[en] We report on the realization of an integer quantum Hall system with superconducting electrodes. Graphene was contacted to niobium electrodes that show a critical field of about 4 tesla, where electronic transport passes mainly through quantum Hall edge-states and bulk transport is largely suppressed. We find a magnetic field range of more than one tesla where well developed quantum Hall plateaus coexist with superconductivity in the leads. In high magnetic fields with the electrodes in the normal state we observe plateaus at G=νe²/h for ν=2, 4, and 10. Reducing the magnetic field to below the upper critical field of the electrodes, the conductance on the plateaus shows a sudden increase. Whereas the conductance on the ν=2 plateau increases only by 10%, the increase on the ν=6 and ν=10 plateau is considerably larger with 60% and 80%, respectively. We attribute this conductance enhancement to multiple Andreev reflection processes along the graphene-superconductor interface, that lead to the formation of Andreev edge-states. The observed conductance enhancement of the ν=6 and 10 plateaus is consistent with a doubling of the conductance contribution of the second and third edge-states. We attribute the small conductance increase on the ν=2 plateau to the special nature of the zero energy Landau level, that makes the corresponding edge-state sensitive to the structure of the graphene edge.

[en] We present nonlinear transport measurements on a carbon nanotube contacted to superconducting electrodes. At low temperatures, the nanotube acts as a quantum dot with intermediate contact transparencies. While level spacing and the charging energy dominate, the contact transparencies are large enough for higher order processes like the Kondo effect to occur. With superconducting electrodes, signs of the superconducting gap become visible in nonlinear transport, and additional subgap features due to multiple Andreev reflections appear. In addition, we see a striking difference between even and odd charge states, with the first Andreev process at V=Δ/e being strongly enhanced in states with odd electron number. Although direct signs of the Kondo effect, as the Kondo ridge around zero bias, are suppressed, we argue that this even-odd asymmetry is due to a hidden Kondo resonance that, due to contact asymmetries, survives only on one contact, and leads to strongly enhanced transport at V=Δ/e. We find good agreement of our data with a single impurity Anderson model solved in a Slave Boson mean field approach

[en] We present a method to establish inner point contacts with dimensions as small as 100 nm on hexagonal boron nitride (hBN) encapsulated graphene heterostructures by pre-patterning the top-hBN in a separate step prior to dry-stacking. 2- and 4-terminal field effect measurements between different lead combinations are in qualitative agreement with an electrostatic model assuming point-like contacts. The measured contact resistances are 0.5–1.5 kΩ per contact, which is quite low for such small contacts. By applying a perpendicular magnetic field, an insulating behaviour in the quantum Hall regime was observed, as expected for inner contacts. The fabricated contacts are compatible with high mobility graphene structures and open up the field for the realization of several electron optical proposals

[en] Purpose: Radiotherapy plays a major role in the treatment strategy of childhood sarcomas. Consequences of treatment are likely to affect the survivor's quality of life significantly. We investigated the feasibility of spot-scanning proton therapy (PT) for soft tissue tumors in childhood. Methods and Materials: Sixteen children with soft tissue sarcomas were included. Median age at PT was 3.3 years. In 10 children the tumor histology was embryonal rhabdomyosarcoma. All tumors were located in the head or neck, parameningeal, or paraspinal, or pelvic region. In the majority of children, the tumor was initially unresectable (Intergroup Rhabdomyosarcoma Study [IRS] Group III in 75%). In 50% of children the tumors exceeded 5 cm. Fourteen children had chemotherapy before and during PT. Median total dose of radiotherapy was 50 cobalt Gray equivalent (CGE). All 16 children were treated with spot-scanning proton therapy at the Paul Scherrer Institute, and in 3 children the PT was intensity-modulated (IMPT). Results: After median follow-up of 1.5 years, local control was achieved in 12 children. Four children failed locally, 1 at the border of the radiation field and 3 within the field. All 4 children died of tumor recurrence. All 4 showed unfavorable characteristic either of site or histopathology of the tumor. Acute toxicity was low, with Grade 3 or 4 side effects according to Radiation Therapy Oncology Group/European Organization for Research and Treatment of Cancer (RTOG/EORTC) criteria occurring in the bone marrow only. Conclusions: Proton therapy was feasible and well tolerated. Early local control rates are comparable to those being achieved after conventional radiotherapy. For investigations on late effect, longer follow-up is needed