Abstract
The chiral pwave superconductor is the archetypal example of a state of matter that supports nonAbelian anyons, a highly desired type of exotic quasiparticle. With this, it is foundational for the distant goal of building a topological quantum computer. While some candidate materials for bulk chiral superconductors exist, they are subject of an ongoing debate about their actual paring state. Here we propose an alternative route to chiral superconductivity, consisting of the surface of an ordinary superconductor decorated with a twodimensional lattice of magnetic impurities. We furthermore identify a promising experimental platform to realize this proposal.
Introduction
The chiral pwave superconductor in two dimensions (2D) and the closely related fractional quantum Hall Pfaffian state at filling fraction ν=5/2 are the archetypal examples of topologically ordered states of matter that support nonAbelian anyonic excitations^{1,2,3}. The theoretical exploration of these states has shaped our understanding of topological order and is foundational for the distant goal of building a topological quantum computer^{4,5,6}. In contrast to the ν=5/2 fractional quantum Hall state, however, to date chiral pwave superconductivity has not been confirmed in any experimental system. The most prominent candidate system, superconducting Sr_{2}RuO_{4} (ref. 7), has been the subject of an ongoing debate about its actual paring state^{8,9,10,11}.
Chiral superconductors break timereversal symmetry. This hinders the formation of Cooper pairs, since orbital (and possibly paramagnetic) pairbreaking effects can come into play. Depairing is also the main hurdle for realizing a line of proposals in which layered heterostructures involving ferromagnets and swave superconductors are used to build an artificial 2D pwave superconductor^{12,13,14,15}. The guiding principle for these proposals is to design a band structure with a single normalstate Fermi surface with no spindegeneracy. If Rashba spinorbit coupling is present so that the states on this Fermi surface are not fully spinpolarized, and if swave superconductivity is proximityinduced in such a system, the effective pairing near the single Fermi surface is equivalent to that of a chiral pwave superconductor.
In one dimension (1D), based on the principle of combining spinorbit coupling and externally applied magnetic fields, various groups have proposed engineering artificial realizations of pwave superconductors^{16,17,18,19,20,21,22}. An experimental realization of these proposals employing semiconductor nanowires with strong spinorbit coupling^{23} has reported Majorana fermion signatures. In this setup, the externally applied magnetic field has to be rather small (to avoid suppressing superconductivity). As the phasespace for the existence of a topological superconductor is controlled by the Zeeman gap^{24}, these systems require a delicate balance of the parameters involved (spinorbit coupling, magnetic field and chemical potential) in order to create the topological superconductor.
Recently, a 1D topological superconductor was realized in a system that is quite distinct but employs similar microscopic ingredients—spinorbit coupling, ferromagnetism and swave superconductivity^{25}. A chain of magnetic Fe atoms is deposited on the surface of an swave superconductor with strong spin–orbit interactions. The Fe chain is ferromagnetically ordered^{25} with a large magnetic moment, which takes the role of the magnetic field in the nanowire experiments. Unlike previous proposals, this magnetic field is mostly localized on the Fe chain, with small leakage outside. Superconductivity is not destroyed along the chain. In this setup, the energy scale of the exchange coupling of the Fe atoms is typically much larger than that of the Rashba spinorbit coupling and the superconducting pairing. The ferromagnetically ordered Fe atoms induce localized Shiba states within the gap of the superconductor^{26,27,28}. The hybridization of these states forms the band structure of a 1D pwave superconductor that supports Majorana end states^{29,30}. Because the Fe bands are fully spin split, no additional control over the chemical potential is necessary. A similar scenario applies when the Fe orbitals are magnetic but itinerant^{24}.
In this article, we point out that this strategy can also be successful in 2D. Magnetic adatoms on the surface of a superconductor with strong spinorbit coupling, when arranged in a 2D lattice, can yield a 2D topological chiral pwave superconductor whose chiral Majorana edge modes can be observed in scanning tunneling microscope measurements. To shed light on the rich range of possibilities, we analyse the topological properties of a system with dense local moments that are exchange coupled to a model 2D superconductor, demonstrating that topological superconductors with higher Chern numbers, and consequently multiple chiral Majorana edge channels, can easily occur. We are also able to analyse the model’s dilute magnetic impurity limit analytically and obtain numerical topological phase diagrams for intermediate impurity concentrations. Based on densityfunctionaltheory (DFT) calculations, we further propose realizing 2D topological chiral pwave superconductors experimentally by depositing transition metal adatoms on superconducting Pb. The type of magnetic ion can be varied to access different strengths of the magnetic moment. In the case of Fe adatoms on a Pb (111) surface, we show that strong magnetic order in general leads to an odd number of 2D Fermi surface segments. As a consequence the proximityinduced superconducting phases can have nonzero Chern numbers and chiral Majorana edge modes.
