Covalency and Low Dimensionality in Superconducting Pairing

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This award supports theoretical research in condensed matter physics. Discovery or synthesis of new materials with unexpected and fascinating properties is a continuing, perhaps accelerating, process exemplified by the discovery in 2001 of superconductivity in MgB2 at a critical temperature of 40K. Superconductivity at almost twice the critical temperature of other intermetallics, in a simple anisotropic compound without d-electrons broke all the known rules for intermetallic superconductors. Such discoveries attract great interest amongst materials physicists, and invariably strongly influence subsequent developments in materials physics, both intellectual and practical.This project focuses on gaining a deep understanding and computational theory of this new class of superconductors, for which covalency and low dimensionality play a critical role. This projected class contains, in addition to MgB2: the closely related Li(1-x)BC system (predicted critical temperature well above 40K); electron-doped chloronitrides typified by Na(x)HfNCl (Tc=25K); the carbon dumbbell system Y2C2Br2 (Tc up to 12K) and a newly revived C2 dumbbell material yttrium sesquicarbide Y2C3 (Tc = 18K); and finally elemental Li under high pressure (Tc = 20K). These systems display strong covalency, two-dimensionality, or both. Research interactions in this area of research are strongly interdisciplinary between condensed matter physicists, solid state chemists, and materials scientists.The multi-pronged research plan includes (1) first principles density-functional based linear response theory, to obtain bonding characteristics, phonon frequencies, electron-phonon matrix elements, and finally Tc itself; (2) first principles molecular dynamics simulations, to characterize the dynamics of ions in situations of extremely strong coupling; (3) diagrammatic techniques to evaluate perturbative corrections to conventional Migdal-Eliashberg theory for interacting electron-phonon systems; and (4) construction of effective thermal distribution functions in systems with strong anisotropy. The objective of this project is to obtain a unified microscopic understanding, based on first principles (parameter-free) computational methods, that is detailed enough to provide not only an understanding of, but also a predictive capability for, the observed behavior.The wide range of materials physics issues that are involved here range from fundamental electronic structure theory, to large-scale dynamical simulations, to interaction and transfer of energy between the electronic and lattice degrees of freedom, to the diagrammatic perturbation theory of the interaction. The combination of these issues, together with close interactions with experimentalists doing synthesis and characterization, make this an ideal training ground for broadly educated computational physicists. The materials research atmosphere at UC-Davis generally, and in the Physics Department in particular, allow students and postdocs alike to become immersed in exciting materials research in preparation for careers in academia, research laboratories and industry. Undergraduates will also participate in the research.%%%This award supports theoretical research in condensed matter physics. Discovery or synthesis of new materials with unexpected and fascinating properties is a continuing, perhaps accelerating, process exemplified by the discovery in 2001 of superconductivity in magnesium di-boride at a critical temperature of 40K. Superconductivity at almost twice the critical temperature of other intermetallic compounds, in a simple anisotropic compound broke all the known rules for intermetallic superconductors. Such discoveries attract great interest amongst materials physicists, and invariably strongly influence subsequent developments in materials physics, both intellectual and practical.The wide range of materials physics issues that are involved here range from fundamental electronic structure theory, to large-scale dynamical simulations, to interaction and transfer of energy between the electronic and lattice degrees of freedom, to the diagrammatic perturbation theory of the interaction. The combination of these issues, together with close interactions with experimentalists doing synthesis and characterization, make this an ideal training ground for broadly educated computational physicists. The materials research atmosphere at UC-Davis generally, and in the Physics Department in particular, allow students and postdocs alike to become immersed in exciting materials research in preparation for careers in academia, research laboratories and industry. Undergraduates will also participate in the research.***