Probing intraband excitations in ${\mathrm{ZrTe}}_5$: A high-pressure infrared and transport study

Zirconium pentatetelluride, ${\mathrm{ZrTe}}_5$, shows remarkable sensitivity to hydrostatic pressure. In this work we address the high-pressure transport and optical properties of this compound, on samples grown by flux and chemical vapor transport. The high-pressure resistivity is measured up to 2 GPa, and the infrared transmission up to 9 GPa. The dc conductivity anisotropy is determined using a microstructured sample. Together, the transport and optical measurements allow us to discern band parameters with and without the hydrostatic pressure, in particular the Fermi level, and the effective mass in the less conducting, out-of-plane direction. The results are interpreted within a simple two-band model characterized by a Dirac-type, linear in-plane band dispersion, and a parabolic out-of-plane dispersion.

Figure 1

Orthorhombic structure of ${\mathrm{ZrTe}}_5$ in which zirconium atoms (green balls) are surrounded by tellurium atoms (yellow balls). The unit cell is shown by a solid line. The $b$ axis points between the planes, the zirconium chains run along the $a$ axis [16].

Figure 2

(a) Ambient pressure resistivity of ${\mathrm{ZrTe}}_5$, measured along the $a$ axis, is shown as a function of temperature for both flux- and CVT-grown samples (sample A and B, respectively). Dashed vertical lines denote the resistivity peak temperature $T^{*}$. (b) Resistivity measured under several high pressures for (b) sample A and (c) for sample B. Full lines show resistivity for increasing the pressure, while dotted lines are curves for releasing the pressure. (d) Pressure dependence of the temperature $T^{*}$ where resistivity has a peak. Dotted lines are guides to the eye.

Figure 3

Reflectivity (a), optical conductivity ${\sigma }_1$ (b), and optical transmission (c) are shown as a function of light frequency for sample B, at 25 K, with incident light polarized in the $a\text{−}c$ plane. In each panel, a Drude-Lorentz fit of reflectivity is shown in dotted lines, resulting in a calculated transmission and ${\sigma }_1$. Absorption onset or optical gap is marked by an arrow in each panel. It is visible as a hump in the reflectivity, a half-step in the optical conductivity ${\sigma }_1$. Inset in (c) shows the first derivative of transmission, in which a kink corresponds to the absorption onset.

Figure 4

(a) Infrared transmission at 25 K for sample B, taken at a series of pressures up to 9 GPa. Pressure values are in GPa, next to each transmission curve. Blue circles mark the position of a kink in the first derivative, which is taken as an approximate optical gap. (b) Extracted approximate value of the optical gap, or Pauli blocking edge, as a function of pressure. Dotted line serves to extract the parameter $\alpha$, as described in the text. (c) Pressure dependence of an infrared phonon at 23 meV ($185{\mathrm{cm}}^{-1})$ for ambient pressure.

Figure 5

Anisotropy of resistivity determined on a CVT-grown sample, prepared by focused ion beam technique. (a) Electron microscope picture of a microstructured sample prepared by focused ion beam technique. Top part shows the crystal from which two lamellas are shaped and cut. Bottom part shows a sample whose plane contains $a$ and $b$ axes. (b) Resistivity measured on sample B for all three crystal directions.

Figure 6

(a) Electronic band structure determined by DFT, which considers spin-orbit coupling. (b) Schematic zoom around the $\mathrm{\Gamma }$ point shows model bands ${\epsilon }_1$ and ${\epsilon }_2$. (c) Numerically determined resistivity as a function of temperature, at three different pressures. Dotted vertical lines indicate the positions of the resistivity peak.