Results
Model Hamiltonian
We first present a model system that bears a number of generic features of superconductor surfaces with ferromagnetically ordered magnetic adatoms (see Fig. 1). To render the analytical calculations tractable, we consider a Hamiltonian that models only the surface layer of a bulk 3D swave superconductor on which the swave superconducting order parameter Δ is induced from the bulk. On this superconducting layer, we model the magnetic impurities as classical spins whose only interaction with the electrons in the superconductor is through Zeemanlike couplings^{26,27,28}. Employing a tightbinding description on a 2D square lattice Λ that is spanned by the primitive lattice vectors and , we consider the meanfield Hamiltonian
Here, is a spinor of the creation operators for electrons at site r with spin ↑, ↓, and σ_{i},i=1,2,3, are the three Pauli matrices. We denote by t the nearest neighbor hopping integral in the superconductor, μ the chemical potential, and α the strength of the Rashba spinorbit coupling. Classical magnetic moments ferromagnetically aligned normal to the plane in the σ_{3} direction are positioned on a sublattice Λ* of Λ and are exchangecoupled to electrons via the term proportional to J. Drawing experience from the 1D situation and the abinitio calculations presented below, the physically relevant hierarchy of energy scales that we consider here is given by
Dense impurity limit
As a warmup, it is instructive to consider the simplest situation in which each lattice site is coupled to a magnetic moment, i.e., Λ=Λ*. This limit is representative of system with selfassembled islands of magnetic adatoms. Its consideration allows us to highlight the difference between the regime in equation (2), and that of small J that has been studied previously^{12,14,15}. In particular, it is possible to access a phase with Chern number 2 in the large J limit. For the case Λ=Λ* and α=0, the Hamiltonian in equation (1) is gapless at zero energy with nodal lines in momentum space defined by (see Supplementary Note 1)
where ɛ_{k}=2t(cos k_{1}+cos k_{2})−μ. Adding spinorbit coupling α≠0 will generically lift these degeneracy lines and gap the spectrum, except if the degeneracy occurs at the four inversionsymmetric momenta [k_{ij}=(1−i, 1−j)π/2 with i, j=±] where the spinorbit coupling vanishes. The occurrence of these nodal points is a signature of transitions between phases characterized by different Chern numbers. The critical chemical potentials are therefore determined by equation (3) with k=k_{ij}, which yields
Around each of the nodal points in the threedimensional kμ parameter space, we can reduce the Hamiltonian to an effective twoband model and expand it to linear order in the deviations δ k from k_{ij} and the deviations δμ from μ_{ijλ} yielding
Here the Pauli matrices act on a subspace defined by the two bands that satisfy condition in equation (3). In the kμ space, the Hamiltonian in equation (5) is a Weyl Hamiltonian that is characterized by a unit topological charge i × j × λ=±1. As the chemical potential ramps through a critical point at μ_{ijλ}, the Chern number associated with the original Hamiltonian in equation (1) changes precisely by the value of the topological charge, or the total topological charges when multiple nodal points occur at the same μ. Therefore, by increasing μ and assuming , the system exhibits phases with the Chern numbers (see Supplementary Note 1)
where we have neglected the trivial phases with μ falling outside the band width. Hence, with homogeneous magnetization, the superconductor may already exhibit Chern number equal to 2. In cases where magnetic impurities are spaced more sparsely, that is, if Λ* is a sublattice of Λ, even higher Chern numbers can be obtained.
