Complete gate control of supercurrent in graphene p-n junctions

In a conventional Josephson junction of graphene, the supercurrent is not turned off even at the charge neutrality point, impeding further development of superconducting quantum information devices based on graphene. Here we fabricate bipolar Josephson junctions of graphene, in which a p-n potential barrier is formed in graphene with two closely spaced superconducting contacts, and realize supercurrent ON/OFF states using electrostatic gating only. The bipolar Josephson junctions of graphene also show fully gate-driven macroscopic quantum tunnelling behaviour of Josephson phase particles in a potential well, where the confinement energy is gate tuneable. We suggest that the supercurrent OFF state is mainly caused by a supercurrent dephasing mechanism due to a random pseudomagnetic field generated by ripples in graphene, in sharp contrast to other nanohybrid Josephson junctions. Our study may pave the way for the development of new gate-tuneable superconducting quantum information devices.open114344sciescopu

How to realize a robust practical Majorana chain in a quantum dot-superconductor linear array

Semiconducting nanowires in proximity to superconductors are promising experimental systems for Majorana fermions, which may ultimately be used as building blocks for topological quantum computers. A serious challenge in the experimental realization of the Majorana fermions is the supression of topological superconductivity by disorder. We show that Majorana fermions protected by a robust topological gap can occur at the ends of a chain of quantum dots connected by s-wave superconductors. In the appropriate parameter regime, we establish that the quantum dot/superconductor system is equivalent to a 1D Kitaev chain, which can be tuned to be in a robust topological phase with Majorana end modes even in the case where the quantum dots and superconductors are both strongly disordered. Such a spin-orbit coupled quantum dot - s-wave superconductor array provides an ideal experimental platform for the observation of non-Abelian Majorana modes.Comment: 8 pages; 3 figures; version 2: Supplementary material updated to include more general proof for localized Majorana fermion

ZnO/Mg0.2Zn0.8O coaxial nanorod heterostructures for high-performance electronic nanodevice applications

We report on fabrication and electrical characteristics of field effect transistors (FETs) based on ZnO/Mg0.2Zn0.8O coaxial nanorod heterostructures. As compared to bare ZnO nanorod FETs, coaxial nanorod heterostructure FETs exhibited the enhanced mobility (similar to 110 cm(2)/V s), superior subthreshold swing (similar to 200 mV/decade), and negligibly small hysteresis to demonstrate very stable operation of high-performance nanorod FETs. In situ surface passivation and carrier confinement effects provided by heteroepitaxially grown Mg0.2Zn0.8O shell layer are presumably responsible for the highly enhanced device performance.open111825sciescopu

Modulation doping in ZnO nanorods for electrical nanodevice applications

We introduce a modulation-doping method to control electrical characteristics of ZnO nanorods. Compared with a conventional homogeneous doping method, the modulation-doping method generates localized doping layers along the circumference in ZnO nanorods, useful for many device applications. Here, we investigated electrical, structural, and optical characteristics of Ga-doped ZnO nanorods with the dopant modulation layers. Electrical conductivity of ZnO nanorods was controlled by changing either dopant mole fraction or the number of modulation-doped layers. Furthermore, the modulation-doped nanorod field effect transistors exhibited precisely controlled conductance in the order of magnitude without degradation of electron mobility. The effects of the doping on structural and optical characteristics of the nanorods are also discussed.open112126sciescopu

Controlled epitaxial growth modes of ZnO nanostructures using different substrate crystal planes

A combined experimental and theoretical investigation has clarified the nanometre-scale vapour-phase epitaxial growth of ZnO nanostructures on different crystal planes of GaN substrates. Under typical growth conditions, ZnO nanorods grow perpendicular to the GaN(0001) plane, but thin flat films form on GaN(10 (1) over bar1), (10 (1) over bar0) and (1 (1) over bar 20). High-resolution X-ray diffraction data and transmission electron microscopy confirm the heteroepitaxial relationship between the ZnO nanostructures and GaN substrates. These results are consistent with first-principles theoretical calculations, indicating that the ZnO surface morphologies are mainly influenced by highly anisotropic GaN/ZnO interface energies. As a result of the large surface energy gradients, different ZnO nanostructures grow by preferential heteroepitaxial growth on different facets of regular GaN micropattern arrays. High-resolution transmission electron microscopy shows that ZnO nanotubes develop epitaxially on micropyramid tips, presumably as a result of enhanced nucleation and growth about the edges.open113031sciescopu

