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تسجيل مستخدم جديدObservation of Double Weyl Phonons in Parity-Breaking FeSi
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Condensed matter systems have now become a fertile ground to discover emerging topological quasi-particles with symmetry protected modes. While many studies have focused on Fermionic excitations, the same conceptual framework can also be applied to bosons yielding new types of topological states. Motivated by the recent theoretical prediction of double-Weyl phonons in transition metal monosilicides [Phys. Rev. Lett. 120, 016401 (2018)], we directly measured the phonon dispersion in parity-breaking FeSi using inelastic x-ray scattering. By comparing the experimental data with theoretical calculations, we make the first observation of double-Weyl points in FeSi, which will be an ideal material to explore emerging Bosonic excitations and its topologically non-trivial properties.
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Topological states of electrons and photons have attracted significant interest recently. Topological mechanical states also being actively explored, have been limited to macroscopic systems of kHz frequency. The discovery of topological phonons of a
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In 1929, H. Weyl proposed that the massless solution of Dirac equation represents a pair of new type particles, the so-called Weyl fermions [1]. However the existence of them in particle physics remains elusive for more than eight decades. Recently,
significant advances in both topological insulators and topological semimetals have provided an alternative way to realize Weyl fermions in condensed matter as an emergent phenomenon: when two non-degenerate bands in the three-dimensional momentum space cross in the vicinity of Fermi energy (called as Weyl nodes), the low energy excitation behaves exactly the same as Weyl fermions. Here, by performing soft x-ray angle-resolved photoemission spectroscopy measurements which mainly probe bulk band structure, we directly observe the long-sought-after Weyl nodes for the first time in TaAs, whose projected locations on the (001) surface match well to the Fermi arcs, providing undisputable experimental evidence of existence of Weyl fermion quasiparticles in TaAs.
We measured the reflectivity of a single crystal of FeSi from the far-infrared to the visible region (50-20000 wavenumber), varying the temperature between 4 and 300 K. The optical conductivity function was obtained via Kramers-Kronig analysis. We ob
served a dirty metal-like behavior at room temperature and the opening of a gap of 70 meV at low temperature. The results of a group theoretical analysis of the lattice vibrational modes are presented and compared to the experimental data. Of the five optical phonons expected for this material only four have been observed in the far-infrared region.
Using femtosecond time-resolved X-ray diffraction, we investigate optically excited coherent acoustic phonons in the Weyl semimetal TaAs. The low symmetry of the (112) surface probed in our experiment enables the simultaneous excitation of longitudin
al and shear acoustic modes, whose dispersion closely matches first-principles calculations and previously measured elastic properties. We find an asymmetry in the spectral lineshape of the longitudinal mode that is notably absent from the shear mode, suggesting a time-dependent frequency chirp that is likely driven by photoinduced carrier diffusion. Our study underscores the benefit of using off-axis crystal orientations when optically exciting shear deformations in topological semimetals, allowing one to transiently change their crystal structure and potentially their topological properties.
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