Electrical transport in nanothick ${\mathrm{ZrTe}}_5$ sheets: From three to two dimensions

${\mathrm{ZrTe}}_5$ is a newly discovered topological material. Shortly after a single layer ${\mathrm{ZrTe}}_5$ had been predicted to be a two-dimensional topological insulator, a handful of experiments have been carried out on bulk ${\mathrm{ZrTe}}_5$ crystals, which however suggest that its bulk form may be a three-dimensional topological Dirac semimetal. We report a transport study on ultrathin ${\mathrm{ZrTe}}_5$ flakes down to 10 nm. A significant modulation of the characteristic resistivity maximum in the temperature dependence by thickness has been observed. Remarkably, the metallic behavior, occurring only below about 150 K in bulk, persists to over 320 K for flakes less than 20 nm thick. Furthermore, the resistivity maximum can be greatly tuned by ionic gating. Combined with the Hall resistance, we identify contributions from a semiconducting and a semimetallic band. The enhancement of the metallic state in thin flakes are a consequence of shifting of the energy bands. Our results suggest that the band structure sensitively depends on the film thickness, which may explain the divergent experimental observations on bulk materials.

Figure 1

Large bilayer flakes by exfoliation. (a) Optical micrograph showing a few large size flakes. The contrast is enhanced to show the flakes. (b) AFM image of one of the flakes. The line profile (blue line) indicates a bilayer.

Figure 2

Time evolution of the surface morphology. (a) AFM image for a freshly cleaved flake. By the thickness, it can be seen that there are a single layer area and a bilayer area. (b) AFM image for another freshly cleaved single layer flake. The line profile indicates a similar surface roughness for the substrate and the flake. (c) AFM image for the sample in (b) after exposed in ambient air for 48 h.

Figure 3

Exfoliated thin ${\mathrm{ZrTe}}_5$ flakes. (a) HRTEM image of a thin flake. Top-right, the electron diffraction looking down the [010] direction. The lattice constants are estimated as $a=0.400\pm 0.002$ nm, $c=1.382\pm 0.002$ nm. (b) Raman spectra at regions of different thickness, measured by AFM. Spectra are shifted for clarity. Inset: The optical image of the measured sample. (c) AFM image of a flake. (d) The line profile shows steps, of which the height corresponds to the layer distance. On the right side, a single layer can be identified by the step height.

Figure 4

Temperature dependence of resistivity for thin ${\mathrm{ZrTe}}_5$ flakes. (a) Resistivity as a function of temperature for flakes of a series of thicknesses. For clarity, not all curves are shown. More data can be found in the Supplemental Material [34]. (b) Temperature of the maximum resistivity $T_{\mathrm{p}}$ versus $t$ for flakes thicker than 20 nm.

Figure 5

Hall resistivity. (a) Hall resistivity as a function of magnetic field at 1.5 K for a few flakes of different thickness prepared in the same batch. The dashed lines are best fits to a two-band model. (b) Carrier density as a function of $t$. Different colors represent different fabrication batches. Solid and open symbols represent $n_1$ and $n_2$, respectively. Since the Hall of the thinnest sample in each batch is linear, the corresponding $n_1$ is directly calculated from the Hall slope.

Figure 6

Gate dependence of the resistivity peak. (a)–(c) Resistivity as a function of temperature at different gate voltages for three samples with $t=14$, 28, and 40 nm, respectively. In (b), the four curves on the top are shifted up in $y$ axis so as to show the change of $T_{\mathrm{p}}$. In (c), the top two curves are shifted. The peak temperature $T_{\mathrm{p}}$ is marked by pink *. (d)–(f) $T_{\mathrm{p}}$ versus $n$ for the samples in (a)–(c), respectively. $n$ is obtained from the low field Hall coefficient. In the vicinity of the transition, the Hall coefficient can be very small as it must change its sign. Thus, $n$ cannot be correctly calculated. Instead, it is interpolated from the $n$ versus the gate voltage $V_{\mathrm{TG}}$ relatively away from the transition point (see the Supplemental Material [34]).

Figure 7

Quantum oscillations of a 15 nm flake at $V_{\mathrm{BG}}=70$ V. (a) Magetoresistance. (b) Resistivity oscillations as a function of $1/B$ after subtracting a smooth background. (c) Damping of the oscillation amplitude with temperature for Landau levels of $n=3$, 4, 5, and 6. Solid lines are fits to $R_{\mathrm{T}}$, see the text.

Figure 8

Schematic two-band model. (a) For thicker flakes, two bands cross the Fermi level. One is a semimetallic, while the other is semiconducting. (b) For thin flakes, only the semimetallic band remains at the Fermi level due to band shifting.

Figure 9

Temperature dependence of low field $R_{\mathrm{H}}$ at different gate voltages for a 40 nm flake. Inset: Replot of $R_{\mathrm{H}}$ at $V_{\mathrm{TG}}=0$ V.