Metal nanoclusters as size-resolved probes of quantum materials and phenomena

锂是最轻的金属元素。它的用途很广泛：电池、医药、冶金和烟火、核电站和武器、肥皂和润滑脂等。它甚至是一种超导体：当压缩时，它携带的电流没有电阻。由于锂原子很轻，所以表现出明显的量子效应。例如，它们的量子“抖动”使得锂金属的热膨胀异常高。了解锂的量子行为，寻找其新的表现形式，并将其用于未来的用途，这是有趣和重要的。这项研究的重点是锂的纳米团簇颗粒：几十到数百个原子的聚集体，可以逐个原子精确地选择大小。通过研究这种微观的金属液滴，研究小组可以精确地调整它们的性质，并选择最有希望的量子配置。该项目的目标是(I)详细探索锂的热膨胀；(Ii)产生受热时变得有磁性的锂纳米团簇(这是不寻常的，特别是因为大部分锂是非磁性的)；以及(Iii)创造同时呈扁平和拉长形状的纳米团簇，这是一种独特的纯量子效应。团队中的研究生和本科生接受广泛的科学和领导技能培训。此外，该主题还用于卓有成效的外展项目。该项目包括尺寸、同位素和温度控制的金属纳米团簇粒子的补充实验。它的目的是揭示纳米尺度和量子-固体行为界面上的新现象。最适合这一目的的材料是锂，它表现出独特的行为，因为它是最轻的金属元素，具有最小的电子核心。对单个自由粒子的测量使人们有可能关注具有精确定义的大小和组成的系统的内在性质，并追踪从纳米尺度到整体的新出现的物理性质。这项研究探索了三种固有的量子现象：(I)温度、同位素质量和团簇大小与热膨胀的关系，作为离子在纳米尺度上零点运动的表现；(Ii)纳米团簇经历量子形状振荡的能力，即同时处于两种不同形状的线性叠加中；以及(Iii)温度诱导纳米团簇中的磁性：随着温度的升高出现高自旋超顺磁态。实验方法利用了自由纳米团簇的光电子能谱、光吸收和Stern-Gerlach偏转。这些结果可作为电子结构和晶格结构微观理论的基准。潜在的应用领域包括光学纳米电子学、量子传感和磁存储与检测。该项目为研究生提供内在跨学科领域的实验和理论方面的培训，鼓励本科生参与，并作为外展活动的富有成效的资源。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL ABSTRACTLithium is the lightest metallic element. Its uses are broad: in batteries, in medication, in metallurgy and pyrotechnics, in nuclear power plants and weapons, in soaps and greases, etc. It is even a superconductor: when compressed, it carries electrical current without resistance. By being so light, lithium atoms display conspicuous quantum effects. For example, their quantum “jiggling” makes the thermal expansion of lithium metal unusually high. It is interesting and important to understand lithium’s quantum behavior, to search for its novel manifestations, and to harness it for future use. This research focuses on nanoclusters particles of lithium: aggregates of tens to hundreds of atoms which can be size-selected with atom-by-atom accuracy. By studying such microscopic metallic droplets, the research team can precisely tune their properties and select the most promising quantum configurations. The project’s aim is to (i) explore in detail the thermal expansion of lithium; (ii) produce lithium nanoclusters which become magnetic when heated (this is unusual, especially since bulk lithium is nonmagnetic); and (iii) create nanoclusters which are simultaneously oblate and prolate in shape, a unique and purely quantum effect. The graduate and undergraduate students on the team receive training in a wide range of scientific and leadership skills. In addition, the subject matter is used for fruitful outreach programs.TECHNICAL ABSTRACTThis project encompasses complementary experiments on size-, isotope-, and temperature-controlled metal nanocluster particles. It aims to reveal novel phenomena lying at the interface of nanoscale and quantum-solid behavior. The optimal material for this purpose is lithium, which displays distinctive behavior stemming from it being the lightest metallic element with the smallest electron core. Measurements on individual free particles make it possible to focus on the intrinsic properties of systems with a precisely defined size and composition and to trace the emergent physical properties from the nanoscale to the bulk. The research explores three inherently quantum phenomena: (i) Temperature, isotope mass, and cluster size dependences of thermal expansion as manifestations of the zero-point motion of ions at the nanoscale; (ii) The ability of nanoclusters to undergo quantum shape oscillations, i.e., to be in a linear superposition of two different shapes at the same time; and (iii) Temperature-induced magnetism in nanoclusters: the appearance of high-spin superparamagnetic states with increasing temperature. The experimental approach employs photoemission, photoabsorption, and Stern-Gerlach deflection of free nanoclusters. The results can serve as benchmarks for microscopic theories of electronic and lattice structure. Potential areas of applications include optical nanoelectronics, quantum sensing, and magnetic storage and detection. The project offers graduate student training in experimental and theoretical aspects of an inherently interdisciplinary field, encourages involvement by undergraduates, and serves as a fruitful resource for outreach activities.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.