van Pj Piet Hall

Scattering of 20.4 MeV polarized protons from even-mass zinc isotopes

Measurements are reported on polarized-proton scattering from the even-mass zinc isotopes at 20.4 MeV beam energy. The elastic and first-order inelastic processes are represented well by standard theory. The analysis of the two-phonon multiplets presents difficulties which may indicate a change in the nuclear structure between isotopes or the occurrence of an energy-dependent interference process.

Polarized-proton scattering by even-mass germanium and selenium isotopes at 22 MeV incident energy

Elastic and inelastic scattering of 22.3 MeV polarized protons from even-mass nuclides 70-76Ge and 76-80Se has been studied. The elastic data have been analysed by optical-model techniques; also the elastic and inelastic data together have been investigated using a coupled-channels formalism. In the latter case the original optical-model parameters had to be modified in order to obtain an adequate description of the elastic scattering process. The 21+ and 31- states are described satisfactorily by the coupled channels formalism in a vibrational model. This model represents the 02+ states better than expected, and the 22+ and 41+ states only if the 2+ and 4+ levels are treated as mixtures of one-phonon and two-phonon components. Alternatively, a rotational description of the 21+, 22+ and 41+ states was moderately successful. Some features may be understood qualitatively from the interacting-boson approximation (IBA-2).

The (p,d) reaction on 56Fe at 24.6 MeV

The authors have studied the (p,d) reaction on 56Fe, initiated by a 24.6 MeV polarised proton beam. Cross sections and analysing powers were obtained for nine states in the final nucleus 55Fe. These were compared with DWBA and CRC calculations, confirming spin and parity assignments. As opposed to the situation in 58Ni(p,d) no large two-step contributions were necessary for obtaining agreement with calculations.

Time-resolved optical second harmonic generation study of the electric field dynamics at a metal-semiconductor interface

The time evolution of the electric field at a Schottky barrier interface due to above bandgap excitation is directly measured using time-resolved optical second harmonic generation (SHG). The results are found to be in good qualitative agreement with Monte Carlo simulations. Sub-picosecond decay times are observed that are dependent on the excitation powers.

Coherent plasmon-surface phonon oscillations in a GaAs Schottky barrier sample

Excitation of a biased GaAs-Au Schottky barrier sample by intense fs laser pulses leads to coherent oscillations that are probed by time-resolved second harmonic generation experiments. The observed oscillations agree very well with the results of MC simulations and appear to be plasma oscillations modified by surface phonons.

The (p,d) reaction on 58Ni and 56Fe at 24.6 MeV

The (p,t) reaction on 56Fe and 58Ni initiated by 24.6 MeV polarised protons has been investigated. The reaction process is described in terms of simultaneous and sequential neutron transfer. For all five final states studied the sequential process competes strongly with simultaneous transfer of a neutron pair. Theoretical values for the spectroscopic amplitudes were derived from shell-model wavefunctions; these seem to give a reliable description of the reaction process.

Coherent plasmon-phonon oscillations in a GaAs schottky barrier sample

We have observed coherent plasmon–phonon oscillations in a biased GaAs–Au Schottky barrier sample excited by intense fs laser pulses using time resolved Second Harmonic Generation. The results are compared with Monte-Carlo simulations. To account for the observed dependence of the frequency on the excitation density it was necessary to separate the electric field in two components: one due to the space charge and the other following from the lattice polarization. In this way a coupling between the plasmon and the LO phonons is obtained, which resulted in a nearly perfect description of the experimentally observed frequencies.

Low field Monte-Carlo calculations in heterojunctions and quantum wells

We present results of low-field Monte-Carlo calculations and compare them with experimental results. The negative absolute mobility of minority electrons in p-type quantum wells, as found in recent experiments, is described quite well.

Time-of-flight experiments on 2D electron gases

Transport of electrons in GaAs/AlGaAs modulation-doped heterostructures is studied by means of the Time-of-Flight technique. By applying a constant background illumination, we are able to distinguish between lateral transport processes and effects of perpendicular transport across the GaAs/AlGaAs interface. It is shown that the initial part of the photocurrent signal is due to lateral transport only. This part of the signal is further investigated as a function of sample length and applied electric field.