Faculty of Physics and Earth Science
MQF-Group
MaReMaS-Seminar
Magnetic Resonance and Materials Science
Tuesday, 4th June 2013, 11:00 am, SR 225, Linnéstr. 5
Pressure-Induced Electron Localization/Delocalization Effects in
Alkali and Rare Earth Metals
Professor Dr. James Schilling
Department of Physics – Washington University, St. Louis (USA)
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High pressure experiments play a significant role in the field of superconductivity in a number of ways, three of the
most important being to expand the number of known superconductors, enhance their transition temperatures Tc to
record values, and test theories of superconductivity. One of the physically most interesting, but least appreciated,
roles of extreme pressures in both superconductivity and magnetism is to bring the ion cores so close together that the
conduction electrons are forced into interstitial sites, thereby experiencing enhanced localization. This 'interstitial
localization' yields conditions favorable both for itinerant magnetism and superconductivity. The highly compressible
alkali metals afford the ideal conditions to realize this anomalous conduction electron state. In fact, ferromagnetism has
been predicted for K metal near 20 GPa pressure. The effect of interstitial localization on the superconducting state
have been shown to be enormous. The value of Tc for elemental Li, for example, increases from a lowly 0.4 mK to
above 14 K under 30 GPa pressure, an increase in Tc of more than four orders of magnitude! The nearly free electron
behavior prized by all alkali metals is completely lost under sufficient pressure. In contrast to the behavior of the
conduction electrons in alkali metals under pressure, the 4f electrons responsible for magnetism in the lanthanides are
believed to undergo various forms of delocalization that may, or may not be, responsible for the volume collapse
experienced by most lanthanides at a critical pressure Pc. Three possible forms of 4f delocalization are: (1) change of
valence through promotion of a 4f electron into the conduction band, (2) increasing hybridization of the 4f electrons
with the conduction electrons leading to a Kondo volume collapse, (3) transition of the 4f electrons through orbital
overlap from a highly localized to an itinerant 4f state. Recent synchrotron spectroscopic and electrical resistivity
studies under pressures approaching 1 Mbar are discussed which shed light on the mechanisms responsible for the
volume collapse in selected lanthanides.
Prof. Dr. Jürgen Haase