Spin-subband populations and spin polarization of quasi two-dimensional carriers under in-plane magnetic field

Under an in-plane magnetic field, the density of states of quasi two-dimensional carriers deviates from the occasionally stereotypic step-like form both quantitatively and qualitatively. For the first time, we study how this affects the spin-subband populations and the spin-polarization as functions of the temperature, T, and the in-plane magnetic field, B, for narrow to wide dilute-magnetic-semiconductor quantum wells. We examine a wide range of material and structural parameters, focusing on the quantum well width, the magnitude of the spin-spin exchange interaction, and the sheet carrier concentration. Generally, increasing T, the carrier spin-splitting, U, decreases, augmenting the influence of the minority-spin carriers. Increasing B, U increases and accordingly carriers populate majority-spin subbands while they abandon minority-spin subbands. Furthermore, in line with the density of states modification, all energetically higher subbands become gradually depopulated. We also indicate the ranges where the system is completely spin-polarized.Comment: 7 pages, 6 figures, Physical Review B (in press

Quasi two-dimensional carriers in dilute-magnetic-semiconductor quantum wells under in-plane magnetic field

Due to the competition between spatial and magnetic confinement, the density of states of a quasi two-dimensional system deviates from the ideal step-like form both quantitatively and qualitatively. We study how this affects the spin-subband populations and the spin-polarization as functions of the temperature, $T$, and the in-plane magnetic field, $B$, for narrow to wide dilute-magnetic-semiconductor quantum wells. We focus on the quantum well width, the magnitude of the spin-spin exchange interaction, and the sheet carrier concentration dependence. We look for ranges where the system is completely spin-polarized. Increasing $T$, the carrier spin-splitting, $U_{o\sigma}$, decreases, while increasing $B$, $U_{o\sigma}$ increases. Moreover, due to the density of states modification, all energetically higher subbands become gradually depopulated.Comment: 3 pages, 3 figure

Empirical LCAO parameters for π\pi molecular orbitals in planar organic molecules

We present a parametrization within a simplified LCAO model (a type of Hueckel model) for the description of $\pi$ molecular orbitals in organic molecules containing $\pi$-bonds between carbon, nitrogen, or oxygen atoms with $sp^2$ hybridization, which we show to be quite accurate in predicting the energy of the highest occupied $\pi$ orbital and the first $\pi$-$\pi*$ transition energy for a large set of organic compounds. We provide four empirical parameter values for the diagonal matrix elements of the LCAO description, corresponding to atoms of carbon, nitrogen with one $p_z$ electron, nitrogen with two $p_z$ electrons, and oxygen. The bond-length dependent formula (proportional to $1/d^2$) of Harrison is used for the non-diagonal matrix elements between neighboring atoms. The predictions of our calculations have been tested against available experimental results in more than sixty organic molecules, including benzene and its derivatives, polyacenes, aromatic hydrocarbons of various geometries, polyenes, ketones, aldehydes, azabenzenes, nucleic acid bases and others. The comparison is rather successful, taking into account the small number of parameters and the simplicity of the LCAO method, involving only $p_z$ atomic orbitals, which leads even to analytical calculations in some cases.Comment: 20 pages, 6 tables, 65 planar organic molecule

Purely orbital diamagnetic to paramagnetic fluctuation of quasi two-dimensional carriers under in-plane magnetic field

An external magnetic field, $H$, applied parallel to a quasi two-dimensional system modifies quantitatively and qualitatively the density of states. Using a self-consistent numerical approach, we study how this affects the entropy, $S$, the free energy, $F$, and the magnetization, $M$, for different sheet carrier concentrations, $N_s$. As a prototype system we employ III-V double quantum wells. We find that although $M$ is mainly in the opposite direction of $H$, the system is not linear. Surprisingly $\partial M / \partial H$ swings between negative and positive values, i.e., we predict an entirely orbital diamagnetic to paramagnetic fluctuation. This phenomenon is important compared to the ideal de Haas-van Alphen effect i.e. the corresponding phenomenon under perpendicular magnetic field.Comment: 4 pages, 6 figure

Quasi-periodic and fractal polymers: Energy structure and carrier transfer

We study the energy structure and the coherent transfer of an extra electron or hole along aperiodic polymers made of $N$ monomers, with fixed boundaries, using B-DNA as our prototype system. We use a Tight-Binding wire model, where a site is a monomer (e.g., in DNA, a base pair). We consider quasi-periodic (Fibonacci, Thue-Morse, Double-Period, Rudin-Shapiro) and fractal (Cantor Set, Asymmetric Cantor Set) polymers made of the same monomer (I polymers) or made of different monomers (D polymers). For all types of such polymers, we calculate the HOMO and LUMO eigenspectrum, the HOMO-LUMO gap and the density of states. We examine the mean over time probability to find the carrier at each monomer, the frequency content of carrier transfer (Fourier spectra, weighted mean frequency of each monomer, total weighted mean frequency of the polymer), and the pure mean transfer rate $k$. Our results reveal that there is a correspondence between the degree of structural complexity and the transfer properties. I polymers are more favorable for charge transfer than D polymers. We compare $k(N)$ of quasi-periodic and fractal sequences with that of periodic sequences (including homopolymers) as well as with randomly shuffled sequences. Finally, we discuss aspects of experimental results on charge transfer rates in DNA with respect to our coherent pure mean transfer rates.Comment: 19 pages, 13 figures. arXiv admin note: text overlap with arXiv:1808.0561