CAREER: Excitons, electron-hole plasmas, and electron-hole liquids in the time domain

该职业奖支持二维材料中发现的新型相变的理论和计算研究和教育。相变在我们周围很常见；例如，在水的沸腾中，或者在将水蒸气冷凝成雨的相反过程中。在某些半导体材料中，可能会发生非常相似的相变。当暴露在连续激光束的能量下时，材料中的电子从正常的量子态被激发到另一个量子态。然而，处于新状态的激发态电子对它留下的“空穴”（即它原来所在的位置）产生了有效的吸引力。如果有足够的电子被泵入激发态，它们和它们的空穴对应物就会经历气-液型相变。虽然这种相变的成分是电子和空穴，但它的特征与水中的相变相似。然而，一个显著的区别是，这发生在超快的时间尺度上（纳秒），它涉及到驱动激光。重要的是，由于这些成分显然是量子物体，因此产生了许多令人兴奋和有趣的特性。所讨论的材料形成二维薄片，这意味着它们可能被整合到柔性电子产品中。最重要的是，现在有可能在纳秒内将它们的行为从直接的半导体（通常是透明材料）转变为金属和反射的物体。这在太阳能光收集和通过光子学绕过摩尔定律方面有很好的应用前景。该项目涉及算法和软件的大量开发，这些将作为开源代码分发给社区。除了研究之外，该职业奖还将通过与美国物理教师协会（AAPT）合作，将其成功的“Bootstrap for Computational Modeling In Physics First”研讨会转变为远程学习课程，支持美国STEM教育的改善。该研讨会将计算建模整合到八年级和九年级的物理课程中，并且通常亲自参加。开发远程学习版本将能够接触到更多的参与者，特别是来自农村地区的参与者，这些地区可能无法随时获得长时间的面对面讲习班。由于计算在STEM工作场所发挥着越来越大的作用，因此早期的准备和接触将提高学生的整体能力。这个版本的研讨会将首先在北卡罗来纳州部署，并将通过AAPT在全国进一步传播。该职业奖支持超快非平衡相变的理论和计算研究和教育。研究小组将研究过渡金属二硫族化合物在室温下高光激发下向电子空穴液体（EHL）的转变。由于相变的超快性质，时域方法至关重要。该项目将确定创建和维持EHL的条件，其对外部刺激和邻近耦合紧急订单的响应，其可调性以及其运输和转移动力学。这种理解将使电子空穴液体在新型器件中的应用成为可能，特别是在太阳能收集和通过光子学绕过摩尔定律的应用中。该项目通过非平衡科学的理论研究和计算方法开发，为高影响力科学提供了机会。这个问题的一个重要方面是，它必须在时域内解决，而不是诉诸通常的平衡分析技术。PI将通过非平衡多体理论和与实验团体的合作来解决这一挑战。研究小组将考虑不依赖遍历性的非平衡相变的概念，通过在时域的破对称状态中进行自洽计算。通过这些方法，研究小组将重新评估我们如何看待量子材料中的非平衡过程和相变，并打破对平衡概念的依赖。整个项目中创建的算法和软件将作为开源代码分发到社区。除了研究之外，该职业奖还将通过与美国物理教师协会（AAPT）合作，将其成功的“Bootstrap for Computational Modeling In Physics First”研讨会转变为远程学习课程，支持美国STEM教育的改善。该研讨会将计算建模整合到八年级和九年级的物理课程中，并且通常亲自参加。开发远程学习版本将能够接触到更多的参与者，特别是来自农村地区的参与者，这些地区可能无法随时获得长时间的面对面讲习班。由于计算在STEM工作场所发挥着越来越大的作用，因此早期的准备和接触将提高学生的整体能力。这个版本的研讨会将首先在北卡罗来纳州部署，并将通过AAPT在全国进一步传播。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

NONTECHNICAL SUMMARYThis CAREER award supports theoretical and computational research and education in a new type of phase transition found in two-dimensional materials. Phase transitions are commonly found around us; for example, in the boiling of water, or in the reverse process of condensing water vapor into rain. In some semiconducting materials, a remarkably similar phase transition may occur. When exposed to the energy a continuous laser beam, electrons in the material get excited from their normal quantum state into another. However, the excited electron in its new state feels an effective attraction to the "hole" it left behind, i.e. the place where it used to be. If enough electrons get pumped into the excited state, they - and their hole counterparts - can undergo a gas-to-liquid-type phase transition. This phase transition, although its constituents are electrons and holes, has characteristics similar to the one found in water. However, a significant difference is that this happens on ultrafast time-scales (nanoseconds), and it involves a driving laser. Importantly, because the constituents are manifestly quantum objects, a host of exciting and interesting properties arise.The materials in question form two-dimensional sheets, which means they may be incorporated into flexible electronics. On top of this, there is now the possibility of switching their behavior within nanoseconds from being a straightforward semiconductor (which are typically transparent materials) to being an object that is metallic and reflecting. This has promising applications for solar-light harvesting and for bypassing Moore's law through photonics. The project involves substantial development of algorithms and software, which will be disseminated to the community as open-source code.In addition to the research, this CAREER award will support the improvement of STEM education in the U.S. by partnering with the American