Superconductivity In Graphite Intercalation Compounds Research Articles

Direct evidence for a metallic interlayer band in Rb-intercalated bilayer graphene

We have fabricated Rb-intercalated bilayer graphene on 6H-SiC(0001) and performed low-energy electron diffraction (LEED) and high-resolution angle-resolved photoemission spectroscopy (ARPES). In LEED experiments, we observed a 2 $\ifmmode\times\else\texttimes\fi{}$ 2 pattern identical to that of the bulk graphite intercalation compound C${}_{8}$Rb. The existence of a 2 $\ifmmode\times\else\texttimes\fi{}$ 2 Rb layer is also confirmed by ARPES, where band folding due to the periodic potential of intercalated atoms was observed. All these results indicate that Rb atoms are intercalated between the two graphene monolayers in a regular manner similar to that of bulk C${}_{8}$Rb. Further, we observed a parabolic metallic band at the Brillouin-zone center, analogous to the so-called interlayer band, which is thought to play an important role for the superconductivity in graphite intercalation compounds.

Panet al.Reply:

In their comment Calandra \textit{et al} \cite{Calandra}, assert two points: (1) the estimate of charge transfer from Li to graphene layers in LiC$_6$ in our letter \cite{Pan2011c} is incorrect because of the three dimensional (3D) character of the electronic structure in bulk LiC$_6$; (2) our main claim that the superconductivity in graphite intercalation compounds (GICs) is graphene-sheet-driven is therefore invalid.

Comparative study of the phonons in nonsuperconducting BaC6and superconducting CaC6using 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 BaC${}_{6}$, 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 CaC${}_{6}$. Additional IXS intensity is observed below 15 meV in both BaC${}_{6}$ and CaC${}_{6}$. It has been previously suggested that this additional inelastic intensity may arise from defect or vacancy modes not predicted by theory [d'Astuto 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.

Electronic Structure of SuperconductingKC8and NonsuperconductingLiC6Graphite 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 SuperconductingCaC6using Angle-Resolved Photoemission Spectroscopy

We present the first angle-resolved photoemission studies of electronic structure in CaC6, a superconducting graphite intercalation compound with T_{c}=11.6 K. We find that, contrary to theoretical models, the electron-phonon coupling on the graphene-derived Fermi sheets with high-frequency graphene-derived phonons is surprisingly strong and anisotropic. The shape of the Fermi surface is found to favor a dynamical intervalley nesting via exchange of high-frequency phonons. Our results suggest that graphene sheets play a crucial role in superconductivity in graphite intercalation compounds.

Unified Model for Superconductivity in Graphite Intercalation Compounds: Prediction of Optimum Tc and Suggestion for Its Realization

Based on the model that was successfully applied to KC 8 , RbC 8 , and CsC 8 in 1982, we have calculated the superconducting transition temperature T c for CaC 6 and YbC 6 to find that the same model reproduces the observed T c in those compounds as well, indicating that it is a unified model for superconductivity in the graphite intercalation compounds with T c ranging over three orders of magnitude. Further enhancement of T c well beyond 10 K is also predicted.

Mechanism of Superconductivity in Graphite Intercalation Compounds Including CaC6

On the basis of the model that was successfully applied to KC8, RbC8, and CsC8 about a quarter century ago, we have implemented a quantitative calculation of the superconducting transition temperature T c to find that the observed values for T c in both CaC6 and YbC6 are also reproduced in the model, indicating that the model is a unified one for superconductivity in the graphite intercalation compounds with T c ranging three orders of magnitudes.

Electron-phonon interaction in graphite intercalation compounds

Motivated by the recent discovery of superconductivity in Ca- and Yb-intercalated graphite (CaC$_{6}$ and YbC$_{6}$) and from the ongoing debate on the nature and role of the interlayer state in this class of compounds, in this work we critically study the electron-phonon properties of a simple model based on primitive graphite. We show that this model captures an essential feature of the electron-phonon properties of the Graphite Intercalation Compounds (GICs), namely, the existence of a strong dormant electron-phonon interaction between interlayer and $\pi ^{\ast}$ electrons, for which we provide a simple geometrical explanation in terms of NMTO Wannier-like functions. Our findings correct the oversimplified view that nearly-free-electron states cannot interact with the surrounding lattice, and explain the empirical correlation between the filling of the interlayer band and the occurrence of superconductivity in Graphite-Intercalation Compounds.

A two-band electron-phonon model for superconductivity in graphite intercalation compounds

A two-band electron-phonon model for superconductivity in graphite intercalation compounds is developed. A new mechanism for the relaxation times due to interband scattering of intraband pairs is proposed for a superconductor with a two-component order parameter. Two distinct relaxation times τ1 and τ2 of the order parameters are predicted for C6K and C8K.

Superconductivity in graphite intercalation compounds.

Alkali-metal graphite intercalation compounds undergo a normal-to-superconducting transition at a critical temperature ${\mathit{T}}_{\mathit{c}}$ that increases with increasing alkali-metal concentration. Furthermore, the temperature dependence of the critical magnetic field ${\mathit{H}}_{\mathit{c}2}$ is linear down to very low temperatures. A two-band model, which considers coupling between a three-dimensional band and an almost two-dimensional electronic energy band, and which was successfully applied to explain superconductivity in ${\mathrm{C}}_{8}$K, is used to interpret recently reported data. In particular, we show that the increase in ${\mathit{T}}_{\mathit{c}}$ as the alkali-metal concentration increases, the linear temperature dependence of ${\mathit{H}}_{\mathit{c}2}$, and the anisotropy in its value, are consistent with the two-band model.

