Superconductivity and pseudogap states studied by microwave conductivity measurements of λ-(BEDT-TSF) 2GaCl 4

Abstract

We have investigated the microwave response at 45 GHz in an organic superconductor λ-(BEDT-TSF) 2GaCl 4 with T c = 4.8 K. 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 + i σ 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 50 K, σ 1 c (parallel to the c-axis) deviates downward from σ dc c , 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 FeCl 4 salt with the anomalous metallic states, which shows a negative σ 2 in contrast to the GaCl 4 salt. Just the opposite of ground states in between the GaCl 4 and FeCl 4 salts may result in the contrasting anomalous metallic states with different precursory phenomena with opposite signs of σ 2.