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

University Of Southern California, Los Angeles CA

Investigators

Abstract

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 California Alkali 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.