| 1. |
- Dutta, Paramita, et al.
(författare)
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Finite bulk Josephson currents and chirality blockade removal from interorbital pairing in magnetic Weyl semimetals
- 2020
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 101:6
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic Weyl semimetals (WSMs) have been presumed to be immune to proximity-induced spin-singlet superconducting pairing due to a chirality blockade of the regular Andreev reflection. In this work, we study all possible superconducting pairing induced in a WSM sandwiched between two conventional superconductors in a Josephson junction setup. We confirm that, while conventional intraorbital s-wave pairing is present on the surface of the WSM, it cannot propagate into the bulk due to the chirality blockade. However, interorbital s-wave pairing, with both even-frequency spin-singlet and odd-frequency mixed-spin-triplet symmetry, propagates into the bulk of the WSM, as do several p-wave symmetries. To demonstrate the importance of these finite interorbital and p-wave pair amplitudes in an experimental setup, we calculate the Josephson current and find a finite and even increasing current when the chirality blockade effect for the conventional intraorbital pairing is enhanced.
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| 2. |
- Dutta, Paramita, et al.
(författare)
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Superconductivity in spin-3/2 systems : Symmetry classification, odd-frequency pairs, and Bogoliubov Fermi surfaces
- 2021
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Ingår i: Physical Review Research. - : American Physical Society. - 2643-1564. ; 3:3
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Tidskriftsartikel (refereegranskat)abstract
- The possible symmetries of the superconducting pair amplitude is a consequence of the fermionic nature of the Cooper pairs. For spin-1/2 systems this leads to the SPOT = -1 classification of superconductivity, where S, P, O, and T refer to the exchange operators for spin, parity, orbital, and time between the paired electrons. However, this classification no longer holds for higher spin fermions, where each electron also possesses a finite orbital angular momentum strongly coupled with the spin degree of freedom, giving instead a conserved total angular moment. For such systems, we here instead introduce the JPT = -1 classification, where J is the exchange operator for the z component of the total angular momentum quantum numbers. We then specifically focus on spin-3/2 fermion systems and several superconducting cubic half-Heusler compounds that have recently been proposed to be spin-3/2 superconductors. By using a generic Hamiltonian suitable for these compounds we calculate the superconducting pair amplitudes and find finite pair amplitudes for all possible symmetries obeying the JPT = -1 classification, including all possible odd-frequency (odd-omega)) combinations. Moreover, one of the very interesting properties of spin-3/2 superconductors is the possibility of them hosting a Bogoliubov Fermi surface (BFS), where the superconducting energy gap is closed across a finite area. We show that a spin3/2 superconductor with a pair potential satisfying an odd-gap time-reversal product and being noncommuting with the normal-state Hamiltonian hosts both a BFS and has finite odd-omega) pair amplitudes. We then reduce the full spin-3/2 Hamiltonian to an effective two-band model and show that odd-omega) pairing is inevitably present in superconductors with a BFS and vice versa.
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| 3. |
- Khatibi, Zahra, et al.
(författare)
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Strain impacts on commensurate bilayer graphene superlattices : Distorted trigonal warping, emergence of bandgap and direct-indirect bandgap transition
- 2019
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Ingår i: Diamond and related materials. - : ELSEVIER SCIENCE SA. - 0925-9635 .- 1879-0062. ; 92, s. 228-234
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Tidskriftsartikel (refereegranskat)abstract
- Due to low dimensionality, the controlled stacking of graphene films and their electronic properties are susceptible to environmental changes including strain. The strain-induced modification of the electronic properties such as the emergence and modulation of bandgaps crucially depends on the stacking of the graphene films, However, to date, only the impact of strain on electronic properties of Bernal and AA-stacked bilayer graphene has been extensively investigated in theoretical studies. Exploiting density functional theory and tight-binding calculation, we investigate the impacts of in-plane strain on two different classes of commensurate twisted bilayer graphene (TBG) which are even/odd under sublattice exchange (SE) parity. We find that the SE odd TBG remains gapless whereas the bandgap increases for the SE even TBG when applying equibiaxial tensile strain. Moreover, we observe that for extremely large mixed strains both investigated TBG superstructures demonstrate direct-indirect bandgap transition.
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| 4. |
- Löthman, Tomas, et al.
