Suggestions for Future Research

Some progress has been made toward answering most of the questions that were asked in the first chapter of this work. The one question that continues to be elusive is the most basic one, why is C₈K superconducting? In the coming years experimentalists could help to pin matters down by extending the study of GIC superconductivity to such compounds as C₈K_(1-x)Rb_x and C₆Ba since, according to available theories,[4,234] these compounds should be superconducting. The C₈K_(1-x)Rb_x system is particularly interesting since several phonon modes soften dramatically for x 2/3.[183,219] The data in Figure illustrate the softening of the acoustic modes discovered by Neumann and coworkers.[183] These authors attribute the anomaly to a composition-dependent charge transfer. All other things begin equal, conventional theories of electron-phonon coupling would tend to predict a maximum of T_c versus x for the C₈K_(1-x)Rb_x system near this minimum in the phonon frequencies. Yet the T_c for x = 1.0 ( C₈Rb) is only 26 mK,[133] compared to the T_c = 150 mK for x = 0.0 ( C₈K).[141] Investigation of T_c as a function of stoichiometry in this system is clearly an important test of theories of GIC superconductivity.

  
Figure: a) Softening of the elastic constant C₃₃ as a function of composition in the C₈K_(1-x)Rb_x system. From Ref. [183]. The elastic constant was obtained from a fit to the acoustic branch of the phonon system. The phonons were observed using inelastic neutron scattering. Similar softening of the M-point optic modes has been seen using Raman scattering.[219] b) T_c versus x in the C₈K₍1-x)Rb_x system. Only the endpoint compounds have been characterized.

The high-pressure experiments, which show T_c of C₄K to be as high as 2 K,[12] also offer an important clue. The structure of C₄K is currently uncertain, but it is believed to include a double intercalant layer of K atoms, much like C₄KHg.[13] These experiments offer another fundamental test of the theories of GIC superconductivity.

Despite the progress reported here in studies of the KHg-GIC's, some questions about these materials remain. In general, it would be advisable to repeat many of the experiments that have already been performed (resistivity, susceptibility, specific heat) on specimens whose T_c has been measured. Comparative studies of gold and pink specimens are of particular interest because of the possibility of the detection of a charge-density wave. More work on hydrogenation of C₄KHg is also needed; studies as a function of hydrogen uptake are of particular interest.

In order to interpret these experiments a lot more theoretical work is needed, particularly band-structure calculations for input to the microscopic critical field models. As always in the field of superconductivity, experiment and theory have a stimulating partnership. In the coming years our understanding of both low-temperature and high-temperature superconductivity will continue to increase.