| 1. |
- Arouca, Rodrigo, et al.
(författare)
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Topological superconductivity enhanced by exceptional points
- 2023
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 108:6
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Tidskriftsartikel (refereegranskat)abstract
- Majorana zero modes (MZMs) emerge as edge states in topological superconductors and are promising for topological quantum computation, but their detection has so far been elusive. Here we show that non-Hermiticity can be used to obtain dramatically more robust MZMs. The enhanced properties appear as a result of an extreme instability of exceptional points to superconductivity, such that even a vanishingly small superconducting order parameter already opens a large energy gap, produces well-localized MZMs, and leads to strong superconducting pair correlations. Our work thus illustrates the large potential of enhancing electronic ordering, here in the form of topological superconductivity, using non-Hermitian exceptional points.
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| 2. |
- Awoga, Oladunjoye A., et al.
(författare)
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Mitigating disorder-induced zero-energy states in weakly coupled superconductor-semiconductor hybrid systems
- 2023
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:18
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Tidskriftsartikel (refereegranskat)abstract
- Disorder has appeared as one of the main mechanisms to induce topologically trivial zero-energy states in superconductor-semiconductor systems, thereby challenging the detection of topological superconductivity and Majorana bound states. Here, we demonstrate that, for disorder in any part of the system, the formation of disorder-induced trivial zero-energy states can, to a large extent, be mitigated by keeping the coupling between the semiconductor and superconductor weak. The only exception is strong disorder in the semiconductor, where instead the strong-coupling regime is somewhat more robust against disorder. Furthermore, we find that the topo-logical phase in this weak-coupling regime is robust against disorder, with a large and well-defined topological gap which is highly beneficial for topological protection. Our work shows the advantages and disadvantages of weak and strong couplings under disorder, important for designing superconductor-semiconductor hybrid structures.
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| 3. |
- Awoga, Oladunjoye A., et al.
(författare)
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Robust topological superconductivity in weakly coupled nanowire-superconductor hybrid structures
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 105:14
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the role of the coupling between a spin-orbit coupled semiconductor nanowire and a conventional s-wave superconductor on the emergence of the topological superconducting phase with Majorana bound states in an applied magnetic field. We show that when the coupling is strong, the topological phase transition point is very sensitive to the size of the superconductor and in order to reach the topological phase a strong magnetic field is required, which can easily be detrimental to superconductivity. Moreover, the induced energy gap separating the Majorana bound states and other quasiparticles in the topological phase is substantially suppressed compared to the gap at zero field. In contrast, in the weak-coupling regime, we find that the situation is essentially the opposite, with the topological phase emerging at much lower magnetic fields and a sizable induced energy gap in the topological phase, which can also be controlled by the chemical potential of the superconductor. Furthermore, we show that the weak-coupling regime does not generally allow for the formation of topologically trivial zero-energy states at the wire end points, in stark contrast to the strong-coupling regime, where such states are found for a wide range of parameters. Our results thus put forward the weak-coupling regime as a promising route to mitigate the most unwanted problems present in nanowires for realizing topological superconductivity and Majorana bound states.
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| 4. |
- Awoga, Oladunjoye, et al.
(författare)
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Supercurrent Detection of Topologically Trivial Zero-Energy States in Nanowire Junctions
- 2019
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Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 0031-9007 .- 1079-7114. ; 123:11
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Tidskriftsartikel (refereegranskat)abstract
- We report the emergence of zero-energy states in the trivial phase of a short nanowire junction with a strong spin-orbit coupling and magnetic field, formed by strong coupling between the nanowire and two superconductors. The zero-energy states appear in the junction when the superconductors induce a large energy shift in the nanowire, such that the junction naturally forms a quantum dot, a process that is highly tunable by the superconductor width. Most importantly, we demonstrate that the zero-energy states produce a re shift in the phase-biased supercurrent, which can be used as a simple tool for their unambiguous detection, ruling out any Majorana-like interpretation.
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| 5. |
- Baldo, Lucas, et al.
(författare)
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Zero-frequency supercurrent susceptibility signatures of trivial and topological zero-energy states in nanowire junctions
- 2023
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Ingår i: Superconductors Science and Technology. - : Institute of Physics (IOP). - 0953-2048 .- 1361-6668. ; 36:3
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Tidskriftsartikel (refereegranskat)abstract
- We propose a method to distinguish between trivial and topological, Majorana, zero-energy states (ZESs) in both short and long superconductor-normal-superconductor junctions based on Rashba nanowires using phase-biased equilibrium transport measurements. In particular, we show how the sawtooth profile of the supercurrent, due to the Majorana oscillation suppression in the topological phase for sufficiently long superconductor regions, leads to a strong signal in its zero-frequency susceptibility for a phase difference of ? = pi. This signal is notably insensitive to the chemical potential in the normal region, while trivial ZESs only causes signals in the susceptibility that is highly varying with the chemical potential, thus turning gating of the normal region into a simple experimental control knob. Furthermore, we obtain that, by tuning the junction transparency, critical currents in both short and long junctions undergo a reduction in the number of oscillations as a function of magnetic field only in the topological phase, an effect that find to be intimately linked to Majorana non-locality. Finally, we show that our results also hold at finite temperatures, thus highlighting their potential measurability under realistic experimental conditions.
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| 6. |
- Cayao, Jorge, et al.
