Electronic structures of [111]-oriented free-standing InAs and InP nanowires

We report on a theoretical study of the electronic structures of the [111]-oriented, free-standing, zincblende InAs and InP nanowires with hexagonal cross sections by means of an atomistic $sp^{3}s^{*}$, spin-orbit interaction included, nearest-neighbor, tight-binding method. The band structures and the band state wave functions of these nanowires are calculated and the symmetry properties of the bands and band states are analyzed based on the $C_{3v}$ double point group. It is shown that all bands of these nanowires are doubly degenerate at the $\Gamma$-point and some of these bands will split into non-degenerate bands when the wave vector $k$ moves away from the $\Gamma$-point as a manifestation of spin-splitting due to spin-orbit interaction. It is also shown that the lower conduction bands of these nanowires all show simple parabolic dispersion relations, while the top valence bands show complex dispersion relations and band crossings. The band state wave functions are presented by the spatial probability distributions and it is found that all the band states show $2\pi/3$-rotation symmetric probability distributions. The effects of quantum confinement on the band structures of the [111]-oriented InAs and InP nanowires are also examined and an empirical formula for the description of quantization energies of the lowest conduction band and the highest valence band is presented. The formula can simply be used to estimate the enhancement of the band gaps of the nanowires at different sizes as a result of quantum confinement.Comment: 9 pages, 8 figures. arXiv admin note: substantial text overlap with arXiv:1502.0756

Self-orthogonal modules over coherent rings

AbstractLet R be a left coherent ring, S any ring and RωS an (R,S)-bimodule. Suppose ωS has an ultimately closed FP-injective resolution and RωS satisfies the conditions: (1) ωS is finitely presented; (2) The natural map R→ End(ωS) is an isomorphism; (3) ExtSi(ω,ω)=0 for any i≥1. Then a finitely presented left R-module A satisfying ExtRi(A,ω)=0 for any i≥1 implies that A is ω-reflexive. Let R be a left coherent ring, S a right coherent ring and RωS a faithfully balanced self-orthogonal bimodule and n≥0. Then the FP-injective dimension of RωS is equal to or less than n as both left R-module and right S-module if and only if every finitely presented left R-module and every finitely presented right S-module have finite generalized Gorenstein dimension at most n

Analysis of coupling vibration characteristics of electrically driven pile hammer linkage system

In order to solve the problem that electromechanical coupling had influences on the control effect of multi-pile hammers linkage, the coupling influence rules were studied. With the complex system of hammer-pile-soil being simplified, the dynamic model was constructed for the two pile hammers vibration system. The mathematical equations of systemic electromechanical coupling were established. Based on the Hamilton principle, synchronous operation conditions and system stability were established. Simulation model was developed with MATLAB/Simulink for numerical simulation. The electromechanical coupling processes and the basic system rules were obtained under different electrical motors’ speeds, the initial phase differences, soil parameters and fixing parameters. It could be found that electromechanical coupling might result in self-synchronization under given conditions. Finally, the mathematical model’s validity, theoretical derivation and simulation results were proved by some experiments. The analytical conclusions of electromechanical coupling rules provide the theoretical evidence for making control strategy on electric control linkage mode, and the basis for related engineering applications and experiments

Topological energy gaps in the [111]-oriented InAs/GaSb and GaSb/InAs core-shell nanowires

The [111]-oriented InAs/GaSb and GaSb/InAs core-shell nanowires have been studied by the $8\times 8$ Luttinger-Kohn $\vec{k}\cdot\vec{p}$ Hamiltonian to search for non-vanishing fundamental gaps between inverted electron and hole bands. We focus on the variations of the topologically nontrivial fundamental gap, the hybridization gap, and the effective gap with the core radius and shell thickness of the nanowires. The evolutions of all the energy gaps with the structural parameters are shown to be dominantly governed by quantum size effects. With a fixed core radius, a topologically nontrivial fundamental gap exists only at intermediate shell thicknesses. The maximum gap is $\sim 4.4$ meV for GaSb/InAs and $\sim 3.5$ meV for InAs/GaSb core-shell nanowires, and for the GaSb/InAs core-shell nanowires the gap persists over a wider range of geometrical parameters. The intrinsic reason for these differences between the two types of nanowires is that in the shell the electron-like states of InAs is more delocalized than the hole-like state of GaSb, while in the core the hole-like state of GaSb is more delocalized than the electron-like state of InAs, and both features favor stronger electron-hole hybridization. Since similar features of the electron- and hole-like states have been found in nanowires of other materials, it could serve as a common rule to put the hole-like state in the core while the electron-like state in the shell of a core-shell nanowire to achieve better topological properties.Comment: 10 pages, 10 figure

Planktonic Rotifers in a Subtropical Shallow Lake: Succession, Relationship to Environmental Factors, and Use as Bioindicators

Changes in the density and species composition of planktonic rotifers as well as their relationship to several environmental variables were studied at Dadian Lake, a shallow subtropical lake, which was completely dredged and reconstructed. Samples were taken monthly (2006–2009) at five stations. The total rotifer abundance exponentially declined and reached a relatively stable stage in 2009. Polyarthra dolichoptera and Trichocerca pusilla dominated the rotifer community in most seasons. TN, TP, and CODMn went down at the beginning of the monitoring period, rebounded in the second winter, and then decreased and reached a stable state in 2009. CCA showed that the most significant variations were caused by fluctuations in temperature, CODMn, SRP, and NO2-N. The rotifer community experienced a two-stage succession and the difference of species between the stages was exhibited during warm seasons. GAMs indicated that the selected factors were responsible for 64.8% of the total rotifer abundance variance and 16.5~64.3% of the variances of individual species abundance. Most of the environmental parameters had effects on rotifer abundance that could only be described by complicated curves, characterised by unimodality and bimodality instead of linearity. Our study highlighted the temperature influence on rotifer species composition and total abundance in subtropical lakes

Thermoelectric Property Studies of Nanostructured Bulk Materials: Si1-xGex, In4Se3-x, and Zn4Sb3

Thesis advisor: Zhifeng RenThermoelectric materials have attracted a lot of research interests because of their promising applications in solid-state cooling and power generation. The low ZTs of the current available thermoelectric materials have restricted the device efficiency, and thus the wide application of the thermoelectric technique. We propose a nanocomposite approach to improve ZT by reducing lattice thermal conductivity. The nanocomposite approach was first applied to n-type Si. Since there is no point defect scattering from Ge in pure Si, hence it provides an opportunity to study the scattering of grain boundaries. We found that the thermal conductivity is reduced by a factor of 10 in nanostructured Si in comparison with bulk crystalline Si. By adding 5 at% Ge, the thermal conductivity is further reduced by a factor of 2, thereby leading to a thermoelectric figure of merit 0.95 for Si95Ge5, similar to that of large grained Si80Ge20 alloys. Moreover, thermoelectric properties of In4Se3-x and Zn4Sb3 were investigated. Extremely low thermal conductivity values of 0.41 and 0.69 Wm-1K-1 were obtained in In4Se2.2 and Zn4Sb3 nanocomposites respectively, leading to peak ZTs of 1 and 1.3.Thesis (PhD) — Boston College, 2011.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Physics