| 1. |
- Mashkoori, Mahdi, et al.
(författare)
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Impact of topology on the impurity effects in extended s-wave superconductors with spin-orbit coupling
- 2019
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 99:1
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the impact of topology on the existence of impurity subgap states in a time-reversal-invariant superconductor with an extended s-wave pairing and strong spin-orbit coupling. By simply tuning the chemical potential, we access three distinct phases: topologically trivial s-wave, topologically nontrivial s(+/-)-wave, and nodal superconducting phase. For a single potential impurity, we find subgap impurity bound states in the topological phase, but notably no subgap states in the trivial phase. This is in sharp contrast with the expectation that there would be no subgap state in the presence of potential impurities in s-wave superconductors. These subgap impurity states have always finite energies for any strength of the potential scattering and, subsequently, the superconducting gap in the topological s(+/-)-wave phase survives but is attenuated in the presence of finite disorder. By creating islands of potential impurities, we smoothly connect the single impurity results to topological edge states of impurity islands. On the other hand, magnetic impurities lead to the formation of Yu-Shiba-Rusinov states in both the trivial and topological phases, which even reach zero energy at certain scattering strengths. We thus propose that potential impurities can be a very valuable tool to detect time-reversal-invariant topological superconductivity.
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| 2. |
- Wojek, Bastian M., et al.
(författare)
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Band inversion and the topological phase transition in (Pb,Sn)Se
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:16, s. 161202-
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Tidskriftsartikel (refereegranskat)abstract
- The recent discovery of a topological phase transition in IV-VI narrow-gap semiconductors has revitalized the decades-old interest in the bulk band inversion occurring in these materials. Here we systematically study the (001) surface states of Pb1-xSnxSe mixed crystals by means of angle-resolved photoelectron spectroscopy in the parameter space 0 <= x <= 0.37 and 300 K >= T >= 9 K. Using the surface-state observations, we monitor directly the topological phase transition in this solid solution and gain valuable information on the evolution of the underlying fundamental band gap of the system. In contrast to common model expectations, the band-gap evolution appears to be nonlinear as a function of the studied parameters, resulting in the measuring of a discontinuous band-inversion process. This finding signifies that the anticipated gapless bulk state is in fact not a stable configuration and that the topological phase transition therefore exhibits features akin to a first-order transition.
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| 3. |
- 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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| 4. |
- Awoga, Oladunjoye A., et al.
(författare)
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Domain walls in a chiral d-wave superconductor on the honeycomb lattice
- 2017
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 96:1
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Tidskriftsartikel (refereegranskat)abstract
- We perform a fully self-consistent study of domain walls between different chiral domains in chiral d(x2-y2) +/- id(xy) -wave superconductors with an underlying honeycomb lattice structure. We investigate domain walls along all possible armchair and zigzag directions and with a finite global phase shift across the domain wall, in addition to the change of chirality. For armchair domain walls we find the lowest domain wall energy at zero global phase shift, while the most favorable zigzag domain wall has a finite global phase shift dependent on the doping level. Belowthe van Hove singularity the armchair domain wall is most favorable, while at even higher doping the zigzag domain wall has the lowest energy. The domain wall causes a local suppression of the superconducting order parameter, with the superconducting recovery length following a universal curve for all domain walls. Moreover, we always find four subgap states crossing zero energy and well localized to the domain wall. However, the details of their energy spectrum vary notably, especially with the global phase shift across the domain wall.
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| 5. |
- 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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| 6. |
- 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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| 7. |
- Awoga, Oladunjoye A., et al.
(författare)
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Superconductivity and magnetism in the surface states of ABC-stacked multilayer graphene
- 2023
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Ingår i: Physical Review B. - 2469-9950. ; 108:14
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Tidskriftsartikel (refereegranskat)abstract
- ABC-stacked multilayer graphene (ABC-MLG) exhibits topological surface flat bands with a divergent density of states, leading to many-body instabilities at charge neutrality. Here, we explore electronic ordering within a mean-field approach with full generic treatment of all spin-isotropic, two-site charge density and spin interactions up to next-nearest-neighbor (NNN) sites. We find that surface superconductivity and magnetism are significantly enhanced over bulk values. We find spin-singlet s wave and unconventional NNN bond spin-triplet f wave to be the dominant superconducting pairing symmetries, both with a full energy gap. By establishing the existence of ferromagnetic intrasublattice interaction (J2<0) we conclude that the f-wave state is favored in ABC-MLG, in sharp contrast to bulk ABC-graphite where chiral d- or p-wave states, together with s-wave states, display stronger ordering tendencies albeit not achievable at charge neutrality. We trace this distinctive surface behavior to the strong sublattice polarization of the surface flat bands. We also find competing ferrimagnetic order, fully consistent with density functional theory (DFT) calculations. The magnetic order interpolates between sublattice ferromagnetism and antiferromagnetism, but only with the ratio of the sublattice magnetic moments (R) being insensitive to the DFT exchange correlation functional. We finally establish the full phase diagram by constraining the interactions to the R value identified by DFT. We find f-wave superconductivity being favored for all weak to moderately strong couplings J2 and as long as J2 is a sufficiently large part of the full interaction mix. Gating ABC-MLG away from charge neutrality further enhances the f-wave state over the ferrimagnetic state, establishing ABC-MLG as a strong candidate for f-wave superconductivity.
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| 8. |
- Awoga, Oladunjoye, et al.
(författare)
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Disorder robustness and protection of Majorana bound states in ferromagnetic chains on conventional superconductors
- 2017
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 95:18
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Tidskriftsartikel (refereegranskat)abstract
- Majorana bound states (MBSs) are well established in the clean limit in chains of ferromagnetically aligned impurities deposited on conventional superconductors with finite spin-orbit coupling. Here we show that these MBSs are very robust against disorder. By performing self-consistent calculations we find that the MBSs are protected as long as the surrounding superconductor show no large signs of inhomogeneity. We also find that longer chains offer more stability against disorder for the MBSs, albeit the minigap decreases, as do increasing strengths of spin-orbit coupling and superconductivity.
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| 9. |
- Awoga, Oladunjoye, et al.
(författare)
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Probing unconventional superconductivity in proximitized graphene by impurity scattering
- 2018
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:21
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate how potential impurities are a very powerful tool for determining the pairing symmetry in graphene, proximity coupled to a spin-singlet superconductor. All d-wave states are characterized by subgap resonances, with spatial patterns clearly distinguishing between nodal and chiral d-wave symmetry, while s-wave states have no subgap resonances. We also find strong supergap impurity resonances associated with the normal state Dirac point. Subgap and supergap resonances only interact at very low doping levels, then causing suppression of the supergap resonances.
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| 10. |
- 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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