Ubiquitous Non-Majorana Zero-Bias Conductance Peaks in Nanowire Devices

Chen, Jun; Woods, Benjamin D.; Yu, Peng; Hocevar, Moïra; Car, Diana; Plissard, Sébastien; Bakkers, Erik P. A. M.; Stãnescu, Tudor D.; Frolov, Sergey

doi:10.1103/physrevlett.123.107703

Cited by 135 publications

(81 citation statements)

References 46 publications

(64 reference statements)

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“…A crucial question is whether single-subband or fewsubbands occupancy can be obtained by applying a negativeenough gate potential. The relevance of this question becomes clear in the context of realizing topological superconductivity and Majorana zero modes (MZMs) using hybrid SM-SC structures, as high occupancy is not only detrimental to the stability of MZMs (as a result of being associated with smaller topological gaps), but also generates ubiquitous trivial low-energy states in the presence of disorder or system inhomogeneities [27][28][29][30]33,36,47,48 . To answer this question, we first notice that, in principle, the conduction bands will eventually become completely depleted for a large enough value of | V g |.…”

Section: Resultsmentioning

confidence: 99%

“…The reduced inter-subband energy separation associated with high subband occupancy makes the system susceptible to perturbations such as disorder and inhomogeneous effective potentials due to the presence of multiple gates, strain, etc. If the subbands are not well separated, these perturbations lead to inter-subband mixing 33,52 , which, in turn, induces (topologically-trivial) low-energy Andreev bound states (ABSs) 36 . Note that this type of near-zero energy states are generic in class D systems 37,53 and were explicitly shown to emerge in proximitized nanowires (in the presence of spin-orbit coupling and nonzero Zeeman field) within a large window of local gate potentials 33,36 .…”

Section: Resultsmentioning

confidence: 99%

“…If the subbands are not well separated, these perturbations lead to inter-subband mixing 33,52 , which, in turn, induces (topologically-trivial) low-energy Andreev bound states (ABSs) 36 . Note that this type of near-zero energy states are generic in class D systems 37,53 and were explicitly shown to emerge in proximitized nanowires (in the presence of spin-orbit coupling and nonzero Zeeman field) within a large window of local gate potentials 33,36 . We emphasize that the emergence of this type of low-energy states does not require the explicit presence of disorder, as multiple subband occupancy leads to an effective random matrix Hamil- tonian in the presence of any perturbation that couples the subbands (e.g., applied gate potentials, strain, etc.).…”

Section: Resultsmentioning

confidence: 99%

“…While some of the ABS-producing mechanisms such as, for example, long-range inhomogeneous potentials (e.g., soft confinement) and inhomogeneous paring, can be understood within a strictly one dimensional model, other mechanisms rely on or are strongly enhanced by the presence of multiple occupied subbands in wires with finite thickness. For example, the inter-band coupling mechanism produces lowenergy ABSs though mixing multiple subbands in the presence of, e.g., an inhomogeneous electrostatic potential 33,36 . Note that this is a specific example of trivial low-energy states that emerge generically in systems with only particle-hole symmetry 37 (i.e.…”

Section: Introductionmentioning

confidence: 99%

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Subband occupation in semiconductor-superconductor nanowires

2020

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Subband occupancy (i.e. the number of occupied subbands or energy levels in the semiconductor) is a key physical parameter characterizing the topological properties of superconductor-semiconductor hybrid systems in the context of the search for non-Abelian Majorana zero modes. We theoretically study the subband occupation of semiconductor nanowire devices as function of the applied gate potential, the semiconductor-superconductor (SM-SC) work function difference, and the surface charge density by solving self-consistently the Schrödinger-Poisson equations for the conduction electrons of the semiconductor nanowire. Realistic surface charge densities, which are responsible for band bending, are shown to significantly increase the number of occupied subbands, making it difficult or impossible to reach a regime where only a few subbands are occupied. We also show that the energy separation between subbands is significantly reduced in the regime of many occupied subbands, with highly detrimental consequences for the realization and observation of robust Majorana zero modes. As a consequence, the requirements for the realization of robust topological superconductivity and Majorana zero modes should include a low value of the chemical potential, consistent with the occupation of only a few subbands. Finally, we show that the local density of states on the exposed nanowire facets provides a powerful tool for identifying a regime with many occupied subbands and is capable of providing additional critical information regarding the feasibility of Majorana physics in semiconductor-superconductor devices. In our work, we address both InAs/Al and InSb/Al superconductor-nanowire hybrid systems of current experimental interest.arXiv:1910.04362v1 [cond-mat.mes-hall]

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Subband occupation in semiconductor-superconductor nanowires

