  Semimetallic tungsten ditelluride (WTe2) displays an extremely large
non-saturating magnetoresistance (XMR), which is the subject of intense
interest. This phenomenon is thought to arise from the combination of perfect
n-p charge compensation with low carrier densities in WTe2 and presumably
details of its band structure. Recently, "spin texture" induced by strong
spin-orbital coupling (SOC) has been observed in WTe2 by angle-resolved
photoemission spectroscopy (ARPES). This provides a mechanism for protecting
backscattering for the states involved and thus was proposed to play an
important role in the XMR of WTe2. Here, based on our density functional
calculations for bulk WTe2, we found a strong Rashba spin-orbit effect in the
calculated band structure due to its non-centrosymmetric structure. This splits
bands and two-fold spin degeneracy of bands is lifted. A prominent Umklapp
interference pattern (a spectroscopic feature with involving reciprocal lattice
vectors) can be observed by scanning tunneling microscopic (STM) measurements
on WTe2 surface at 4.2 K. This differs distinctly from the surface atomic
structure demonstrated at 77 K. The energy dependence of Umklapp interference
shows a strong correspondence with densities of states integrated from ARPES
measurement, manifesting a fact that the bands are spin-split on the opposites
side of Gamma point. Spectroscopic survey reveals the ratio of electron/hole
asymmetry changes alternately with lateral locations along b axis, providing a
microscopic picture for double-carrier transport of semimetallic WTe2. The
calculated band structure and Fermi surface is further supported by our ARPES
results and Shubnikov-de Haas (SdH) oscillations measurements.
