Enhancement of Superconductivity and Evidence of Structural Instability in Intercalated GraphiteCaC6under High Pressure

Abstract

We measured the temperature dependent resistivity, varrho(T), of the intercalated graphite superconductor CaC6 as a function of pressure up to 16 GPa. We found a large linear increase of critical temperature, Tc, from the ambient pressure value 11.5 K up to 15.1 K, the largest value for intercalated graphite, at 7.5 GPa. At approximately 8 GPa, a jump of varrho and a sudden drop of Tc down to approximately 5 K indicates the occurrence of a phase transition. Our data analysis suggests that a pressure-induced phonon softening related to an in-plane Ca phonon mode is responsible for the Tc increase and that higher pressures greater, similar8 GPa lead to a structural transition into a new phase with a low Tc less, similar3 K.

Superconductivity in graphite intercalation compounds

“…For YbC6 this occurs at approximately 2 GPa whilst in CaC6 this occurs at approximately 8 GPa. In the case of CaC6 there was a suggestion [35] that the decrease can be attributed to a structural transition and a subsequent paper confirmed an order to disorder transition at this pressure with no apparent change in space group [36]. This onset of disorder is consistent with the interpretation of the increase in residual resistivity reported in [35].…”

“…In order to search for any trends several groups have made a number of measurements to explore the impact of pressure. Figure 1.4 presents the pressure phase diagrams for both YbC6 [34] and CaC6 [35]. In addition the pressure dependence of Tc in SrC6 is presented in the inset plot in This difference is the point at which the transition temperature begins to decrease.…”

Superconductivity forCaC6to 32 GPa hydrostatic pressure

“…The measured T c (P )-dependence is seen to be reversible and not depend on the temperature at which the pressure is changed. Within experimental error the rate of increase, dT c /dP ≈ +0.40(1) K/GPa, is the same for both samples, but 5 -10 % less than that reported previously in experiments where less hydrostatic pressure media were used [3,4,6].…”

“…This relatively large positive value of the initial slope dT c /dP stands in contrast to the negative dependence normally found in s, p-electron metals which superconduct at ambient pressure [5]. An electrical resistivity study on CaC 6 to 16 GPa using Fluorinert pressure medium reported that T c initially increases under pressure at the rate ∼ +0.5 K/GPa, reaching a maximum value of 15.1 K before dropping abruptly at 9 GPa to ∼ 5 K, apparently due to a first-order structural phase transition [6]. Recent room-temperature synchrotron x-ray diffraction studies to 13 GPa using helium or argon pressure media report the onset of reversible peakbroadening above 9 GPa which is interpreted as giving evidence for an unusual orderdisorder phase tranformation with no change in space group symmetry from R3m rhombohedral [7].…”

Charge density waves in the graphene sheets of the superconductor CaC6

“…The ground states of graphitic materials can be metallic, semi-metallic or semiconducting, depending on the allotrope form or intercalated species, and some have even been shown to superconduct 2,3 with transition temperatures (T c ) above 10 K (ref. 4,5).…”