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

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.

5 pages, 3 figures

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Cited by in corpus (2)

• Weak metal-metal transition in the vanadium oxytelluride Rb${}_{1-\delta }$V${}_2$Te${}_2$O
• Commensurate lattice distortion in the layered titanium oxypnictides Na${}_2$Ti${}_{2}P{n}_2$O ($Pn=$ As, Sb) determined by X-ray diffraction