Mossbauer second-order Doppler shift and the recoilless fraction of Sn-119 in superconductors

The second-order Doppler shift (SODS) of the Mossbauer spectra in a superconductor, delta(SC), is calculated for the first time. The shift depends on the fourth power of the ratio of the gap energy of the superconductor to that of the matrix element of the electronphonon interaction. At T = T-c, where the gap vanishes, the shift reduces to that in the normal state. The relative values of the SODS and the isomer shifts are found in several superconductors. The isomer shift for Sn-119 in EuBa2 Cu2.98Sn0.02O7-delta has been found. The recoilless fraction of the Sn-119 in EuBa2Cu2.98Sn0.02O7-delta is compared with a previous theory. It is found that the ordinary BCS type gap agrees with the experimental measurements of the recoilless fraction only when the gap is modified to include a lattice distortion in the superconducting state.