Electrical transport and anisotropic superconducting properties in single crystalline and dense polycrystallineMgB2

The transport properties of dense polycrystalline and single-crystalline ${\mathrm{MgB}}_2$ platelets have been measured to determine the effects of grain size on the upper critical field $H_{c2\mathrm{}}$ and the anisotropy parameter. The $H_{c2\mathrm{}}$ in polycrystalline ${\mathrm{MgB}}_2$ is enhanced and the magnetic-field-induced broadening is significantly reduced with increasing grain size, probably due to reduced contamination at the grain boundaries. Single-crystalline ${\mathrm{MgB}}_2$ exhibits remarkable anisotropy in $H_{c2\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$ and irreversibility field $H^{*}\mathrm{}\left(T\right)\mathrm{}$ with anisotropy $\gamma {=H}_{c2\mathrm{}}^{\mathrm{ab}}{/H}_{c2\mathrm{}}^c\sim 3\pm \mathrm{0.2.}$ The large broadening of the superconducting transition for the $\mathbf{H}||c$ direction indicates a significant suppression of $H^{*}\mathrm{}\left(T\right)\mathrm{}$ below $H_{c2\mathrm{}}^c,$ giving $H^{*c}\mathrm{}\left(T\right)\mathrm{}\sim {0.71H}_{c2\mathrm{}}^c\mathrm{}\left(T\right).$