Spin and orbital hybridization at specifically nested Fermi surfaces in URu$_2$Si$_2$

Abstract: The Fermi surface (FS) nesting properties of URu$2$Si$_2$ are analyzed with particular focus on their implication for the mysterious hidden order phase. We show that there exist two Fermi surfaces that exhibit a strong nesting at the antiferromagnetic wavevector, $\boldsymbol{Q}_0$=(0,\,0,\,1). The corresponding energy dispersions fulfill the relation $\epsilon{1}(\boldsymbol{k})$=$- \epsilon_{2} (\boldsymbol{k}\pm \boldsymbol{Q}_0)$ at eight FS hotspot lines. The spin-orbital characters of the involved $5f$ states are {\it distinct} ($j_z$=$\pm$5/2 {\it vs.} $\pm$3/2) and hence the degenerate Dirac crossings are symmetry protected in the nonmagnetic normal state. Dynamical symmetry breaking through an Ising-like spin and orbital excitation mode with $\Delta j_z$=$\pm$1 induces a hybridization of the two states, causing substantial FS gapping. Concomitant spin and orbital currents in the uranium planes give rise to a rotational symmetry breaking.