First-principles study of magnetism, lattice dynamics, and superconductivity in LaFeSiH_x

The structural, electronic, magnetic, and vibrational properties of LaFeSiH_x for x between 0 and 1 are investigated using density functional calculations. We find that the electronic and magnetic properties are strongly controlled by the hydrogen concentration x in LaFeSiHx. While fully hydrogenated LaFeSiH has a striped antiferromagnetic ground state, the underdoped LaFeSiH_x for x ≦ 0.75 , is not magnetic within the virtual crystal approximation or with explicit doping of supercells. The antiferromagnetic configuration breaks the symmetry of Fe d-orbitals and increases electron-phonon coupling up to 50%, especially for modes in the 20-50 meV range that are associated with Fe atomic movement. We find competing nearest and next-nearest neighbor exchange interactions and significant spin-phonon coupling, qualitatively similar but smaller in magnitude compared those found in LaOFeAs superconductors. The superconducting T_c for antiferromagnetic LaFeSiH_x, assuming conventional superconductivity via McMillan's equation, therefore is computed to be 2-10 K, in contrast to T_c≈0 for the nonmagnetic material. We also predict that the LaFeSiH_x could be a good proton conductor due to phase stability with a wide range of hydrogen concentration x < 1.