First-principles analysis of electron correlation, spin ordering and phonons in the normal state ofFeSe1 − x

Abstract

We present first-principles density-functional-theory-based calculations todetermine the effects of the strength of on-site electron correlation, magneticordering, pressure and Se vacancies on phonon frequencies and electronic structure ofFeSe1 − x. The theoretical equilibrium structure (lattice parameters) ofFeSe depends sensitively on the value of the Hubbard parameterU of on-site correlation and magnetic ordering. Our results suggest that there is a competitionbetween different antiferromagnetic states due to comparable magnetic exchange couplingsbetween first- and second-neighbor Fe sites. As a result, a short range order of stripeantiferromagnetic type is shown to be relevant to the normal state of FeSe at lowtemperature. We show that there is a strong spin–phonon coupling in FeSe (comparable toits superconducting transition temperature) as reflected in large changes in thefrequencies of certain phonons with different magnetic ordering, which is used toexplain the observed hardening of a Raman-active phonon at temperatures (∼100 K) where magnetic ordering sets in. The symmetry of the stripe antiferromagnetic phasepermits an induced stress with orthorhombic symmetry, leading to orthorhombic strain as asecondary order parameter at the temperature of magnetic ordering. The presence of Sevacancies in FeSe gives rise to a large peak in the density of states near the Fermi energy,which could enhance the superconducting transition temperature within the BCS-likepicture.