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Scattering processes in quantum materials emerge as resonances in electronic transport, including confined modes, Andreev states, and Yu-Shiba-Rusinov states. However, in most instances, these resonances are driven by a single scattering mechanism. Here we show the appearance of resonances due to the combination of two simultaneous scattering mechanisms, one from superconductivity and the other from graphene p-n junctions. These resonances stem from Andreev reflection and Klein tunneling that occur at two different interfaces of a hole-doped region of graphene formed at the boundary with superconducting graphene due to proximity effects from Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_{8+\delta}$. The resonances persist with gating the from p$^+$-p and p-n configurations. The suppression of the oscillation amplitude above the bias energy comparable to the induced superconducting gap indicates the contribution from Andreev reflection. Our experimental measurements are supported by quantum transport calculations in such interfaces, leading to analogous resonances. Our results put forward a hybrid scattering mechanism in graphene/high-temperature superconductor heterojunctions of potential impact for graphene-based Josephson junctions., Comment: 6 pages, 4 figures, submitted

Photovoltaics has two main processes: Optical absorption and power conversion. In photosynthesis, the two equivalent processes are optical absorption and chemical conversion. Whereas in the latter, the two processes are carried out by distinct proteins, in conventional photovoltaic diodes, the two processes are convoluted because the optical and transport paths are the same, leading to inefficiencies. Here, we separate the site and direction of light absorption from those of power generation to show that semiconducting single-walled carbon nanotubes (s-SWCNTs) provide an artificial system that models photosynthesis in a tandem geometry. Using different s-SWCNT chiralities, we implement an energy funnel in dual-gated p-n diodes. This enables the capture of photons from multiple regions of the solar spectrum and the funneling of photogenerated excitons to the smallest bandgap s-SWCNT layer, where they become free carriers. As a result, we demonstrate an increase in the magnitude and spectral response of photocurrent by adding more s-SWCNT layers of different bandgaps without a corresponding deleterious increase in the dark leakage current.

The use of an energy funnel or hierarchy to efficiently transfer photoexcitations is not new. In photosynthetic systems, whether in plants, algae, or photosynthetic bacteria, to achieve high efficiency, special light-harvesting protein complexes convert sunlight to excitons. High efficiency is achieved by temporarily storing excitons in these complexes before shuttling them to the reaction center where the conversion to usable energy takes place. For added efficiency, photosynthetic systems have adopted an energy hierarchy that funnels excitations from the light-harvesting complexes to the reaction center. The use of an energy funnel to increase the efficiency of photovoltaic devices, however, has not been widely adopted, due in part to limited materials compatibility and energy landscape. An energy funnel can absorb over a broad solar spectrum while minimizing the volume of material needed to construct a device, which can reduce the dark leakage current. Carbon nanotubes (CNTs) provide an ideal system to explore this new feature since a single material system can provide a wide range of energy levels. Depending on the diameter, excitonic levels in CNTs span from the visible to near-infrared spectrum. Furthermore, the device design can take advantage of the high aspect ratio of CNTs, to efficiently transport both free carriers and excitons along the nanotube. Here, we describe our efforts to develop photovoltaic p-n diodes using energy hierarchy. We use a network of CNTs in a p-n junction configuration where the transport is mostly along the CNTs. To form the p-n junction, we use buried split gates that are biased independently to form the p- and n-doped regions. The use of split gates is an extension of our earlier studies on single CNT p-n diodes. We use different chirality CNTs to implement the energy hierarchy. Specifically, the network is structured to funnel excitons by placing larger bandgap semiconducting nanotubes on top of smaller bandgap CNTs. We discuss features that show enhanced exciton to photocurrent generation in these devices compared to devices without the energy hierarchy.

Electron microscopes are ubiquitous across the scientific landscape and have been improved to achieve ever smaller beam spots, a key parameter that determines the instrument's resolution. However, the traditional techniques to characterize the electron beam have limited effectiveness for today's instruments. Consequently, there is an ongoing need to develop detection technologies that can potentially measure the smallest electron beam, which is valuable for the continual advancement of microscope performance. We report on a new electron beam detector based on a single-wall carbon nanotube. The nanotubes are atomically smooth, have a well-defined diameter that is similar in size to the finest electron probes, and can be used to directly measure the beam profile. Additionally, by rotating the nanotube in a plane perpendicular to the beam path and scanning the beam at different angles, we can apply tomographic reconstruction techniques to determine the spatial intensity profile of an electron beam accurately.