Coherent strain in the defect-free crystal structure results in high hole-mobilities. In contrast to the group III–V materials used in most previous work, we use Ge–Si core–shell nanowires consisting of a monocrystalline Ge ⟨110⟩ core with a diameter of ∼15 nm and a Si shell thickness of 2.5 nm covered by a native SiO 2. Strong efforts have been made to improve these interfaces, that is, induce a hard gap, using epitaxially grown Al (20,21) or specialized surface treatments methods, (22,23) resulting in much better resolved Majorana signatures. (15,16) The resulting quasiparticle poisoning decoheres Majorana states since they will participate in braiding operations (17−19) and additionally obscure the Majorana signatures at zero energy. The first reports showing these zero-bias conductance peaks in InAs and InSb nanowires (8−14) suffered from sizable subgap conductivity attributed to inhomogeneities in the nanowire–superconductor interface. (4−7) Signatures of Majorana fermions are expected to arise as a conductance peak at zero bias and finite magnetic fields. Majorana fermions require a topological superconducting material, which in practice can be realized by coupling a conventional s-wave superconductor to a one-dimensional nanowire with high spin–orbit coupling and g-factor. The discovery that Majorana fermions offer a route toward an inherently topologically protected fault-tolerant quantum computer (1−3) marked the beginning of a quickly growing field of research to achieve their experimental realization. ![]() The small size of these diffusion-induced superconductors inside nanowires may be of special interest for applications requiring high magnetic fields in arbitrary direction. In another device where the diffusion of Al rendered the nanowire completely metallic, a superconductor with a much higher critical temperature ( T C = 2.9 K) and critical field ( B C = 3.4 T) is found. We can therefore selectively switch either superconductor to the normal state by tuning the temperature and the magnetic field and observe that the additional superconductor induces a proximity supercurrent in the semiconducting part of the nanowire even when the Al is in the normal state. Next to Al, we find a superconductor with lower critical temperature ( T C = 0.9 K) and a higher critical field ( B C = 0.9–1.2 T). ![]() This is realized by annealing devices at 180 ☌ during which aluminum interdiffuses and replaces the germanium in a section of the nanowire. A hard gap requires a highly homogeneous tunneling heterointerface between the superconducting contacts and the semiconducting nanowire. We show a hard superconducting gap in a Ge–Si nanowire Josephson transistor up to in-plane magnetic fields of 250 mT, an important step toward creating and detecting Majorana zero modes in this system.
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