Dresden 2003 – wissenschaftliches Programm
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SYEC: Materials under extreme conditions
SYEC 1: Extreme Conditions I
SYEC 1.1: Hauptvortrag
Mittwoch, 26. März 2003, 14:00–14:30, HSZ/01
New high-pressure phases of simple metals — •Niels E. Christensen — Department of Physics and Astronomy, University of Aarhus DK-8000 Aarhus C, Denmark
Recent x-ray diffraction experiments on alkali metals under high pressure have provided new insight in pressure-induced structural transformations. New structures have been identified, and some of these have surprising properties such as low coordination numbers [1]. Theoretical studies, some of which are described here (see also [2]), using ab initio methods to calculate electronic and structural properties provide support for the analysis of these experiments and also predict new properties as well as new phases [1-3]. For instance, at extreme pressures Li as well as Na become "poor" metals in the sense that some phases have very low values of the density of states at the Fermi level. Sodium may even become an insulator.
The light as well as the heavy alkali metals undergo a softening of the lattice related to electronic s → p and s → d transfer, respectively. In the case of Cs this is predicted to produce a phonon softening which affects the thermodynamic properties significantly, including some of the phases to have negative thermal expansion coefficients [4].
For some time there have been speculations about the possibility of superconductivity in the lightest metal, Li. However, at ambient pressure it was found experimentally that Tc for Li is less than 0.1 mK. The situation may be quite different when external pressure is applied. Calculations [5] suggest that the phases Li-fcc, Li-hR1 and Li-cI16 may be superconductors for pressures in the range 20 to 60 GPa, that Tc may have a maximum of ≈ 50 K, and that the Tc values are enhanced by the lattice softening near the pressure-induced structural transformations. Experiments [6] have now confirmed that Li is a superconductor at 20-80 GPa, but our calculations seem to overestimate Tc unless the Coulomb pseudopotential parameter µ* is larger than 0.3. New calculations show that also Na may become a superconductor under pressure.
[1] M. Hanfland et al., Nature 408, 174 (2000).
[2] J. B. Neaton and N. W. Ashcroft, Nature 400, 141 (1999); Phys. Rev. Lett. 86, 2830 (2001).
[3] N. E. Christensen and D. L. Novikov, Solid State Commun. 119, 477 (2001).
[4] N. E. Christensen et al., Phys. Rev. B 61, R3764 (2000).
[5] N. E. Christensen and D. L. Novikov, Phys. Rev. Lett. 86, 1861 (2001).
[6] K. Shimizu et al., Nature 419, 597 (2002); V. Struzhkin et al., Science 298, 1212 (2002).