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Atomic manipulation and interface engineering techniques have provided a novel approach to custom-designing topological superconductors and the ensuing Majorana zero modes, representing a new paradigm for the realization of topological quantum computing and topology-based devices. Magnet-superconductor hybrid (MSH) systems have proven to be experimentally suitable to engineer topological superconductivity through the control of both the complex structure of its magnetic layer and the interface properties of the superconducting surface. Here, we demonstrate that two-dimensional MSH systems containing a magnetic skyrmion lattice provide an unprecedented ability to control the emergence of topological phases. By changing the skyrmion radius, which can be achieved experimentally through an external magnetic field, one can tune between different topological superconducting phases, allowing one to explore their unique properties and the transitions between them. In these MSH systems, Josephson scanning tunneling spectroscopy spatially visualizes one of the most crucial aspects underlying the emergence of topological superconductivity, the spatial structure of the induced spin-triplet correlations.
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The noncentrosymmetric Half Heusler compound YPtBi exhibits superconductivity below a critical temperature T_c = 0.77 K with a zero-temperature upper critical field H_c2(0) = 1.5 T. Magnetoresistance and Hall measurements support theoretical predictions that this material is a topologically nontrivial semimetal having a surprisingly low positive charge carrier density of 2 x 10^18 cm^-3. Unconventional linear magnetoresistance and beating in Shubnikov-de Haas oscillations point to spin-orbit split Fermi surfaces. The sensitivity of magnetoresistance to surface roughness suggests a possible contribution from surface states. The combination of noncentrosymmetry and strong spin-orbit coupling in YPtBi presents a promising platform for the investigation of topological superconductivity.
In recent years, signatures of Majorana fermions have been demonstrated experimentally in several superconducting systems. However, finding systems which can be scaled up to accommodate a large number of Majorana fermions for quantum computation remains a major challenge for experimentalists. In a recent work [1], signatures of a pair of Majorana zero modes (MZMs) were found in a new experimental platform formed by EuS islands deposited on top of a gold wire which were made superconducting through proximity coupling to a superconductor. In this work, we provide a theoretical explanation for how MZMs can be formed in EuS/Au/superconductor heterostructures. This simple experimental setup provides a new route for realizing a large number of Majorana fermions for quantum computations.
We consider a new model system supporting Majorana zero modes based on semiconductor nanowires with a full superconducting shell. We demonstrate that, in the presence of spin-orbit coupling in the semiconductor induced by a radial electric field, the winding of the superconducting order parameter leads to a topological phase supporting Majorana zero modes. The topological phase persists over a large range of chemical potentials and can be induced by a predictable and weak magnetic field piercing the cylinder. The system can be readily realized in semiconductor nanowires covered by a full superconducting shell, opening a pathway for realizing topological quantum computing proposals.
Superconducting topological crystalline insulators (TCI) are predicted to host new topological phases protected by crystalline symmetries, but available materials are insufficiently suitable for surface studies. To induce superconductivity at the surface of a prototypical TCI SnTe, we use molecular beam epitaxy to grow a heterostructure of SnTe and a high-Tc superconductor Fe(Te,Se), utilizing a buffer layer to bridge the large lattice mismatch between SnTe and Fe(Te,Se). Using low-temperature scanning tunneling microscopy and spectroscopy, we measure a prominent spectral gap on the surface of SnTe, and demonstrate its superconducting origin by its dependence on temperature and magnetic field. Our work provides a new platform for atomic-scale investigations of emergent topological phenomena in superconducting TCIs.
We present an in-depth classification of the topological phases and Majorana fermion (MF) excitations that arise from the bulk interplay between unconventional multiband spin-singlet superconductivity and various magnetic textures. We focus on magnetic texture crystals with a periodically-repeating primitive cell of the helix, whirl, and skyrmion types. Our analysis is relevant for a wide range of layered materials and hybrid devices, and accounts for both strong and weak, as well as crystalline topological phases. We identify a multitude of accessible topological phases which harbor flat, uni- or bi-directional, (quasi-)helical, or chiral MF edge modes. This rich variety of MFs originates from the interplay between topological phases with gapped and nodal bulk energy spectra, with the resulting types of spectra and MFs controlled by the size of the pairing and magnetic gaps.
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