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Register a new userTunable Klein-like tunneling of high-temperature superconducting pairs into graphene
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Superconductivity can be induced in a normal material via the leakage of superconducting pairs of charge carriers from an adjacent superconductor. This so-called proximity effect is markedly influenced by graphene unique electronic structure, both in fundamental and technologically relevant ways. These include an unconventional form of the leakage mechanism the Andreev reflection and the potential of supercurrent modulation through electrical gating. Despite the interest of high-temperature superconductors in that context, realizations have been exclusively based on low-temperature ones. Here we demonstrate gate-tunable, high-temperature superconducting proximity effect in graphene. Notably, gating effects result from the perfect transmission of superconducting pairs across an energy barrier -a form of Klein tunneling, up to now observed only for non-superconducting carriers- and quantum interferences controlled by graphene doping. Interestingly, we find that this type of interferences become dominant without the need of ultra-clean graphene, in stark contrast to the case of low-temperature superconductors. These results pave the way to a new class of tunable, high-temperature Josephson devices based on large-scale graphene.
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The bias dependence of spin injection in graphene lateral spin valves is systematically studied to determine the factors affecting the tunneling spin injection efficiency. Three types of junctions are investigated, including MgO and hexagonal boron nitride (hBN) tunnel barriers and direct contacts. A DC bias current applied to the injector electrode induces a strong nonlinear bias dependence of the nonlocal spin signal for both MgO and hBN tunnel barriers. Furthermore, this signal reverses its sign at a negative DC bias for both kinds of tunnel barriers. The analysis of the bias dependence for injector electrodes with a wide range of contact resistances suggests that the sign reversal correlates with bias voltage rather than current. We consider different mechanisms for nonlinear bias dependence and conclude that the energy-dependent spin-polarized electronic structure of the ferromagnetic electrodes, rather than the electrical field-induced spin drift effect or spin filtering effect of the tunnel barrier, is the most likely explanation of the experimental observations.
We show that in gapped bilayer graphene, quasiparticle tunneling and the corresponding Berry phase can be controlled such that it exhibits features of single layer graphene such as Klein tunneling. The Berry phase is detected by a high-quality Fabry-P{e}rot interferometer based on bilayer graphene. By raising the Fermi energy of the charge carriers, we find that the Berry phase can be continuously tuned from $2pi$ down to $0.68pi$ in gapped bilayer graphene, in contrast to the constant Berry phase of $2pi$ in pristine bilayer graphene. Particularly, we observe a Berry phase of $pi$, the standard value for single layer graphene. As the Berry phase decreases, the corresponding transmission probability of charge carriers at normal incidence clearly demonstrates a transition from anti-Klein tunneling to nearly perfect Klein tunneling.
We study the zero temperature conductance and magnetoconductance of ballistic textit{p-n} junctions in Weyl semimetals. Electron transport is mediated by Klein tunneling between textit{n}- and textit{p}- regions. The chiral anomaly that is realized in Weyl semimetals plays a crucial role in the magnetoconductance of the junction. With the exception of field orientations where the angle between $mathbf{B}$ and the junction plane is small, magnetoconductance is positive and linear in $B$ at both weak and strong magnetic fields. In contrast, magnetoconductance in conventional textit{p-n} junctions is always negative.
We assess independently the impact of high-temperature substrate annealing and metal deposition conditions on the coherence of transmon qubits in the standard 2D circuit-QED architecture. We restrict our study to devices made with aluminum interdigital capacitors on sapphire substrates. We record more than an order-of-magnitude improvement in the relaxation time of devices made with an annealed substrate, independent of whether a conventional evaporator or molecular beam epitaxy chamber was used to deposit the aluminum. We also infer similar levels of flux noise and photon shot noise through dephasing measurements on these devices. Our results indicate that substrate annealing plays a primary role in fabricating low-loss qubits, consistent with the hypothesis that substrate-air and substrate-metal interfaces are essential factors limiting the qubit lifetimes in superconducting circuits.
Graphene electrons feature a pair of massless Dirac cones of opposite pseudospin chirality at two valleys. Klein tunneling refers to the intriguing capability of these chiral electrons to penetrate through high and wide potential barrier. The two valleys have been treated independently in the literature, where time reversal symmetry dictates that neither the normal incidence transmission nor the angle-averaged one can have any valley polarization. Here we show that, when intervalley scattering by barrier is accounted, graphene electrons normally incident at a superlattice barrier can experience a fully valley-selective Klein tunneling, i.e. perfect transmission in one valley, and perfect reflection in the other. Intervalley backscattering creates staggered pseudospin gaps in the superlattice barrier, which, combined with the valley contrast in pseudospin chirality, determines the valley polarity of Klein tunneling. The angle averaged transmission can have a net valley polarization of 20% for a 5-period barrier, and exceed 75% for a 20-period barrier. Our finding points to an unexpected opportunity to realize valley functionalities in graphene electronics.
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