(författare)
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Nematic superconductivity in magic-angle twisted bilayer graphene from atomistic modeling
- 2021
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Bilayer graphene with small internal twist angles develops large scale moiré patterns with flat energy bands hosting both correlated insulating states and superconductivity. The large system size and intricate band structure have however hampered investigations into the properties of the superconducting state. Here, we use full-scale atomistic modeling with local electronic interactions and find a highly inhomogeneous superconducting state with nematic ordering on both the atomic and moiré lattice length scales. More specifically, we obtain locally anisotropic real-valued d-wave pairing with a nematic vector winding throughout the moiré pattern, generating a three-fold ground state degeneracy. Despite the d-wave nature, the superconducting state has a full energy gap, which we further tie to a π-phase interlayer coupling. The superconducting nematicity is easily detected through signatures in the local density of states. Our results show not only that atomistic modeling is essential for twisted bilayer graphene but also that the superconducting state is necessarily distinctly different from that of the high-temperature cuprate superconductors, despite similarity in electronic interactions.
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| 5. |
- Löthman, Tomas, et al.
(författare)
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Nematic superconductivity in magic-angle twisted bilayer graphene from atomistic modeling
- 2022
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Ingår i: Communications Physics. - : Springer Nature. - 2399-3650. ; 5:1
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Tidskriftsartikel (refereegranskat)abstract
- Twisted bilayer graphene (TBG) develops large moire patterns at small twist angles with flat energy bands hosting domes of superconductivity. The large system size and intricate band structure have however hampered investigations into the superconducting state. Here, using full-scale atomistic modelling with local electronic interactions, we find at and above experimentally relevant temperatures a highly inhomogeneous superconducting state with nematic ordering on both atomic and moire length scales. The nematic state has a locally anisotropic real-valued d-wave pairing, with a nematic vector winding throughout the moire pattern, and is three-fold degenerate. Although d-wave symmetric, the superconducting state has a full energy gap, which we tie to a pi-phase interlayer coupling. The superconducting nematicity is further directly detectable in the local density of states. Our results show that atomistic modeling is essential and also that very similar local interactions produce very different superconducting states in TBG and the high-temperature cuprate superconductors.
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| 6. |
- Mashkoori, Mahdi, et al.
(författare)
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Detrimental effects of disorder in two-dimensional time-reversal invariant topological superconductors
- 2023
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 107:1
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Tidskriftsartikel (refereegranskat)abstract
- The robustness against local perturbations, as long as the symmetry of the system is preserved, is a distinctive feature of topological quantum states. Magnetic impurities and defects break time-reversal invariance and, consequently, time-reversal invariant (TRI) topological superconductors are fragile against this type of disorder. Nonmagnetic impurities, however, preserve time-reversal symmetry and one naively expects a TRI topological superconductor to persist in the presence of nonmagnetic impurities. In this work, we study the effect of nonmagnetic disorder on a TRI topological superconductor with extended s-wave pairing, which can be engineered at the interface of an Fe-based superconductor and a strongly spin-orbit coupled Rashba layer. We model two different types of nonmagnetic random disorder and analyze both the bulk density of states and edge state spectrum. Contrary to naive expectations, we find that the disorder strongly affects the topological phase by closing the energy gap, while trivial superconducting phases remain stable and fully gapped. The disorder phase diagram reveals a strong expansion of a nodal phase with increasing disorder. We further show the decay of the helical Majorana edge states in the topological phase and how they eventually disappear with increasing disorder. These results alter our understanding of effects of impurities and disorder on TRI topological phases and may help explain the difficulty of experimental observation of TRI topological superconductors.
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| 7. |
- 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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| 8. |
- Parhizgar, Fariborz, et al.
