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Electron spins in Si/SiGe quantum wells suffer from nearly degenerate conduction band valleys, which compete with the spin degree of freedom in the formation of qubits. Despite attempts to enhance the valley energy splitting deterministically, by engineering a sharp interface, valley splitting fluctuations remain a serious problem for qubit uniformity, needed to scale up to large quantum processors. Here, we elucidate and statistically predict the valley splitting by the holistic integration of 3D atomic-level properties, theory and transport. We find that the concentration fluctuations of Si and Ge atoms within the 3D landscape of Si/SiGe interfaces can explain the observed large spread of valley splitting from measurements on many quantum dot devices. Against the prevailing belief, we propose to boost these random alloy composition fluctuations by incorporating Ge atoms in the Si quantum well to statistically enhance valley splitting. Spin qubits in Si/SiGe quantum dots suffer from variability in the valley splitting which will hinder device scalability. Here, by using 3D atomic characterization, the authors explain this variability by random Si and Ge atomic fluctuations and propose a strategy to statistically enhance the valley splitting

Semiconductor nanowires have opened new research avenues in quantum transport owing to their confined geometry and electrostatic tunability. They have offered an exceptional testbed for superconductivity, leading to the realization of hybrid systems combining the macroscopic quantum properties of superconductors with the possibility to control charges down to a single electron. These advances brought semiconductor nanowires to the forefront of efforts to realize topological superconductivity and Majorana modes. A prime challenge to benefit from the topological properties of Majoranas is to reduce the disorder in hybrid nanowire devices. Here we show ballistic superconductivity in InSb semiconductor nanowires. Our structural and chemical analyses demonstrate a high-quality interface between the nanowire and a NbTiN superconductor that enables ballistic transport. This is manifested by a quantized conductance for normal carriers, a strongly enhanced conductance for Andreev-reflecting carriers, and an induced hard gap with a significantly reduced density of states. These results pave the way for disorder-free Majorana devices. Disorder has been a prime challenge to study the topological properties in a hybrid system. Here, Zhang et al . report ballistic superconductivity in InSb nanowires interfacing with a NbTiN superconductor, paving the way for disorder-free Majorana devices.

Nanowires are promising platforms for realizing ultra-compact light sources for photonic integrated circuits. In contrast to impressive progress on light confinement and stimulated emission in III-V and II-VI semiconductor nanowires, there has been no experimental demonstration showing the potential to achieve strong cavity effects in a bottom-up grown single group-IV nanowire, which is a prerequisite for realizing silicon-compatible infrared nanolasers. Herein, we address this limitation and present an experimental observation of cavity-enhanced strong photoluminescence from a single Ge/GeSn core/shell nanowire. A sufficiently large Sn content ( ~ 10 at%) in the GeSn shell leads to a direct bandgap gain medium, allowing a strong reduction in material loss upon optical pumping. Efficient optical confinement in a single nanowire enables many round trips of emitted photons between two facets of a nanowire, achieving a narrow width of 3.3 nm. Our demonstration opens new possibilities for ultrasmall on-chip light sources towards realizing photonic-integrated circuits in the underexplored range of short-wave infrared (SWIR). Group-IV nanowires hold great promise for building ideal light sources for photonic integrated circuits. This study presents an observation of cavity resonances in a single GeSn nanowire, laying the foundation for realizing group-IV nanowire lasers.

