Superconductivity and pseudogap states studied by microwave conductivity measurements of λ-(BEDT-TSF)2GaCl4

We have investigated the microwave response at 45 GHz in an organic superconductor λ-(BEDT-TSF)2GaCl4 with Tc = 4.8 K. We determine the μ0Hc2–T phase diagram from microwave loss and find that the superconducting state is in the pure limit (l/ξGL ∼ 10). Although the real part of the complex conductivity (=σ1 + iσ2) does not show a coherence peak just below Tc, the London penetration depth completely saturates at low temperatures down to T/Tc = 0.2, which may provide an evidence for a conventional s-wave pairing. In the metallic state below about 50 K, σ1c (parallel to the c  -axis) deviates downward from σdcc, while σ2, which should be zero in a conventional metal, increases exponentially toward Tc. In spite of the fact that the Hagen–Rubens limit is well satisfied as far as the dc conductivity is concerned, a Drude model is unable to explain the large positive σ2. In order to explain such anomalies in the metallic state, we propose a possible existence of so-called a pseudogap near a Fermi level. The anomalous increase of the positive σ2 may be attributed to an appearance of pre-formed electron pairs in the pseudogap state. This appearance can be regarded as a precursor to the superconducting transition. Such a precursory phenomenon has been observed also in the isostructural FeCl4 salt with the anomalous metallic states, which shows a negative σ2 in contrast to the GaCl4 salt. Just the opposite of ground states in between the GaCl4 and FeCl4 salts may result in the contrasting anomalous metallic states with different precursory phenomena with opposite signs of σ2.