Conductance Quantization at Zero Magnetic Field in InSb Nanowires

Abstract

Ballistic electron transport is a key requirement for existence of a topological phase transition in proximitized InSb nanowires. However, measurements of quantized conductance as direct evidence of ballistic transport have so far been obscured due to the increased chance of backscattering in one dimensional nanowires. We show that by improving the nanowiremetal interface as well as the dielectric environment we can consistently achieve conductance quantization at zero magnetic field. Additionally, studying the sub-band evolution in a rotating magnetic field reveals an orbital degeneracy between the second and third sub-bands for perpendicular fields above 1 T.

Hybrid superconductor-quantum point contact devices using InSb nanowires

“…19 Additionally, numerical simulations also support a degeneracy from crossing between subbands 2 and 3 of InSb nanowires emerging around 2T. 19 We observe conductance quantization consistent with a QPC having rotational symmetry in all of our InSb QPC devices measured at high bias. Figures 3A and B show the zero-magnetic field conductance quantization for two different length QPCs.…”

“…15 A recent advance in nanowire synthesis has produced epitaxy of superconducting aluminum to InAs nanowires, [16][17] but residual disorder in the InAs nanowire results in unintentional quantum confined regions in these wires. 17 InSb NWs, in contrast, have higher electron mobility 9,15,18 and, as evidenced by clear demonstrations of quantized conductance, 10,19 less intrinsic disorder than InAs NWs. InSb NWs also have large Lande g factors of ~ 40-50 19 , compared to values of ~5-10 in InAs; [20][21][22] this allows for lower magnetic fields required to induce topological superconductivity.…”

“…In contrast, 1D constrictions with rotational symmetry, such as nanowires and nanotubes, possess a different subband structure, which results in different conductance quantization. 19,25,26,27 Rotational symmetry of the nanowire should result in a total or near degeneracy of the 2 nd -3 rd and 4 th -5 th subbands. 19,26,27 Figure 2C shows conductance vs gate voltage measurements for a nanowire QPC having a channel length of 150 nm at 10 mV applied bias.…”

“…1,2,4,9 Additionally, disorder can produce zero-bias conductance signatures similar to Majorana bound states. 11 While the typical signature of ballistic 1D transport-quantized conductance-has been observed in InSb nanowires at high magnetic field 9 and more recently at zero-field, 19 clear demonstrations of ballistic transport at lower fields (< 1T, i.e., before the expected onset of topological superconductivity) in hybrid nanowire-superconductor systems are lacking. Hence, in order to clearly demonstrate topological superconductivity and remove alternative mechanisms for observing zero bias conduction peaks, quantized conductance should be observed at zero magnetic field in NWs proximity coupled to superconductors.…”

“…The imperfect quantization of the 2 G 0 plateau is likely related to remaining disorder in the device. 19 The plateau emerging 4 near 2G 0 indicates there is a finite energy difference in the spacing of the 2 nd and 3 rd subband that is not resolved for high bias scans.Finally, we discuss the proximity effect of the superconducting leads to the InSb nanowire. As shown in Figure 4A, we observe a gate tunable modulation of conduction centered between V SD ≈ ± 250 µV.…”