(författare)
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Diamagnetic and paramagnetic Meissner effect from odd-frequency pairing in multiorbital superconductors
- 2021
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:5
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Tidskriftsartikel (refereegranskat)abstract
- The Meissner effect is one of the defining properties of superconductivity, with a conventional superconductor completely repelling an external magnetic field. In contrast to this diamagnetic behavior, odd-frequency superconducting pairing has often been seen to produce a paramagnetic Meissner effect, which instead makes the superconductor unstable due to the attraction of magnetic field. In this paper, we study how both even- and odd-frequency superconducting pairing contributes to the Meissner effect in a generic two-orbital superconductor with a tunable odd-frequency pairing component. By dividing the contributions to the Meissner effect into intra- and interband processes, we find that the odd-frequency pairing actually generates both dia- and paramagnetic Meissner responses, determined by the normal-state band structure. More specifically, for materials with two electronlike (holelike) low-energy bands, we find that the odd-frequency interband contribution is paramagnetic but nearly canceled by a diamagnetic odd-frequency intraband contribution. Combined with a diamagnetic even-frequency contribution, such superconductors thus always display a large diamagnetic Meissner response to an external magnetic field, even in the presence of large odd-frequency pairing. For materials with an inverted, or topological, band structure, we find the odd-frequency interband contribution to instead be diamagnetic and even the dominating contribution to the Meissner effect in the near-metallic regime. Taken together, our results show that odd-frequency pairing in multiorbital superconductors does not generate a destabilizing paramagnetic Meissner effect and can even generate a diamagnetic response in topological materials.
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| 9. |
- Parhizgar, Fariborz, et al.
(författare)
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Highly tunable time-reversal-invariant topological superconductivity in topological insulator thin films
- 2017
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Ingår i: Scientific Reports. - : Nature Publishing Group. - 2045-2322. ; 7
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Tidskriftsartikel (refereegranskat)abstract
- We study time-reversal-invariant topological superconductivity in topological insulator (TI) thin films including both intra-and inter-surface pairing. We find a nontrivial topology for multiple different configurations. For intra-surface pairing a p-phase difference between the intra-surface pairing states is required. We show that in this case the resulting topological phase is highly tunable by both an applied electric field and varied chemical potential. For spin-singlet inter-surface pairing, a sign-changing tunnel coupling present in many TI thin films is needed, and again, the topology can be tuned by electric field or doping. Notably, we find that the required inter-surface pairing strength for achieving nontrivial topology can still be subdominant compared to the intra-surface pairing. Finally, for spin-triplet intersurface pairing we prove that the superconducting state is always topological nontrivial. We show that thin films of Cu-doped Bi2Se3 will likely host such spin-triplet inter-surface pairing. Taken together, these results show that time-reversal-invariant topological superconductivity is common in superconducting TI thin films and that the topological phase and its Kramers pair of Majorana edge modes is highly tunable with an applied electric field and varied chemical potential.
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| 10. |
- Parhizgar, Fariborz, et al.
(författare)
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Large Josephson current in Weyl nodal loop semimetals due to odd-frequency superconductivity
- 2020
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Ingår i: NPJ QUANTUM MATERIALS. - : NATURE PUBLISHING GROUP. - 2397-4648. ; 5:1
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Tidskriftsartikel (refereegranskat)abstract
- Weyl nodal loop semimetals (WNLs) host a closed nodal line loop Fermi surface in the bulk, protected zero-energy flat band, or drumhead, surface states, and strong spin-polarization. The large density of states of the drumhead states makes WNL semimetals exceedingly prone to electronic ordering. At the same time, the spin-polarization naively prevents conventional superconductivity due to its spin-singlet nature. Here we show the complete opposite: WNLs are extremely promising materials for superconducting Josephson junctions, entirely due to odd-frequency superconductivity. By sandwiching a WNL between two conventional superconductors we theoretically demonstrate the presence of very large Josephson currents, even up to orders of magnitude larger than for normal metals. The large currents are generated both by an efficient transformation of spin-singlet pairs into odd-frequency spin-triplet pairing by the Weyl dispersion and the drumhead states ensuring exceptionally proximity effect. As a result, WNL Josephson junctions offer unique possibilities for detecting and exploring odd-frequency superconductivity.
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| 11. |
- Parhizgar, Fariborz, et al.
(författare)
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Unconventional proximity-induced superconductivity in bilayer systems
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:18, s. 184517-
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Tidskriftsartikel (refereegranskat)abstract
- We study the proximity-induced superconducting state in a general bilayer-conventional s-wave superconductor hybrid structure. For the bilayer we include a general parabolic dispersion, Rashba spin-orbit coupling, and finite-layer tunneling as well as the possibility to apply a bias potential and a magnetic Zeeman field, in order to address experimentally relevant bilayer systems, ranging from topological insulator thin films to generic double quantum well systems. By extracting the proximity-induced anomalous Green's function in the bilayer, we show a very rich structure for the superconducting pairing, including different spin states and odd-frequency pairing. Equal-spin spin-triplet (p(x) +/- ip(y))-wave pairing is induced in both layers in the presence of a finite spin-orbit coupling and opposite-spin spin-triplet s-wave pairing with odd-frequency dependence appears for an applied magnetic Zeeman field. Finite interlayer pairing is also generally present in the bilayer. The interlayer pairing can be either even or odd in the layer index, with a complete reciprocity between parity in frequency and in layer index. We also find that a bilayer offers the possibility of sign reversal of the superconducting order parameters, both between the two layers and between multiple Fermi surfaces.
