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Register a new userElectronic transport and quantum Hall effect in bipolar graphene p-n-p junction
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We have developed a device fabrication process to pattern graphene into nanostructures of arbitrary shape and control their electronic properties using local electrostatic gates. Electronic transport measurements have been used to characterize locally gated bipolar graphene $p$-$n$-$p$ junctions. We observe a series of fractional quantum Hall conductance plateaus at high magnetic fields as the local charge density is varied in the $p$ and $n$ regions. These fractional plateaus, originating from chiral edge states equilibration at the $p$-$n$ interfaces, exhibit sensitivity to inter-edge backscattering which is found to be strong for some of the plateuas and much weaker for other plateaus. We use this effect to explore the role of backscattering and estimate disorder strength in our graphene devices.
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We report on the fabrication and transport studies of a single-layer graphene p-n junction. Carrier type and density in two adjacent regions are individually controlled by electrostatic gating using a local top gate and a global back gate. A functionalized Al203 oxide that adheres to graphene and does not significantly affect its electronic properties is described. Measurements in the quantum Hall regime reveal new plateaus of two-terminal conductance across the junction at 1 and 3/2 times the quantum of conductance, e2/h, consistent with theory.
Owing to a linear and gapless band structure and a tunability of the charge carrier type, graphene offers a unique system to investigate transport of Dirac Fermions at p-n junctions (PNJs). In a magnetic field, combination of quantum Hall physics and the characteristic transport across PNJs leads to a fractionally quantized conductance associated with the mixing of electron-like and hole-like modes and their subsequent partitioning. The mixing and partitioning suggest that a PNJ could be used as an electronic beam-splitter. Here we report the shot noise study of the mode mixing process and demonstrate the crucial role of the PNJ length. For short PNJs, the amplitude of the noise is consistent with an electronic beam-splitter behavior, whereas, for longer PNJs, it is reduced by the energy relaxation. Remarkably, the relaxation length is much larger than typical size of mesoscopic devices, encouraging using graphene for electron quantum optics and quantum information processing.
We study photodetection in graphene near a local electrostatic gate, which enables active control of the potential landscape and carrier polarity. We find that a strong photoresponse only appears when and where a p-n junction is formed, allowing on-off control of photodetection. Photocurrents generated near p-n junctions do not require biasing and can be realized using submicron gates. Locally modulated photoresponse enables a new range of applications for graphene-based photodetectors including, for example, pixilated infrared imaging with control of response on subwavelength dimensions.
The motion of massless Dirac-electrons in graphene mimics the propagation of photons. This makes it possible to control the charge-carriers with components based on geometrical-optics and has led to proposals for an all-graphene electron-optics platform. An open question arising from the possibility of reducing the component-size to the nanometer-scale is how to access and understand the transition from optical-transport to quantum-confinement. Here we report on the realization of a circular p-n junction that can be continuously tuned from the nanometer-scale, where quantum effects are dominant, to the micrometer scale where optical-guiding takes over. We find that in the nanometer-scale junction electrons are trapped in states that resemble atomic-collapse at a supercritical charge. As the junction-size increases, the transition to optical-guiding is signaled by the emergence of whispering-gallery modes and Fabry-Perot interference. The creation of tunable junctions that straddle the crossover between quantum-confinement and optical-guiding, paves the way to novel design-architectures for controlling electronic transport.
We demonstrate high-frequency mechanical resonators in ballistic graphene p-n junctions. Fully suspended graphene devices with two bottom gates exhibit ballistic bipolar behavior after current annealing. We determine the graphene mass density and built-in tension for different current annealing steps by comparing the measured mechanical resonant response to a simplified membrane model. We consistently find that after the last annealing step the mass density compares well with the expected density of pure graphene. In a graphene membrane with high built-in tension, but still of macroscopic size with dimensions 3 $times$ 1 $mu m^{2}$, a record resonance frequency of 1.17 GHz is observed after the final current annealing step. We further compare the resonance response measured in the unipolar with the one in the bipolar regime. Remarkably, the resonant signals are strongly enhanced in the bipolar regime. This enhancement is caused in part by the Fabry-Perot resonances that appear in the bipolar regime and possibly also by the photothermoelectric effect that can be very pronounced in graphene p-n junctions under microwave irradiation.
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