M. Ellerby

Superconductivity in the intercalated graphite compounds C6Yb and C6Ca

Low dimensionality is generally considered as a necessary ingredient for high superconducting transition temperatures. Surprisingly, perhaps, systems based on graphite1,2,3 have received little attention in this context. Introducing metal atoms between the carbon layers can tune the interlayer spacing and charging of the graphite host through a variety of electronic ground states. One such ground state is superconductivity3, which is not present in pure graphite. Here we report the discovery of superconductivity in the intercalation compounds C6Yb and C6Ca, with transition temperatures of 6.5 and 11.5xa0K, respectively. These critical temperatures are unprecedented in graphitic systems and have not been explained by a simple phonon mechanism for the superconductivity. This discovery has already stimulated several proposals for the superconducting mechanism4,5,6 that range from coupling by wayxa0ofxa0the intercalant phonons through to acoustic plasmons. Itxa0alsoxa0points towards the potential of superconductivity in systems such as carbonxa0nanotubes.

Electronic Structure of Superconducting KC8 and Nonsuperconducting LiC6 Graphite Intercalation Compounds: Evidence for a Graphene-Sheet-Driven Superconducting State

We have performed photoemission studies of the electronic structure in LiC(6) and KC(8), a nonsuperconducting and a superconducting graphite intercalation compound, respectively. We have found that the charge transfer from the intercalant layers to graphene layers is larger in KC(8) than in LiC(6), opposite of what might be expected from their chemical composition. We have also measured the strength of the electron-phonon interaction on the graphene-derived Fermi surface to carbon derived phonons in both materials and found that it follows a universal trend where the coupling strength and superconductivity monotonically increase with the filling of graphene π(*) states. This correlation suggests that both graphene-derived electrons and graphene-derived phonons are crucial for superconductivity in graphite intercalation compounds.

Anisotropic Electron-Phonon Coupling and Dynamical Nesting on the Graphene Sheets in Superconducting CaC6 using Angle-Resolved Photoemission Spectroscopy

Superconductivity in graphite intercalated compounds has been studied for more than 40 years and it is still not fully understood, despite the recent progress and the discovery of relatively high Tc superconductivity in CaC6 and YbC6. Recent studies now suggest that the electron-phonon coupling is most likely responsible for pairing and that the intercalant-derived electronic states and vibrations play the dominant role. Here, we present the first studies of electronic structure in CaC6, a superconductor with Tc=11.6 K. Using angle-resolved photoemission spectroscopy, we find that, contrary to theoretical models, the EPC on the graphene-derived Fermi sheets is surprisingly strong, reflecting the interaction with high-frequency graphene-derived vibrations. Thus, in addition to the amazing properties in the charge-neutral state, graphene sheets also show surprises in the heavily doped regime: they may support strong pairing interactions and lead to superconductivity in compounds in which they are building blocks.

Superconductivity in graphite intercalation compounds

The field of superconductivity in the class of materials known as graphite intercalation compounds has a history dating back to the 1960s (Dresselhaus and Dresselhaus, 1981; Enoki et al., 2003). This paper recontextualizes the field in light of the discovery of superconductivity in CaC6 and YbC6 in 2005. In what follows, we outline the crystal structure and electronic structure of these and related compounds. We go on to experiments addressing the superconducting energy gap, lattice dynamics, pressure dependence, and how these relate to theoretical studies. The bulk of the evidence strongly supports a BCS superconducting state. However, important questions remain regarding which electronic states and phonon modes are most important for superconductivity, and whether current theoretical techniques can fully describe the dependence of the superconducting transition temperature on pressure and chemical composition.

Magnetic structure and field-dependent properties of CeCu5

A study of the magnetic properties of the hexagonal compound CeCu5 reveals antiferromagnetic order below TN=3.9 K. The magnetic structure comprises moments along the c-axis with a propagation vector k=(0,0, 1/2 ). The presence of the Kondo interaction reduces the magnitude of the moments from about 0.42 mu B, expected for a mod +or- 1/2 ) doublet, to about 0.36 mu B, observed from elastic neutron scattering experiments.

