84 research outputs found
First-principles envelope-function theory for lattice-matched semiconductor heterostructures
In this paper a multi-band envelope-function Hamiltonian for lattice-matched
semiconductor heterostructures is derived from first-principles norm-conserving
pseudopotentials. The theory is applicable to isovalent or heterovalent
heterostructures with macroscopically neutral interfaces and no spontaneous
bulk polarization. The key assumption -- proved in earlier numerical studies --
is that the heterostructure can be treated as a weak perturbation with respect
to some periodic reference crystal, with the nonlinear response small in
comparison to the linear response. Quadratic response theory is then used in
conjunction with k.p perturbation theory to develop a multi-band effective-mass
Hamiltonian (for slowly varying envelope functions) in which all interface
band-mixing effects are determined by the linear response. To within terms of
the same order as the position dependence of the effective mass, the quadratic
response contributes only a bulk band offset term and an interface dipole term,
both of which are diagonal in the effective-mass Hamiltonian. Long-range
multipole Coulomb fields arise in quantum wires or dots, but have no
qualitative effect in two-dimensional systems beyond a dipole contribution to
the band offsets.Comment: 25 pages, no figures, RevTeX4; v3: final published versio
Homogeneously derived transit timings for 17 exoplanets and reassessed TTV trends for WASP-12 and WASP-4
We homogeneously analyse ∼3.2 × 105 photometric measurements for ∼1100 transit lightcurves belonging to 17 exoplanet hosts. The photometric data cover 16 years 2004–2019 and include amateur and professional observations. Old archival lightcurves were reprocessed using up-to-date exoplanetary parameters and empirically debiased limb-darkening models. We also derive self-consistent transit and radial-velocity fits for 13 targets. We confirm the nonlinear TTV trend in the WASP-12 data at a high significance, and with a consistent magnitude. However, Doppler data reveal hints of a radial acceleration about ( − 7.5 ± 2.2) m/s/yr, indicating the presence of unseen distant companions, and suggesting that roughly 10 per cent of the observed TTV was induced via the light-travel (or Roemer) effect. For WASP-4, a similar TTV trend suspected after the recent TESS observations appears controversial and model-dependent. It is not supported by our homogeneus TTV sample, including 10 ground-based EXPANSION lightcurves obtained in 2018 simultaneously with TESS. Even if the TTV trend itself does exist in WASP-4, its magnitude and tidal nature are uncertain. Doppler data cannot entirely rule out the Roemer effect induced by possible distant companions
Effect of Dopants on Zirconia Stabilization—An X-ray Absorption Study: I, Trivalent Dopants
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65562/1/j.1151-2916.1994.tb06964.x.pd
Amplitude of oscillations and the nonlinear frequency shift upon excitation of a surface-helical instability in semiconductor plates
Nonlinear theory of surface-helical instability of a semiconductor plasma. II. Calculation of linear parameters
Calculation of the linear parameters of a surface-helical instability in semiconductor plates
Nonlinear effects of the surface-helical instability in intrinsic or nearly intrinsic Ge
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