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The interplay of Dirac physics and induced superconductivity at the interface of a 3D topological insulator (TI) with an s-wave superconductor (S) provides a new platform for topologically protected quantum computation based on elusive Majorana modes. To employ such S–TI hybrid devices in future topological quantum computation architectures, a process is required that allows for device fabrication under ultrahigh vacuum conditions. Here, we report on the selective area growth of (Bi,Sb)2Te3 TI thin films and stencil lithography of superconductive Nb for a full in situ fabrication of S–TI hybrid devices via molecular-beam epitaxy. A dielectric capping layer was deposited as a final step to protect the delicate surfaces of the S–TI hybrids at ambient conditions. Transport experiments in as-prepared Josephson junctions show highly transparent S–TI interfaces and a missing first Shapiro step, which indicates the presence of Majorana bound states. To move from single junctions towards complex circuitry for future topological quantum computation architectures, we monolithically integrated two aligned hardmasks to the substrate prior to growth. The presented process provides new possibilities to deliberately combine delicate quantum materials in situ at the nanoscale.

One prominent hallmark of topological semimetals is the existence of unusual topological surface states known as Fermi arcs. Nevertheless, the Fermi-arc superconductivity remains elusive. Here, we report the critical current oscillations from surface Fermi arcs in Nb-Dirac semimetal Cd 3 As 2 -Nb Josephson junctions. The supercurrent from bulk states are suppressed under an in-plane magnetic field ~0.1 T, while the supercurrent from the topological surface states survives up to 0.5 T. Contrary to the minimum normal-state conductance, the Fermi-arc carried supercurrent shows a maximum critical value near the Dirac point, which is consistent with the fact that the Fermi arcs have maximum density of state at the Dirac point. Moreover, the critical current exhibits periodic oscillations with a parallel magnetic field, which is well understood by considering the in-plane orbital effect from the surface states. Our results suggest the Dirac semimetal combined with superconductivity should be promising for topological quantum devices. Superconductivity through the topological surface states of a Dirac semimetal remains elusive. Here, the authors fabricate a Nb-Cd 3 As 2 -Nb Josephson junction and observe supercurrent oscillations from surface Fermi arcs of Cd 3 As 2

An array of superconducting islands placed on a normal metal film offers a tunable realization of nanopatterned superconductivity. This system enables investigation of the nature of competing vortex states and phase transitions between them. A square array creates the eggcrate potential in which magnetic field–induced vortices are frozen into a vortex insulator. We observed a vortex insulator–vortex metal transition driven by the applied electric current and determined critical exponents that coincided with those for thermodynamic liquid-gas transition. Our findings offer a comprehensive description of dynamic critical behavior and establish a deep connection between equilibrium and nonequilibrium phase transitions.

Edge supercurrent has attracted great interest recently due to its crucial role in achieving and manipulating topological superconducting states. Proximity-induced superconductivity has been realized in quantum Hall and quantum spin Hall edge states, as well as in higher-order topological hinge states. Non-Hermitian skin effect, the aggregation of non-Bloch eigenstates at open boundaries, promises an abnormal edge channel. Here we report the observation of broad edge supercurrent in Dirac semimetal Cd 3 As 2 -based Josephson junctions. The as-grown Cd 3 As 2 nanoplates are electron-doped by intrinsic defects, which enhance the non-Hermitian perturbations. The superconducting quantum interference indicates edge supercurrent with a width of ~1.6 μm and a magnitude of ~1 μA at 10 mK. The wide and large edge supercurrent is inaccessible for a conventional edge system and suggests the presence of non-Hermitian skin effect. A supercurrent nonlocality is also observed. The interplay between band topology and non-Hermiticity is beneficial for exploiting exotic topological matter. The non-Hermitian skin effect, or localization of eigenstates at the boundary of a non-Hermitian system, has been intensively studied. Chu et al. observe a large and wide edge supercurrent in the Dirac semimetal Cd 3 As 2 -based Josephson junctions, which is consistent with the non-Hermitian skin effect.

