No abstract

We have measured the noise temperature of a single, sensitive superconducting NbN hot electron bolometer (HEB) mixer in a frequency range from 1.6 to 5.3 THz, using a setup with all the key components in vacuum. By analyzing the measured receiver noise temperature using a quantum noise (QN) model for HEB mixers, we confirm the effect of QN. The QN is found to be responsible for about half of the receiver noise at the highest frequency in our measurements. The β-factor (the quantum efficiency of the HEB) obtained experimentally agrees reasonably well with the calculated value.

We show with angle-resolved photoemission spectroscopy that a new energy band appears in the electronic structure of electron doped hydrogenated monolayer graphene (H-graphene). Its occupation can be controlled with the hydrogen amount and allows for tuning of graphene's doping level. Our calculations of the electronic structure of H-graphene suggest that this state is largely composed from hydrogen 1s orbitals and remains extended for low H coverages despite the random chemisorption of H. Further evidence for the existence of a hydrogen state is provided by X-ray absorption studies of undoped H-graphene which are clearly showing the emergence of an additional state in the vicinity of the π * -resonance.