There is an increasing interest in the properties of nanoscopic objects. Yet little is known about the properties of the very smallest metal particles, or clusters, which consist of only a few atoms. It is possible to produce these extremely small particles and to study their properties in a vacuum chamber. This individual investigator award supports a project that will produce a beam of small metallic clusters, which are subsequently cooled to extremely low temperatures. Beams of these particles are then deflected in electric and magnetic fields. The magnetic deflections provide critical information about the magnetic properties of the clusters, which can be compared with magnetic properties of bulk materials. Deflections in electric fields show whether these small metal particles conduct electricity or not. There is some evidence that certain metal clusters have properties akin to superconductivity even at room temperature. These fundamental studies will provide important information about what makes a metal a metal. They will also indicate whether specific metal clusters may be useful for nanotechnology applications such as magnetic recording. Graduate and undergraduate students will learn a variety of experimental techniques that will prepare them for future positions in academia or industry. The training they receive will provide them with the skills needed to become forefront researchers in the properties of nanoscopic electronic objects. This award receives funding from the Division of Materials Research and the Division of Physics.
This individual investigator award supports a project that addresses the fundamental question of how metallic properties develop in metallic clusters as a function of size. Beams of small metal clusters with from one to several hundred atoms are produced in a cryogenic cluster source using the pulsed laser vaporization method and their properties are studied in high vacuum as they fly from the source to a position sensitive mass analyzer that can simultaneously measure both the mass and the positions of the clusters. Clusters with temperatures of about 20 K are essentially in their electronic and vibrational ground states and therefore are ideally suited for studying the ground state properties of the material. In the bulk, metals do not tolerate a voltage difference and therefore bulk metal objects cannot have electric dipole moments. Hence electric dipole measurements of small clusters will provide information on the emergence of this important metallic property. Molecular beam electron spin resonance measurements of the magnetic moments of paramagnetic clusters will trace the size evolution of the g-values in small alkali clusters and in niobium clusters. In the latter case these measurements may provide insight into a possible connection between a ferroelectric state and superconductivity. Graduate and undergraduate students will learn a variety of experimental techniques that will prepare them for future positions in academia or industry. The training the receive will provide them with the skills needed to become forefront researchers in the properties of nanoscopic electronic objects. This award receives funding from the Division of Materials Research and the Division of Physics.
A bulk metal is assembly of very large number of atoms (on the order of at least one billion times one billion). It can have many properties: it conducts electricity and, it is ductile, it can be magnetic, and so forth. Condensed matter physics is about measuring and understanding these properties and comparing one metal with the next to understand the differences between them. In cluster physics we take a special approach to gain this knowledge. We consider how a specific property develops. Much like embryology teaches about the development of living beings, by tracing its development from a single cell to a fully developed organism, cluster physics measures a specific property in a cluster of atoms with a precise number of atoms, and follows the evolution of that property as a function of that number. For example, an iron atom has magnetic properties that are very different from a bulk magnet. When several iron atoms are put together they start behaving more and more like a bulk magnet. Our measurements have shown how this actually happens. We produce the clusters by using a laser pulse to vaporize a minute amount of metal. This vapor contains clusters of all sizes. Our apparatus is able to very precisely sort the clusters by size and measure how they are affected by magnetic fields. This allows us to quantify their magnetic properties. We are able to do this at extremely low temperatures (about 20 degrees above absolute zero), which is advantageous to bring out the more subtle features. We have found that bulk magnetism requires several takes several iron atoms in are a cluster. Up to that size, the magnetism is intermediate between that of an atom and the bulk. We find this to be true not only for iron, but for all magnetic materials that we have measured, showing that there is a general principle involved. This principle is important input for condensed matter physicists to formulate more refined theories of magnetism, which in turn is critical for the development of new magnetic materials. And magnetic materials are extremely important in technology from hard disk drives to electric motors. More surprising is that some clusters of some materials are very different then what we see in the bulk. For example, we have found that very small gold clusters are actually magnetic. Moreover, the magnetism comes in two varieties, one where the gold cluster is attracted to the magnet and the other where it is strongly repulsed! This has never been seen before and may be very important in nanoscience. In the course of our investigations we have also shed light on more subtle magnetic properties, that are related to the resistivity of magnetic materials and other effects. While these effects do not have direct applications potential, they do provide vital input in the fundamental understanding of magnetism in general on all size scales, from the atom to the bulk.