We solved the Hamiltonian in equation (1) numerically for the case of one magnetic impurity every 2 × 2 and 3 × 3 plaquettes of the square lattice, and show the phase diagrams in Fig. 2. From the phase diagrams, we can read off three general features. First, at small chemical potentials around the band bottom, where the Fermi wavelengths are larger than or comparable to the lattice spacing of Λ*, the sequence of Chern numbers (−1, 0, +2) always occurs when J/t is not too large—this universal feature corresponds to the dense limit that we have discussed above; Second, at larger chemical potentials, more than two Fermi surfaces can exist in the reduced Brillouin zone defined by Λ* as the Fermi wavelengths are significantly smaller than the lattice spacing of Λ*, as a consequence higher Chern numbers can occur (for example, 8 in Fig. 2a and 15 in Fig. 2b) but the tradeoff is an overall smaller induced gap; Third, the phases with different Chern numbers are generally separated by lines (in the 2D parameter space) defined by conditions similar to equation (4), and across each specific line the change of Chern numbers is a constant determined essentially the same way as in our preceding analysis.
Dilute impurity limit
To better understand the dilute impurity limit, we complement our results on the lattice by a calculation in which we treat the underlying 2D superconductor in the continuum limit and consider sparsely distributed Shiba impurities that are arranged in a square lattice on top of it. This allows us to derive an effective twoband model for the hybridizing Shiba states. This effective Hamiltonian represents a chiral pwave superconductor in the appropriate parameter regime.
The strategy of our derivation is inspired by the calculation for 1D Shiba chains of Pientka et al.^{29} (for details, see Methods and Supplementary Note 2). We start from a 4 × 4 Bogoliubovde Gennes (BdG) Hamiltonian
that acts on 4spinor valued wave functions , , with ξ_{k}=k^{2}/(2m)−μ+α(k_{1}σ_{2}−k_{2}σ_{1}). In addition, the magnetic impurities are represented by the Hamiltonian
where S_{3}=σ_{3}⊗τ_{0}, with τ_{0} the identity matrix acting on particlehole space. If we restrict the wavefunction Ψ(r) to the locations r*∈Λ* of the impurities, we obtain the selfconsistency equation
for the Fourier transforms , where the momentum q∈[0,2π)^{2} now belongs to the Λ* Brillouin zone and we have set the impurity spacing to unity.
We need two more steps to reduce equation (9) to an effective twoband model for the Shiba states, assuming they are deep in the superconducting gap and dilute compared with the Fermi wavelength. First, the lefthand side is expanded to linear order in the energy E to cast the equation in the form of the timeindependent Schroedinger equation. Second, we project the effective Hamiltonian into the eigenstates of an isolated Shiba impurity on every site, given by and (ref. 29). We obtain the effective twoband Hamiltonian
where
are defined in terms of the functions
In the equation above, ξ is the coherence length of the 2D superconductor (without the magnetic impurities), is its Fermi velocity, are the Fermi wavevectors for its two spinsplit bands, and is a dimensionless parameter. In the deep Shiba limit in which the projection in the states Ψ_{+} and Ψ_{−} is justified, we have η∼1.
The Hamiltonian in equation (10) represents the effective superconductor formed by the Shiba bound states within the gap of the underlying swave superconductor. Similar to the case of the effective twoband model in equation (5), this Hamiltonian can have nodal points in kμspace at which the Chern number changes. However, unlike in equation (5), the nodes can occur at any point in the Brillouin zone, making an analytic treatment intractable. In addition, the validity of equation (10) requires a selfconsistency that permits the lowenergy expansion of equation (9). Therefore, instead of computing the Chern numbers in an extended parameter space (for examples of Chern numbers along several linecuts of the parameter space, see Supplementary Fig. 1), we focus on information that can be obtained at special points of the Brillouin zone at infinitesimal energy. To that end, observe that the Hamiltonian in equation (10) has C_{4} rotational symmetry. Thus, any gap closing at points other than the C_{4}symmetric momenta k=(0, 0) and k=(π, π) changes the Chern number by an even integer due to the symmetryimposed multiplicity of the nodal points. By expanding the Hamiltonian into Dirac form around the C_{4}symmetric momenta, we obtain the expression (see Supplementary Note 2)
for the parity of the Chern number. The numerical evaluation of this equation is shown in Fig. 3 in the form of a phase diagram.
Helical magnetic order
We have also performed calculations for magnetic orders other than simple ferromagnetism. In particular, the case where the magnetic configurations corresponds to 2D helices is related to previous studies on 1D helices^{31,32,33,34,35,36,37,38,39,40}. We obtained criteria for such a system to be fully gapped by proximity effect, and found that the fullygapped superconducting phases can be generically topologically nontrivial. The results and phase diagrams are presented in Supplementary Note 3 and Supplementary Figs 2–4.
