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

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

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

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

Tunneling properties of in-situ-fabricated intrinsic Josephson junctions in Bi2Sr2CaCu2O8+x single crystals

Slacks of intrinsic Josephson junctions were fabricated on the surface of Bi2Sr2CaCu2O8+x single crystals, using photolithography and Ar-ion etching. The number of junctions in a stack was controlled by the etching time, while the c-axis I-V and R versus T curves were measured in-situ in a vacuum chamber for T down to similar to 13 K. The tunneling resistance and the I-V curves are scaled by the surface junction resistance. The superconductivity of the surface conducting plane in contact with a Au electrode is weakened by the proximity to the normal metal. In a low-bias region, main tunneling properties of a junction are not affected by the presence of other junctions in a stack. (C) 2000 Elsevier Science B.V. All rights reserved.X11sciescopu

Progressive disappearance of the c-axis tunneling barrier in Bi2Sr2CaCu2O8+x single crystals near T-c

We observed an enhancement of the differential conductance dI/dV around the zero bias in junctions of Au/Bi2Sr2CaCu2O8+x(Bi2212) single crystals near the superconducting transition temperature of Bi2212 crystals with the tunneling current along the c axis. We attribute such an enhancement to an Andreev reflection (AR) between the surface Cu-O bilayer, which is in the normal state, and the next superconducting bilayer below the crystal surface. The continuous evolution from a gap-like depression to the AR peak structure of the dI/dV curves around the zero bias indicates weakening of the barrier strength of the non-superconducting layers between adjacent Cu-O bilayers as the temperature is raised up to T-c. We suggest that the fact can be conveniently used to investigate the possible existence of preformed pairs in the pseudogap state above T-c.X111sciescopuskciothe