Electron correlation effects on excited state behavior of carbon-based semiconductors.

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Sumitendra Mazumdar from the University of Arizona is supported by the Chemical Theory, Models and Computational Methods program of the Chemistry Division and the Condensed Matter and Materials Theory program of the Division of Materials Research in an award for theoretical and computational research and education on (1) electron correlation effects on carbon-based semiconductors, and (2) layered organic charge-transfer solids and cobaltates. In (1), the principal investigator (PI) aims to understand (a) spin-triplet excitations in carbon-based semiconductors, (b) determine their roles in photoinduced charge-transfer, if any, and (c) determine the relative ordering of one- versus two-photon states in graphene molecular fragments. Carbon-based semiconductors of interest include those in which the triplet is very close in energy to the optical singlet exciton, such as wide single-walled carbon nanotubes and graphene nanoribbons; and systems in which the triplet is considerably below the optical singlet exciton and which are therefore candidates for singlet fission. In (b), the goal is to determine whether photoinduced charge-transfer involving spin-triplets gives enhanced yields of charge carriers. Excited state ordering in graphene molecular fragments will be investigated theoretically in (c). Occurrence of the lowest two-photon state below the lowest one-photon optical state will indicate strong electron-electron interactions in graphene. In (2) the PI will theoretically investigate (a) broken symmetries in the semiconducting states proximate to superconductivity in organic charge-transfer solids, and (b) layered cobaltates. The PI has proposed a paired-electron crystal (PEC) phase within the correlated-electron description of layered charge-transfer solids for moderate lattice frustration, and has suggested that the quantum spin liquid like behavior in charge-transfer solids is due to transition to the PEC. Excitations of the PEC will be investigated to understand the thermodynamic behavior of charge-transfer solids. In (b) the PI will investigate layered cobaltates in which the cobalt ions form an isotropic triangular lattice and the carrier-concentration can be tuned over a wide range. Cobaltates with large (small) carrier concentration exhibit behavior associated with weak (strong) correlations. The PI will perform numerical calculations on finite triangular lattice clusters to understand this unusual carrier concentration-dependent electronic behavior.This award supports theoretical and computational research and education seeking to understand (1) the interaction of light with carbon-based semiconductors and nanostructures, and (2) the consequences of strong electron correlations and lattice frustration on layered organic and inorganic materials. Strong interactions between electrons lead to unusual optical properties and exotic phases in the above materials. The PI uses computer simulations and theory to gain insight at a microscopic level. During the past two decades carbon-based semiconductors have evolved from laboratory curiosities to new optical materials, while the layered organic materials and cobaltates to be investigated by the PI have strong similarities with the high temperature copper oxide based superconductors. The work supported by this award will contribute to the intellectual foundation from which new technologies based on nanoscale optoelectronics will emerge, and also provide a theoretical basis for understanding unconventional superconductivity. The proposed theoretical research will probe issues that are highly relevant for the device physics of carbon-based semiconductors. Graduate students and the postdoctoral fellow working on the project will gain experience at the interface of physics, chemistry and optics. Undergraduate students will be involved in the research. Developing scientific potential within underrepresented communities is one goal of this program. The PI has established a Bridge Program between the University of Arizona and several Minority Serving Institutions in the southwest to expose underrepresented U.S. minority students to summer research. One Bridge student will work in the PI's group each summer. The PI has a long track record of international collaborations. These collaborations will be continued.