Experiments in Education: Studies of the Not So Simple Alkali Metals

教育实验：不那么简单的碱金属的研究

• 批准号: 0124422
• 负责人: Gerd Bergmann
• 金额: $ 30万
• 依托单位: University of Southern California
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-01-01 至 2004-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0124422&HistoricalAwards=false
• 关键词: Experiments; Education; Studies; So; Simple

This experimental project will address a major theme in condensed matter physics, the effect of the Coulomb interaction in an otherwise band-free electron system, that is the alkali metals, largely in thin film form or with transition metal impurities. The latter systems are excellent probes because they introduce a strong local exchange repulsion between the d-states of the impurities coupling to the conduction electrons. However, there are several difficulties to overcome for the experimental investigations: (a) transition metal impurities do not dissolve in alkali hosts and (b) the magnetization of the transition metal impurities is very small and difficult to measure. In this project the method of quenched condensation is used to prepare alkali films with small concentrations of 3d and 4d impurities. The magnetization as a function of temperature and magnetic field is measured with the method of the anomalous Hall effect. Presently there are no other laboratories in the world that can perform this task. The experimental goal is two-fold: (a) to determine the effect of the magnetic impurities on the polarization of the host and (b) to determine the magnetic moment of the impurities These experiments should stimulate renewed theoretical effort to rationalize the currently poorly understood measurements. The experiments introduce graduate students to the technique of high vacuum and low temperature physics with sophisticated evaporation methods. During the period of the last several undergraduate students also performed these difficult experiments, co-authored several publications in major physics journals, presented their results at American Physical Society meetings, and won first and second prizes for undergraduate research at the University of Southern CaliforniaAlkali metals are a special group of metals. The wave length of the conduction electrons is larger than the atomic distance so that the atomic ions hardly disturb the motion of the electrons. Furthermore the volume of the atomic ions occupies only a fraction of the total volume. This makes the alkali metals the best representation of the "jelly" model in which the electrons move undisturbed by the ions through the metal and interact only with each other through their electrical charge, the repulsive Coulomb interaction. Therefore the alkali metals are the ideal system to study fundamental aspects of Coulomb interaction, which underlies many of the properties exhibited by solids: ferro-magnetism, high temperature superconductivity, etc. The effect of the Coulomb interaction can be probed in the alkali metals by introducing transition metal impurities (for example atoms of iron, cobalt, nickel, vanadium, titanium,..). These impurities introduce locally an additional strong Coulomb repulsion which pulls on the electrons like the bow of a violinist pulls on the string of a violin. The resulting magnetic moments reveal information about the energetic state of the electrons and the effect of the Coulomb interaction on them. However, for experimental investigations there are several difficulties to overcome: (a) transition metal impurities do not dissolve in alkali hosts and (b) the magnetization of the impurities is very small and difficult to measure. Two techniques, "quenched condensation" and the "anomalous Hall effect" overcome these experimental hurdles. Presently there are no other laboratories in the world that can perform this task. The experiments introduce graduate students to the techniques of high vacuum and low temperature physics with sophisticated evaporation methods. During the period of the last grant several undergraduate students also performed these difficult experiments, co-authored several publications in leading physics journals, presented their results at American Physical Society meetings, and won first and second prizes for undergraduate research at the University of Southern California.

该实验项目将解决凝聚态物理学的一个主要主题，即库仑相互作用在其他无带电子系统中的影响，即碱金属，主要是薄膜形式或过渡金属杂质。 后一种系统是很好的探针，因为它们在与传导电子耦合的杂质的d态之间引入了强的局部交换排斥。然而，对于实验研究有几个困难需要克服：（a）过渡金属杂质不溶于碱基质;（B）过渡金属杂质的磁化强度非常小，难以测量。本计画利用急冷凝聚法制备低浓度3d、4d杂质的硷性薄膜。用反常霍尔效应方法测量了磁化强度随温度和磁场的变化。目前，世界上没有其他实验室可以完成这项任务。实验的目标是双重的：（a）以确定的影响的磁性杂质对主机的极化和（B），以确定磁矩的杂质。这些实验应该刺激新的理论努力，合理化目前知之甚少的测量。 该实验向研究生介绍了高真空和低温物理技术与复杂的蒸发方法。在过去的几个本科生期间也进行了这些困难的实验，共同撰写了几篇论文在主要的物理学杂志上，在美国物理学会会议上介绍了他们的结果，并获得了一等奖和二等奖的本科研究在南加州大学碱金属是一个特殊的金属组。传导电子的波长大于原子距离，使得原子离子几乎不干扰电子的运动。此外，原子离子的体积只占总体积的一小部分。这使得碱金属成为“果冻”模型的最佳代表，在该模型中，电子不受离子的干扰而穿过金属，并且仅通过它们的电荷（排斥库仑相互作用）相互作用。因此，碱金属是研究库仑相互作用的基本方面的理想系统，库仑相互作用是固体所表现出的许多性质的基础：铁磁性，高温超导性等。通过引入过渡金属杂质（例如铁，钴，镍，钒，钛等），可以在碱金属中探测库仑相互作用的影响。这些杂质在局部引入了一个额外的强库仑排斥力，就像小提琴家拉小提琴的弦一样拉着电子。由此产生的磁矩揭示了电子的能量状态和库仑相互作用对它们的影响的信息。 然而，对于实验研究，有几个困难要克服：（a）过渡金属杂质不溶于碱性基质和（B）杂质的磁化强度非常小，难以测量。两种技术，“淬火冷凝”和“反常霍尔效应”克服了这些实验障碍。 目前，世界上没有其他实验室可以完成这项任务。 这些实验向研究生介绍了高真空和低温物理技术以及复杂的蒸发方法。 在最后一次拨款期间，几名本科生也进行了这些困难的实验，在领先的物理学期刊上共同撰写了几篇论文，在美国物理学会会议上介绍了他们的结果，并获得了南加州大学本科生研究的一等奖和二等奖。