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We report on optical reflectivity experiments performed on Cd3As2 over a broad range of photon energies and magnetic fields. The observed response clearly indicates the presence of 3D massless charge carriers. The specific cyclotron resonance absorption in the quantum limit implies that we are probing massless Kane electrons rather than symmetry-protected 3D Dirac particles. The latter may appear at a smaller energy scale and are not directly observed in our infrared experiments.

Zirconium pentatelluride was recently reported to be a 3D Dirac semimetal, with a single conical band, located at the center of the Brillouin zone. The cone's lack of protection by the lattice symmetry immediately sparked vast discussions about the size and topological/trivial nature of a possible gap opening. Here we report on a combined optical and transport study of ZrTe5, which reveals an alternative view of electronic bands in this material. We conclude that the dispersion is approximately linear only in the a-c plane, while remaining relatively flat and parabolic in the third direction (along the b axis). Therefore, the electronic states in ZrTe5 cannot be described using the model of 3D Dirac massless electrons, even when staying at energies well above the band gap 2∆ = 6 meV found in our experiments at low temperatures. arXiv:1905.00280v2 [cond-mat.mes-hall]

The detailed optical properties of BaFe2As2 have been determined over a wide frequency range above and below the structural and magnetic transition at TN ≃ 138 K. A prominent in-plane infrared-active mode is observed at 253 cm −1 (31.4 meV) at 295 K. The frequency of this vibration shifts discontinuously at TN ; for T < TN the frequency of this mode displays almost no temperature dependence, yet it nearly doubles in intensity. This anomalous behavior appears to be a consequence of orbital ordering in the Fe-As layers.

BiTeI is a giant Rashba spin splitting system, in which a noncentrosymmetric topological phase has recently been suggested to appear under high pressure. We investigated the optical properties of this compound, reflectivity and transmission, under pressures up to 15 GPa. The gap feature in the optical conductivity vanishes above p ∼ 9 GPa and does not reappear up to at least 15 GPa. The plasma edge, associated with intrinsically doped charge carriers, is smeared out through a phase transition at 9 GPa. Using high-pressure Raman spectroscopy, we follow the vibrational modes of BiTeI, providing additional clear evidence that the transition at 9 GPa involves a change of crystal structure. This change of crystal structure possibly inhibits the high-pressure topological phase from occurring. DOI: 10.1103/PhysRevLett.112.047402 PACS numbers: 78.20.hb, 62.50.-p, 78.30.Am, 78.40.Fy Interest in the noncentrosymmetric semiconductor BiTeI surged when it was found that this compound hosts the largest known Rashba spin splitting in bulk form [1][2][3]. While this material is structurally related to the recently discovered bismuth chalcogenide topological insulators [4,5], it is an insulator of the common variety at ambient pressure. Recent first-principles band structure calculations suggested that BiTeI undergoes a transition to the topological insulating phase under pressure [6], through which BiTeI would become the first example of noncentrosymmetric topological insulator. Moreover, such a bandstructure topology change realizes a remarkable example of topological phase transition. While several examples of topological phase transitions occurring upon varying chemical composition have been reported in the literature [7][8][9], the pressure-induced transition in BiTeI would present the advantage of being controllable and reversible.Optical conductivity is well suited to probe the band structure of BiTeI under pressure. In this Letter, we determine the high-pressure optical properties by measuring transmission and reflectivity of BiTeI up to 15 GPa. We follow the optical gap under pressure and find that it decreases monotonically until 9 GPa. At this pressure the plasma edge associated with the doped carriers is strongly broadened due to a sudden increase of σ 1 ðωÞ at the plasma frequency. Above this pressure the gap feature in the optical conductivity has disappeared, and it does not reappear to the highest pressure reached. The high-pressure phase appears to be metallic. Using Raman spectroscopy, we observe a sudden change in the number and frequency of the vibrational modes at 9 GPa, which shows that a structural transition occurs at this pressure.Single crystals of BiTeI were grown by the floating zone method, starting from the stoichiometric ratio of metallic bismuth, tellurium and bismuth iodide. The unit cell of BiTeI is composed of triple layers, Te-Bi-I, stacked along the polar c-axis [1]. The triple layers are bound by a weak van der Waals interaction. The structure is described by the noncentrosymmetric space...