Extracting the diffusivity ratio from point contact Andreev reflection spectroscopy and upper critical field measurements in MbB_2

摘要

The diffusivity ratio 77, which measures the relative intraband scattering in the π and σ bands in MgB_2 has been determined by fitting the H_(c2)(T) at T ～ T_c and by Point Contact Andreev Reflection. We find a satisfactory agreement between the values for η obtained by both methods for c-axis orientated MgB_2 thin films. Point contact Andreev Reflection was then applied to bulk MgB_2 containing Mg vacancies. Spectra obtained in zero field indicate a distribution of the two gaps Δ_(σ,π) but no merging of the values with increased magnesium deficiency. Spectra fitted as a function of field are consistent with an increase in π intraband scattering with increasing magnesium deficiency. Measurement of the point contact Andreev reflection spectra as a function of temperature revealed features not immediately expected from current theoretical models.