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Hybrid structures synthesized from di erent materials have attracted considerable attention because they may allow not only combination of the functionalities of the individual constituents but also mutual control of their properties. To obtain such a control an interaction between the components needs to be established. For coupling the magnetic properties, an exchange interaction has to be implemented which typically depends on wavefunction overlap and is therefore short-ranged, so that it may be compromised across the hybrid interface. Here we study a hybrid structure consisting of a ferromagnetic Co layer and a semiconducting CdTe quantum well, separated by a thin (Cd,Mg)Te barrier. In contrast to the expected p-d exchange that decreases exponentially with the wavefunction overlap of quantum well holes and magnetic atoms, we find a long-ranged, robust coupling that does not vary with barrier width up to more than 30 nm. We suggest that the resulting spin polarization of acceptor-bound holes is induced by an e ective p-d exchange that is mediated by elliptically polarized phonons. E xchange interactions are the origin for correlated magnetism in condensed matter with multi-faceted behaviour such as ferro-, antiferro-or ferrimagnetism. In magnetic semiconductors (SCs), the exchange occurs between free charge carriers and localized magnetic atoms 1-4 and is determined by their wavefunction overlap. To control this overlap, hybrid structures consisting of a ferromagnetic (FM) layer and a semiconductor quantum well (QW) are appealing objects because they allow wavefunction engineering. Furthermore, the mobility of QW carriers will not be reduced by inclusion of magnetic ions in the same spatial region in these systems.More specifically, for a two-dimensional hole gas (2DHG, the p-system) in a QW the overlap of the hole wavefunction with the magnetic atoms in a nearby ferromagnetic layer (the d-system) is believed to result in p-d exchange interaction 5-8 . This exchange interaction may cause strong coupling between the SC and FM spin systems 9 , through which the ferromagnetism of the unified system, as evidenced by its hysteresis loop, can be tuned. In particular, the 2DHG spin system becomes polarized in the effective magnetic field from the p-d exchange 5,8 . Recently 10 , it was shown that in addition to this equilibrium 2DHG polarization there is an alternative mechanism involving spin-dependent capture of carriers from the SC into the FM. For ferromagnetic (Ga,Mn)As on top of an (In,Ga)As QW, electron capture induces electron spin polarization in the QW, representing a dynamical effect in contrast to the exchange-induced equilibrium polarization.Here we study a different FM/QW hybrid, consisting of a Co layer and a CdTe II-VI semiconductor QW, separated by a nanometrethick barrier. Owing to the negligible hole tunnelling through the barrier, this hybrid combination shows mostly a quasi-equilibrium proximity effect due to p-d exchange interaction between magnetic atoms and holes in the QW. Surprisingly, howev...

We have measured ultraviolet photoemission spectra of quasi-size-selected silver clusters at T 40 K grown in the nanopits of a graphite surface. The Fermi-level onset observed shows distinct deviations from the steplike shape typical for metals. A simple model that takes into account the finite lifetime of the photohole (corresponding to a charged cluster) can explain these deviations by the clustersubstrate interaction on a femtosecond time scale, and hence provide an explanation for the differences between photoemission spectra of free and deposited clusters in general. [S0031-9007(98)07729-1]

We have performed scanning tunneling microscopy ͑STM͒ in ultrahigh vacuum and transmission electron microscopy ͑TEM͒ on silver and gold clusters grown in preformed nanometer-sized pits on the surface of highly oriented pyrolytic graphite. We describe the preparation method and evaluate the three-dimensional shape of the clusters using a combination of STM and TEM applied to the same cluster sample. The nanometer-sized pits were essential to fix the clusters in position when using STM. The influence of the tip shape on the STM imaging of nanometer-sized clusters is discussed.

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Metal nanoparticles supported by thin films are important in the fields of molecular electronics, biotechnology and catalysis, among others. Penetration of these nanoparticles through their supporting films can be undesirable in some circumstances but desirable in others, and is often considered to be a diffusive process. Here, we demonstrate a mechanism for the penetration of thin films and other nanoscopic barriers that is different from simple diffusion. Silver clusters that are soft-landed onto a monolayer of C(60) supported by gold sink through the monolayer in a matter of hours. However, the clusters are stable when landed onto two monolayers of C(60) supported on gold, or on one monolayer of C(60) supported on graphite. With backing from atomistic calculations, these results demonstrate that a metallic substrate exerts attractive forces on metallic nanoparticles that are separated from the substrate by a single monolayer.

The energetic position of the Au(111) Shockley surface state is compared before and after adsorbing different rare gas monolayers (Ar, Kr, and Xe). We used ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS) in combination to get more complete information by using the advantages of both methods. For determining the energetic position and the effective mass of the surface state in UPS an analytic mathematical method is used, which takes the finite angular resolution of the analyzer into account. We performed STS scans for the pure Au(111) surface as well as covered with a monolayer Kr and Xe. For an accurate analysis it is possible to use an extended Kronig-Penney model to take into account the influence of the 23ϫ ͱ 3 reconstruction. We found that the first monolayer of a rare gas induces shifts of around 50-150 meV increasing with the gas atomic number, whereas a second monolayer has only a small influence of about 3-18 meV. Using an image potential model it is possible to characterize these shifts qualitatively. For a semiquantitative analysis the phase accumulation model is applied. Within this model we can describe the experimental data roughly with a Coulomb potential changing in dependence of the electron affinity and the dielectric constant of the rare gas.

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