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We measure the current due to electrons tunneling through the ground state of hydrogenic Si donors placed in a GaAs quantum well in the presence of a magnetic field tilted at an angle to the plane of the well. The component of B parallel to the direction of current compresses the donor wave function. By measuring the current as a function of the perpendicular component of B, we probe how the magnetocompression affects the spatial form of the wave function and observe directly the transition from Coulombic to magnetic confinement at high fields.

Extending a key observation made by Meyers et al. (Meyers et al., J Chem Phys, 1992, 97, 2750, a strategy for the systematic design of conducting polymers with tailor-made bandgaps and carrier effective masses is described and quantumchemically implemented. Such strategy relies on the construction of alternating binary copolymers from well-characterized parent polymers, in such a manner that those electronic parameters can be phenomenologically predicted from the composition of the copolymer. Illustrative calculations for three types of alternating copolymers built from five parent p-conjugated polymers demonstrate the plausibility of the methodology and the internal consistency of its computational implementation. Specifically, it is shown that the bandgaps of copolymers built from parent monomers with similar chemical structures exhibit nearly linear behaviors as functions of composition, whereas the bandgaps of copolymers with dissimilar parent monomers exhibit nearly monotonic deviations from linearity. On the other hand, the electron and hole effective masses of copolymers with similar parent monomers do not show a significant dependence on composition, whereas for copolymers with dissimilar parent monomers these quantities also display nearly monotonic deviations from linearity. A qualitative rationalization of these trends in terms of the strengths of the inter-parent-monomer interactions, which bears an intriguing resemblance to the behavior of the vapor

A conceptually appealing and computationally economical course-grained molecular-orbital (MO) theory for extended quasi-linear molecular heterostructures is presented. The formalism, which is based on a straightforward adaptation, by including explicitly the vacuum, of the envelope-function approximation widely employed in solid-state physics, leads to a mapping of the three-dimensional single-particle eigenvalue equations into simple one-dimensional hole and electron Schrödinger-like equations with piecewise-constant effective potentials and masses. The eigenfunctions of these equations are envelope MO's in which the short-wavelength oscillations present in the full MO's, associated with the atomistic details of the molecular potential, are smoothed out automatically. The approach is illustrated by calculating the envelope MO's of high-lying occupied and low-lying virtual π states in prototypical nanometric heterostructures constituted by oligomers of polyacetylene and polydiacetylene. Comparison with atomistic electronic-structure calculations reveals that the envelope-MO energies agree very well with the energies of the π MO's and that the envelope MO's describe precisely the long-wavelength variations of the π MO's. This envelope MO theory, which is generalizable to extended systems of any dimensionality, is seen to provide a useful tool for the qualitative interpretation and quantitative prediction of the single-particle quantum states in mesoscopic molecular structures and the design of nanometric molecular devices with tailored energy levels and wavefunctions.PACS numbers: 36.20.Kd, 71.20.Rv,

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