Quantum-classical spin hybrids composed of physical system with complimentary characteristics have enabled novel capabilities and functionalities within the realm of existing technology. One half of such hybrid systems is the quantum impurity spin with small spin quantum number such that its description is governed by the counter-intuitive laws of quantum mechanics. The other is a classical magnet with large spin quantum number such that its dynamics can be captured within the framework of classical physics. Such hybrids give rise to possibilities where controlling the degrees of freedom in one system can be leveraged to control dynamics in the other. Leveraging the demonstrated spintronic tools of classical magnet dynamics, we demonstrate two significant steps towards realizing a quantum network for information processing applications. One, a theoretically designed regime where electrical control of non-linear magnetization dynamics of a nanomagnet provides a local, coherent, and low-power drive to manipulate a coupled quantum impurity spin without introducing additional decoherence. Another, where we demonstrate via a joint theoretical and experimental effort, the electrical tuning of interaction between electrically-controlled propagating magnons in an extended magnet and a quantum impurity spin. The merits of such a hybrid system provide pathways to overcome the bottlenecks associated with local controllability of individual quantum spins in a quantum network and modulate the interaction mediating the two subsystems.
Graphene photodetectors’ intrinsically low responsivity (sensitivity) has been a long-standing issue that overshadows graphene’s other excellent optical properties as a photodetection material. The key to improving the graphene photodetector responsivity lies in enhancing the photothermoelectric (PTE) effect, which has already been demonstrated to be the dominant photocarrier generation mechanism. To maximize the PTE current, one would need a strong optically-induced temperature gradient to overlap with a graphene p-n junction spatially. Here, the temperature gradient drives the charge carrier movement, while the graphene p-n junction separates the different charge carrier types (electrons and holes) and makes them drift in opposite directions. In this work, we show that these two conditions can be met simultaneously in a meticulously designed device, combining a gap plasmon structure and a pair of split-gates. The gap plasmon structure absorbs 71% of incident light creating localized heating (thereby large temperature gradient), and the split-gates create a p-n junction at the center of the localized thermal gradient. We fabricated a graphene photodetector with the proposed configuration, and experimentally verified the dominance of PTE effect in photocurrent generation in good agreement with theoretical calculations. More importantly, we obtained a responsivity 70 times higher than the previously reported value from a similar device without plasmon-enhancement. Moreover, originating from the combination of gap plasmon-enhanced optical absorption and optimized p-n junction, our responsivity is 5~7 times higher than reported values for other graphene photodetectors with different types of plasmon-enhancement and no junction control.
Due to its high charge carrier mobility, broadband light absorption, and ultrafast carrier dynamics, graphene is a promising material for the development of high-performance photodetectors. Graphene-based photodetectors have been demonstrated to date using monolayer graphene operating in conjunction with either metals or semiconductors. Most graphene devices are fabricated on doped Si substrates with SiO2 dielectric used for back gating. Here, we demonstrate photodetection in graphene field effect phototransistors fabricated on undoped semiconductor (SiC) substrates. The photodetection mechanism relies on the high sensitivity of the graphene conductivity to the local change in the electric field that can result from the photo-excited charge carriers produced in the back-gated semiconductor substrate. We also modeled the device and simulated its operation using the finite element method to validate the existence of the field-induced photoresponse mechanism and study its properties. Our graphene phototransistor possesses a room-temperature photoresponsivity as high as ~7.4 A/W, which is higher than the required photoresponsivity (1 A/W) in most practical applications. The light power-dependent photocurrent and photoresponsivity can be tuned by the source-drain bias voltage and back-gate voltage. Graphene phototransistors based on this simple and generic architecture can be fabricated by depositing graphene on a variety of undoped substrates, and are attractive for many applications in which photodetection or radiation detection is sought.
Graphene has been demonstrated to be a promising photodetection material because of its atomic-thin nature, broadband and uniform optical absorption, etc. Photovoltaic and photothermoelectric, which are considered to be the main contributors to photo current/voltage generation in graphene, enable photodetection through driving electrons via built-in electric field and thermoelectric power, respectively. Graphene photovoltaic/photothermoelectric detectors are ideal for ultrafast photodetection applications due to the high carrier mobilities in graphene and ultrashort time the electrons need to give away heat. Despite all the advantages for graphene photovoltaic/photothermoelectric detectors, the sensitivity in such detectors is relatively low, owing to the low optical absorption in the single atomic layer. In the past, our research group has used delicately designed snowflake-like fractal metasurface to realize broadband photovoltage enhancement in the visible spectral range, on SiO2 thin film backed by Si substrates. We have also demonstrated that the enhancement from the proposed fractal metasurface is insensitive to the polarization of the incident light. In this current work, we have carried out experiments of the same fractal metasurface on transparent SiO2 substrates, and obtained higher enhancement factor on the fractal metasurface than that achieved on SiO2/Si substrates. Moreover, the device allows more than 70% of the incident light to transmit during the detection, enabling photodetection in the optical path without any significant distortion. Another possibility to make use of the large portion of transmitted light is to stack multiple such devices along the optical path to linearly scale up the sensitivity.
