Energy Gap in Superconducting Fullerides: Optical and Tunneling Studies

Abstract

Tunneling and optical transmission studies have been performed on superconducting samples of Rb3C60. At temperatures much below the superconducting transition temperature Tc the energy gap is 2∆ = 5.2 ± 0.2meV, corresponding to 2∆/kBTc = 4.2. The low temperature density of states, and the temperature dependence of the optical conductivity resembles the BCS behavior, although there is an enhanced "normal state" contribution. The results indicate that this fulleride material is an s-wave superconductor, but the superconductivity cannot be described in the weak coupling limit. PACS: 74.70.Wz 74.25.Gz 74.50.+r

Fullerenes and carbon derivatives: From insulators to superconductors

“…2 was also found by the latest tunnelling experiment on pressed pellet polycrystalline sample by Koller et al [144]. The same group also investigated the optical transmission on thin ® lm and found the optical properties to be consistent with the BCS prediction but with a gap of about 4 .…”

“…2kB Tc. Even though this coincidental agreement between optical and tunnelling data was claimed to be indicative of a strong-coupling limit, Koller et al [144] also estimated that superconductivity is mediated by the low-lying vibrational intramolecular mode of the C60 molecule (at about 210 cm -1 ) in partial agreement with the Eliashberg calculation discussed above (® gure 30) [26]. Nonetheless, these latter results were not obtained on a single crytal.…”

“…Nonetheless, these latter results were not obtained on a single crytal. The resistivity of the ® lm su ers from granularity e ects, leading to an anomalous increasing behaviour of q ( T) for T ® Tc [144]. Moreover, it is well known that polycrystalline samples often exhibit large Tc transition widths and low superconducting fractions.…”

Thermodynamic Properties of the Superconducting State in the K3C60Fulleride

“…In the case of the complete substitution of the carbon isotope 12 C by 13 C the isotope coefficient (α) is equal to 0.3 ± 0.06 [8]. On the other hand, the very high value of α equal to 1.43 can be achieved for the partial substitution [9].…”

“…Second, the order parameter has a pure s-wave symmetry [11][12][13][14]. This fact can be explained in a natural way by taking into consideration an assumption that the electron-phonon interaction is responsible for the superconducting phase induction.…”