Shape transition from InAs quantum dash to quantum dot on InP(3 1 1)A

We report on the shape transition from InAs quantum dashes to quantum dots (QDs) on lattice-matched GaInAsP on InP(3 1 1)A substrates. InAs quantum dashes develop during chemical-beam epitaxy of 3.2 monolayers InAs, which transform into round InAs QDs by introducing a growth interruption without arsenic flux after InAs deposition. The shape transition is solely attributed to surface properties, i.e., increase of the surface energy and symmetry under arsenic deficient conditions. The round QD shape is maintained during subsequent GaInAsP overgrowth because the reversed shape transition from dot to dash is kinetically hindered by the decreased ad-atom diffusion under arsenic flux.     

Charge manipulation and imaging of the Mn acceptor state in GaAs by cross-sectional scanning tunneling microscopy

An individual Mn acceptor in GaAs is mapped by cross-sectional scanning tunneling microscopy (X-STM) at room temperature and a strongly anisotropic shape of the acceptor state is observed. An acceptor state manifests itself as a cross-like feature which we attribute to a valence hole weakly bound to the Mn ion forming the (Mn²⁺3d⁵+hole) complex. We propose that the observed anisotropy of the Mn acceptor wavefunction is due to the d-wave present in the acceptor ground state.     

The Peierls transition of Perylene radical cation salts with 2:1 stoichiometry containing tetrahydrofuran

Crystal structure, microwave conductivity and static magnetic susceptibility were analyzed for the conducting perylene (PE) radical cation salts of nominal composition (PE)₂(PF₆)_1–x (AsF₆) _x ·2/3 THF for 0x1. Crystallographic phase transitions in the high-temperature metal phase were also characterized by differential scanning calorimetry and electron spin resonance. The Peierls transition occuring at 100–120 K (depending onx) and the opening of the energy gap in the semiconductor phase were determined from the magnetic data. The dependence of these properties on the size of the complex anions and the packing of the PE molecules is discussed.     

A novel solid support for the synthesis of 3-aminoalkylated oligonucleotides

A novel improved controlled pore glass (CPG) support based on the 2-(hydroxymethyl)-6-nitrobenzoyl (HMNB) protecting group was developed for the synthesis of 3^′-aminoalkylated oligonucleotides. The release of oligonucleotides with free 3^′-amino groups from the support is complete within 2 h at 55 °C in concentrated ammonia.     

Asymmetry of velocity distributions in peripheral reactions with heavy ions at Fermi energies

The asymmetry of velocity distributions of projectile-like fragments produced in heavy ion collisions is considered. The calculations performed in the transport model approach (Vlasov kinetic equation with the collision term) are compared with the experimental data for the ²²Ne (40MeV/nucleon) + ⁹Be and ¹⁸O (35 MeV/nucleon) + ⁹Be(¹⁸¹Ta) reactions. It is found that the velocity distributions contain two components: a direct component centered at the beam velocity and a dissipative component at lower energies, leading to asymmetry of velocity distributions. The direct component is interpreted empirically within the Goldhaber model, and the centroids and widths σ₀ of the distributions for each fragment are extracted. It is found that value of σ₀ derived from experimental data is smaller by a factor of 2 than the theoretical one. The dissipative (also called deep inelastic) component is described well by the transport calculations. It is shown that the ratio of yields of direct and dissipative components, which determines the asymmetry of velocity distributions, depends on shape of the deflection function. Original Russian Text ? T.I. Mikhailova, B. Erdemchimeg, G. Kaminski, A.G. Artyukh, M. Colonna, M. Di Toro, I.N. Mikhailov, Yu.M. Sereda, H.H. Wolter, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 6, pp. 898–903.     

Real-space measurement of the potential distribution inside organic semiconductors

We demonstrate that the soft nature of organic semiconductors can be exploited to directly measure the potential distribution inside such an organic layer by scanning-tunneling microscope (STM) based spectroscopy. Keeping the STM feedback system active while reducing the tip-sample bias forces the tip to penetrate the organic layer. From an analysis of the injection and bulk transport processes it follows that the tip height versus bias trace obtained in this way directly reflects the potential distribution in the organic layer.     

Quantum beats in absorption

Modulations in the absorption of quasi-monochromatic light from atoms prepared in a coherent superposition of atomic states are derived semiclassically. Explicit expressions are given for the case of fine or hyperfine levels and for the photoionization of Rydberg states.