(författare)
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Andreev spectrum and supercurrents in nanowire-based SNS junctions containing Majorana bound states
- 2018
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Ingår i: Beilstein Journal of Nanotechnology. - : BEILSTEIN-INSTITUT. - 2190-4286. ; 9, s. 1339-1357
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Tidskriftsartikel (refereegranskat)abstract
- Hybrid superconductor-semiconductor nanowires with Rashba spin-orbit coupling are arguably becoming the leading platform for the search of Majorana bound states (MBSs) in engineered topological superconductors. We perform a systematic numerical study of the low-energy Andreev spectrum and supercurrents in short and long superconductor-normal-superconductor junctions made of nanowires with strong Rashba spin-orbit coupling, where an external Zeeman field is applied perpendicular to the spin-orbit axis. In particular, we investigate the detailed evolution of the Andreev bound states from the trivial into the topological phase and their relation with the emergence of MBSs. Due to the finite length, the system hosts four MBSs, two at the inner part of the junction and two at the outer one. They hybridize and give rise to a finite energy splitting at a superconducting phase difference of p, a wellvisible effect that can be traced back to the evolution of the energy spectrum with the Zeeman field: from the trivial phase with Andreev bound states into the topological phase with MBSs. Similarly, we carry out a detailed study of supercurrents for short and long junctions from the trivial to the topological phases. The supercurrent, calculated from the Andreev spectrum, is 2 pi-periodic in the trivial and topological phases. In the latter it exhibits a clear sawtooth profile at a phase difference of pi when the energy splitting is negligible, signalling a strong dependence of current-phase curves on the length of the superconducting regions. Effects of temperature, scalar disorder and reduction of normal transmission on supercurrents are also discussed. Further, we identify the individual contribution of MBSs. In short junctions the MBSs determine the current-phase curves, while in long junctions the spectrum above the gap (quasi-continuum) introduces an important contribution.
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| 7. |
- Cayao, Jorge, et al.
(författare)
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Bulk Bogoliubov Fermi arcs in non-Hermitian superconducting systems
- 2023
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 107:10
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Tidskriftsartikel (refereegranskat)abstract
- We consider a non-Hermitian superconducting system by coupling a conventional superconductor to a ferromagnet lead and demonstrate the emergence of exceptional points when an external Zeeman field is applied. We discover that, depending on the non-Hermiticity and the Zeeman field, the exceptional points mark the ends of lines with zero real energy, thus giving rise to topologically protected and highly tunable bulk Fermi arcs, which we coin bulk Bogoliubov Fermi arcs due to their superconducting nature. We show that these bulk Bogoliubov Fermi arcs are the non-Hermitian counterparts of the Hermitian topological phase transition but are much more prevalent and also experimentally detectable through large spectral signatures.
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| 8. |
- Cayao, Jorge, et al.
(författare)
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Confinement-induced zero-bias peaks in conventional superconductor hybrids
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:13
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Tidskriftsartikel (refereegranskat)abstract
- Majorana bound states in topological superconductors have been predicted to appear in the form of zero-bias conductance peaks of height 2e(2)/h, which represents one of the most studied signatures so far. Here, we show that quasi-zero-energy states, similar to Majorana bound states, can naturally form in any superconducting hybrid junction due to confinement effects, in the absence of spin fields and, thus, without relation to topology. Remarkably, these topologically trivial quasi-zero-energy states produce zero-bias conductance peaks, that could be similar to Majorana signatures, but develop a different peak height (4e(2)/h) and are less stable under gating or depletion of the confined region. Our results put forward confinement as an alternative mechanism to explain the ubiquitous presence of trivial zero-bias peaks and quasi-zero-energy states in superconductor hybrids.
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| 9. |
- Cayao, Jorge, PhD, 1986-, et al.
(författare)
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Controllable odd-frequency Cooper pairs in multisuperconductor Josephson junctions
- 2024
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 109:20
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Tidskriftsartikel (refereegranskat)abstract
- We consider Josephson junctions formed by multiple superconductors with distinct phases and explore the formation of nonlocal or intersuperconductor pair correlations. We find that the multiple superconductor nature offers an additional degree of freedom that broadens the classification of pair symmetries, enabling nonlocal evenand odd -frequency pairings that can be highly controlled by the superconducting phases and the energy of the superconductors. Specially, when the phase difference between two superconductors is n , their associated nonlocal odd -frequency pairing is the only type of intersuperconductor pair correlations. Finally, we show that these nonlocal odd -frequency Cooper pairs dominate the nonlocal conductance via crossed Andreev reflections, which constitutes a direct evidence of odd -frequency pairing.
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| 10. |
- Cayao, Jorge, et al.
(författare)
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Distinguishing trivial and topological zero-energy states in long nanowire junctions
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:2
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The emergence of zero-energy states in nontopological superconductors represents an inevitable problem that obscures the proper identification of zero-energy Majorana bound states (MBSs) and prevents their use as topologically protected qubits. In this Research Letter we investigate long superconductor-normal-superconductor junctions where trivial zero-energy states, robust over a large range of parameters, appear as a result of helicity and confinement in the normal region. We demonstrate that both equilibrium supercurrents and critical currents are sensitive to variations in the length of the superconductor regions in the topological phase hosting MBSs but, remarkably, no such length dependence exists when robust, but trivial, zero-energy states are present. This strikingly different response originates from the nonlocal nature of the MBSs, and we therefore propose it as a simple protocol for distinguishing between trivial and topological zero-energy states.
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