Topological superconductors hosting Majorana zero modes are of great interest for both fundamental physics and potential quantum computing applications. In this work, we investigate the transport properties of the intrinsic magnetic topological insulator MnBi2Te4 (MBT). In normal transport measurements, we observe the presence of chiral edge channels, though with deviations from perfect quantization due to factors such as non-uniform thickness, domain structures, and the presence of quasi-helical edge states. Subsequently, we fabricate superconducting junctions using niobium leads on MBT exfoliated flakes, which show an onset of supercurrent with clear Josephson coupling. The interference patterns in the superconducting junctions reveal interesting asymmetries, suggesting changes in the magnetic ordering of the MBT flakes under small applied magnetic fields. Moreover, the modulation of the critical current by magnetic field reveals a SQUID-like pattern, suggesting the presence of supercurrent through the quasi-helical edge states.Topological superconductors hosting Majorana zero modes are of great interest for both fundamental physics and potential quantum computing applications. Here, the intrinsic and Josephson junction transport properties of magnetic topological insulator MnBi2Te4 are investigated, revealing superconducting interference patterns that suggest the presence of supercurrent through quasi-helical edge states.

Intrinsic magnetic topological insulators (IMTIs) have a non-trivial band topology in combination with magnetic order. This potentially leads to fascinating states of matter, such as quantum anomalous Hall (QAH) insulators and axion insulators. One of the theoretically predicted IMTIs is VBi2Te4, but experimental evidence of this material is lacking so far. Here, we report on our attempts to synthesise VBi2Te4 by molecular beam epitaxy (MBE). X-ray diffraction reveals that in the thermodynamic phase space reachable by MBE, there is no region where VBi2Te4 is stably synthesised. Moreover, scanning transmission electron microscopy shows a clear phase separation to Bi2Te3 and VTe2 instead of the formation of VBi2Te4. We suggest the phase instability to be due to either the large lattice mismatch between VTe2 and Bi2Te3 or the unfavourable valence state of vanadium.

The material BaBiO3 is known for its insulating character. However, for thin films, in the ultra-thin limit, metallicity is expected because the oxygen octahedra breathing mode will be suppressed as reported recently. Here, we confirm the influence of the oxygen breathing mode on the size of the band gap. The electronic properties of a BaBiO3 thickness series are studied using in-situ scanning tunneling microscopy. We observe a wide-gap (EG > 1.2 V) to small-gap (EG ≈ 0.07 eV) semiconductor transition as a function of a decreasing BaBiO3 film thickness. However, even for an ultra-thin BaBiO3 film, no metallic state is present. The dependence of the band gap size is found to be coinciding with the intensity of the Raman response of the breathing phonon mode as a function of thickness.

The study of the exotic properties of the surface states of topological insulators requires defect-free and smooth surfaces. This work aims to study the enhancement of the surface morphology of optimally doped, high-crystalline (Bi0.4Sb0.6)2Te3 films deposited by molecular beam epitaxy on Al2O3 (001) substrates. Atomic force microscopy shows that by employing an in situ thermal post anneal, the surface roughness is reduced significantly, and transmission electron microscopy reveals that structural defects are diminished substantially. Thence, these films provide a great platform for the research on the thickness-dependent properties of topological insulators.

A 4 π -periodic supercurrent through a Josephson junction can be a consequence of the presence of Majorana bound states. A systematic study of the radio frequency response for several temperatures and frequencies yields a concrete protocol for examining the 4 π -periodic contribution to the supercurrent. This work also reports the observation of a 4 π -periodic contribution to the supercurrent in BiSbTeSe 2 -based Josephson junctions. As a response to irradiation by radio frequency waves, the junctions showed an absence of the first Shapiro step. At high irradiation power, a qualitative correspondence to a model including a 4 π -periodic component to the supercurrent is found.

Ultrathin films of the ternary topological insulator (Bi0.4Sb0.6)2Te3 are fabricated by molecular beam epitaxy. Although it is generally assumed that the ternary topological insulator tellurides grow by van der Waals epitaxy, our results show that the influence of the substrate is substantial and governs the formation of defects, mosaicity, and twin domains. For this comparative study, InP (111)A, Al2O3 (001), and SrTiO3 (111) substrates were selected. While the films deposited on lattice-matched InP (111)A show van der Waals epitaxial relations, our results point to a quasi-van der Waals epitaxy for the films grown on substrates with a larger lattice mismatch.