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

来自亚利桑那大学的Sumitendra Mazumdar获得了化学部化学理论、模型和计算方法计划以及材料研究部凝聚态物质和材料理论计划的支持，该奖项旨在表彰(1)碳基半导体的电子关联效应，以及(2)层状有机电荷转移固体和钴盐的理论和计算研究和教育。在(1)中，首席研究员(PI)的目标是了解(A)碳基半导体中的自旋三重态激发，(B)确定它们在光致电荷转移中的作用，以及(C)确定石墨烯分子片段中单光子和双光子的相对有序性。感兴趣的碳基半导体包括那些三重态在能量上非常接近光学单态激子的系统，如宽的单壁碳纳米管和石墨烯纳米带；以及三重态显著低于光学单态激子的系统，因此是单态裂变的候选者。在(B)中，目标是确定涉及自旋三重态的光致电荷转移是否提高了电荷载流子的产额。石墨烯分子碎片中的激发态有序将在(C)中进行理论研究。最低双光子态出现在最低单光子光学态以下，表明石墨烯中存在较强的电子-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电性的半导体态的对称性破缺，以及(B)层状钴盐。PI在层状电荷转移固体的关联电子描述中提出了一种成对电子晶体(PEC)相来描述适度的晶格受挫，并认为电荷转移固体中的类量子自旋液体行为是由于向PEC的转变。为了了解电荷转移固体的热力学行为，我们将研究PEC的激发。在(B)中，PI将研究层状钴酸盐，其中钴离子形成各向同性的三角形晶格，载流子浓度可以在很大范围内调节。具有大(小)载流子浓度的钴酸盐表现出与弱(强)关联相关的行为。PI将对有限的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，试图了解(1)光与碳基半导体和纳米结构的相互作用，以及(2)强电子关联和晶格受挫对层状有机和无机材料的影响。电子之间的强烈相互作用导致了上述材料中不寻常的光学性质和奇特的相。PI使用计算机模拟和理论来获得微观层面的洞察。在过去的二十年里，碳基半导体已经从实验室的好奇演变为新的光学材料，而PI将要研究的层状有机材料和钴酸盐与高温铜氧化物超导体有很大的相似之处。该奖项支持的工作将有助于为基于纳米级光电子学的新技术的出现奠定智力基础，并为理解非常规超导提供理论基础。拟议的理论研究将探索与碳基半导体的器件物理高度相关的问题。参与该项目的研究生和博士后将在物理、化学和光学方面获得经验。本科生将参与这项研究。在代表性不足的社区中开发科学潜力是该计划的目标之一。国际学生联合会在亚利桑那大学和西南部的几所少数族裔服务机构之间建立了一个桥梁计划，让代表不足的美国少数族裔学生接触暑期研究。每年夏天，一名桥牌学生将在PI的小组中工作。国际和平协会在国际合作方面有着长期的记录。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.