46 research outputs found
Electronic States in Diffused Quantum Wells
In the present study we calculate the energy values and the spatial
distributions of the bound electronic states in some diffused quantum wells.
The calculations are performed within the virtual crystal approximation, spin dependent empirical tight-binding model and the surface Green
function matching method. A good agreement is found between our results and
experimental data obtained for AlGaAs/GaAs quantum wells with thermally induced
changes in the profile at the interfaces. Our calculations show that for
diffusion lengths {\AA} the transition (C3-HH3) is not
sensitive to the diffusion length, but the transitions (C1-HH1), (C1-LH1),
(C2-HH2) and (C2-LH2) display large "blue shifts" as L_{D} increases. For
diffusion lengths {\AA} the transitions (C1-HH1) and (C1-LH1)
are less sensitive to the L_{D} changes than the (C3-HH3) transition. The
observed dependence is explained in terms of the bound states spatial
distributions.Comment: ReVTeX file, 7pp., no macros, 4 figures available on the reques
Electron and hole states in quantum-dot quantum wells within a spherical 8-band model
In order to study heterostructures composed both of materials with strongly
different parameters and of materials with narrow band gaps, we have developed
an approach, which combines the spherical 8-band effective-mass Hamiltonian and
the Burt's envelope function representation. Using this method, electron and
hole states are calculated in CdS/HgS/CdS/H_2O and CdTe/HgTe/CdTe/H_2O
quantum-dot quantum-well heterostructures. Radial components of the wave
functions of the lowest S and P electron and hole states in typical quantum-dot
quantum wells (QDQWs) are presented as a function of radius. The 6-band-hole
components of the radial wave functions of an electron in the 8-band model have
amplitudes comparable with the amplitude of the corresponding 2-band-electron
component. This is a consequence of the coupling between the conduction and
valence bands, which gives a strong nonparabolicity of the conduction band. At
the same time, the 2-band-electron component of the radial wave functions of a
hole in the 8-band model is small compared with the amplitudes of the
corresponding 6-band-hole components. It is shown that in the CdS/HgS/CdS/H_2O
QDQW holes in the lowest states are strongly localized in the well region
(HgS). On the contrary, electrons in this QDQW and both electron and holes in
the CdTe/HgTe/CdTe/H_2O QDQW are distributed through the entire dot. The
importance of the developed theory for QDQWs is proven by the fact that in
contrast to our rigorous 8-band model, there appear spurious states within the
commonly used symmetrized 8-band model.Comment: 15 pages, 5 figures, E-mail addresses: [email protected],
[email protected]
General boundary conditions for the envelope function in multiband k.p model
We have derived general boundary conditions (BC) for the multiband envelope
functions (which do not contain spurious solutions) in semiconductor
heterostructures with abrupt heterointerfaces. These BC require the
conservation of the probability flux density normal to the interface and
guarantee that the multiband Hamiltonian be self--adjoint. The BC are energy
independent and are characteristic properties of the interface. Calculations
have been performed of the effect of the general BC on the electron energy
levels in a potential well with infinite potential barriers using a coupled two
band model. The connection with other approaches to determining BC for the
envelope function and to the spurious solution problem in the multiband k.p
model are discussed.Comment: 15 pages, 2 figures; to be published in Phys. Rev. B 65, March 15
issue 200
Vegetation and peat characteristics of restiad bogs on Chatham Island (Rekohu), New Zealand
Restiad bogs dominated by Sporadanthus traversii on Chatham Island, New Zealand, were sampled to correlate vegetation patterns and peat properties, and to compare with restiad systems dominated by Sporadanthus ferrugineus and Empodisma minus in the Waikato region, North Island, New Zealand. Classification and ordination resulted in five groups that reflected a disturbance gradient. The largest S. traversii group, which comprised plots from central, relatively intact bogs, had the lowest levels of total nitrogen (mean 1.20 mg cm-3), total phosphorus (mean 0.057 mg cm-3), total potassium (mean 0.083 mg cm-3), and available phosphorus (mean 18.6 ÎŒg cm-3). Modification by drainage, stock, and fires resulted in a decline of S. traversii and an increase of Gleichenia dicarpa fern cover, together with elevated peat nutrient levels and higher bulk density. Compared with peat dominated by Sporadanthus ferrugineus or Empodisma minus in relatively unmodified Waikato restiad bogs, Chatham Island peat under S. traversii has significantly higher total potassium, total nitrogen, available phosphorus, bulk density, and von Post decomposition indices, and significantly lower pH. Sporadanthus traversii and Empodisma minus have similar ecological roles in restiad bog development, occupying a relatively wide nutrient range, and regenerating readily from seed after fire. Despite differences in root morphology, S. traversii and E. minus are the major peat formers in raised restiad bogs on Chatham Island and in Waikato, respectively, and could be regarded as ecological equivalents
