Understanding nematicity and band structure of FeSe by first-principles study and tight-binding model

We systematically investigate the effects of spin order and orbital polarization on the electronic and magnetic structures of nematic FeSe using the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi>LDA</a:mi> <a:mo>+</a:mo> <a:mi>U</a:mi> </a:mrow> </a:math> method. While our calculations with on-site Coulomb repulsion <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mi>U</b:mi> <b:mo>∼</b:mo> <b:mn>4.5</b:mn> <b:mspace width="0.28em"/> <b:mi>eV</b:mi> </b:mrow> </b:math> can account for both the nonmagnetic state at ambient pressure and the phase transition to a stripe antiferromagnetic state at moderate pressure as observed experimentally, neither static spin order nor static orbital polarization can describe the experimental band dispersions from angle-resolved photoemission spectroscopy (ARPES) studies, due to the underestimate of quantum fluctuations. By employing a particle swarm optimization algorithm, we construct a tight-binding model with inclusion of correlation effects through seeking appropriate renormalization factors of kinetic energies, which successfully reproduce the ARPES results at various temperatures, highlighting the importance of correlations in FeSe.