2020

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“…Unless otherwise specified, the values of effective parameters in Eq. (2) are [8,29,39,[73][74][75][76]] m * = 0.015 m e (for the effective mass), where m e is the electron rest mass, = 0.2 meV (for the proximity-induced SC gap), μ = 1 meV (for the chemical potential), α = 0.5 eVÅ (for the spin-orbit coupling), and the length of the nanowire L = 1 μm [28,30,33,34] (for the short wire) or 3 μm (for the long wire). (This choice of parameters corresponds approximately to the InSb-Al hybrid SC-SM systems.)…”

Section: A Minimal Effective Modelmentioning

confidence: 99%

Physical mechanisms for zero-bias conductance peaks in Majorana nanowires

Pan

Sarma

2020

Phys. Rev. Research

222

205

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Motivated by the need to understand and simulate the ubiquitous experimentally observed zero-bias conductance peaks in superconductor-semiconductor hybrid structures, we theoretically investigate the tunneling conductance spectra in one-dimensional nanowires in proximity to superconductors in a systematic manner taking into account several different physical mechanisms producing zero-bias conductance peaks. The mechanisms we consider are the presence of quantum dots, inhomogeneous potential, random disorder in the chemical potential, random fluctuations in the superconducting gap, and in the effective g factor with the self-energy renormalization induced by the parent superconductor in both short (L ∼ 1 μm) and long nanowires (L ∼ 3 μm). We classify all foregoing theoretical results for zero-bias conductance peaks into three types: the good, the bad, and the ugly, according to the physical mechanisms producing the zero-bias peaks and their topological properties. We find that, although the topological Majorana zero modes are immune to weak disorder, strong disorder ("ugly") completely suppresses topological superconductivity and generically leads to trivial zero-bias peaks. Compared qualitatively with the extensive existing experimental results in the superconductor-semiconductor nanowire structures, we conclude that most current experiments are likely exploring trivial zero-bias peaks in the "ugly" situation dominated by strong disorder. We also study the nonlocal end-to-end correlation measurement in both the short and long wires, and point out the limitation of the nonlocal correlation in ascertaining topological properties particularly when applied to short wires. Although we present results for "good" and "bad" zero-bias peaks, arising respectively from topological Majorana bound states and trivial Andreev bound states, strictly for the sake of direct comparison with the "ugly" zero-bias conductance peaks arising from strong disorder, the main goal of the current work is to establish with a very high confidence level the real physical possibility that essentially all experimentally observed zero-bias peaks in Majorana nanowires are most likely ugly, i.e., purely induced by strong disorder, and are as such utterly nontopological. Our work clearly suggests that an essential prerequisite for any future observation of topological Majorana zero modes in nanowires is a substantial materials improvement of the semiconductor-superconductor hybrid systems leading to much cleaner wires.

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Electronic Structure of InAs and InSb Surfaces: Density Functional Theory and Angle‐Resolved Photoemission Spectroscopy

et al. 2022

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The electronic structure of surfaces plays a key role in the properties of quantum devices. However, surfaces are also the most challenging to simulate and engineer. Here, the electronic structure of InAs(001), InAs(111), and InSb(110) surfaces is studied using a combination of density functional theory (DFT) and angle‐resolved photoemission spectroscopy (ARPES). Large‐scale first principles simulations are enabled by using DFT calculations with a machine‐learned Hubbard U correction [npj Comput. Mater. 6, 180 (2020)]. To facilitate direct comparison with ARPES results, a “bulk unfolding” scheme is implemented by projecting the calculated band structure of a supercell surface slab model onto the bulk primitive cell. For all three surfaces, a good agreement is found between DFT calculations and ARPES. For InAs(001), the simulations clarify the effect of the surface reconstruction. Different reconstructions are found to produce distinctive surface states, which may be detected by ARPES with low photon energies. For InAs(111) and InSb(110), the simulations help elucidate the effect of oxidation. Owing to larger charge transfer from As to O than from Sb to O, oxidation of InAs(111) leads to significant band bending and produces an electron pocket, whereas oxidation of InSb(110) does not. The combined theoretical and experimental results may inform the design of quantum devices based on InAs and InSb semiconductors, for example, topological qubits utilizing the Majorana zero modes.

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Ubiquitous Non-Majorana Zero-Bias Conductance Peaks in Nanowire Devices

Cited by 135 publications

References 46 publications

Subband occupation in semiconductor-superconductor nanowires

Subband occupation in semiconductor-superconductor nanowires

Physical mechanisms for zero-bias conductance peaks in Majorana nanowires

Electronic Structure of InAs and InSb Surfaces: Density Functional Theory and Angle‐Resolved Photoemission Spectroscopy

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