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Ion conduction is of prime importance for solid-state reactions in ionic systems, and for devices such as high-temperature batteries and fuel cells, chemical filters and sensors. Ionic conductivity in solid electrolytes can be improved by dissolving appropriate impurities into the structure or by introducing interfaces that cause the redistribution of ions in the space-charge regions. Heterojunctions in two-phase systems should be particularly efficient at improving ionic conduction, and a qualitatively different conductivity behaviour is expected when interface spacing is comparable to or smaller than the width of the space-charge regions in comparatively large crystals. Here we report the preparation, by molecular-beam epitaxy, of defined heterolayered films composed of CaF2 and BaF2 that exhibit ionic conductivity (parallel to the interfaces) increasing proportionally with interface density--for interfacial spacing greater than 50 nanometres. The results are in excellent agreement with semi-infinite space-charge calculations, assuming a redistribution of fluoride ions at the interfaces. If the spacing is reduced further, the boundary zones overlap and the predicted mesoscopic size effect is observed. At this point, the single layers lose their individuality and an artificial ionically conducting material with anomalous transport properties is generated. Our results should lead to fundamental insight into ionic contact processes and to tailored ionic conductors of potential relevance for medium-temperature applications.

From the degree of circular polarization of the time-resolved radiative recombination of 2D electrons with photoexcited holes bound to acceptors we have measured the magnetic field dependencies of the electron spin polarization for various fractional (n 2͞3, 3͞5, 4͞7, 2͞5, 3͞7, 4͞9, 8͞5, 4͞3, and 7͞5) and composite fermions (n 1͞2, 1͞4, and 3͞2) states. The Fermi energies of these composite fermion states are measured for the first time and the corresponding value of the composite fermion density of states mass at n 1͞2 is found to be about 4 times heavier than the previously reported values of the "activation" mass. [S0031-9007 (99)09059-6] PACS numbers: 71.10.Pm, 73.40.HmThe existence of unusual new quasiparticles, composite fermions (CFs), assembled from several magnetic flux quanta and an electron, has been demonstrated in various experiments on two-dimensional (2D) electron systems in high magnetic fields [1-3]. These CFs were introduced theoretically [4,5] to explain the fractional quantum Hall effect (FQHE). According to this theory, at half filling of the Landau level (n 1͞2), two flux quanta are attached to an electron to form a CF, which moves in zero effective magnetic field, since external field is compensated by attached fluxes. The CF concentration is equal to that of 2D electrons and the system of these quasiparticles can be characterized by a Fermi wave vector and a Fermi energy. Any deviation of the magnetic field from exactly half filling of the Landau level results in the appearance of an effective magnetic field, which quantizes the CF motion and splits their energy into Landau levels. Every integer filling of the CF Landau levels corresponds to a specific FQHE state. The recent experiments [6-8] not only supported the validity of this theoretical concept but demonstrated also the semiclassical behavior of these strange quasiparticles.

We define two laterally gated small quantum dots with less than 15 electrons in an Aharonov-Bohm geometry in which the coupling between the two dots can be changed. We measure Aharonov-Bohm oscillations for weakly coupled quantum dots. In an intermediate coupling regime we study molecular states of the double dot and extract the magnetic field dependence of the coherently coupled states.

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