Gating Field Research Articles

Atomic-scale visualization of the interlayer Rydberg exciton complex in moiré heterostructures

Excitonic systems, facilitated by optical pumping, electrostatic gating or magnetic field, sustain composite particles with fascinating physics. Although various intriguing excitonic phases have been revealed via global measurements, the atomic-scale accessibility towards excitons has yet to be established. Here, we realize the ground-state interlayer exciton complexes through the intrinsic charge transfer in monolayer YbCl3/graphite heterostructure. Combining scanning tunneling microscope and theoretical calculations, the excitonic in-gap states are directly profiled. The out-of-plane excitonic charge clouds exhibit oscillating Rydberg nodal structure, while their in-plane arrangements are determined by moiré periodicity. Exploiting the tunneling probe to reflect the shape of charge clouds, we reveal the principal quantum number hierarchy of Rydberg series, which points to an excitonic energy-level configuration with unusually large binding energy. Our results demonstrate the feasibility of mapping out the charge clouds of excitons microscopically and pave a brand-new way to directly investigate the nanoscale order of exotic correlated phases.

Robust Anti-Ambipolar Behavior and Gate-Tunable Rectifying Effect in van der Waals p–n Junctions

Anti-ambipolar behavior has been observed in multilayer two-dimensional semiconductors and many p–n junctions. However, temperatures in the range of tens of kelvin are usually required, and there are gaps in the understanding of the underlying physics, most notably the roles of the space charge region and van der Waals (vdW) heterojunction. Here, we systematically investigate the unique transport and transfer characteristics of vertically stacked vdW contacted p-WSe2/n-SnS2 and p-WSe2/n-MoS2 junctions. Interestingly, robust anti-ambipolar behavior is observed at low and room temperature for both darkened and illuminated conditions. A peak-to-valley current ratio exceeding 104 and on-state gating field ranging from −0.3 × 106 to 0.57 × 106 V/cm are achieved at room temperature in darkness. Moreover, the rectifying effect and the type of majority charge carrier of the WSe2/SnS2 junction can be tuned by an electric gate. Robust Raman quenching over the whole interface induced by strong charge transfer is also observed. Based on an equivalent circuit model, a Poisson and drift-diffusion simulation, and experiments with local illumination, we highlight the role of the van der Waals heterojunction. We show that the observed unique anti-ambipolar behavior is due to the recombination of electrons and holes at the space charge region rather than carrier diffusion in the turned-off metal-oxide-semiconductor field-effect transistor (MOSFET). A larger vdW barrier and interface recombination at the vdW barrier are key to having a strong on/off ratio especially at room temperature. The p–n vdW heterostructure plays the roles of a complementary MOSFET connecting with a heterojunction diode, not only a ternary logic inverter but also a tunable multiple-gain amplifier in wide-range gating fields. The width of the middle logic plateau can reach 45 V in a WSe2/MoS2 junction.

Electronic structures, charge transfer, and charge order in twisted transition metal dichalcogenide bilayers

Moir\'e superlattices of transition metal dichalcogenide (TMD) bilayers have been shown to host correlated electronic states, which arise from the interplay of long wavelength moir\'e potential and long-range Coulomb interaction. Here, we theoretically investigate structural relaxation and single-particle electronic structure of twisted TMD homobilayer. From the large-scale density functional theory calculation and continuum model with layer degrees of freedom, we find that the out-of-plane gating field creates a tunable charge transfer gap at the Dirac point between the first and second moir\'e valence bands. We further study the charge orders at the fractional band fillings. In the flat band limit, we find from Monte Carlo simulations a series of charge-ordered insulating states at various fillings $n=\frac{1}{4},\frac{1}{3},\frac{1}{2},\frac{2}{3},1$. We predict that gating field induces a phase transition between different electron crystals at fixed filling $n=\frac{1}{2}$ or $\frac{2}{3}$. At half-filling $n=1$, the ground state is a Mott insulator with electronically driven ferroelectricity. This work demonstrates that TMD homobilayer provides a powerful platform for the investigation of tunable charge transfer insulator and charge orders.

A tunable Fabry-Pérot quantum Hall interferometer in graphene.

Electron interferometry with quantum Hall edge channels in semiconductor heterostructures can probe and harness the exchange statistics of anyonic excitations. However, charging effects present in semiconductors often obscured the Aharonov-Bohm interference in quantum Hall interferometers and make advanced charge screening strategies necessary. Here, we show that high-mobility monolayer graphene constitutes an alternative material system, not affected by charging effects, to perform Fabry-Pérot quantum Hall interferometry in the integer quantum Hall regime. In devices equipped with gate-tunable quantum point contacts acting on the edge channels of the zeroth Landau level, we observe high-visibility Aharonov-Bohm interference widely tunable through electrostatic gating or magnetic field, in agreement with theory. A coherence length of 10 μm at a temperature of 0.02 K allows us to further achieve coherently-coupled double Fabry-Pérot interferometry. In future, quantum Hall interferometry with graphene devices may enable investigations of anyonic excitations in fractional quantum Hall states.

