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Here we report, based on a systematic theoretical study and a detailed experimental characterization, the existence of topological nodal-line states in the non-centrosymmetric compound PbTaSe2 with strong spin-orbit coupling. While the degeneracy points in Weyl semimetals are robust against any perturbation that preserves translational symmetry, nodal lines require protection by additional crystalline symmetries such as mirror reflection.

The result of this decomposition is demonstrated with L-band Pi-SAR images taken over the city of Niigata, Japan.read more read lessĪbstract: Topological semimetals can support one-dimensional Fermi lines or zero-dimensional Weyl points in momentum space, where the valence and conduction bands touch. A four-component decomposition algorithm is developed to deal with a general scattering case. An appropriate choice among the symmetric or asymmetric volume scattering covariance matrices allows us to make a best fit to the measured data. A modification of probability density function for a cloud of dipole scatterers yields asymmetric covariance matrices. In addition, asymmetric volume scattering covariance matrices are introduced in dependence of the relative backscattering magnitude between HH and VV. This term is relevant for describing man-made targets in urban area scattering. This helix scattering term is added to take account of the co-pol and the cross-pol correlations which generally appear in complex urban area scattering and disappear for a natural distributed scatterer. Helix scattering power is added as the fourth component to the three-component scattering model which describes surface, double bounce, and volume scattering. This scheme includes and extends the three-component decomposition method introduced by Freeman and Durden dealing with the reflection symmetry condition that the co-pol and the cross-pol correlations are close to zero. The covariance matrix approach is used to deal with the nonreflection symmetric scattering case. Abstract: A four-component scattering model is proposed to decompose polarimetric synthetic aperture radar (SAR) images.