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Register a new userDelocalized states in three-terminal superconductor-semiconductor nanowire devices
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We fabricate three-terminal hybrid devices with a nanowire segment proximitized by a superconductor, and with two tunnel probe contacts on either side of that segment. We perform simultaneous tunneling measurements on both sides. We identify some states as delocalized above-gap states observed on both ends, and some states as localized near one of the tunnel barriers. Delocalized states can be traced from zero to finite magnetic fields beyond 0.5 T. In the parameter regime of delocalized states, we search for correlated subgap resonances required by the Majorana zero mode hypothesis. While both sides exhibit ubiquitous low-energy features at high fields, no correlation is inferred. Simulations using a one-dimensional effective model suggest that delocalized states may belong to lower one-dimensional subbands, while the localized states originate from higher subbands. To avoid localization in higher subbands, disorder may need to be further reduced to realize Majorana zero modes.
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Conductance at zero source-drain voltage bias in InSb nanowire/NbTiN superconductor devices exhibits peaks that are close to a quantized value of $2e^2/h$. The nearly quantized resonances evolve in the tunnel barrier strength, magnetic field and magnetic field orientation in a way consistent with Majorana zero modes. Our devices feature two tunnel probes on both ends of the nanowire separated by a 400 nm nanowire segment covered by the superconductor. We only find nearly quantized zero bias peaks localized to one end of the nanowire, while conductance dips are observed for the same parameters on the other end. This undermines the Majorana explanation as Majorana modes must come in pairs. We do identify states delocalized from end to end near zero magnetic field and at higher electron density, which is not in the basic Majorana regime. We lay out procedures for assessing the nonlocality of subgap wavefunctions and provide a classification of nanowire bound states based on their localization.
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