Material proposal
We complement our simple model considerations with a specific material proposal to realize a superconductor with nonzero Chern numbers. For that, we consider transition metal atoms, in particular Fe, deposited on the (111) surface of Pb, a strong typeII superconductor. The same combination of materials, but a different surface of Pb, was used in the experimental realization for the 1D pwave superconductor^{25}. The Pb atoms on the (111) surface form a triangular lattice. Through abinitio calculations that assume a ferromagnetic alignment of the Fe magnetic moments and include spinorbit coupling, we compared the relaxation energies for various densities and arrangements of Fe adatoms on the Pb (111) surface, and found that a deposition with one Fe atom in each triangular plaquette is particularly favorable (see Supplementary Figs 5 and 6). In this case, the Fe atoms form a honeycomb lattice in which the atoms sit at different heights in each sublattice (see Fig. 4a). We further performed DFT calculations of the electronic structure. The resulting (not spindegenerate) Fermi surface and band structure restricted to the Fe dorbitals is shown in Fig. 4b,c. Critically, we find an odd number of Fermi surfaces (for examples of the Fermi surfaces at several Fermi energies, see Supplementary Fig. 7). The Chern number of the corresponding BdG Hamiltonian is indeed nonzero over a large range of the chemical potential (see Fig. 4d).
Discussion
In conclusion, we have proposed a versatile platform for realizing chiral superconductors in 2D. We have obtained analytically the topological phase diagram (Chern number and gaps of the superconductor) of the dilute and dense limit, and numerically evaluated the phase diagram in the intermediate regime. We then showed through a more realistic abinitio calculation that ferromagnetically ordered Fe atoms on the (111) surface of Pb in the dense limit could give rise to a chiral superconductor. To find flat islands of magnetic adatoms on the Pb substrate, however, is currently an experimental challenge because under standard growth conditions the magnetic adatoms tend to ball up instead of making flat islands on the (111) surface of Pb (see Supplementary Fig. 5).
The presence of a 2D chiral superconductor could be established experimentally by tunneling into the chiral Majorana modes, whose number is equal to the Chern number of the phase, and which would take place only on the edge of a 2D thin island of Fe on the surface of Pb. Such a technique has been used to image 1D topological edge states of bismuth bilayers in the absence of superconductivity^{41}. Similar observations of quasiparticle edge modes inside the superconducting gap will be a strong signature of topological superconductivity proposed in this paper.
Methods
Effective Hamiltonian in the dilute impurity limit
We outline the general formalism by which an effective Hamiltonian for bound states of a Shiba lattice can be derived in the limit where the impurity spacing is large compared with the spacial extend of the bound states of an isolated impurity. We want to derive the lowenergy effective theory for a Hamiltonian of the general form
where H_{0} is the original superconducting BdG Hamiltonian which is gapped (≈Δ) around zero energy, and H_{1} is the Hamiltonian for a collection of deltafunction impurities at positions r_{n}, where n takes values in a set c, for example, a lattice. Here, V_{n} are matrices associated with the local degrees of freedom (such as spin and particlehole) which can induce ingap states. We implicitly keep r as a ddimensional vector, so that the formalism is applicable for systems in any dimension d (the same applies to r_{n}, n, m, k, q, R and n_{b} below).
We start with the Schrödinger equation for bound state wave functions Ψ
It follows that
where G_{0}(E)=(E−H_{0})^{−1} is the Green function.
Because H_{1} is composed of deltafunctions for a small set c, G_{0}H_{1} is nonzero only in the columns corresponding to c, thus equation 17 is equivalent to
In the simplest case, if c contains only one single point, labelled by 0, then equation 18 implies
The energy of the excitations can be obtained by solving
In more complicated cases, the following equation, again implied by equation 18, can serve as the starting point to extract an effective Hamiltonian
In addition, if ∀n: V_{n}=V_{0} and r_{n}=nR (n∈^{d} with d the dimension), equation (22) can be transformed to kspace:
where
n_{b} can be interpreted as the band index when the kspace is folded into the Brillouin zone defined by [−π/R, π/R). The application of this formalism to the Hamiltonian in equation (7) is detailed in Supplementary Note 2 and results in the Hamiltonian in equation (10).