On the superconductivity of graphite intercalation compounds with sodium

The graphite intercalation compounds with sodium C 2Na and C 3Na have been synthesized at high pressure. All the samples studied — high-pressure phases — are superconductors. In C 2Na (P = 35 kbar) the critical temperature of superconducting transition amounts 5 K.

Model for superconductivity in graphite intercalation compounds

The observed superconductivity in the stage-1 graphite---alkali-metal intercalation compounds (GIC's) is modeled using both graphite $\ensuremath{\pi}$ bands and intercalate $s$ bands. The anisotropy observed in the superconducting properties is explained in terms of the anisotropy of the Fermi surfaces of the GIC's.

Superconductivity of graphite intercalation compounds — Stage and pressure dependence of anisotropy

Superconductivity of graphite intercalation compounds with KHg, RbHg and KTl 1.5 was studied with a particular emphasis on the two-dimensional anisotropy. The stage and pressure dependence of the critical field clearly manifests the way the two-dimensionality is controlled by changing the layer separations. It is argued that the major part in the superconductivity of this class of materials is played by electrons in the intercalant band rather than those in the graphitic band.

Theory of superconductivity in graphite intercalation compounds

The superconducting transition temperature, T c, of the first stage alkali-graphite intercalation compounds (GICs), especially of C 8K, has been calculated, assuming that the three-dimensional (3D) alkali-like electrons contribute to superconductivity. We suggest that the McMillan formula for T c is applicable to the GIC systems, in contrast to a previous theoretical treatment by Takada. Thus the mass enhancement factor of the 3D electrons due to the electron-phonon interactions is calculated from the first principle. It is confirmed that the interaction with the longitudinal phonon modes in which the carbon and alkali metal atoms vibrate along the c-axis is very strong, while that with the phonon modes in which all the atoms vibrate along the layers is screened by the two-dimensional, graphite-like electrons. The former mechanism is characteristic of GICs and this results in superconductivity in GICs. The latter, i.e., the screening effect due to the 2D electrons, is mainly responsible for the reduction of T c by pressure, recently observed in alkali-amalgam GICs.

Superconductivity of graphite intercalation compounds with alkali-metal amalgams

Superconductivity of the alkali-metal amalgam graphite intercalation compounds of stage 1 (${\mathrm{C}}_{4}$KHg, ${\mathrm{C}}_{4}$RbHg) and stage 2 (${\mathrm{C}}_{8}$KHg, ${\mathrm{C}}_{8}$RbHg) has been studied as well as that of the pristine amalgams (KHg, RbHg). The transition temperatures are 0.73, 0.99, 1.90, and 1.40 K for ${\mathrm{C}}_{4}$KHg, ${\mathrm{C}}_{4}$RbHg, ${\mathrm{C}}_{8}$KHg, and ${\mathrm{C}}_{8}$RbHg, respectively. The critical-field anisotropy ratio $\frac{{H}_{c2}^{\ensuremath{\perp}}}{{H}_{c2}^{\ensuremath{\parallel}}}$ is about 10 for the stage 1 and about 15 to 40 for the stage 2. It is argued that electrons in the intercalant bands rather than the graphitic bands play the main role in the superconductivity. An interesting feature is that the stage-2 compound, which has a lower density of states at the Fermi level, has a higher transition temperature than the corresponding stage-1 compound.

Superconductivity in Graphite Intercalation Compounds

ABSTRACTSuperconductivity in graphite intercalation compounds, C4nMHg (M:K, Rb, n=1,2) and C4KTl1.5 is studied in ambient as well as elevated pressures. It is argued that the major role in the superconductivity of this class of materials is played by electrons in the intercalant bands rather than those in the graphite bands. It is experimentally demonstrated that the two-dimensional anisotropy of the superconductivity is controlled by stage and pressure.

Dimensionality of electronic structure and superconductivity of graphite intercalation compounds

Two different models of charge distribution in GICs have come into use: (1) a 2D-model of N graphite sheets having non-uniform charge distribution for Nth stage and (2) a 3D-model of uniformly charge graphite layers having a superlattice of intercalate layers. The former model has been adopted successfully to explain optical spectroscopy, etc. of acceptor-type GICs and the latter has proved to give a fair understanding of magneto-oscillatory phenomena of alkali metal GICs. Further criteria on these models are discussed in connection with the experiments of de Haas-van Alphen (dHvA) effect on GICs. The band calculation of C 8K by Kamimura's group is compared with the dHvA experiments on C 8K and C 8Rb. Newly measured superconducting characteristics of C 8K and C 8KHg are analyzed with the use of 3D anisotropic effective mass model or of 2D Josephson coupling model.

Comparison between ferromagnetism and superconductivity of intercalation compounds

A comparison between the ferromagnetism and superconductivity of graphite intercalation compounds suggests the absence of either phase transition in a two dimensional array.