High pressure experiments play a significant role in the field of superconductivity in a number of
ways [1], one of the most important being to expand the number of known superconductors and enhance
their transition temperatures Tc to record values. 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 [2]. This 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 localization. In fact, ferromagnetism has been predicted for
K metal near 20 GPa pressure [3]. The effects of this localization on the superconducting state have been
shown to be enormous. For example, the value of Tc for elemental Li increases from a lowly 0.4 mK to
above 14 K under 30 GPa pressure [4], 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
magnetic 4f electrons 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 the 4f delocalization are: (1) change of valence through the promotion of a
4f electron into the conduction band, (2) increasing hybridization of the 4f electrons with the
conductions 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 [5].
Acknowledgments. The authors are grateful for research support by the National Science Foundation under Grant
No. DMR-1104742 and by the Carnegie/DOE Alliance Center (CDAC) through NNSA/DOE Grant No. DEFC52-08NA28554. Work at Argonne is supported by the US Department of Energy (DOE), Office of Science,
Office of Basic Energy Sciences, under Contract No. DE-AC-02-06CH11357. Many thanks are due Neil Ashcroft
for his encouragement and for many fruitful discussions.
1.
2.
3.
4.
5.
J. S. Schilling, Ch. 11 in: "Handbook of High Temperature Superconductivity: Theory and Experiment", editor J. R.
Schrieffer, associate editor J. S. Brooks (Springer Verlag, Hamburg, 2008), p. 427; for a review of superconductivity in
the alkali metals, see: J. S. Schilling, High Pres. Res. 26, 145 (2006).
J. B. Neaton and N. W. Ashcroft, Nature 400, 141 (1999); ibid., Phys. Rev. Lett. 86, 2830 (2001).
C. J. Pickard and R. J. Needs, Phys. Rev. Lett. 107, 087201 (2011).
S. Deemyad and J. S. Schilling, Phys. Rev. Lett. 91, 167001 (2003).
G. Fabbris, T. Matsuoka, J. Lim, R. L. Mardegan, K. Shimizu, D. Haskel, and J. S. Schilling (in preparation).