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

这个实验项目将解决凝聚态物理中的一个主要主题，库仑相互作用在一个原本没有能带的电子系统中的影响，也就是碱金属，主要是薄膜形式的或带有过渡金属杂质的。后一种系统是很好的探测器，因为它们在耦合到传导电子的杂质的d态之间引入了强烈的局域交换排斥。然而，对于实验研究来说，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很小，很难测量。在本项目中，采用淬冷冷凝法制备了3d和4d杂质浓度较低的碱性薄膜。用反常霍尔效应的方法测量了磁化强度随温度和磁场的变化。目前，世界上还没有其他实验室可以完成这项任务。实验的目标有两个：(A)确定磁性杂质对主体极化的影响和(B)确定杂质的磁矩，这些实验应该刺激新的理论努力，使目前知之甚少的测量合理化。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。在过去的一段时间里，几名本科生也进行了这些困难的实验，在主要物理期刊上与人合著了几篇论文，在美国物理学会的会议上展示了他们的结果，并在南加州大学的本科生研究中获得了一等奖和二等奖。传导电子的波长大于原子距离，因此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“凝胶”模型的最佳代表，在该模型中，电子在金属中的运动不受离子的干扰，并且只通过它们的电荷，即排斥库仑相互作用而相互作用。因此，碱金属是研究库仑相互作用基本方面的理想体系，库仑相互作用是固体表现出的许多性质的基础：铁磁性、高温超导等。通过引入过渡金属杂质(例如铁、钴、镍、钒、钛等原子)，可以在碱金属中探索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大的库仑斥力，它拉动电子，就像小提琴家的弓拉小提琴的弦一样。由此产生的磁矩揭示了有关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，有几个困难需要克服：(A)过渡金属杂质不溶于碱基质，以及(B)杂质的磁化强度很小，很难测量。“猝灭凝聚”和“反常霍尔效应”这两种技术克服了这些实验障碍。目前，世界上还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。在最后一次资助期间，几名本科生也进行了这些困难的实验，在领先的物理期刊上与人合著了几篇论文，在美国物理学会的会议上展示了他们的结果，并在南加州大学的本科生研究中获得了一等奖和二等奖。

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.