Acceptor binding energies in GaN and AlN
We employ effective mass theory for degenerate hole-bands to calculate the
acceptor binding energies for Be, Mg, Zn, Ca, C and Si substitutional acceptors
in GaN and AlN. The calculations are performed through the 66
Rashba-Sheka-Pikus and the Luttinger-Kohn matrix Hamiltonians for wurtzite (WZ)
and zincblende (ZB) crystal phases, respectively. An analytic representation
for the acceptor pseudopotential is used to introduce the specific nature of
the impurity atoms. The energy shift due to polaron effects is also considered
in this approach. The ionization energy estimates are in very good agreement
with those reported experimentally in WZ-GaN. The binding energies for ZB-GaN
acceptors are all predicted to be shallower than the corresponding impurities
in the WZ phase. The binding energy dependence upon the crystal field splitting
in WZ-GaN is analyzed. Ionization levels in AlN are found to have similar
`shallow' values to those in GaN, but with some important differences, which
depend on the band structure parameterizations, especially the value of crystal
field splitting used.Comment: REVTEX file - 1 figur
Development of an eight-band theory for quantum-dot heterostructures
We derive a nonsymmetrized 8-band effective-mass Hamiltonian for quantum-dot
heterostructures (QDHs) in Burt's envelope-function representation. The 8x8
radial Hamiltonian and the boundary conditions for the Schroedinger equation
are obtained for spherical QDHs. Boundary conditions for symmetrized and
nonsymmetrized radial Hamiltonians are compared with each other and with
connection rules that are commonly used to match the wave functions found from
the bulk kp Hamiltonians of two adjacent materials. Electron and hole energy
spectra in three spherical QDHs: HgS/CdS, InAs/GaAs, and GaAs/AlAs are
calculated as a function of the quantum dot radius within the approximate
symmetrized and exact nonsymmetrized 8x8 models. The parameters of dissymmetry
are shown to influence the energy levels and the wave functions of an electron
and a hole and, consequently, the energies of both intraband and interband
transitions.Comment: 36 pages, 10 figures, E-mail addresses: [email protected],
[email protected]
Development of an in vitro protocol for a difficult-to-propagate endemic Australian dryland sedge species Mesomelaena pseudostygia (Cyperaceae)
In vitro propagation for Mesomelaena pseudostygia a difficult-to-propagate dryland sedge species (Cyperaceae) endemic to Western Australia is described. Multiple avenues to in vitro propagation were investigated: shoot culture, organogenesis and somatic embryogenesis, with zygotic embryos as initiation material. The highest multiplication rate for shoots was 3.4â±â1.0 after 6 wk on basal medium (1/2 strength Murashige and Skoog) with 2.5 ÎŒM kinetin and 0.5 ÎŒM 6-benzylaminopurine. Shoots achieved peak rooting (83%) following a pulse treatment on basal medium containing 10 ÎŒM indolebutyric acid and 2 ÎŒM α-naphthaleneacetic acid for 7 wk, followed by transfer to medium (without growth regulators) for a further 7 wk. Alternatively, in vitro grown shoots were pulse treated on basal medium with both 100 ÎŒM indolebutyric acid and 20 ÎŒM α-naphthaleneacetic acid for 1 wk then placed in Rockwool plugs (under propagation house conditions) for another 7 wk resulting in 63% root induction. Rooted plantlets were also successfully transferred to potting mixture either in Rockwool plugs or bare rooted and maintained in propagation house conditions with â„95% survival after 7 wk. These results indicate that micropropagation of M. pseudostygia is feasible for small to medium scale restoration purposes. The highest frequency of callus induction was from cultured zygotic embryos on basal medium with 5 ÎŒM α-naphthaleneacetic acid, whereas 2,4-dichlorophenoxacetic acid (2 or 5 ÎŒM) produced the largest callus sizes. A low frequency of shoot regeneration occurred in zygotic callus tissues in basal medium treatments containing cytokinin (kinetin or thidiazuron at 1 ÎŒM). A small proportion (<20%) of zygotic embryo callus explants from 2,4-dichlorophenoxyacetic acid treatments were found to be embryogenic, firstly developing embryo-like structures after 2 wk on basal medium (minus plant growth hormones), that continued to develop with approximately one in twenty germinating after a further 4 wk on basal medium to form small plantlets. Further optimisation is needed to improve somatic embryogenesis efficiency for mass propagation