using a common 3D printing Stereolithography process and photopolymer resin. Results An idealised geometry of a cerebral aneurysm was printed to demonstrate its applicability for PIV experimentation. The material was shown to have a refractive index of 1.51, which can be refractive matched with a mixture of de-ionised water with ammonium thiocyanate (NH4SCN). The images were of a quality that after applying common PIV pre-processing techniques and a PIV cross-correlation algorithm, the results produced were consistent within the aneurysm when compared to previous studies. Conclusions This study presents an alternative low-cost option for 3D printing of a flow phantom suitable for flow visualization simulations. The use of 3D printed flow phantoms reduces the complexity, time and effort required compared to conventional investment casting methods by removing the necessity of a multi-part process required with investment casting techniques