The Center for Materials Physics and Technology performs basic and applied research on functional, structural, biological, and electronic material systems. Research includes the study of the fundamental physics and properties of materials and systems across wide ranges of length and time scales. The Center pioneers new methods for studying these systems including original experimental techniques for the development of electronic devices, as well as the development of new computational methods for modeling systems. The Center develops innovative scientific and engineering solutions for systems ranging from the atomic scale through the macroscopic, and from basic physics through the prototyping of devices for naval applications.
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Recent Publications
2015

Structure and Stoichiometry in Supervalent Doped Li7La3Zr2O12 

Chemistry of Materials 
2015 
Show abstract »The oxide garnet material L(i)7La(3)Zr(2)O(12) shows remarkably high ionic conductivity when doped with supervalent ions that are charge compensated by Li vacancies and is currently one of the best candidates for development of a technologically relevant solid electrolyte. Determination of optimal dopant concentration, however, has remained a persistent problem due to the extreme difficulty of establishing the actual (as compared to nominal) stoichiometry of intentionally doped materials and by the fact that it is still not entirely clear what level of lattice expansion/contraction best promotes. ionic diffusion. By combining careful synthesis, neutron diffraction, highresolution Xray diffraction (XRD), Raman measurements, and density functional theory calculations, we show that structure and stoichiometry are intimately related such that the former can in many cases be used as a gauge of the latter. We show that different Livacancy creating supervalent ions (Al3+ vs Ta5+) affect the structure very differently, both in terms of the lattice constant, which is easily measurable, and hi terms of the local structure, which can be difficult or impossible to access experimentally but may have important ramifications for conduction. We carefully correlate the lattice constant to dopant type/concentration via Vegard's law and then further correlate these quantities to relevant local structural parameters. Our work opens the possibility of developing a codopant scheme that optimizes the Li vacancy concentration and the lattice size simultaneously.

Temperature and MagneticField Dependence of Radiative Decay in Colloidal Germanium Quantum Dots 

Nano Letters 
2015 
Show abstract »We conduct spectroscopic and theoretical studies of photoluminescence (PL) from Ge quantum dots (QDs) fabricated via colloidal synthesis. The dynamics of latetime PL exhibit a pronounced dependence on temperature and applied magnetic field, which can be explained by radiative decay involving two closely spaced, slowly emitting exciton states. In 3.5 nm QDs, these states are separated by ~ 1 meV and are characterized by ~ 82 mu s and ~ 18 mu s lifetimes. By using a fourband formalism, we calculate the fine structure of the indirect bandedge exciton arising from the electronhole exchange interaction and the Coulomb interaction of the Gpoint hole with the anisotropic charge density of the Gammapoint electron. The calculations suggest that the observed PL dynamics can be explained by phononassisted recombination of excitons thermally distributed between the lowerenergy "dark" state with the momentum projection J = ± 2 and a higher energy "bright" state with J = ± 1. A fairly small difference between lifetimes of these states is due to their mixing induced by the exchange term unique to crystals with a highly symmetric cubic lattice such as Ge.

The Adaptive Buffered Force QM/MM Method in the CP2K and AMBER Software Packages 

Journal of Computational Chemistry 
2015 
Show abstract »The implementation and validation of the adaptive buffered force (AdBF) quantummechanics/molecularmechanics (QM/MM) method in two popular packages, CP2K and AMBER are presented. The implementations build on the existing QM/MM functionality in each code, extending it to allow for redefinition of the QM and MM regions during the simulation and reducing QMMM interface errors by discarding forces near the boundary according to the buffered forcemixing approach. New adaptive thermostats, needed by forcemixing methods, are also implemented. Different variants of the method are benchmarked by simulating the structure of bulk water, water autoprotolysis in the presence of zinc and dimethylphosphate hydrolysis using various semiempirical Hamiltonians and density functional theory as the QM model. It is shown that with suitable parameters, based on force convergence tests, the AdBF QM/MM scheme can provide an accurate approximation of the structure in the dynamical QM region matching the corresponding fully QM simulations, as well as reproducing the correct energetics in all cases. Adaptive unbuffered forcemixing and adaptive conventional QM/MM methods also provide reasonable results for some systems, but are more likely to suffer from instabilities and inaccuracies. (c) 2015 Wiley Periodicals, Inc.

Unconventional Fermi Surface in an Insulating State 

Science 
2015 
Show abstract »Insulators occur in more than one guise; a recent finding was a class of topological insulators, which host a conducting surface juxtaposed with an insulating bulk. Here, we report the observation of an unusual insulating state with an electrically insulating bulk that simultaneously yields bulk quantum oscillations with characteristics of an unconventional Fermi liquid. We present quantum oscillation measurements of magnetic torque in highpurity single crystals of the Kondo insulator SmB6, which reveal quantum oscillation frequencies characteristic of a large threedimensional conduction electron Fermi surface ~ the metallic rare earth hexaborides such as PrB6 and LaB6. The quantum oscillation amplitude strongly increases at low temperatures, appearing strikingly at variance with conventional metallic behavior.