Stark shift and field induced tunneling in doped quantum wells with arbitrary potential profiles

The energies and resonance widths of single doped quantum wells consisting of AlGaAs/GaAs with rectangular and annealing induced diffusion modified shapes are calculated under an uniform electric field using the stabilization method. The electronic structure is calculated without an electric field in the finite temperature density functional theory with exchange-correlation potential treated in the local density approximation. Our scheme for solving the Schrodinger and Poisson equations is based on the Fourier series method. The electric field is added to the self-consistent potential and energies are obtained as a function of the combined width of the well and barriers. This yields us the stabilization graph from which the energies and resonance widths at different field strengths are extracted using the Fermi Golden rule.published_or_final_versio

Electric field effect on the diffusion modified AlGaAs/GaAs single quantum well

The electron subband energies and wave functions in an interdiffusion-induced Al xGa 1-xAs/GaAs/Al xGa 1-xAs single quantum well are calculated in the presence of the dc electric field using the finite difference method. The mean lifetimes are obtained from the time-dependent probability of tunneling of the wave packet out of the well by the applied electric field. The effect of the applied electric field on the subband energies in the well is the same as in the as-grown square quantum well when the interdiffusion length is below 20 Å. In the well with higher diffusion length the barrier height reduces so that the wave function tunnels out of the well. The linear and nonlinear intersubband absorption coefficients and the change in the real part of the index of refraction are calculated with the applied electric field at 100 kV/cm and without the field in both the as-grown square well and the diffusion modified well with the interdiffusion length at 20 Å. © 1996 American Institute of Physics.published_or_final_versio

Temperature dependence of the positronium work function in aluminum

The electron and positron chemical potentials and their volume derivatives are calculated in Al using the first-principles pseudopotential technique and the frozen-core approximation method, respectively. The positron deformation potential of -8.03 eV obtained by this method agrees very well with that found in other first-principles calculations. The variation of the positronium work function with temperature is calculated by including the thermal lattice vibrations into the electron and positron potentials. It is observed that the temperature dependence of the positronium work function does not arise entirely from the volume expansion of the lattice as has been suggested in the past. Proper inclusion of the nonzero-temperature derivative of the positronium work function at constant volume leads to good agreement between the positron deformation potential as obtained from positronium work function data and present theoretical values. © 1994 The American Physical Society.published_or_final_versio

Electron-positron momentum density in diamond, Si, and Ge

The electron-positron momentum densities in diamond. Si, and Ge are calculated using a first-principles method. Comparison of the theoretical momentum densities with the experiment shows that the electronpositron correlation effects are very important in Si and Ge, while this effect is negligible in diamond because the electrons are tightly bound. Our analysis shows that only the upper two bands, which consist of the sp3 hybridized orbitals, contribute to the structures at the low-momentum region of the momentum density. Diamond does not show these structures at the low-momentum region is due to its smaller lattice constant and weak electron-positron correlation effects.published_or_final_versio

Electron-positron momentum distributions and positron lifetime in semiconductors in the generalized gradient approximation

The positron annihilation characteristics have been calculated taking the electron-positron correlation in the generalized gradient approximation (GGA). The calculated electron-positron momentum distributions in Si along the [110] direction in the GGA scheme agree very well with the experiment. The comparison of anisotropies of the momentum distributions along different crystal directions with the theory shows that only the GGA scheme gives the exact values. The enhancement factor for the valence electrons in the electron-positron momentum density is found to be weakly dependent on the momentum. The positron lifetimes in group IV, III-V, and II-VI semiconductors agree very well with the previous calculations and the experiment.published_or_final_versio

Positron effective mass in silicon

The positron effective mass in Si is obtained from the first-principles calculations along various crystallographic directions. The effect of electron-positron correlation on the band mass is examined in this work. A positron pseudopotential scheme is worked out to calculate the isotropic band mass without explicitly solving the band energy. The effective mass 1.46m obtained as a sum of band mass and the positron-plasmon interaction compares very well with 1.5m obtained from the positron mobility data. © 1995 The American Physical Society.published_or_final_versio

Au/GaAs interface annealing study by positron-lifetime spectroscopy

Structural changes of annealed Au contacts on semi-insulating GaAs have been observed by conventional positron lifetime-spectroscopy in which a significant fraction of positrons are drifted from a Na22 source to the contact by an electric field. For annealing temperatures below 200 °C it is found that the interface traps positrons into microvoids with a characteristic positron lifetime of 380±10 ps. For annealing in the range of 300-400 °C a 150±20 ps additional component appears in the lifetime spectra that is attributed to positrons annihilating from AuGa phases at the interface. The most likely explanation for this sudden onset of the positron component in the AuGa phases is that the changes in the GaAs near surface electron chemical potential, brought about by annealing, affect the interfacial dipole in such a way as to allow a favorable potential barrier for positron penetration into the metallic phase. © 1995 The American Physical Society.published_or_final_versio

EL2 deep-level transient study in semi-insulating GaAs using positron-lifetime spectroscopy

Positron lifetime measurements performed on Au/GaAs samples at room temperature with an applied square-wave ac bias show a frequency dependent interlace related lifetime intensity that peaks around 0.4 Hz. The observation is explained by the ionization of the deep-donor level EL2 to EL2+ in the GaAs region adjacent to the Au/GaAs interface, causing a transient electric field to be experienced by positrons drifting towards the interface. Without resorting to temperature scanning or any Arrhenius plot the EL2 donor level is found to be located 0.80±0.01±0.05 eV below the conduction-band minimum, where the first error estimate is statistical and the second systematic. The result suggests positron annihilation may, in some instances, act as an alternative to capacitance transient spectroscopies in characterizing deep levels in both semiconductors and semi-insulators.published_or_final_versio

EL2 deep donor state in semi-insulating GaAs revealed by frequency dependent positron mobility measurements

Positron mobility measurements carried out on semi-insulating GaAs, using the Doppler shift in annihilation radiation technique, show a sharp transition from a high mobility value ∼120 cm2 V-1 s-1 to a lower value ∼45 V-1 s-1 just below room temperature. The temperature of the transition is found to be dependent on the frequency of the applied AC bias. We show that this effect is an artifact due to the thermal ionization of the EL2 deep donor state, which in its ionized state forms a positive space charge that causes the positron to experience large electric fields. This observation suggests a new positron annihilation-based deep-level transient technique applicable to semi-insulating materials. © 1994 The American Physical Society.published_or_final_versio

Order of Magnitude Smaller Limit on the Electric Dipole Moment of the Electron

The Standard Model of particle physics is known to be incomplete. Extensions to the Standard Model, such as weak-scale supersymmetry, posit the existence of new particles and interactions that are asymmetric under time reversal (T) and nearly always predict a small yet potentially measurable electron electric dipole moment (EDM), d_e, in the range of 10^(−27) to 10^(−30) e·cm. The EDM is an asymmetric charge distribution along the electron spin (S) that is also asymmetric under T. Using the polar molecule thorium monoxide, we measured d_e = (–2.1±3.7_(stat)±2.5_(syst)) × 10−29 e·cm. This corresponds to an upper limit of ❘d_e❘ < 8.7 × 10^(−29) e·cm with 90% confidence, an order of magnitude improvement in sensitivity relative to the previous best limit. Our result constrains T-violating physics at the TeV energy scale