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

来自亚利桑那大学的Sumitendra Mazumdar得到了化学部的化学理论，模型和计算方法计划以及材料研究部的凝聚态物质和材料理论计划的支持，该计划获得了理论和计算研究和教育奖（1）碳基半导体上的电子相关效应，以及（2）层状有机电荷转移固体和钴酸盐。在（1）中，主要研究者（PI）旨在了解（a）碳基半导体中的自旋三重态激发，（B）确定它们在光诱导电荷转移中的作用（如果有的话），以及（c）确定石墨烯分子碎片中单光子态与双光子态的相对顺序。感兴趣的碳基半导体包括其中三重态在能量上非常接近光学单重态激子的那些，例如宽的单壁碳纳米管和石墨烯纳米带;以及其中三重态显著低于光学单重态激子并且因此是单重态裂变的候选者的系统。在（B）中，目标是确定涉及自旋三重态的光诱导电荷转移是否给出电荷载流子的提高的产率。石墨烯分子碎片中的激发态有序将在（c）中进行理论研究。在最低单光子光学状态之下的最低双光子状态的出现将指示石墨烯中的强电子-电子相互作用。在（2）中，PI将从理论上研究（a）有机电荷转移固体中接近超导性的半导体态的对称性破缺，以及（B）层状钴酸盐。PI在层状电荷转移固体的相关电子描述中提出了一个配对电子晶体（PEC）相，用于适度的晶格挫折，并建议电荷转移固体中的量子自旋液体行为是由于过渡到PEC。将研究PEC的激发以了解电荷转移固体的热力学行为。在（B）中，PI将研究层状钴酸盐，其中钴离子形成各向同性三角形晶格，并且载流子浓度可以在宽范围内调节。具有大（小）载流子浓度的钴酸盐表现出与弱（强）相关性相关的行为。PI将对有限三角形晶格团簇进行数值计算，以了解这种不寻常的载流子浓度依赖的电子行为。该奖项支持理论和计算研究和教育，旨在了解（1）光与碳基半导体和纳米结构的相互作用，以及（2）强电子相关性和层状有机和无机材料的晶格挫折的后果。电子之间的强相互作用导致上述材料中不寻常的光学性质和奇异相。PI使用计算机模拟和理论来获得微观层面的洞察力。在过去的二十年里，碳基半导体已经从实验室的好奇心发展到新的光学材料，而PI研究的层状有机材料和钴酸盐与高温氧化铜基超导体有很强的相似性。该奖项所支持的工作将为基于纳米光电子学的新技术的出现奠定知识基础，并为理解非常规超导性提供理论基础。拟议的理论研究将探讨与碳基半导体器件物理高度相关的问题。研究生和从事该项目的博士后研究员将获得物理，化学和光学界面的经验。本科生将参与研究。在代表性不足的社区开发科学潜力是该计划的目标之一。PI在亚利桑那大学和西南部的几个少数民族服务机构之间建立了一个桥梁计划，让代表性不足的美国少数民族学生参加夏季研究。一个桥梁的学生将在PI的组每年夏天工作。PI拥有长期的国际合作记录。这些合作将继续下去。

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.