Terahertz-optical properties of a bismuth ferrite single crystal

Transmission through a (100)-oriented single crystal of bismuth ferrite (BiFeO3) was studied at frequencies of 0.485-0.765 THz with a continuous-wave source and in the temperature range of 30-350 K as a function of polarization angle. The fine splitting of spin-wave modes that could be detected with our vector network analyzer-based spectrometer is shown to depend on polarization angle. The crystal shows strong, temperature-dependent, linear dichroism. The interference pattern produced in the sample slab revealed a huge birefringence. The fine splitting dependence on polarization angle suggests correlation with the observed dichroism and the birefringence

Radial Spin Texture of the Weyl Fermions in Chiral Tellurium

Trigonal tellurium, a small-gap semiconductor with pronounced magneto-electric and magneto-optical responses, is among the simplest realizations of a chiral crystal. We have studied by spin- and angle-resolved photoelectron spectroscopy its unconventional electronic structure and unique spin texture. We identify Kramers-Weyl, composite, and accordionlike Weyl fermions, so far only predicted by theory, and show that the spin polarization is parallel to the wave vector along the lines in k space connecting high-symmetry points. Our results clarify the symmetries that enforce such spin texture in a chiral crystal, thus bringing new insight in the formation of a spin vectorial field more complex than the previously proposed hedgehog configuration. Our findings thus pave the way to a classification scheme for these exotic spin textures and their search in chiral crystals