Point Contact Andreev Reflection and the measurement of spin polarization - towards higher fields and temperatures

Summary form only given. In this work, PCAR spectra are measured in the standard needle-anvil configuration, where the superconducting tip lands on the film of investigated material, for the case of Nb-Ti. Alternatively, for the case of MgB
₂
, the superconductor is in thin film form and the investigated material is shaped into a needle, in order to form the point contacts. The conventional low-field and temperature PCAR spectra are interpreted using the modified Blonder-Tinkham-Klapwijk [4] (mBTK) theory allowing for the extraction of a number of fitted parameters superconducting proximity gap (Δ1), barrier strength (Z), spin polarization (P), and effective electronic temperature (Te). The analysis of the high-field and high-temperature PCAR spectra is currently qualitative. The superconducting parameters of both Nb-Ti and MgB
₂
represent substantial improvement over the traditionally used Nb. The critical temperature of Nb-Ti is 9.5 K, with an upper critical field of about 9 T at 4.2 K, whereas the critical temperature of the MgB
₂
film is 32.8(8) K, with an upper critical field in excess of 14 T at 4.2 K. Reproducible spin polarization measurements have been possible with Nb-Ti superconducting tips in magnetic fields of up to 7 T.