We have investigated the electric-field-strength dependence of the reduced effective masses of the minibands in a GaAs/AlAs superlattice embedded in a p-i-n diode structure with use of electroreflectance spectroscopy. Frantz-Keldysh (FK) oscillations are observed in the energy region between the n = 1 heavy hole to electron transition at the mini-Brillouin-zone center (kz = 0: Γ point) and that at the edge (kz = π/D: π point), where D is the superlattice period. Analyzing the FK oscillations, we evaluate the reduced effective masses at the Γ and π points as a function of electric field strength. The evaluated reduced effective masses are consistent with the values estimated from the calculated miniband dispersion relations in a low electric field regime. It is found that the reduced effective masses tend to be heavier at a given electric field strength. Considering the electric-field-strength dependence of eigenstates calculated using a transfer-matrix method, we conclude that an increase of the reduced effective masses originates from weak localization of the electron envelope function in the transformation process from the miniband to Stark-ladder states in Wannier-Stark localization.

We have developed spin-resolved resonant electron energy-loss spectroscopy with the primary energy of 0.3–1.5 keV, which corresponds to the core excitations of 2p–3d absorption of transition metals and 3d–4f absorption of rare-earths, with the energy resolution of about 100 meV using a spin-polarized electron source as a GaAs/GaAsP strained superlattice photocathode. Element- and spin-selective carrier and valence plasmons can be observed using the resonance enhancement of core absorptions and electron spin polarization. Furthermore, bulk-sensitive electron energy-loss spectroscopy spectra can be obtained because the primary energy corresponds to the mean free path of 1–10 nm. The methodology is expected to provide us with novel information about elementary excitations by resonant inelastic x-ray scattering and resonant photoelectron spectroscopy.