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

Abstract: We have investigated the microwave response at 45GHz in an organic superconductor λ-(BEDT-TSF)2GaCl4 with T c =4.8K. We determine the μ 0 H c2–T phase diagram from microwave loss and find that the superconducting state is in the pure limit (l/ξ GL ∼10). Although the real part of the complex conductivity (=σ 1 +iσ 2) does not show a coherence peak just below T c, the London penetration depth completely saturates at low temperatures down to T/T c =0.2, which may provide an evidence for a conventional s-wave pairing. In the metallic state below about 50K, (parallel to the c-axis) deviates downward from , while σ 2, which should be zero in a conventional metal, increases exponentially toward T c. 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 σ 2. 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 σ 2 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 σ 2 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 σ 2. [Copyright &y& Elsevier]