Magnetic doping of semiconductor nanostructures is actively pursued for applications in magnetic memory and spin-based electronics. Central to these efforts is a drive to control the interaction strength between carriers (electrons and holes) and the embedded magnetic atoms. In this respect, colloidal nanocrystal heterostructures provide great flexibility through growth-controlled 'engineering' of electron and hole wavefunctions in individual nanocrystals. Here, we demonstrate a widely tunable magnetic sp-d exchange interaction between electron-hole excitations (excitons) and paramagnetic manganese ions using 'inverted' core-shell nanocrystals composed of Mn(2+)-doped ZnSe cores overcoated with undoped shells of narrower-gap CdSe. Magnetic circular dichroism studies reveal giant Zeeman spin splittings of the band-edge exciton that, surprisingly, are tunable in both magnitude and sign. Effective exciton g-factors are controllably tuned from -200 to +30 solely by increasing the CdSe shell thickness, demonstrating that strong quantum confinement and wavefunction engineering in heterostructured nanocrystal materials can be used to manipulate carrier-Mn(2+) wavefunction overlap and the sp-d exchange parameters themselves.

We demonstrate that not only the static structural properties but also the crystal stability and pressure-induced phase transformations in solids can be accurately described employing an ab initio pseudopotential method within the local density-functional formalism.Using atomic numbers of constituent elements and a subset of crystal structures as the only input information, the calculated structural properties of Si and Ge are in excellent agreement with experiment.-1--2- I. INTRODUCTIONIn this paper, we present an ab initio microscopic study of the static structural properties and other important structural properties including crystal stability and phase transformation of Si and Ge. Part of the results have been 1 previously reported. The method is based on a pseudopotential approach and uses the local density-functional approximation 2 which has also been used in all-electron calculations of static 3 structural studies of metals.We choose Si and Ge as our prototypes since they are the most studied semiconductors experimentally.Both have the (cubic) diamond structure and are found to undergo a semiconductor-4 metal phase transformation under pressure.Using the xray diffraction technique, the transformed phases have been and 100 kbar for Ge. 9In addition to the diamond and B-tin phases, a hexagonal diamond phase has been made lO for Si at room temperature and atmospheric pressure using a sequence of high-pressure and high-temperature treatments. This phase is semiconducting and has the same density as the (cubic) diamond phase. The Since this form has not been found in nature and no large crystals have been prepared,10 it is ~etastable with respect to the diamond phase. A similar structural form has not been found in Ge.There are other metastable phases of Si and Ge 10 11 (a bcc form,with 16 atoms per unit cell for Si and Ge and ,-.. 12 a tetra gonal form with 12 atoms per unit cell for Ge )i these will not be considered in the present study.There are interesting relations between the general phase transformation in semiconductors and other crystalline properties.Jamieson has related 5 the transition pressure (P t ) and the ;..../ atomic volume change (~V) in the phase transformation to the fundamental 'energy gap (E g ), and he ,obtained an empirical rule -of P t ~V = Eg/2 for Group IV elements and iso-row III-V compounds. Although this rule is less accurate when later refined experimental data is considered, the trend is still correct. This is consistent with the physical picture that the bigger the energy gap is, the more stabilized the structure is. Phillips13 has suggested that ionicity may be an important parameter in characterizing the phase transformation. He noted that the rocksalt structure becomes more favorable as the highpressure phase with increasing ionicity. The covalent counterpart ' ;:. of the rocksalt structure, that is, the simple cubic structure, is included in the present study.Pressure-induced phase transformations in tetrahedrally coordinated semiconductors have previously been ...

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