Orbital-enhanced Warping Effect in P\textsubscript{x},P\textsubscript{y}-derived Rashba Spin Splitting of Monatomic Bismuth Surface Alloy Surface Alloy

Abstract

Spin-split Rashba bands have been exploited to efficiently control the spin degree of freedom of moving electrons, which possesses a great potential in frontier applications of designing spintronic devices and processing spin-based information. Given that intrinsic breaking of inversion symmetry and sizeable spin-orbit interaction, two-dimensional (2D) surface alloys formed by heavy metal elements exhibit a pronounced Rashba-type spin splitting of the surface states. Here, we have revealed the essential role of atomic orbital symmetry in the hexagonally warped Rashba spin-split surface state of $\sqrt{3}\times \sqrt{3}R30^{\circ }$ BiCu${}_2$ monatomic alloy by scanning tunneling spectroscopy (STS) and density functional theory (DFT). From $\mathrm{d}I/\mathrm{d}U$ spectra and calculated band structures, three hole-like Rashba-split bands hybridized from distinct orbital symmetries have been identified in the unoccupied energy region. Because of the hexagonally deformed Fermi surface, quasi-particle interference (QPI) mappings have resolved scattering channels opened from interband transitions of \textit{p${}_x,$p${}_y$}($m_j=1/2$) band. In contrast to the \textit{s,p${}_z$}-derived band, the hexagonal warping predominately is accompanied by substantial out-of-plane spin polarization $S_z$ up to 24\% in the dispersion of \textit{p${}_x$,p${}_y$}($m_j=1/2$) band with an in-plane orbital symmetry.