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This is a copy of the slides presented at the meeting but not formally written up for the volume.
Description: Semiconductor physics contains a rich body of theory and working designs. However, their material properties seem to be reaching their limits. Perovskite oxides on the other hand have abundant physical properties, but are still under active investigation. The advent of RHEED-monitoring of pulsed laser deposition allows for the fabrication of structures with single unit cell (4 Å) thick layers. In this way we may be able to fabricate quantum well structures for both applications and fundamental investigations. Superlattices of the Mott insulator LaTiO3 (LTO) and the band gap insulator SrTiO3 (STO) form such a structure. The superlattices are metallic, both as-grown and post-annealed . This has been attributed to the existence of metallic states at the interfaces between LTO and STO . At these interfaces the electron density is found to extend about 10 Å into the STO. However, theoretically, the required length scale for quantum confinement is of the order of 4 Å. A possible way to increase this confinement is to use a buffer material with a larger band gap than that of LTO (similar to semiconductor band gap engineering) and/or with a lower dielectric constant . LaAlO3 (LAO) is such a material (ΔELAO = 5.6 eV vs. ΔESTO = 3.2 eV, εLAO = 24 vs. εSTO = 300). Here we report on the growth of LTO/LAO superlattices on STO substrates. As-grown superlattices of LTO/LAO are metallic, while post-annealing turns them insulating. This may be explained from a disorder-order transition in a 2D Mott-Hubbard model . XPS and EELS measurements of the titanium valence show interesting differences for LTO layers close to and far away from the sample surface. The former, for thin LAO capping layers, show the presence of Ti4+ while the latter only have Ti3+. Hard XPS of samples with varying capping layer thickness shows an exponential dependence of the Ti3+ contents on a length scale of about 5 unit cells.  A. Ohtomo et al., Nature 419, 378-380 (2002).  S. Okamoto & A.J. Millis, Phys. Rev. B 70, 075101 (2004).  D. Heidarian & N. Trivedi, Phys. Rev. Lett. 93, 126401 (2004).
Besides the importance of the actinide dioxide series as a nuclear fuel, the magnetic properties of these compounds at low temperatures are particularly interesting. Their surprisingly varied physical properties at low temperatures stimulate continuing interest for both theory and experiment. Recently, we have performed 17O-NMR studies for the first time on Pu and Amcontaining dioxide systems, (Pu1-xAmx)O2. For the x=0.09 sample, a temperature-dependent NMR line broadening has been observed at low temperatures. By comparing the experimental data with the results of NMR line simulations, we have estimated the effective moment of Am ions to be Peff=1.38 μB. The value suggests the 5f5 (Am4+) state of the Am ion in PuO2. For the x=1 (=AmO2) sample, on the other hand, our 17O-NMR data provide the first microscopic evidence for a phase transition at 8.5 K as a bulk property in this system. A spectrum with a triangular line shape indicates that the internal field is distributed very nearly randomly in the ordered state.
The use of polyimides in integrated circuits to planarize complex topography and the need to decrease the dimensions of electronic devices have motivated us to gain a better understanding of the details of the polymer microenvironment. UV/Vis absorption experiments suggest that intramolecular charge transfer (ICT) exists and therefore might utilized as intrinsic, nonperturbing probes. The transfer of charge is believed to occur upon exposure of ultraviolet light as a result of donation from the phenyl ring (from the diamine fragment) to the adjacent imide ring. Furthermore, the transfer of charge should be a function of the torsional angle between the planes of the donor and acceptor segments.
In this study, we first investigate the nature of the ICT by conducting UV/Vis absorption measurements on a series of model compounds. A comparative analysis of 10−5 M solutions of the models in dioxane provides evidence that intramolecular charge transfer occurs when the compounds are exposed to 264 nm (ultraviolet) light. Similarly, a comparison of an analogous series of polyimides demonstrates that the same phenomenon occurs in the more complex polymeric systems when spin-cast films are exposed to 326 nm (ultraviolet) light
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