We present an experimental study of hybrid turnstiles with high charging energies in comparison to the superconducting gap. The device is modeled with the sequential tunneling approximation. The backtunneling effect is shown to limit the amplitude of the gate drive and thereby the maximum pumped current of the turnstile. We compare results obtained with sine and square wave drive and show how a fast rise time can suppress errors due to leakage current. Quantized current plateaus up to 160 pA are demonstrated.
We demonstrate shadow evaporation-based fabrication of high-quality ultrasmall normal metal -- insulator -- superconductor tunnel junctions where the thickness of the superconducting electrode is not limited by the requirement of small junction size.
The junctions are formed between a film of manganese-doped aluminium acting as the normal conducting electrode, covered by a thicker, superconducting layer of pure Al. We characterize the junctions by sub-gap current--voltage measurements and charge pumping measurements in a gate-driven hybrid single-electron transistor, operated as a turnstile for single electrons. The technique allows to advance towards turnstiles with close to ideally thermalized superconducting reservoirs, prerequisite for reaching metrological current quantization accuracy in a hybrid turnstile. We further present an alternative way to realize small junctions with thick Al leads based on multi-angle deposition. The work enables the future investigation of turnstiles based on superconductors other than Al, and benefits various other Al tunnel junction devices for which quasiparticle thermalization is essential.
We perform in-situ detection of individual electrons pumped through a single-electron turnstile based on ultrasmall normal metal - insulator - superconductor tunnel junctions. In our setup, limited by the detector bandwidth, at low repetition rates w
e observe errorless sequential transfer of up to several hundred electrons through the system. At faster pumping speeds up to 100 kHz, we show relative error rates down to 10^-3, comparable to typical values obtained from measurements of average pumped current in non-optimized individual turnstiles. The work constitutes an initial step towards a self-referenced current standard realized with metallic single-electron turnstiles, complementing approaches based on semiconductor quantum dot pumps. It is the first demonstration of on-chip pumping error detection at operation frequencies exceeding the detector bandwidth, in a configuration where the average pumped current can be simultaneously measured. The scheme in which electrons are counted from the superconducting lead of the turnstile, instead of direct probing of the normal metal island, also enables studies of fundamental higher-order tunneling processes in the hybrid structures, previously not in reach with simpler configurations.
Graphene p-n junctions provide an ideal platform for investigating novel behavior at the boundary between electronics and optics that arise from massless Dirac fermions, such as whispering gallery modes and Veselago lensing. Bilayer graphene also hos
ts Dirac fermions, but they differ from single-layer graphene charge carriers because they are massive, can be gapped by an applied perpendicular electric field, and have very different pseudospin selection rules across a p-n junction. Novel phenomena predicted for these massive Dirac fermions at p-n junctions include anti-Klein tunneling, oscillatory Zener tunneling, and electron cloaked states. Despite these predictions there has been little experimental focus on the microscopic spatial behavior of massive Dirac fermions in the presence of p-n junctions. Here we report the experimental manipulation and characterization of massive Dirac fermions within bilayer graphene quantum dots defined by circular p-n junctions through the use of scanning tunneling microscopy-based (STM) methods. Our p-n junctions are created via a flexible technique that enables realization of exposed quantum dots in bilayer graphene/hBN heterostructures. These quantum dots exhibit sharp spectroscopic resonances that disperse in energy as a function of applied gate voltage. Spatial maps of these features show prominent concentric rings with diameters that can be tuned by an electrostatic gate. This behavior is explained by single-electron charging of localized states that arise from the quantum confinement of massive Dirac fermions within our exposed bilayer graphene quantum dots.
We investigate a hybrid structure consisting of $20pm4$ implanted $^{31}$P atoms close to a gate-induced silicon single electron transistor (SiSET). In this configuration, the SiSET is extremely sensitive to the charge state of the nearby centers, tu
rning from the off state to the conducting state when the charge configuration is changed. We present a method to measure fast electron tunnel rates between donors and the SiSET island, using a pulsed voltage scheme and low-bandwidth current detection. The experimental findings are quantitatively discussed using a rate equation model, enabling the extraction of the capture and emission rates.
We use quantum detector tomography to investigate the detection mechanism in WSi nanowire superconducting single photon detectors (SSPDs). To this purpose, we fabricated a 250nm wide and 250nm long WSi nanowire and measured its response to impinging
photons with wavelengths ranging from $lambda$ = 900 nm to $lambda$ = 1650 nm. Tomographic measurements show that the detector response depends on the total excitation energy only. Moreover, for energies Et > 0.8eV the current energy relation is linear, similar to what was observed in NbN nanowires, whereas the current-energy relation deviates from linear behaviour for total energies below 0.8eV.
A. Kemppinen
,S. Kafanov
,Yu. A. Pashkin
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(2009)
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"Experimental investigation of hybrid single-electron turnstiles with high charging energy"
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Antti Kemppinen
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