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

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

Microstructure and thermal stability of Fe, Ti and Ag implanted Yttria-stabilized zirconia

Yttria-stabilized zirconia (YSZ) was implanted with 15 keV Fe or Ti ions up to a dose of 8×1016 at cm−2. The resulting “dopant” concentrations exceeded the concentrations corresponding to the equilibrium solid solubility of Fe2O3 or TiO2 in YSZ. During oxidation in air at 400° C, the Fe and Ti concentration in the outermost surface layer increased even further until a surface layer was formed of mainly Fe2O3 and TiO2, as shown by XPS and ISS measurements. From the time dependence of the Fe and Ti depth profiles during anneal treatments, diffusion coefficients were calculated. From those values it was estimated that the maximum temperature at which the Fe- and Ti-implanted layers can be operated without changes in the dopant concentration profiles was 700 and 800° C, respectively. The high-dose implanted layer was completely amorphous even after annealing up to 1100° C, as shown by scanning transmission electron microscopy. Preliminary measurements on 50 keV Ag implanted YSZ indicate that in this case the amorphous layer recrystallizes into fine grained cubic YSZ at a temperature of about 1000° C. The average grain diameter was estimated at 20 nm, whereas the original grain size of YSZ before implantation was 400 nm. This result implies that the grain size in the surface of a ceramic material can be decreased by ion beam amorphisation and subsequent recrystallisation at elevated temperatures

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

Исследование и верификация скоростных моделей земной коры методами математического моделирования и активной сейсмологии

The article discusses a comparison of theoretical seismograms for two velocity models of the Earth's crust and P-wave arrival times estimated from experimental vibration seismograms for the 400-km long section of the Baikal–Ulaanbaatar profile. The theoretical seismograms were obtained by mathematical simulation of wave fields using the Earth's crust velocity models based on the data of the BEST and PASSCAL experiments. Vibration seismograms were obtained by measuring the wave field of a CVO-100 vibrator in the SB RAS Southern Baikal polygon. In the experiments, the vibration seismograms show that arrival times in the P-wave group correspond to the values for waves of large amplitudes in the theoretical seismograms. The P-wave arrival times in the theoretical seismograms of the BEST experiment are compared to the values in the experimental vibration seismograms for the 400-km long section of the Baikal–Ulaanbaatar profile. This comparison shows that the arrival times of maximum amplitude waves correspond to the theoretical hodographs of waves with velocities of 6.25–6.80 km/sec in the BEST experiment velocity model. At the same time, the experimental data set does not contain arrival times corresponding to longitudinal waves with Vp=7.25 km/sec, which are related to an assumed layer (more than 10 km thick) in the lower crust for the BEST experiment velocity model. In the experiments, the P-wave arrival times in the vibration seismograms correspond to the P-wave arrival times in the theoretical seismograms of the PASSCAL experiment throughout the entire 400-km long section of the Baikal–Ulaanbaatar profile. It is thus confirmed that the average values of the wave velocities in the PASSCAL velocity model have been reliably estimated. It should be noted that the experimental values of the arrival times of the first wave in the P-wave group are in agreement with the first arrival times in the hodographs of the theoretical seismograms for the velocity model in the PASSCAL experiment considering the distances from the source in a range of 65–380 km.Представлены результаты сравнения теоретических сейсмограмм для двух скоростных моделей земной коры и данных о временах вступлений Р-волн на экспериментальных вибрационных сейсмограммах на 400-километровом участке профиля Байкал – Улан-Батор. Теоретические сейсмограммы получены методами математического моделирования волновых полей для скоростных моделей земной коры, построенных по данным экспериментов BEST и PASSCAL. Вибрационные сейсмограммы получены при измерении волнового поля вибратора ЦВО-100 Южно-Байкальского полигона СО РАН. Экспериментальные значения времен вступлений в группе Р-волн на вибрационных сейсмограммах соответствуют значениям на теоретических сейсмограммах для волн большой амплитуды. Cравнение теоретических сейсмограмм эксперимента BEST и данных о временах вступлений Р-волн на экспериментальных вибрационных сейсмограммах на 400-кило­метровом участке профиля Байкал – Улан-Батор показывает, что экспериментальные времена вступлений волн максимальной амплитуды соответствуют теоретическим годографам волн со скоростью 6.25–6.80 км/с скоростной модели эксперимента BEST. Вместе с тем экспериментальные данные не содержат времен вступлений, соответствующих продольным волнам со скоростью Vp=7.25 км/с, связанных с предполагаемым слоем мощностью более 10 км в нижней коре в скоростной модели эксперимента BEST. Экспериментальные значения времен вступлений волн в группе Р-волн вибрационных сейсмограмм находятся в области времен вступлений волн на теоретических сейсмограммах эксперимента PASSCAL на всем протяжении 400-километрового профиля. Это свидетельствует о надежном определении средних значений скоростей волн в скоростной модели эксперимента PASSCAL. Следует отметить, что экспериментальные значения времен вступлений первой волны в группе Р-волн согласуются с временами первых вступлений на годографах теоретических сейсмограмм для скоростной модели эксперимента PASSCAL в диапазоне расстояний от 65 до 380 км от источника