Two-Dimensional Conical Dispersion in ${\mathrm{ZrTe}}_5$ Evidenced by Optical Spectroscopy

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 or trivial nature of a possible gap opening. Here, we report on a combined optical and transport study of ${\mathrm{ZrTe}}_5$, 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 ${\mathrm{ZrTe}}_5$ cannot be described using the model of 3D Dirac massless electrons, even when staying at energies well above the band gap $2\mathrm{\Delta }=6\mathrm{meV}$ found in our experiments at low temperatures.

Figure 1

(a) Orthorhombic unit cell of ${\mathrm{ZrTe}}_5$. (b) Resistivity and thermoelectric power, (c) Hall coefficient, (d) carrier density, and Hall mobility are shown for samples $A$ and $B$. Blue and red dashed vertical lines denote temperatures $T_A^{\prime }$ and $T_B^{\prime }$, respectively. Inset in (c) illustrates the chemical potential shift with temperature.

Figure 2

Reflectance of (a) sample $A$ and (b) sample $B$ as a function of photon energy. Loss function color map for (c) sample $A$ and (d) sample $B$. The data were taken at each 25 K, and interpolated.

Figure 3

(a) ${\sigma }_1(\omega )$ is shown at 5 K for $E\parallel a$, the calculated conductivity ($\sim \sqrt{\omega }$) is detailed in the text. (b) Landau level transition energies obtained from magneto-optical transmission measurements on sample A, at $T=2\mathrm{K}$. Red lines are a fit to Eq. (4); the gap $2\mathrm{\Delta }=6\mathrm{meV}$ is indicated. The two lowest observed transitions are labeled. Inset shows a relative magnetotransmission spectrum for $B=1\mathrm{T}$. (c) Wide frequency range ${\sigma }_1(\omega )$ for the two polarizations in the $a$-$c$ plane, at $T=5\mathrm{K}$; inset shows low-energy details of ${\sigma }_1$. (d) Temperature evolution of ${\sigma }_1(\omega )$ for $E\parallel a$.