First principle calculations
We performed electronic structure calculations within the DFT formalism as implemented in the Vienna ab initio simulation package^{42}. We used the allelectron projector augmented wave^{43,44} basis sets with the generalized gradient approximation of Perdew, Burke and Ernzerhof^{45} to the exchange correlation potential. The Hamiltonian contains scalar relativistic corrections, and the spinorbit coupling was taken into account by the second variation method^{46}.
In this work, we chose the host superconductor to be Pb thin film with a (111) surface, and consider different transition metal adatoms. We started by finding the stable configurations of the adatoms on top of the Pb surface. To this end we have compared the relaxation energy (per atom) for an extensive collection of possible configurations, four of which are shown in Supplementary Fig. 5. Based on this energetic consideration and for simplicity, we adopted the configuration with two adatoms per Pb unit cell (highlighted in Supplementary Fig. 5) in our following simulations. When the transition metal element is chosen to be Fe, the details of the configuration are shown in Supplementary Fig. 6. In this configuration, two species of Fe atoms form a buckled honeycomb structure which gains bonding energy due to the short nearestneighbor distance.
To simulate the composite system and consider the effect of Pb, we used six layers of Pb atoms as substrate in the relaxation calculations with roughly 15 Å vacuum space, taking into account spinorbital coupling and the ferromagnetic alignment of the Fe moments. We performed DFT calculations with the abovementioned stable configuration. Based on the DFT calculations, we constructed the maximally localized Wannier functions^{47,48} for Fe, and obtained a tightbinding model with a band structure that agrees well with the DFT result. We then used the tightbinding model and added a small swave superconducting pairing term to it. We computed the Chern numbers of the thusobtained BogoliubovdeGennes Hamiltonian to be nonvanishing, as shown in Fig. 4. For completeness, we present a few more Fermi surfaces with different values of E_{F} in Supplementary Fig. 7 to complement Fig. 4.
Additional information
How to cite this article: Li, J. et al. Twodimensional chiral topological superconductivity in Shiba lattices. Nat. Commun. 7:12297 doi: 10.1038/ncomms12297 (2016).
References
 1
Moore, G. & Read, N. Nonabelions in the fractional quantum hall effect. Nucl. Phys. B 360, 362–396 (1991).
 2
Read, N. & Green, D. Paired states of fermions in two dimensions with breaking of parity and timereversal symmetries and the fractional quantum Hall effect. Phys. Rev. B 61, 10267 (2000).
 3
Ivanov, D. A. Nonabelian statistics of halfquantum vortices in pwave superconductors. Phys. Rev. Lett. 86, 268 (2001).
 4
Kitaev, A. Y. Faulttolerant quantum computation by anyons. Ann. Phys. 303, 2–30 (2003).
 5
Nayak, C., Simon, S., Stern, A., Freedman, M. & Das Sarma, S. Nonabelian anyons and topological quantum computation. Rev. Mod. Phys. 80, 1083–1159 (2008).
 6
Tewari, S., Das Sarma, S., Nayak, C., Zhang, C. & Zoller, P. Quantum computation using vortices and majorana zero modes of a p x+ip y superfluid of fermionic cold atoms. Phys. Rev. Lett. 98, 010506 (2007).
 7
Das Sarma, S., Nayak, C. & Tewari, S. Proposal to stabilize and detect halfquantum vortices in strontium ruthenate thin films: nonabelian braiding statistics of vortices in a p x+ip y superconductor. Phys. Rev. B 73, 220502 (2006).
 8
Mackenzie, A. P. & Maeno, Y. The superconductivity of Sr2RuO4 and the physics of spintriplet pairing. Rev. Mod. Phys. 75, 657–712 (2003).
 9
Raghu, S., Kapitulnik, A. & Kivelson, S. A. Hidden quasionedimensional superconductivity in Sr2RuO4 . Phys. Rev. Lett. 105, 136401 (2010).
 10
Wang, Q. H. et al. Theory of superconductivity in a threeorbital model of Sr2RuO4 . EPL (Europhys. Lett.) 104, 17013 (2013).
 11
Scaffidi, T., Romers, J. C. & Simon, S. H. Pairing symmetry and dominant band in Sr2RuO4 . Phys. Rev. B 89, 220510 (2014).
