Flux pinning and critical currents in polycrystalline MgB2

The results of accurate electrical resistivity and ac susceptibility measurements for a well-characterized, single phase MgB2 sample are reported. The onset of electrical resistivity in magnetic fields B≤1 T is thermally activated R∼exp(−U(T,B)/kBT) with U0(B)=U(T=0,B)∝1/B for B≥0.2T. The pinning potential U0(B=1 T) is similar to that for YBa2Cu3O7 compound, but the region of flux liquid is quite narrow and the upper critical field Bc2 fairly low. High-resolution ac susceptibility measurements show homogeneous superconducting behavior (no Josephson coupled grains) and yield true zero-field critical current density Jc∼(1−T/Tc)n with n≈1.2. The comparison of literature results for Jc with our ones shows pronounced mass density dependence of Jc in bulk untextured MgB2 samples.