Topological insulators (TIs), with their helically spin-momentum-locked topological surface states (TSS), are considered promising for spintronics applications. Several recent experiments in TIs have demonstrated a current induced electronic spin polarization that may be used for all-electrical spin generation and injection. Here, we report spin potentiometric measurements in TIs that have revealed a long-lived persistent electron spin polarization even at zero current. Unaffected by a small bias current and persisting for several days at low temperature, the spin polarization can be induced and reversed by a large “writing” current applied for an extended time. While the exact mechanism responsible for the observed long-lived persistent spin polarization remains to be better understood, we speculate on possible roles played by nuclear spins hyperfine coupled to TSS electrons and dynamically polarized by the spin-helical “writing current”. Such an electrically controlled persistent spin polarization with unprecedented long lifetime could enable a rechargeable spin battery and rewritable spin memory for potential applications in spintronics and quantum information.
Graphene has been demonstrated to be a promising photo-detection material because of its ultra-broadband absorption, compatibility with CMOS technology, and dynamic tunability. There are multiple known photo-detection mechanisms in graphene, among which the photovoltaic effect has the fastest response time thus is the prioritized candidate for ultrafast photodetector. There have been numerous efforts to enhance the intrinsically low sensitivity in graphene photovoltaic detectors using metallic plasmonic structures, but such plasmonic enhancements are mostly narrowband and polarization dependent. In this work, we propose a gold Cayley-tree fractal metasurface design that has a multi-band resonance, to realize broadband and polarization-insensitive plasmonic enhancement in graphene photovoltaic detectors. When illuminated with visible light, the fractal metasurface exhibits multiple hotspots at the metal-graphene interface, where the electric field of the incident electromagnetic wave is enhanced and contributes to generating excess electron-hole pairs in graphene. The large metal-graphene interface length in the fractal metasurface also helps to harvest at a higher efficiency the electron-hole pairs by built-in electric field due to metal-graphene potential gradient. To demonstrate the concept, we carried out experiment using Ar-Kr CW laser, an optical chopper, and lock-in amplifier. We obtained experimentally an almost constant ten-fold enhancement of photocurrent generated on the fractal metasurface compared to that generated on the plain gold-graphene edge, at all tested wavelengths (488 nm, 514 nm, 568 nm, and 647 nm). We also observed an unchanged photoresponse with respect to incident light polarization angles, which is a result of the highly symmetric geometry of the fractal metasurface.
Topological insulators (TI) have emerged as a new class of quantum materials with many novel and unusual properties.
In this article, we will give a brief review of the key electronic properties of topological insulators, including the
signatures for the unusual electronic transport properties of their characteristic topological surface states (TSS). We will
then discuss how these novel properties and physics may be utilized for TI-based energy efficient devices, such as lowpower-
consumption electronics and high performance thermo-electrics. Furthermore, going beyond conventional singleparticle,
charge-based transport, to utilize coherent many-body coherent ground states such as excitonic condensates
(EC), new and intriguing functionalities previously unexplored in electronic and energy devices may be realized with the
potential to dramatically improve the energy efficiency.
We exploit the dependence of the electrical conductivity of graphene on a local electric field, which can be abruptly
changed by charge carriers generated by ionizing radiation in an absorber material, to develop novel highperformance
radiation sensors for detection of photons and other kinds of ionizing radiation. This new detection
concept is implemented by configuring graphene as a field effect transistor (FET) on a radiation-absorbing undoped
semiconductor substrate and applying a gate voltage across the sensor to drift charge carriers created by incident
photons to the neighborhood of graphene, which gives rise to local electric field perturbations that change graphene
resistance. Promising results have been obtained with CVD graphene FETs fabricated on various semiconductor
substrates that have different bandgaps and stopping powers to address different application regimes. In particular,
graphene FETs made on SiC have exhibited a ~200% increase in graphene resistance at a gate voltage of 50 V when
exposed to room light at room temperature. Systematic studies have proven that the observed response is a field
effect.
We present a study of the effects of electron-beam irradiation on the Raman spectra and electronic transport
properties of graphene and the operation of graphene field-effect transistors (GFET). Exposure to a 30 keV electronbeam
causes negative shifts in the charge-neutral point (CNP) of the GFET, interpreted as due to n-doping in the
graphene from the interaction of the energetic electron beam with the substrate. The electron beam is seen to also
decrease the carrier mobilities and minimum conductivity of the graphene, as well as increase the intensity of the
Raman D peak, all of which indicate defects generated in the graphene. We also study the relaxation of electronic
properties after irradiation. The findings are valuable for understanding the effects of radiation damage on graphene
and for the development of radiation-hard graphene-based electronics.
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