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# Pressure dependence of the superconducting transition temperature in C$$_6$$Yb and C$$_6$$Ca

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### Abstract

We have studied the evolution, with hydrostatic pressure, of the recently discovered superconductivity in the graphite intercalation compounds C$$_6$$Yb and C$$_6$$Ca. We present pressure-temperature phase diagrams, for both superconductors, established by electrical transport and magnetization measurements. In the range 0-1.2 GPa the superconducting transition temperature increases linearly with pressure in both materials with $$dT_c/dP = +0.39 K/GPa$$ and $$dT_c/dP = +0.50 K/GPa$$ for C$$_6$$Yb and C$$_6$$Ca respectively. The transition temperature in C$$_6$$Yb, which has beenmeasured up to 2.3 GPa, reaches a peak at around 1.8 GPa and then starts to drop. We also discuss how this pressure dependence may be explained within a plasmon pairing mechanism.

### Most cited references5

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### Superconductivity in Graphitic Compounds

(1965)
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### Superconductivity of graphite intercalation compounds with alkali-metal amalgams

(1982)
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### Electronic Collective Modes and Superconductivity in Layered Conductors

(2003)
A distinctive feature of layered conductors is the presence of low-energy electronic collective modes of the conduction electrons. This affects the dynamic screening properties of the Coulomb interaction in a layered material. We study the consequences of the existence of these collective modes for superconductivity. General equations for the superconducting order parameter are derived within the strong-coupling phonon-plasmon scheme that account for the screened Coulomb interaction. Specifically, we calculate the superconducting critical temperature Tc taking into account the full temperature, frequency and wave-vector dependence of the dielectric function. We show that low-energy plasmons may contribute constructively to superconductivity. Three classes of layered superconductors are discussed within our model: metal-intercalated halide nitrides, layered organic materials and high-Tc oxides. In particular, we demonstrate that the plasmon contribution (electronic mechanism) is dominant in the first class of layered materials. The theory shows that the description of so-called quasi-two-dimensional superconductors'' cannot be reduced to a purely 2D model, as commonly assumed. While the transport properties are strongly anisotropic, it remains essential to take into account the screened interlayer Coulomb interaction to describe the superconducting state of layered materials.
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### Author and article information

###### Journal
07 April 2006
###### Article
10.1016/j.physb.2006.01.327
cond-mat/0604204