Plateau Regions for Zero-Bias Peaks within 5% of the Quantized Conductance Value $2e^2/h$

Probing an isolated Majorana zero mode is predicted to reveal a tunneling conductance quantized at $2e^2/h$ at zero temperature. Experimentally, a zero-bias peak (ZBP) is expected and its height should remain robust against relevant parameter tuning, forming a quantized plateau. Here, we report the observation of large ZBPs in a thin InAs-Al hybrid nanowire device. The ZBP height can stick close to $2e^2/h$, mostly within 5% tolerance, by sweeping gate voltages and magnetic field. We further map out the phase diagram and identify two plateau regions in the phase space. Despite the presence of disorder and quantum dots, our result constitutes a step forward toward establishing Majorana zero modes.

Figure 1

(a) Device SEM (false colored). The contacts and gates are Ti/Au ($5/70\mathrm{nm}$ in thickness). The substrate is $p$-doped Si covered by 300 nm thick ${\mathrm{SiO}}_2$, serving as a global BG. (b) Hard gap tunneling spectroscopy at 0 T. (c) $B_z$ dependence of the gap. (d) $B$ scan (aligned with the nanowire) of a ZBP near $2e^2/h$. Lower panel, zero-bias line cut. The pink bar marks the range from 0.95 to 1.05 in the unit of $2e^2/h$ for all figures. (e) “Waterfall” plots of (d). For clarity, every other line cut is shown and two colors are used. (f) $V_{\mathrm{TG}}$ and $V_{\mathrm{BG}}$ scans of the ZBP. (g) Gate and $B$ scans at zero bias. The three-color plots mark red as regions within $\pm 5\%$ of $2e^2/h$. (d),(f),(g) Share the same color bar.

Figure 2

$V_{\mathrm{TG}}=-9.03\mathrm{V}$ for all panels. (a) Zero-bias map in ($B$, $V_{\mathrm{BG}}$) space. Right: the multicolor plot. (b) $B$ scans at four $V_{\mathrm{BG}}$ settings, see the dashed lines in (a). Bottom: zero-bias line cuts, with corresponding colors. (c) $V_{\mathrm{BG}}$ scan at 1.2 T (top) and 1.0 T (middle) with zero-bias line cuts shown in the bottom. (d) Line cuts from (b) at 0 T (dashed) and $\sim 1.73\mathrm{T}$ (solid) with corresponding colors. All panels share the same color bar.

Figure 3

(a) $B$ scan of a ZBP near $2e^2/h$. Lower: zero-bias line cut. (b) Schematic illustration of the angle $\psi$. $B$ is in plane. (c) Angle dependence of the ZBP by fixing the $B$ amplitude at 1.35 T, see the middle black arrow in (a). Lower: zero-bias line cut. (d) Two more rotation scans at 1.2 T (upper) and 1.499 T (middle), see the other two arrows in (a). Lower: zero-bias line cuts. (e) Line cuts from (d) (upper). For clarity, half of the angle range (and every other line cut) is shown. All panels share the same color bar.

Figure 4

(a) Zero-bias map in ($V_{\mathrm{TG}}$, $V_{\mathrm{BG}}$) space at 1.05 T. Gray regions have no data. (b) Replotting (a) using the new axis $V_{\mathrm{A}}$ and $V_B$ (see labeling). Right: three-color plot. (c) Zero-bias maps at 0.3 and 1.35 T in three-color plots. (d),(e) Three additional example scans ($V_{\mathrm{BG}}$, $V_{\mathrm{TG}}$, $B$) for ZBPs within region I. (f),(g) $B_z$ and gate scans of a ZBP in region II. All panels share the same color bar.