Low-field microwave absorption in organic superconductor k-(ET)2[Cu(NCS)2]

Superconductivity of C60 Langmuir−Blodgett Films Doped with Potassium: Low-Field Signal and Electron Spin Resonance Study

“…The broad signal showed a Curie-like temperature dependence until it disappeared above 25 K; the narrow one showed a Pauli-like temperature dependence above 180 K and a Curie-like behavior in the lower temperature range. Similar behavior was observed in bulk K-doped C 60 …”

“…Figure 3 presents the results. The intensity of LFS (Figure 3a) decreased slightly with increasing temperature, until the LFS disappeared at 12.9 K; this indicates T c (onset) ≈ 12.9 K. Compared with the LFS for a bulk K-doped C 60 sample (where LFS is 10 2 larger than background), 11 the intensity of the LFS for the K-doped C 60 LB films is very weak (at best five times larger than the noise level). Even for well-doped state, the LFS is relatively "noisy".…”

“…Figure presents the results. The intensity of LFS (Figure a) decreased slightly with increasing temperature, until the LFS disappeared at 12.9 K; this indicates T c (onset) ≈ 12.9 K. Compared with the LFS for a bulk K-doped C 60 sample (where LFS is 10 2 larger than background), the intensity of the LFS for the K-doped C 60 LB films is very weak (at best five times larger than the noise level). Even for well-doped state, the LFS is relatively “noisy”.

3 LFS and EPR spectra for C 60 LB film sample L1 after 480 min of K-doping: (a) LFS (microwave power P = 10 mW, modulation amplitude H mod = 5 G, receiver gain G = 1 × 10 4 ); (b) ESR spectrum ( P = 0.5 mW, H mod = 0.5 G).

…”

“…Similar behavior was observed in bulk K-doped C 60 . 11 In sample L2, after 690 min of doping, the color of most C 60 LB film strips became black or dark brown. The LFS still exists at and below 12 K (Figure 4a) and is very similar to that of sample L1.…”

“…Due to air sensitivity of the alkali-metal-doped samples and the potential for inhomogeneous doping, it is difficult to study the doping process in situ and to find optimal doping conditions by routine resistivity or SQUID (superconducting quantum interference device) measurements. The low magnetic field microwave absorption, or low-field signal (LFS), due to the Meissner−Ochsenfeld effect, has been detected for high T c superconductors, − organic superconductors, − alkali-metal-doped C 60 superconductors, and K-doped C 60 LB films; the LFS signal has proved to be a sensitive and simple method for the study of negligibly small amounts of superconducting phases in non-superconducting or superconducting matrices and air-sensitive samples …”