M. Budde

Si-H stretch modes of hydrogen-vacancy defects in silicon

Abstract The Si H stretch modes in proton-implanted crystalline silicon have been studied by FTIR spectroscopy. From the annealing behaviour and dose dependence of the stretch-mode intensities, it is found that one complex gives rise to modes at 2121 and 2144 cm −1 , a second to modes at 2166 and 2191 cm −1 , and a third to a single mode at 2222 cm −1 . The isotope shifts of these centres have been obtained from measurements on samples implanted with protons and deuterons. Moreover, the symmetries of the centres have been determined from uniaxial stress measurements. We assign the 2121 and 2144 cm −1 modes to VH 2 , the 2166 and 2191 cm −1 modes to VH 3 , and the 2222 cm −1 mode to VH 4 . Another mode at 2068 cm −1 is tentatively ascribed to VH. The Si H and Si D stretch modes of VH n D m ( n + m ≤ 4) have been calculated with a simple model which has been used to fit all the experimental frequencies. The agreement between fitted and observed frequcies is excellent.

Vacancy-Hydrogen Complexes in Germanium

Local-density-functional pseudopotential theory is used to investigate the structural, electronic and vibrational properties of vacancy-hydrogen complexes in germanium. The results are compared with recent infrared absorption data from proton and deuteron implanted Ge. The acceptor and donor levels of the VHn defects are derived semi-empirically from the relaxed structures.

Mixing of ultracold atomic clouds by merging of two magnetic traps

We demonstrate a method to make mixtures of ultracold atoms that does not make use of a two-species magneto-optical trap. We prepare two clouds of 87Rb atoms in distinct magnetic quadrupole traps and mix the two clouds by merging the traps. For correctly chosen parameters the mixing can be done essentially without loss of atoms and with only minor heating. The basic features of the process can be accounted for by a classical simulation of particle trajectories. Such calculations indicate that mixing of different mass species is also feasible, opening the way for using the method as a starting point for making quantum gas mixtures.