Supercurrent reversal in quantum dots

When two superconductors become electrically connected by a weak link a zero-resistance supercurrent can flow. This supercurrent is carried by Cooper pairs of electrons with a combined charge of twice the elementary charge, e. The 2e charge quantum is clearly visible in the height of Shapiro steps in Josephson junctions under microwave irradiation and in the magnetic flux periodicity of h/2e in superconducting quantum interference devices. Several different materials have been used to weakly couple superconductors, such as tunnel barriers, normal metals, or semiconductors. Here, we study supercurrents through a quantum dot created in a semiconductor nanowire by local electrostatic gating. Due to strong Coulomb interaction, electrons only tunnel one-by-one through the discrete energy levels of the quantum dot. This nevertheless can yield a supercurrent when subsequent tunnel events are coherent. These quantum coherent tunnelling processes can result in either a positive or a negative supercurrent, i.e. in a normal or a pi-junction, respectively. We demonstrate that the supercurrent reverses sign by adding a single electron spin to the quantum dot. When excited states of the quantum dot are involved in transport, the supercurrent sign also depends on the character of the orbital wavefunctions

Ge/Si nanowire mesoscopic Josephson junctions

The controlled growth of nanowires (NWs) with dimensions comparable to the Fermi wavelengths of the charge carriers allows fundamental investigations of quantum confinement phenomena. Here, we present studies of proximity-induced superconductivity in undoped Ge/Si core/shell NW heterostructures contacted by superconducting leads. By using a top gate electrode to modulate the carrier density in the NW, the critical supercurrent can be tuned from zero to greater than 100 nA. Furthermore, discrete sub-bands form in the NW due to confinement in the radial direction, which results in stepwise increases in the critical current as a function of gate voltage. Transport measurements on these superconductor-NW-superconductor devices reveal high-order (n = 25) resonant multiple Andreev reflections, indicating that the NW channel is smooth and the charge transport is highly coherent. The ability to create and control coherent superconducting ordered states in semiconductor-superconductor hybrid nanostructures allows for new opportunities in the study of fundamental low-dimensional superconductivity

Widespread Use of Imaging-Guided PCI in Asia: Time for Extended Application

In recent years, a wealth of clinical data has emerged regarding intravascular imaging involving either intravascular ultrasound or optical coherence tomography. This surge in data has propelled the adoption of intravascular imaging–guided percutaneous coronary intervention (PCI) in daily clinical practice. The findings of current randomized clinical trials regarding imaging guidance have lent strong support to the benefits of intravascular imaging–guided PCI. This holds especially true for the diagnosis and treatment of complex lesions, such as left main disease, diffuse long lesions, chronic total occlusion, severely calcified lesions, bifurcations, and in-stent restenosis, as well as in high-risk patients such as those with acute myocardial infarction or chronic kidney disease. During intravascular imaging–guided PCI, operators attempt to achieve stent optimization for maximized benefits of imaging guidance. This paper provides a comprehensive review on the updated clinical data of intravascular imaging–guided PCI and intravascular ultrasound/optical coherence tomography–derived stent optimization criteria

Discordance Between Angiographic Assessment and Fractional Flow Reserve or Intravascular Ultrasound in Intermediate Coronary Lesions: A Post-hoc Analysis of the FLAVOUR Trial

