Strongly Disordered TiN and NbTiN<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>s</mml:mi></mml:math>-Wave Superconductors Probed by Microwave Electrodynamics

Driessen, E. F. C.; Coumou, P. C. J. J.; Tromp, Rutger R.; Visser, Pieter de; Klapwijk, T. M.

doi:10.1103/physrevlett.109.107003

Cited by 129 publications

(101 citation statements)

References 25 publications

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“…While a qualitative correspondence can be traced between our observations and the experiment, a more quantitative agreement is beyond the goal of our study. This is in part due to a considerable number of unknown parameters (precise L SO , screened gate potential profiles, localization and pair-breaking effects in the bulk superconductor, 35 etc.). Moreover, other features that we find, such as the ZBA splitting oscillations in B, have not been observed.…”

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confidence: 99%

Transport spectroscopy ofNSnanowire junctions with Majorana fermions

2012

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We investigate transport through normal-superconductor nanowire junctions in the presence of spin-orbit coupling and magnetic field. As the Zeeman field crosses the critical bulk value B c of the topological transition, a Majorana bound state (MBS) is formed, giving rise to a sharp zero-bias anomaly (ZBA) in the tunneling differential conductance. We identify novel features beyond this picture in wires with inhomogeneous depletion, such as the appearance of two MBSs inside a long depleted region for B < B c . The resulting ZBA is in most cases weakly split and may coexist with Andreev bound states near zero energy. The ZBA may appear without evidence of a topological gap closing. This latter aspect is more evident in the multiband case and stems from a smooth pinch-off barrier. Most of these features are in qualitative agreement with recent experiments [V. Mourik et al., Science 336, 1003(2012]. We also discuss the rich phenomenology of the problem in other regimes which remain experimentally unexplored. and semiconductors with strong spin-orbit (SO) coupling. 4-8In proximity to s-wave superconductors, these systems behave as topological superconductors (TSs) when the excitation gap is closed and reopened again: as the gap crosses zero, Majorana bound states (MBSs) appear wherever the system interfaces with a nontopological insulator (see Refs. 9 and 10 for reviews).The TS transition occurs when an external Zeeman field B exceeds a critical value B c ≡ μ 2 + 2 defined in terms of the Fermi energy μ and the induced s-wave pairing . 7,8 This prediction has spurred a great deal of experimental activity towards detecting MBSs in hybrid superconductorsemiconductor systems. Indeed, signatures of Majorana detection have been recently reported in Ref. 11. These experiments (and subsequently Refs. 12 and 13) clearly show the emergence of a zero-bias anomaly (ZBA) in differential conductance dI/dV measurements as B increases. It has been predicted that such a ZBA reflects tunneling into the MBS.14-16 Crucially, the emerging ZBA, which signals the TS transition, should be accompanied by a closing and reopening of the excitation gap.17,18 something which is, however, not observed. Other experimental findings, such as ZBA splitting and coexistence of Andreev bound states (ABSs) and MBSs, 11 also need further analysis.Motivated by this, we present here a detailed study of transport through normal-superconductor (NS) junctions containing topological wires. As in the experiment, the wires are tunnel-coupled to the normal reservoir by a pinch-off gate V p (to allow for transport spectroscopy using dI/dV ), and are depleted by an additional gate V d in order to bring B c down to accessible fields (since the induced potential U d lowers the Fermi energy μ → μ − U d ). The depletion profile, however, is necessarily inhomogeneous, since it cannot extend deep into the superconducting side due to efficient screening, see Fig. 1 and Ref. 11. We find that for this class of devices, a number of distinct transport regimes arise as the va...

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confidence: 99%

Transport spectroscopy ofNSnanowire junctions with Majorana fermions

2012

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“…We model the microwave response using a description of the superconducting state, in which the superconductor is homogeneously weakened by the disorder-dependent pairbreaking parameter α, similar to the effect of magnetic impurities. 3 We assume homogeneous superconductivity and describe the superconducting state using the Usadel equation,…”

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Electrodynamic response and local tunneling spectroscopy of strongly disordered superconducting TiN films

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We have studied the electrodynamic response of strongly disordered superconducting TiN films using microwave resonators, where the disordered superconductor is the resonating element in a high-quality superconducting environment of NbTiN. We describe the response assuming an effective pair-breaking mechanism modifying the density of states and compare this to local tunneling spectra obtained using scanning tunneling spectroscopy. For the least disordered film (k F l = 8.7, R s = 13 ), we find good agreement, whereas for the most disordered film (k F l = 0.82, R s = 4.3 k ), there is a strong discrepancy, which signals the breakdown of a model based on uniform properties.

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“…Furthermore, the multilayer structure may show additional physics due to proximity effects. 20 Further study is needed to understand the electrodynamics of these strongly disordered films, 21 an important avenue of research for detector applications.…”

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confidence: 99%