Experimental investigation of electronic interactions in collapsed and uncollapsed phases

The iron-based pnictide LaFe2As2 is not superconducting as synthesized, but it becomes such below Tc ∼ 12 K upon annealing, as a consequence of a structural transition from a phase with collapsed tetragonal crystal structure to an uncollapsed phase. In this work, we carry out specific heat, Raman spectroscopy, and normal state electric and thermoelectric transport measurements in the collapsed and uncollapsed LaFe2As2 phases to gain insight into the electron interactions and their possible role in the superconducting pairing mechanism. Despite clear features of strong electron-phonon coupling observed in both phases, neither the low energy phonon spectra nor the electron-phonon coupling show significant differences between the two phases. Conversely, the Sommerfield constants are significantly different in the two phases, pointing to much higher electron correlation in the superconducting uncollapsed phase and confirming theoretical studies