Reversible-magnetization derived London penetration depth of κ-(BEDT-TTF)2X for X=Cu(NCS)2 and X=Cu[N(CN)2]Br: No indication for unconventional superconductivity

Abstract

In this paper we summarize our recent results on the magnetic penetration depth λ of the title organic superconductors. Using the London model, we determine the temperature dependence of λ from the slope of the linear M(lnH) dependence of the reversible isothermal magnetization. For both salts the in-plane London penetration depth shows only a weak T dependence at low temperature in excellent agreement with a conventional order parameter. Our results are incompatible with any of the unconventional pairing schemes proposed by Kanoda et al. and Le et al.. It is also the intention of this paper to address experimental difficulties in determining the magnetic penetration depth, in particular, the influence of extrinsic factors as temperature dependent flux pinning and demagnetization effects on the various experimental techniques.