Anisotropic superconductivity is the fastest-growing condensed-matter physics subfield of the late 1980's. The primary boom has been in the high-temperature superconductivity industry, which was created seemingly overnight by Bednorz and Müller's astonishing discovery of over-30 K superconductivity in La5-xBaxCu5Oy.[178] The subsequent observation of 90 K superconductivity in YBa2Cu3O7[269] changed the character of superconductivity research forever.
Even before the attention of the physics community was focussed on the new high- Tc materials, the field of superconductivity was far from dormant. There were contributions from superconductivity research to many different disciplines, such as the influence of granular superconductivity research on percolation theory,[191,158] or the influence of the study of Josephson-junction arrays on the theory of two-dimensional phase transitions.[182,88] Despite the many aspects of superconductivity research in the '80's, though, one paradigm has been central throughout: the concept of anisotropic superconductivity. This theme has been manifested in all sorts of systems, from quasi-one-dimensional superconductors like the polymer SNx,[95] to heavy-fermion superconductors with possibly anisotropic pair-states,[225] to artificially layered superconducting superlattices which show a 3D-2D crossover in their critical fields.[209] The unexpected advent of high- Tc materials is in harmony with the theme of anisotropic superconductivity since the oxide superconductors show orientation-dependent properties in many ways similar to those of the superconductors studied earlier in the decade.[188,268,118,173]
The principal topic of this thesis is anisotropic superconductivity of graphite intercalation compounds (GIC's). Graphite intercalation compounds are superlattices formed by the chemical insertion into the graphite matrix of a species called the intercalant.[67,1] GIC's are called donor compounds if the intercalant gives electrons to the carbon layers, and acceptor compounds if the intercalant takes electrons from the carbon layers. None of the acceptor GIC's is known to be superconducting, despite the fact that many of them are metallic.[67,64]
Superconducting GIC's do not have an exotic electron-pairing mechanism, do not show a 3D-2D crossover, and have no foreseeable technological applications. Yet there are many interesting questions about graphite-based superconductors which remain to be answered, especially with regard to their anisotropy. The most obvious of these questions concerns the very existence of superconductivity in the GIC C8K,[102] a compound made from two constituents which are not separately superconducting. C8K is a binary GIC formed by the insertion of potassium atoms between the layers of graphite. The accepted structure of C8K is shown in Figure 1.
Figure 1: Structure of C8K, a
typical stage 1 intercalation compound. Stage n means that n
layers of graphite are present between each pair of
intercalant layers.[67] a) The
layer structure. The lattice constant along the direction
perpendicular to the planes is called Ic in GIC's.
b) Three common in-plane structures. C8K has the
(2 × 2)R0° structure.
The superconductivity of C8K is still under investigation,[5] but in recent years attention has turned more to the ternary GIC superconductors, which have two intercalated components, an alkali metal (denoted M) and a heavy metal. The ternary GIC superconductors are highly anisotropic, with critical fields that vary by as much as a factor of 47 as a function of orientation.[121] The binary GIC superconductors, on the other hand, have critical field anisotropies only on the order of 4.[141] Despite an extensive study of the critical fields of the ternary GIC superconductors by Iye and Tanuma,[120] there are still some fundamental questions about the angular and temperature dependence of the critical fields to be answered.
Unlike the binary GIC superconductors, the MHg-GIC's are synthesized from an intercalant, the MHg amalgam, which is in itself superconducting. The intercalation compounds formed from the MHg amalgams have higher superconducting transition temperatures than the amalgams themselves. In addition, the second-stage MHg-GIC's, which have two carbon layers between the graphite planes, have higher Tc's than the stage 1 GIC's, with one carbon layer between the graphite planes. These facts run counter to usual expectations based on proximity-effect theories,[261,49] which predict the monotonic depression of Tc when a superconducting material is layered with a non-superconducting one like graphite. Therefore the ternary graphite-based superconductors display qualitatively different phenomena than the artificially structured superlattices, which do show behavior qualitatively in agreement with proximity theories.[209]
The ternary GIC superconductors are in many respects similar to the transition metal dichalcogenides (TMDC's) and their intercalation compounds (TMDCIC's). The TMDC's are different from graphite in that they are by themselves superconducting, but alike in their divergence from the predictions of proximity-effect theories. Intercalation of the TMDC's with organic molecules increases their transition temperature Tc,[92,200] despite the fact that the proximity effect should cause the opposite behavior. In graphite-based superconductors, the Tc of the intercalant is enhanced by intercalation, whereas in the TMDC-based superconductors, the Tc of the host is enhanced by intercalation.
The enhancement of Tc in the TMDCIC's that is associated with intercalation is attributed to the suppression of a charge-density wave (CDW) state.[179] Besides suppression through intercalation, CDW's in TMDC's can also be inhibited by hydrogen absorption[179] and the application of small hydrostatic pressures.[172] Both hydrogen sorption and hydrostatic pressure in suppressing the CDW raise Tc in the TMDC's. Application of a small pressure can also raise Tc in the ternary GIC superconductor C4KHg.[55] Recent experiments have also shown that hydrogen sorption increases the transition temperature of C4KHg.[207] These experiments in combination with other similarities between the TMDC's and GIC's suggest the possibility of a CDW in C4KHg.[55]
The fundamental questions in the field of GIC superconductivity are therefore the following: Why is C8K superconducting? Why do the MHg-GIC's have higher Tc's than their intercalants alone? Why do stage 2 MHg-GIC's have higher Tc's than the stage 1? How can one account for the unusual angular and temperature dependence of the critical fields in GIC superconductors? and Why do hydrogenation and pressure dramatically increase the Tc of C4KHg? Why aren't acceptor GIC's superconducting?
These questions will be addressed in the chapters that follow. Chapter 2 is a discussion of non-GIC anisotropic superconductors and the ideas they give regarding the GIC experiments. Chapter 3 explains how the samples used in the new experiments were prepared, and discusses issues in GIC synthesis as they relate to superconductivity. Chapter 4, the heart of this work, is a report of detailed critical field studies on C4KHg, and an account of what these studies reveal about GIC superconductivity. Hydrogenation experiments on C4KHg and the possibility of a CDW suppression of Tc are discussed in Chapter 5. Chapter 6 details attempts to find superconductivity in CsBi-GIC's, and Chapter 7 is a summary.
A Word about Notation: In the GIC literature, it is customary to measure all orientation angles from the graphite c-axis. On the other hand, in the anisotropic superconductivity literature it is customary to measure all angles from the layer planes. The GIC convention has been adopted here, and all formulae taken from papers that used the other convention have been converted. Thus the direction called || in the superconductivity literature is now called _|_ c, and the direction called _|_ in the superconductivity literature is now called || c. While on the subject of confusion, it is worth mentioning that cgs units have been used throughout, the quantity N(0) is the density-of-states for both spin directions, and e is the electronic charge, a positive quantity.