Majorana fermions, particles that are their own antiparticles, are important building blocks for topological quantum computation due to the non-Abelian statistics. The existence of Majorana zero mode (MZM) in the interface between a s-wave superconductor (SC) and a topological insulator (TI) has been predicted [1] and was observed in Bi2Te3/NbSe2 heterostructures experimentally [2]. On the other hand, in contrast to conventional TIs with a single Dirac cone, a topological crystalline insulator (TCI) hosts multiple Dirac cones in the Brillouin zone [3-5]. Recently, a superconducting TCI was predicted to possess multiple MZMs [6, 7], which can encode more qubits in topological quantum computation. To create and manipulate multiple MZMs effectively in a SC/TCI heterostructure, a high-quality homogeneous interface and surface are essential. Unlike SC/TI systems, very few experiments have been performed for SC/TCI systems due to their novelty and difficulties in sample fabrication.
SnTe is known as a typical TCI and owns a rock-salt crystal structure. Unlike van der Waals materials, this type of structure requires lattice matching at the interface of SC/TCI heterostructures, which greatly increases the difficulties of sample fabrication due to the limited material choices. In 2018, a-few-layer-α-Sn(111) (namely stanene) was reported to be a superconductor when grown on PbTe(111) although neither bulk α-Sn nor PbTe shows superconductivity [8, 9]. This enlightened us because SnTe and PbTe have the same rock-salt structure and similar lattice constants, and even better SnTe is a TCI while PbTe is a trivial semiconductor. Therefore, if α-Sn(111) can be grown on SnTe(111) and exhibit superconductivity, it is expected to become a candidate for a SC/TCI heterostructure with multiple MZMs.
In this study, we have successfully fabricated a few-layer-thick single crystal α-Sn(111) on SnTe(111) using the underlayer of Bi2Te3(111)/Si(111) by molecular beam epitaxy. The schematic cross-section image of deposited layers is shown in Figure 1(a). Figure 1(b) shows the temperature dependence of resistance with varying thickness of α-Sn. Superconductivity emerges around 1 K in the samples with more than a 3-layer of α-Sn even though SnTe is not a superconductor at this temperature range. The observed maximum transition temperature is ~ 1.5 K in the sample with 4-layer-α-Sn. Furthermore, we found that our samples show two-dimensional superconductivity checked by measuring critical magnetic fields with rotating the out-of-plane angle of magnetic fields and by analyzing I-V curves with the Berezinskii–Kosterlitz–Thouless (BKT) theory. In the presentation, we will report more details about the superconducting properties including a one- to two-band superconducting transition and a huge in-plane critical magnetic field beyond the Pauli limit.
Reference
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