A Weyl semimetallic state with pairs of nondegenerate Dirac cones in three dimensions was recently predicted to occur in the antiferromagnetic state of the pyrochlore iridates. Here, we show that the THz optical conductivity and temperature dependence of the free carrier response in pyrochlore Eu2Ir2O7 match the predictions for a Weyl semimetal and suggest novel Dirac liquid behavior. The interband optical conductivity vanishes continuously at low frequencies signifying a semimetal. The metal-semimetal transition at TN = 110 K is manifested in the Drude spectral weight, which is independent of temperature in the metallic phase, and which decreases smoothly in the ordered phase. The temperature dependence of the free carrier weight below TN is in good agreement with theoretical predictions for a Dirac material. The data yield a Fermi velocity vF ≈ 4 • 10 7 cm/s, a logarithmic renormalization scale ΛL ≈ 600 K, and require a Fermi temperature of TF ≈ 100 K associated with residual unintentional doping to account for the low temperature optical response and dc resistivity.
We consider theoretically surface plasmon polaritons in Weyl semimetals. These materials contain pairs of band touching points -Weyl nodes -with a chiral topological charge, which induces an optical anisotropy and anomalous transport through the chiral anomaly. We show that these effects, which are not present in ordinary metals, have a direct fundamental manifestation in the surface plasmon dispersion. The retarded Weyl surface plasmon dispersion depends on the separation of the Weyl nodes in energy and momentum space. For Weyl semimetals with broken time-reversal symmetry, the distance between the nodes acts as an effective applied magnetic field in momentum space, and the Weyl surface plasmon polariton dispersion is strikingly similar to magnetoplasmons in ordinary metals. In particular, this implies the existence of nonreciprocal surface modes. In addition, we obtain the nonretarded Weyl magnetoplasmon modes, which acquire an additional longitudinal magnetic-field dependence. These predicted surface plasmon results are observable manifestations of the chiral anomaly in Weyl semimetals and might have technological applications.PACS numbers: 73.20. Mf, 78.68.+m, 71.20.Gj, 03.65.Vf Surface plasmon polaritons (SPPs) are collective electromagnetic and electron-charge excitations that are confined to the surface of a metal or semiconductor. They were proposed in the 1950's [1, 2] and have been observed via electron energy loss spectroscopy [3,4] as well as optically via surface gratings [5] or attenuated total reflection [6]. Over the past decades, SPPs have found widespread technological applications, for example, in surface microscopy [7], for biomolecular detection [8], or lithography [9]. Because SPPs are focused to sizes smaller than the wavelength of light, they hold promise to realize miniaturized plasmon-based optoelectronic devices, and research in creating such plasmonic devices is flourishing [10], with the subject being dubbed "plasmonics" or "nano plasmonics," which is a huge applied physics field in its own right.In this Rapid Communication, we add a fundamental physical aspect to the study of SPPs (and the field of plasmonics), and demonstrate that the surface plasmon polaritons of recently discovered Weyl semimetals (WSMs), which possess topological properties, show a much richer (and unanticipated) structure compared to standard SPPs in ordinary metals and semiconductors. We find that due to the quantum anomalous electrodynamic response of the WSM (which is their hallmark), the retarded Weyl surface plasmon is strongly sensitive to details of the band structure. In particular, we find a geometry in which the SPP is nonreciprocal (i.e., the propagation is unidirectional), even without an applied external magnetic field. In addition, we show that the magnetoplasmon mode displays an additional longitudinal field dependence which is absent in ordinary metals. This can serve as a direct signature of Weyl semimetals in surface measurements. We note that the SPP physics introduced in this work applies...
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