Materials World Network on Rare-Earth and Transition-Metal Nitride Spectroscopic Studies

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An investigation on rare-earth and transition metal nitrides is carried out by a US-New Zealand team. The collaboration consists of a synchrotron spectroscopy group led by Kevin Smith at Boston University and a computational theory group led by Walter R. L. Lambrecht at Case Western Reserve University, with a thin film growth and optical spectroscopies group led by Joe Trodahl and Ben Ruck at the MacDiarmid Institute for Advanced Materials and Nanotechnology at Victoria University in Wellington, New Zealand. The rare-earth nitrides form a closely related family of magnetic semi-conducting or semimetallic materials, exhibiting interesting properties because of the strongly correlated 4f electrons. An array of different spectroscopic techniques, including X-ray emission (XES) and absorption (XAS) spectroscopy, Resonant Inelastic X-ray scattering (RIXS), and Angular Resolved Photoemission Spectroscopy (ARPES) are used to examine films, the latter capped with protective layers or grown in-situ at the beamline. Calculations are carried out of the electronic structure using the LSDA+U approach (Local spin-density approximation complemented with orbital dependent Coulomb and exchange interactions) and implemented in the full-potential linear muffin-tin orbital method. Theoretical analysis of the spectra will identify the need for theoretical developments beyond the mean-field approximation embodied by current LSDA+U. The calculations also tie the electronic structure to magnetic properties by calculation of the magnetic exchange parameters. Magnetic susceptibility measurements as function of temperature are carried out by the NZ group. Selected transition-metal nitrides, which exhibit interesting correlation and magnetic effects arising from the d-electrons are also investigated.Besides their fundamental interest, the magnetic semiconductor and semi-metallic materials may find applications in spintronic technology. This research provides insights into the doping of rare-earth and transition metals in other semiconductors, in particular GaN, or the formation of hybrid structures with such semiconductors for spin-injection and other spin-dependent electronics. The project supports several graduate students, who receive training in an interdisciplinary and international context, including collaborations between spectroscopy and growth issues and theory and experiment. The effort also impacts undergraduate students through the researchers? involvement with undergraduate advising and teaching and their plans to incorporate the research of this proposal in their courses and outreach activities. The international collaboration is anticipated to foster new collaborative research projects. This award is co-funded by the Office of International Science and Engineering.