 12
Sato, M., Takahashi, Y. & Fujimoto, S. Nonabelian topological order in swave superfluids of ultracold fermionic atoms. Phys. Rev. Lett. 103, 020401 (2009).
 13
Lee, P. A. Proposal for Creating a Spinpolarized p x+ip y State and Majorana Fermions. Preprint at http://arxiv.org/abs/0907.2681 (2009).
 14
Sau, J. D., Lutchyn, R. M., Tewari, S. & Das Sarma, S. Generic new platform for topological quantum computation using semiconductor heterostructures. Phys. Rev. Lett. 104, 040502 (2010).
 15
Alicea, J. Majorana fermions in a tunable semiconductor device. Phys. Rev. B 81, 125318 (2010).
 16
Kitaev, A. Y. Unpaired Majorana fermions in quantum wires. PhysicsUspekhi 44, 131–136 (2001).
 17
Lutchyn, R. M., Sau, J. D. & Das Sarma, S. Majorana fermions and a topological phase transition in semiconductorsuperconductor heterostructures. Phys. Rev. Lett. 105, 077001 (2010).
 18
Oreg, Y., Refael, G. & von Oppen, F. Helical liquids and majorana bound states in quantum wires. Phys. Rev. Lett. 105, 177002 (2010).
 19
Potter, A. C. & Lee, P. A. Multichannel generalization of Kitaev’s majorana end states and a practical route to realize them in thin films. Phys. Rev. Lett. 105, 227003 (2010).
 20
Alicea, J., Oreg, Y., Refael, G., von Oppen, F. & Fisher, M. P. A. Nonabelian statistics and topological quantum information processing in 1D wire networks. Nat. Phys. 7, 412–417 (2011).
 21
Halperin, B. I. et al. Adiabatic manipulations of Majorana fermions in a threedimensional network of quantum wires. Phys. Rev. B 85, 144501 (2012).
 22
Stanescu, T. D. & Tewari, S. Majorana fermions in semiconductor nanowires: fundamentals, modeling, and experiment. J. Phys.: Condens. Matter 25, 233201 (2013).
 23
Mourik, V. et al. Signatures of majorana fermions in hybrid superconductorsemiconductor nanowire devices. Science 336, 1003–1007 (2012).
 24
Li, J. et al. Topological superconductivity induced by ferromagnetic metal chains. Phys. Rev. B 90, 235433 (2014).
 25
NadjPerge, S. et al. Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor. Science 346, 602–607 (2014).
 26
Yu, L. Bound state in superconductors with paramagnetic impurities. Acta Phys. Sin. 21, 75–91 (1965).
 27
Shiba, H. Classical spins in superconductors. Prog. Theor. Phys. 40, 435–451 (1968).
 28
Rusinov, A. I. On the theory of gapless superconductivity in alloys containing paramagnetic impurities. Soviet Phys. JETP 29, 1101–1106 (1969).
 29
Pientka, F., Glazman, L. I. & von Oppen, F. Topological superconducting phase in helical Shiba chains. Phys. Rev. B 88, 155420 (2013).
 30
Heimes, A., Mendler, D. & Kotetes, P. Interplay of topological phases in magnetic adatomchains on top of a Rashba superconducting surface. New. J. Phys. 17, 023051 (2015).
 31
Choy, T.P., Edge, J. M., Akhmerov, A. R. & Beenakker, C. W. J. Majorana fermions emerging from magnetic nanoparticles on a superconductor without spinorbit coupling. Phys. Rev. B 84, 195442 (2011).
 32
NadjPerge, S., Drozdov, I. K., Bernevig, B. A. & Yazdani, A. Proposal for realizing Majorana fermions in chains of magnetic atoms on a superconductor. Phys. Rev. B 88, 020407 (2013).
 33
Nakosai, S., Tanaka, Y. & Nagaosa, N. Twodimensional pwave superconducting states with magnetic moments on a conventional swave superconductor. Phys. Rev. B 88, 180503 (2013).
 34
Klinovaja, J., Stano, P., Yazdani, A. & Loss, D. Topological superconductivity and Majorana Fermions in RKKY Systems. Phys. Rev. Lett. 111, 186805 (2013).
