Lower critical field of MgB<f>2</f> measured by Hall probe

We report on the precise determination of the lower critical field (<f>Hc1</f>) of the newly discovered superconductor MgB<f>2</f> measured by Hall probe. Two polycrystalline samples with different grain sizes and one single crystalline sample are compared. By setting a small criterion for the first flux penetration into the sample, <f>Hc1</f> was determined. Based on our measurements, MgB<f>2</f> seems more similar to borocarbides and low-<f>Tc</f> superconductors, which follow BCS temperature dependence, than to high-<f>Tc</f> superconductors. Comparison with two-gap superconductivity is also made. [Copyright &y& Elsevier]