Investigation of Li-doped MgB2

Abstract

We present a systematic characterization ofMg1−xLixB2 polycrystalline samples with nominal Li contentx up to 0.30. We explore the effectiveness of the substitution and investigate its influence onsuperconducting and normal state properties. The structural analyses by neutron and x-raydiffraction indicate that, despite the lattice parameters remaining unchanged within theexperimental accuracy, around 30% of the nominal Li content actually enters the structure.Also the transition temperature is only weakly affected by Li substitution, but its relationshipwith the residual resistivity and magnetoresistivity data is compatible with a picture whereπ bands are filled with holes and/or affected by disorder, as predictedby theory. We also measured the magnetic penetration depthλ by an inductance technique. Data fits based on the standardBardeen–Cooper–Schrieffer two-band model yield the zero-temperature limit of bothλ and superconductinggaps: we find that λ(0) increases weakly and quickly saturates with increasingx, whereasΔπ(0) andΔσ(0) decreasewith x. This analysis suggests that lithium substitution induces disorder mainly in theπ band. Point contact spectroscopy results on samples obtained from the same batch are infull agreement with magnetic penetration depth data on the undoped sample, but givesuperconducting gaps almost insensitive to Li substitution, showing at most a small increase inΔπ(0) that may be related to an inhomogeneous distribution of Li content.