Molecular orbital polarization in Na2Ti2Sb2O: microscopic route to metal-metal transition without spontaneous symmetry breaking

TL;DR

This paper proposes a microscopic mechanism involving spin-orbit coupled molecular orbital polarization to explain a metal-metal transition in Na2Ti2Sb2O that occurs without symmetry breaking, due to partial Fermi surface gapping.

Contribution

It introduces a novel explanation for metal-metal transitions without symmetry breaking, based on momentum-dependent orbital polarization driven by electron interactions.

Findings

Partial Fermi surface gapping observed without density wave order

Orbital polarization sharply increases above a critical interaction

Transition involves gapping of Ti d orbitals while Sb p orbitals remain unaffected

Abstract

Ordered phases such as charge- and spin-density wave state accompany either full or partial gapping of Fermi surface (FS) leading a metal-insulator or metal-metal transition (MMT). However, there are examples of MMT without any signatures of symmetry breaking. One example is Na$_2$Ti$_2$Sb$_2$O, where a partial gapping of FS is observed but a density wave ordering has not been found. Here we propose a microscopic mechanism of such a MMT which occurs due to a momentum dependent spin-orbit coupled molecular orbital polarization. Since a molecular $d$ orbital polarization is present due to a small spin-orbit coupling of Ti, there is no spontaneous symmetry breaking involved. However, a sharp increase of polarization happens above a critical electron interaction which gaps out the $d$ orbtial FS and reduces the density of states significantly, while the rest of FS associated with Sb $p$ orbtials is almost intact across MMT. Experimental implications to test our proposal and applications to other systems are also discussed.