Surface plasmon polaritons in topological Weyl semimetals

Abstract

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...

Negative Refraction in Weyl Semimetals

“…Hereafter, we consider a particular value of the dielectric constant, ε b = 13, which was measured in pyrochlore. [17,22] the Drude dielectric function, which is similar to normal metals (NMs). The appearance of Hall current without external magnetic field is known as anomalous Hall effect given by the second term of Eq.…”

“…As defined before, Ω b = e 2 b/(2π 2 ε 0 ε b ω p ), where b = 0.37Å −1 for pyrochlore [22] and plasmon frequency is given by [17] ω p = 4α 3π…”

“…The presence of a pair of separated Dirac cones is the consequence of symmetry breaking in the WSM, which induces the Hall current, even without magnetic field. [17][18][19] This phenomenon is known as the anomalous Hall effect, which is responsible for the tensor form of the di- * shoufie@flex.phys.tohoku.ac.jp electric function of the WSM [17,23]. In this work, we predict that the EM wave can propagate through WSM even though the frequency is smaller than plasmon frequency.…”

“…[17][18][19][20][21] An example of the WSM is pyrochlore (Eu 2 Ir 2 O 7 ). [17,22] In each cone, the valence and conduction bands coincide at the so-called Weyl nodes. The presence of a pair of separated Dirac cones is the consequence of symmetry breaking in the WSM, which induces the Hall current, even without magnetic field.…”

Chiral magnetic plasmons in anomalous relativistic matter

“…[41] showed that the triangle anomaly implies the existence of a new type of collective excitations that stems from the coupling between the density waves of the chiral and electric charges, which was called the chiral magnetic wave (CMW). Furthermore, it was shown that 3D and 2D topologically nontrivial materials host unusual chiral plasmonic modes confined to their surfaces [44,45] or edges [46]. In view of the relativistic-like quasiparticle dispersion in Dirac and Weyl materials, the main properties of their electromagnetic collective excitations should be qualitatively the same as in relativistic plasmas.…”