Zeeman-Effect-Induced 0-π Transitions in Ballistic Dirac Semimetal Josephson Junctions.

One of the consequences of Cooper pairs having a finite momentum in the interlayer of a Josephson junction is π-junction behavior. The finite momentum can either be due to an exchange field in ferromagnetic Josephson junctions, or due to the Zeeman effect. Here, we report the observation of Zeeman-effect-induced 0-π transitions in Bi_{1-x}Sb_{x}, three-dimensional Dirac semimetal-based Josephson junctions. The large in-plane g factor allows tuning of the Josephson junctions from 0 to π regimes. This is revealed by measuring a π phase shift in the current-phase relation measured with an asymmetric superconducting quantum interference device (SQUID). Additionally, we directly measure a nonsinusoidal current-phase relation in the asymmetric SQUID, consistent with models for ballistic Josephson transport.

4π-periodic Andreev bound states in a Dirac semimetal.

Although signatures of superconductivity in Dirac semimetals have been reported, for instance by applying pressure or using point contacts, our understanding of the topological aspects of Dirac semimetal superconductivity is still developing. Here, we utilize nanoscale phase-sensitive junction technology to induce superconductivity in the Dirac semimetal Bi1-xSbx. Our radiofrequency irradiation experiments then reveal a significant contribution of 4π-periodic Andreev bound states to the supercurrent in Nb-Bi0.97Sb0.03-Nb Josephson junctions. The conditions for a substantial 4π contribution to the supercurrent are favourable because of the Dirac cone's very broad transmission resonances and a measurement frequency faster than the quasiparticle poisoning rate. In addition, we show that a magnetic field applied in the plane of the junction allows tuning of the Josephson junctions from 0 to π regimes. Our results open the technologically appealing avenue of employing the topological bulk properties of Dirac semimetals for topological superconductivity research and topological quantum computer development.

The water equivalence of solid materials used for dosimetry with small proton beams.

Various solid materials are used instead of water for absolute dosimetry with small proton beams. This may result in a dose measurement different to that in water, even when the range of protons in the phantom material is considered correctly. This dose difference is caused by the diverse cross sections for inelastic nuclear scattering in water and in the phantom materials respectively. To estimate the magnitude of this effect, flux and dose measurements with a 177 MeV proton pencil beam having a width of 0.6 cm (FWHM) were performed. The proton flux and the deposited dose in the beam path were determined behind water, lucite, polyethylene, teflon, and aluminum of diverse thicknesses. The number of out-scattered protons due to inelastic nuclear scattering was determined for water and the different materials. The ratios of the number of scattered protons in the materials relative to that in water were found to be 1.20 for lucite, 1.16 for polyethylene, 1.22 for teflon, and 1.03 for aluminum. The difference between the deposited dose in water and in the phantom materials taken in the center of the proton pencil beam, was estimated from the flux measurements, always taking the different ranges of protons in the materials into account. The estimated dose difference relative to water in 15 cm water equivalent thickness was -2.3% for lucite, -1.7% for polyethylene, -2.5% for teflon, and -0.4% for aluminum. The dose deviation was verified by a measurement using an ionization chamber. It should be noted that the dose error is larger when the effective point of measurement in the material is deeper or when the energy is higher.