Background and Objectives: Angiographic assessment of coronary stenosis severity using quantitative coronary angiography (QCA) is often inconsistent with that based on fractional flow reserve (FFR) or intravascular ultrasound (IVUS). We investigated the incidence of discrepancies between QCA and FFR or IVUS, and the outcomes of FFR- and IVUS-guided strategies in discordant coronary lesions. Methods: This study was a post-hoc analysis of the FLAVOUR study. We used a QCA-derived diameter stenosis (DS) of 60% or greater, the highest tertile, to classify coronary lesions as concordant or discordant with FFR or IVUS criteria for percutaneous coronary intervention (PCI). The patient-oriented composite outcome (POCO) was defined as a composite of death, myocardial infarction, or revascularization at 24 months. Results: The discordance rate between QCA and FFR or IVUS was 30.2% (n=551). The QCA-FFR discordance rate was numerically lower than the QCA-IVUS discordance rate (28.2% vs. 32.4%, p=0.050). In 200 patients with ≥60% DS, PCI was deferred according to negative FFR (n=141) and negative IVUS (n=59) (15.3% vs. 6.5%, p<0.001). The POCO incidence was comparable between the FFR- and IVUS-guided deferral strategies (5.9% vs. 3.4%, p=0.479). Conversely, 351 patients with DS <60% underwent PCI according to positive FFR (n=118) and positive IVUS (n=233) (12.8% vs. 25.9%, p<0.001). FFR- and IVUS-guided PCI did not differ in the incidence of POCO (9.5% vs. 6.5%, p=0.294). Conclusions: The proportion of QCA-FFR or IVUS discordance was approximately one third for intermediate coronary lesions. FFR- or IVUS-guided strategies for these lesions were comparable with respect to POCO at 24 months

Physiology- or Imaging-Guided Strategies for Intermediate Coronary Stenosis

Importance: Treatment strategies for intermediate coronary lesions guided by fractional flow reserve (FFR) and intravascular ultrasonography (IVUS) have shown comparable outcomes. Identifying low-risk deferred vessels to ensure the safe deferral of percutaneous coronary intervention (PCI) and high-risk revascularized vessels that necessitate thorough follow-up can help determine optimal treatment strategies. Objectives: To investigate outcomes according to treatment types and FFR and IVUS parameters after FFR- or IVUS-guided treatment. Design, Setting, and Participants: This cohort study included patients with intermediate coronary stenosis from the Fractional Flow Reserve and Intravascular Ultrasound-Guided Intervention Strategy for Clinical Outcomes in Patients With Intermediate Stenosis (FLAVOUR) trial, an investigator-initiated, prospective, open-label, multicenter randomized clinical trial that assigned patients into an IVUS-guided strategy (which recommended PCI for minimum lumen area [MLA] ≤3 mm2or 3 mm2 to 4 mm2with plaque burden [PB] ≥70%) or an FFR-guided strategy (which recommended PCI for FFR ≤0.80). Data were analyzed from November to December 2022. Exposures: FFR or IVUS parameters within the deferred and revascularized vessels. Main Outcomes and Measures: The primary outcome was target vessel failure (TVF), a composite of cardiac death, target vessel myocardial infarction, and revascularization at 2 years. Results: A total of 1619 patients (mean [SD] age, 65.1 [9.6] years; 1137 [70.2%] male) with 1753 vessels were included in analysis. In 950 vessels for which revascularization was deferred, incidence of TVF was comparable between IVUS and FFR groups (3.8% vs 4.1%; P =.72). Vessels with FFR greater than 0.92 in the FFR group and MLA greater than 4.5 mm2or PB of 58% or less in the IVUS group were identified as low-risk deferred vessels, with a decreased risk of TVF (hazard ratio [HR], 0.25 [95% CI, 0.09-0.71]; P =.009). In 803 revascularized vessels, the incidence of TVF was comparable between IVUS and FFR groups (3.6% vs 3.7%; P =.95), which was similar in the revascularized vessels undergoing PCI optimization (4.2% vs 2.5%; P =.31). Vessels with post-PCI FFR of 0.80 or less in the FFR group or minimum stent area of 6.0 mm2or less or with PB at stent edge greater than 58% in the IVUS group had an increased risk for TVF (HR, 7.20 [95% CI, 3.20-16.21]; P <.001). Conclusions and Relevance: In this cohort study of patients with intermediate coronary stenosis, FFR- and IVUS-guided strategies showed comparable outcomes in both deferred and revascularized vessels. Binary FFR and IVUS parameters could further define low-risk deferred vessels and high-risk revascularized vessels