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| 12. |
- Schmidt, Johann, et al.
(författare)
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Odd-frequency superconductivity and Meissner effect in the doped topological insulator Bi2Se3
- 2020
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 101:18
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Tidskriftsartikel (refereegranskat)abstract
- Doped Bi2Se3 is proposed to be a nematic superconductor originating from unusual interorbital pairing. We calculate all induced superconducting pair correlations in Bi2Se3 and discover that intraorbital odd-frequency pairing clearly dominates over a significant range of frequencies. Moreover, we explore the contributions of even- and odd-frequency pairing to the Meissner effect, including separating intra- and interband processes in the response function. Contrary to expectations, and due to interband contributions, we find a diamagnetic Meissner effect from the odd-frequency pairing that stabilizes the superconducting order.
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| 13. |
- Shiranzaei, Mahroo, et al.
(författare)
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Highly tunable magnetic coupling in ultrathin topological insulator films due to impurity resonances
- 2020
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 102:17
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Tidskriftsartikel (refereegranskat)abstract
- We theoretically investigate the carrier-free exchange interaction between magnetic impurities in ultrathin Bi2Se3 topological insulator films by taking into account the low-energy states produced by the impurities. To match with experimental observations of magnetism on the surface of ultrathin topological insulator films, we restrict the calculations to having the chemical potential within the energy gap, with then interband processes mediating the exchange interaction. We find that the locally induced impurity resonances strongly influence the exchange interaction between magnetic moments. In particular, we find a noncollinear alignment to be more favorable than the collinear ferromagnetic alignment preferred when impurity states are ignored and only the pristine topological insulator band structure is considered. As a result, chiral ferromagnetism can easily become favorable over the ferromagnetic phase in these materials. Moreover, we show that by applying an electric field perpendicular to the ultrathin film, the exchange interaction can be drastically enhanced. This generates the possibility of highly tunable magnetism by electric field.
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| 14. |
- Shiranzaei, Mahroo, et al.
(författare)
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Impurity scattering on the surface of topological-insulator thin films
- 2017
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 95:23
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Tidskriftsartikel (refereegranskat)abstract
- We address the electronic structure of the surface states of topological-insulator thin films with embedded local nonmagnetic and magnetic impurities. Using the T -matrix expansion of the real-space Green's function, we derive the local density of electron states and corresponding spin-resolved densities. We show that the effects of the impurities can be tuned by applying an electric field between the surface layers. The emerging magnetic states are expected to play an important role both in the ferromagnetic mechanism of magnetic topological insulators and in its transport properties. In the case of magnetic impurities, we have categorized the possible cases for different spin directions of the impurities as well as the spin direction in which the spin-resolved density of electron states is calculated and have related them to the spin susceptibility of the system.
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| 15. |
- Shiranzaei, Mahroo, et al.
(författare)
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Nonlinear spin susceptibility in topological insulators
- 2018
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 97:18
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Tidskriftsartikel (refereegranskat)abstract
- We revise the theory of the indirect exchange interaction between magnetic impurities beyond the linear response theory to establish the effect of impurity resonances in the surface states of a three-dimensional topological insulator. The interaction is composed of isotropic Heisenberg, anisotropic Ising, and Dzyaloshinskii-Moriya types of couplings. We find that all three contributions are finite at the Dirac point, which is in stark contrast to the linear response theory which predicts a vanishing Dzyaloshinskii-Moriya-type contribution. We show that the spin-independent component of the impurity scattering can generate large values of the Dzyaloshinskii-Moriya-type coupling in comparison with the Heisenberg and Ising types of couplings, while these latter contributions drastically reduce in magnitude and undergo sign changes. As a result, both collinear and noncollinear configurations are allowed magnetic configurations of the impurities.
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