Comparative study of the phonons in nonsuperconducting BaC(6) and superconducting CaC(6) using inelastic x-ray scattering.

The low-energy phonons of two different graphite intercalation compounds (GICs) have been measured as a function of temperature using inelastic x-ray scattering (IXS). In the case of the non-superconductor BaC6, the phonons observed are significantly higher (up to 20%) in energy than those predicted by theory, in contrast to the reasonable agreement found in superconducting CaC6. Additional IXS intensity is observed below 15 meV in both BaC6 and CaC6. It has been previously suggested that this additional inelastic intensity may arise from defect or vacancy modes not predicted by theory [dAstuto et al., Phys. Rev. B 81, 104519 (2010)]. Here it is shown that this additional intensity can arise directly from the polycrystalline nature of the available samples. Our results show that future theoretical work is required to understand the relationship between the crystal structure, the phonons, and the superconductivity in GICs.

Phonons and superconductivity in YbC6 and related compounds

The out-of-plane intercalate phonons of superconducting YbC6 have been measured with inelastic x-ray scattering. Model fits to these data, and previously measured out-of-plane intercalate phonons in graphite intercalation compounds (GICs), reveal surprising trends with the superconducting transition temperature. These trends suggest that superconducting GICs should be viewed as electron-doped graphite.

THE LOW-TEMPERATURE PHASE-DIAGRAM OF UCU5 DETERMINED FROM MAGNETIZATION AND MAGNETORESISTANCE MEASUREMENTS

The low-temperature phase diagram of polycrystalline UCu5 has been determined from magnetization and magnetoresistance measurements. This system undergoes two magnetic transitions at low temperatures. The lower-temperature phase transition at T2=1.16 K, which manifests itself as a sudden jump in the magnetization and electrical resistance with decreasing temperature, shows a temperature hysteresis of about 40 mK for fields up to 12 T. The magnetic properties of this low-temperature phase show irreversible and metastable behaviour, which becomes more dramatic at the lower fields. The critical temperature T2 is observed to increase proportionally with the square of the magnetic field. Our results are discussed in comparison with available specific heat, neutron scattering and nuclear magnetic resonance data, with the aim of determining the actual nature of the low-temperature magnetic phase transition.

The Kondo contribution to the electrical resistivity in UCu5−xNix and the non-Fermi liquid behaviour of UCu4Ni

We report on electrical resistivity measurements performed on polycrystalline samples of UCu5?xNix ?(x = 0.25, 1). In order to extract the Kondo contribution to the resistivity, the experiments were carried out over a wide temperature range (0.4?800 K). From the analysis of our results, we conclude that the Kondo temperature takes values of TK ~ 240?K for x = 1 and TK ~ 245?K for x = 0.25, and that for both Ni concentrations the dominant part of the remarkably high residual resistivity (?(0) ~ 400????cm) corresponds to the Kondo contribution. These results are discussed in comparison with previous analysis of specific heat and magnetic susceptibility data that produced similar values of TK. We interpret our results in terms of disorder-driven non-Fermi liquid behaviour for UCu4Ni, as indicated by the anomalous temperature dependences of the electrical, thermal and magnetic properties previously observed in this compound.

On the relevance of Kondo disorder in the non-Fermi-liquid behavior of UCu4Ni

The electrical, magnetic and thermal properties of UCu4Ni show temperature dependencies suggestive of non-Fermi-liquid (NFL) behavior. In this work, we present an interpretation of these properties in terms of a Kondo disorder model for NFL behavior. The overall magnitude and temperature dependence of the magnetic susceptibility and the specific heat are found to be consistent with this model, assuming reasonable values of the relevant parameters, the average Kondo temperature (TK), the Kondo coupling constant (λ) and the distribution width w. These results are discussed in comparison with those obtained for other NFL systems, as UCu4Pd and UCu3.5Pd1.5.