Association of Physics Teachers (AAPT) to turn their successful "Bootstrap for Computational Modeling in Physics First" workshop into a distance-learning course. The workshop integrates computational modeling into 8th and 9th grade physics courses, and is typically attended in person. The development of a distance-learning version will enable reaching a much larger set of participants, in particular from rural areas which may not have ready access to long in-person workshops. Since computation is playing an increasingly larger role in the STEM workplace, earlier preparation and exposure will improve students' overall capabilities. This version of the workshop will initially be deployed in North Carolina, and will be further disseminated through AAPT across the country.TECHNICAL SUMMARYThis CAREER award supports research and education in theoretical and computational investigations of ultrafast out-of-equilibrium phase transitions. The research team will study the transition into an electron-hole liquid (EHL) that occurs at room temperature upon high photoexcitation in transition-metal dichalcogenides. Due to the ultrafast nature of the phase transition, time-domain approaches are critical. This project will determine the conditions for creating and sustaining the EHL, its response to external stimuli and proximity-coupled emergent orders, its tunability, and its transport and transfer dynamics. This understanding will enable the use of the electron-hole liquid in novel devices, and, in particular, in applications for solar-light harvesting and for bypassing Moore's law through photonics.The project exploits an opportunity for high-impact science through theoretical study and computational method development for nonequilibrium science. One essential aspect of the problem is that it must be tackled in the time domain, rather than resorting to the usual equilibrium analytic techniques. The PI will address this challenge through nonequilibrium many-body theory and through collaboration with the experimental community. The research team will consider the concept of a nonequilibrium phase transition without reliance on ergodicity by performing self-consistent calculations in the broken-symmetry states in the time domain. Through these approaches, the research team will re-evaluate how we think about nonequilibrium processes and phase transitions in quantum materials, and break the reliance on equilibrium concepts. The algorithms and software created throughout the project will be disseminated to the community as open-source code.In addition to the research, this CAREER award will support the improvement of STEM education in the U.S. by partnering with the American Association of Physics Teachers (AAPT) to turn their successful "Bootstrap for Computational Modeling in Physics First" workshop into a distance-learning course. The workshop integrates computational modeling into 8th and 9th grade physics courses, and is typically attended in person. The development of a distance-learning version will enable reaching a much larger set of participants, in particular from rural areas which may not have ready access to long in-person workshops. Since computation is playing an increasingly larger role in the STEM workplace, earlier preparation and exposure will improve students' overall capabilities. This version of the workshop will initially be deployed in North Carolina, and will be further disseminated through AAPT across the country.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.