RESUMO
We investigate the theoretical band structure of organic-inorganic perovskites APbX3 with tetragonal crystal structure. Using D4h point group symmetry properties, we derive a general 16-band Hamiltonian describing the electronic band diagram in the vicinity of the wave-vector point corresponding to the direct band gap. For bulk crystals, a very good agreement between our predictions and experimental physical parameters, as band gap energies and effective carrier masses, is obtained. Extending this description to three-dimensional confined hybrid halide perovskite, we calculate the size dependence of the excitonic radiative lifetime and fine structure. We describe the exciton fine structure of cube-shaped nanocrystals by an interplay of crystal-field and electron-hole exchange interaction (short- and long-range parts) enhanced by confinement. Using very recent experimental results on FAPbBr3 nanocrystals, we extract the bulk short-range exchange interaction in this material and predict its value in other hybrid compounds. Finally, we also predict the bright-bright and bright-dark splittings as a function of nanocrystal size.
RESUMO
The magnetic anisotropy of the ferromagnetic semiconductor (Ga, Mn) (As, P) is studied in a material-specific microscopic k â p approach. We calculate the electronic energy band structure of (Ga, Mn) (As, P) quaternary ferromagnetic alloys using a 40-band k â p model and taking into account the s, p-d exchange interaction and the strain of the (Ga, Mn) (As, P) layer on a GaAs substrate. We determine the variations of the carrier effective masses in the strained (Ga, Mn) (As, P)/GaAs system. The magnetic anisotropy constants obtained from our simulations using a mean-field model are compared with the experimental ones determined by ferromagnetic resonance spectroscopy on a set of samples with constant manganese concentration and varying phosphorus concentration. An excellent quantitative agreement between experiment and theory is found for the uniaxial out-of-plane and cubic in-plane anisotropy parameters.
RESUMO
The initialization of a resident hole spin by the absorption of a circularly polarized light at resonance involves the formation of an excited state called a trion state. For a pure heavy hole, this optical initialization is mediated by the hyperfine electron-nuclear coupling in the trion state. We show here that for a mixed-hole spin an additional mechanism for the optical initialization appears, associated to 'crossed transitions'; it becomes dominant and keeps a high level of hole spin polarization when the magnetic field screens the electron-nuclear interaction. Finally, using a simple model, we obtain a good theoretical agreement with pulsed pump-probe experiments.
RESUMO
We have measured the carrier spin dynamics in p-doped InAs/GaAs quantum dots by pump-probe and time-resolved photoluminescence experiments. We obtained experimental evidence of the hyperfine interaction between hole and nuclear spins. In the absence of an external magnetic field, our calculations based on dipole-dipole coupling between the hole and the quantum dot nuclei lead to a hole-spin dephasing time for an ensemble of dots of 14 ns, in close agreement with experiments.
RESUMO
Collective and single-particle spin-flip excitations of a two-dimensional electron gas in a semimagnetic Cd(1-x)Mn(x)Te quantum well are observed by resonant Raman scattering. Application of a magnetic field splits the spin subbands and a spin polarization is induced in the electron gas. Above 1 T the collective modes, which disperse with the in-plane wave vector, dominate the spectra. The local spin-density approximation provides a good description of our results and enables us to confirm that the energy of the low wave vector collective mode is given by the bare Zeeman energy.