 35
Braunecker, B. & Simon, P. Interplay between classical magnetic moments and superconductivity in quantum onedimensional conductors: toward a selfsustained topological Majorana phase. Phys. Rev. Lett. 111, 147202 (2013).
 36
Vazifeh, M. M. & Franz, M. Selforganized topological state with Majorana fermions. Phys. Rev. Lett. 111, 206802 (2013).
 37
Pöyhönen, K., Westström, A., Röntynen, J. & Ojanen, T. Majorana states in helical Shiba chains and ladders. Phys. Rev. B 89, 115109 (2014).
 38
Pientka, F., Glazman, L. I. & von Oppen, F. Unconventional topological phase transitions in helical Shiba chains. Phys. Rev. B 89, 180505 (2014).
 39
Kim, Y., Cheng, M., Bauer, B., Lutchyn, R. M. & Das Sarma, S. Helical order in onedimensional magnetic atom chains and possible emergence of Majorana bound states. Phys. Rev. B 90, 060401 (2014).
 40
Li, J., Neupert, T., Bernevig, B. A. & Yazdani, A. Manipulating Majorana zero modes on atomic rings with an external magnetic field. Nat. Commun. 7, 10395 (2016).
 41
Drozdov, I. K. et al. Onedimensional topological edge states of bismuth bilayers. Nat. Phys. 10, 664–669 (2014).
 42
Kresse, G. & Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B 47, 558–561 (1993).
 43
Blöchl, P. Projector augmentedwave method. Phys. Rev. B 50, 17953–17979 (1994).
 44
Kresse, G. & Joubert, D. From ultrasoft pseudopotentials to the projector augmentedwave method. Phys. Rev. B 59, 1758–1775 (1999).
 45
Perdew, J., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868 (1996).
 46
Koelling, D. D. & Harmon, B. N. A technique for relativistic spinpolarised calculations. J. Phys. C 10, 3107–3114 (1977).
 47
Marzari, N. & Vanderbilt, D. Maximally localized generalized Wannier functions for composite energy bands. Phys. Rev. B 56, 12847–12865 (1997).
 48
Souza, I., Marzari, N. & Vanderbilt, D. Maximally localized Wannier functions for entangled energy bands. Phys. Rev. B 65, 035109 (2001).
Acknowledgements
We thank F. Pientka for discussions. This work was supported by DARPA SPAWARSYSCEN Pacific N660011114110, and ONRN000141410330. B.A.B. acknowledges support from NSF CAREER DMR0952428, ONRN000141110635, MURI1306082, DARPA under SPAWAR Grant No.: N660011114110, the Packard Foundation, and a Keck grant. J.L. acknowledges support from Swiss National Science Foundation. A.H.M. acknowledges support from the Welch Foundation under grant TBF1473. A.Y. acknowledges support from NSFMRESECDMR0819860 and NSFDMR1104612.
Author information
Affiliations
Contributions
All authors contributed to the conceptual ideas on which the proposal is based and to the writing of the manuscript. J.L. performed the numerical calculations, T.N. and J.L. performed analytical calculations. Z.W. preformed the firstprinciples calculations.
Corresponding author
Ethics declarations
Competing interests
The authors declare no competing financial interests.
Supplementary information
Supplementary Information
Supplementary Figures 17, Supplementary Notes 13 and Supplementary References. (PDF 2149 kb)
Rights and permissions
This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/
About this article
Cite this article
Li, J., Neupert, T., Wang, Z. et al. Twodimensional chiral topological superconductivity in Shiba lattices. Nat Commun 7, 12297 (2016). https://doi.org/10.1038/ncomms12297
Received:
Accepted:
Published:
Further reading

Detecting and distinguishing Majorana zero modes with the scanning tunnelling microscope
Nature Reviews Physics (2021)

Topological superconductivity in skyrmion lattices
npj Quantum Materials (2021)

Aspects of Topological Superconductivity in 2D Systems: Noncollinear Magnetism, Skyrmions, and Higherorder Topology
Journal of Superconductivity and Novel Magnetism (2021)

Electronic and magnetic characterization of epitaxial VSe2 monolayers on superconducting NbSe2
Communications Physics (2020)

Magnetism and ingap states of 3d transition metal atoms on superconducting Re
npj Quantum Materials (2019)
Comments
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.