MÖSSBAUER SECOND-ORDER DOPPLER SHIFT AND THE RECOILLESS FRACTION OF 119Sn IN SUPERCONDUCTORS

The second-order Doppler shift (SODS) of the Mössbauer spectra in a superconductor, δ 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 electron–phonon 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 119 Sn in EuBa 2 Cu 2.98 Sn 0.02 O 7−δ has been found. The recoilless fraction of the 119 Sn in EuBa 2 Cu 2.98 Sn 0.02 O 7−δ 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.