Characteristic features of the temperature dependence of the surface impedance in polycrystalline MgB 2 samples

– The real Rs(T) and imaginary Xs(T) parts of the surface impedance Zs(T) = Rs(T) + iXs(T) in polycrystalline MgB2 samples of different density with the critical temperature Tc ≈ 38 K are measured at the frequency of 9.4 GHz and in the temperature range 5 ≤ T < 200 K. The normal skin-effect condition Rs(T) = Xs(T) at T ≥ Tc holds only for the samples of the highest density with roughness sizes not more than 0.1 µm. For such samples extrapolation T → 0 of the linear at T < Tc/2 temperature dependences λL(T) = Xs(T)/ωµ0 and Rs(T) results in values of the London penetration depth λL(0) ≈ 600Å and residual surface resistance Rres ≈ 0.8 mΩ. In the entire temperature range the dependences Rs(T) and Xs(T) are well described by the modified two-fluid model. The inconclusive situation with the mechanism of superconductivity in MgB 2 [1] is mainly due to the lack of consensus on many important physical quantities in this material. For example, the energy gap ratio 2∆/kT c ranging from 1 to 5 has been reported, which raises the possibility of an anisotropic energy gap or a multiple gap. Indeed, recent tunneling [2] and point contact [3] experiments showed two distinct conductance peaks of different magnitude, and to describe the specific heat data [4] the authors involved either two gaps or an anisotropic gap. At the same time a number of high-frequency measurements in MgB 2 films found a small single gap (the gap ratio is well below the weak coupling value) if the data was fitted by BCS model with isotropic s-wave order parameter [5, 6, 7, 8]. In particular, the exponential temperature dependence of the magnetic field penetration depth at T ≪ T c gave occasion to such fitting [7, 8]. However, in very dense polycrystalline MgB 2 samples the linear ∆λ(T) ∝ T behavior was observed [9, 10]. Other measurements in MgB 2 powders and ceramics provided rather contradictory results: ∆λ(T) dependences were proportional to The estimated values of λ(0) varied from as low as 85 nm in Ref.[12] up to 300 nm in Ref.[6]. Apparently, the data spread indicates that the measured parameters of MgB 2 depend on the sample quality and growth technique. Nevertheless, it is generally accepted that in the ab-plane of MgB 2 the coherence length ξ 0 does not exceed 100Å and, hence, MgB 2 samples characterized …