Journal Article Bardeen-Cooper-Schrieffer theory Migdal-Eliashberg theory Research Support, Non-U.S. Gov't Research Support, U.S. Gov't, Non-P.H.S. high pressure iron-pnictides layered materials low temperature nuclear resonant scattering phonon density of states phonon-modulated superconductivity pressure pressure-stabilized hydrides structural transition superconductivity superconductivity enhancement yttrium hexahydride yttrium hydrides