The coherence of hole spin qubits in germanium planar heterostructures is limited by the hyperfine coupling to the nuclear spin bath due to 29Si $^{29}{\\rm Si}$and 73Ge $^{73}{\\rm Ge}$isotopes. Thus, removing these nuclear spin‐full isotopes is essential to extend the hyperfine‐limited coherence times needed to implement robust quantum processors. This work demonstrates the epitaxial growth of device‐grade nuclear spin‐free 70Ge $^{70}{\\rm Ge}$ /28Si70Ge $^{28}{\\rm Si}^{70}{\\rm Ge}$heterostructures on industrial SiGe buffers while minimizing the amounts of highly purified 70GeH4 $^{70}{\\rm GeH}_4$and 28SiH4 $^{28}{\\rm SiH}_4$used. The obtained 70Ge $^{70}{\\rm Ge}$ /28Si70Ge $^{28}{\\rm Si}^{70}{\\rm Ge}$heterostructures exhibit a dislocation density of 5.3×106cm−2 $5.3 \\times 10^{6}\\nobreakspace \\mathrm{cm}^{-2}$and an isotopic purity exceeding 99.99% $99.99\\%$ , with carbon and oxygen impurities below the detection sensitivity, as revealed by atom probe tomography. Magneto‐transport measurements on gated Hall bars demonstrate effective gate control of hole density in nuclear spin‐free quantum wells. Negative threshold gate voltages confirm the absence of intentional doping in the wells, while Hall and Shubnikov–de Haas analyses yield consistent carrier densities (∼1.4×1011cm−2 $\\sim 1.4 \\times 10^{11}\\nobreakspace \\mathrm{cm}^{-2}$ ) and high mobilities (∼2.4×105cm2/Vs $\\sim 2.4 \\times 10^{5}\\nobreakspace \\mathrm{cm}^{2}/\\mathrm{Vs}$ ). Mobility trends reveal interface‐trap‐limited scattering and percolation concentration below 7×1010cm−2 $7 \\times 10^{10}\\nobreakspace \\mathrm{cm}^{-2}$ . These analyses, along with atomic‐level studies, confirm the high quality of epitaxial 70Ge $^{70}{\\rm Ge}$ /28Si70Ge $^{28}{\\rm Si}^{70}{\\rm Ge}$heterostructures and their relevance as a platform for long‐coherence spin qubits. Hyperfine coupling to 29Si $^{29}{\\rm Si}$and 73Ge $^{73}{\\rm Ge}$nuclear spins limits hole spin‐qubit coherence in Ge heterostructures. We demonstrate device‐grade, nuclear‐spin‐free 70Ge $^{70}{\\rm Ge}$ /28Si70Ge $^{28}{\\rm Si}^{70}{\\rm Ge}$quantum wells grown on industrial SiGe buffers with minimal use of enriched precursors. Atom‐probe and magnetotransport measurements reveal exceptional isotopic purity, low defect density, and high mobilities, establishing a robust platform for long‐coherence spin qubits.

The interface between topological and normal insulators hosts metallic states that appear due to the change in band topology. While topological states at a surface, i.e., a topological insulator-air/vacuum interface, have been studied intensely, topological states at a solid-solid interface have been less explored. Here we combine experiment and theory to study such embedded topological states (ETSs) in heterostructures of GeTe (normal insulator) and Sb 2 Te 3 (topological insulator). We analyse their dependence on the interface and their confinement characteristics. First, to characterise the heterostructures, we evaluate the GeTe-Sb 2 Te 3 band offset using X-ray photoemission spectroscopy, and chart the elemental composition using atom probe tomography. We then use first-principles to independently calculate the band offset and also parametrise the band structure within a four-band continuum model. Our analysis reveals, strikingly, that under realistic conditions, the interfacial topological modes are delocalised over many lattice spacings. In addition, the first-principles calculations indicate that the ETSs are relatively robust to disorder and this may have practical ramifications. Our study provides insights into how to manipulate topological modes in heterostructures and also provides a basis for recent experimental findings [Nguyen et al. Sci. Rep. 6 , 27716 (2016)] where ETSs were seen to couple over thick layers.

This article has been retracted. Please see the Retraction Notice for more detail: https://doi.org/10.1038/s41586-021-03373-x.

Indium antimonide (InSb) nanowires are used as building blocks for quantum devices because of their unique properties, that is, strong spin‐orbit interaction and large Landé g‐factor. Integrating InSb nanowires with other materials could potentially unfold novel devices with distinctive functionality. A prominent example is the combination of InSb nanowires with superconductors for the emerging topological particles research. Here, the combination of the II–VI cadmium telluride (CdTe) with the III–V InSb in the form of core–shell (InSb–CdTe) nanowires is investigated and potential applications based on the electronic structure of the InSb–CdTe interface and the epitaxy of CdTe on the InSb nanowires are explored. The electronic structure of the InSb–CdTe interface using density functional theory is determined and a type‐I band alignment is extracted with a small conduction band offset ( ⩽0.3 eV). These results indicate the potential application of these shells for surface passivation or as tunnel barriers in combination with superconductors. In terms of structural quality, it is demonstrated that the lattice‐matched CdTe can be grown epitaxially on the InSb nanowires without interfacial strain or defects. These shells do not introduce disorder to the InSb nanowires as indicated by the comparable field‐effect mobility measured for both uncapped and CdTe‐capped nanowires. Combining indium antimonide (InSb) nanowires with materials of dissimilar properties, has enabled the engineering of exotic materials, such as topological superconductors. This work explores potential applications of the material combination of InSb with cadmium telluride in the form of core–shell nanowire heterostructures. These heterostructures are studied in terms of growth, epitaxy, electronic structure of their interface, and electric transport properties.