Universal behavior of the bosonic metallic ground state in a two-dimensional superconductor

Anomalous metallic behavior, marked by a saturating finite resistivity much lower than the Drude estimate, has been observed in a wide range of two-dimensional superconductors. Utilizing the electrostatically gated LaAlO3/SrTiO3 interface as a versatile platform for superconductor-metal quantum phase transitions, we probe variations in the gate, magnetic field, and temperature to construct a phase diagram crossing from superconductor, anomalous metal, vortex liquid, to the Drude metal state, combining longitudinal and Hall resistivity measurements. We find that the anomalous metal phases induced by gating and magnetic field, although differing in symmetry, are connected in the phase diagram and exhibit similar magnetic field response approaching zero temperature. Namely, within a finite regime of the anomalous metal state, the longitudinal resistivity linearly depends on the field while the Hall resistivity diminishes, indicating an emergent particle-hole symmetry. The universal behavior highlights the uniqueness of the quantum bosonic metallic state, distinct from bosonic insulators and vortex liquids.

Combinatorial film and high-throughput characterization methods of phase diagram for high-<i>T</i><sub>c</sub> superconductors

<sec> Cuprate and iron-based superconductors are known as the only two types of high-<i>T</i><sub>c</sub> superconductors. The mechanism of high-<i>T</i><sub>c</sub> superconductivity is the most challenging issue in the field. Building accurate high-dimensional phase diagram and exploring key parameters that determine <i>T</i><sub>c</sub>, would be essential to the comprehension of high-<i>T</i><sub>c</sub> mechanism. The electronic phase diagrams of cuprate superconductors show complexity and diversity, for the strong coupling and interplay among lattice, orbital, charge and spin degrees of freedom. It is tough to construct a high-dimensional holographic phase diagram and obtain quantitative laws by traditional research methods. Fortunately, the high-throughput synthesis and fast screening techniques enable to probe the phase diagram via line-by-line or map scanning modes, and thereby are expected to obtain high-dimensional phase diagram and key superconducting parameters in a much efficient way.</sec><sec> In this article, electronic phase diagrams of cuprate superconductors that are obtained mainly by electrical transport measurements, are briefly summarized in the view of cation substitutions, oxygen variation in the parent compounds, electric double-layer gating (electrostatic/electrochemical manipulation) and magnetic field. We introduce the preparation methods for combinatorial film based on the developed pulsed laser deposition and oxide molecular beam epitaxy techniques, as well as corresponding scale-span high-throughput measurement techniques. These high-throughput techniques have been successfully applied in the research of interface superconductivity, quantum phase transition, and so on. The novel high-throughput superconductivity research mode will play an indispensable role in the construction of the high-dimensional holographic phase diagram, the comprehension of high-<i>T</i><sub>c</sub> mechanism, and practical applications of superconductors.</sec>

Living, Self‐Replicating Ferrofluids for Fluidic Transport

AbstractMagnetic actuation offers a means to wirelessly control flow in ferrofluids for applications including microfluidic pumping and targeted drug delivery. Despite the promise of these concepts, practical use of synthetic ferrofluids as actuators of flow frequently requires high concentrations and is hindered by low ferrohydrodynamic coupling efficiency and inhomogeneous flow fields. Inspired by the magnetic properties and hydrodynamic forms displayed by magnetotactic bacteria (MTB), this work studies the use of these microbes as a living, self‐replicating ferrofluid for improved fluidic transport via magnetically coerced rotation. Using multicore iron oxide nanoparticles as a performance benchmark, MTB under rotating magnetic fields are shown to produce more homogeneous and efficient flow. Coupling is enhanced whether the comparison is made in terms of volume of magnetic material or total volume fraction. To clarify the mechanistic role of interactions with boundaries in transport, a computational model is developed and validated experimentally. Applying this model, two distinct and feasible magnetic control strategies are predicted: a rotating gradient field that generates directional flow despite boundaries that promote flow in opposing directions and a magnetostatic gating field that enables spatially selective actuation. The advantageous properties identified for MTB open a design space for these strategies to be realized.

Generation of isolated circularly polarized attosecond pulses by three-color laser field mixing.

We propose and theoretically demonstrate a method to generate the circularly polarized supercontinuum with three-color electric fields. The three-color field is synthesized from an orthogonally polarized two-color (OTC) laser field and an infrared gating field. All driving pulse durations are extended to 40 fs. We demonstrate that the three-color field imposes curved trajectories for ionized electrons and extends the time interval between each harmonic emitting. Through adjusting intensity ratios among three components of the driving field, a nearly circular isolated attosecond pulse can be generated.

Mimicking Sensory Adaptation with Dielectric Engineered Organic Transistors.

Mimicking sensory adaptation with transistors is essential for developing next-generation smart circuits. A key challenge is how to obtain controllable and reversible short-term signal decay while simultaneously maintaining long-term electrical stability. By introducing a buried dynamic-trapping interface within the dielectric layer, an organic adaptive transistor (OAT) with sensory adaptation functionality is developed. The device induces self-adaptive interfacial trapping to enable volatile shielding of the gating field, thereby leading to rapid and temporary carrier concentration decay in the conductive channel without diminishing the mobility upon a fixed voltage bias. More importantly, the device exhibits a fine-tuned decay constant ranging from 50 ms to 5 s, accurately matching the adaptation timescales in bio-systems. This not only suggests promising applications of OATs in flexible artificial intelligent elements, but also provides a strategy for engineering organic devices toward novel biomimetic functions.

Multifarious Transit Gates for Programmable Delivery of Bio-functionalized Matters.

Programmable delivery of biological matter is indispensable for the massive arrays of individual objects in biochemical and biomedical applications. Although a digital manipulation of single cells has been implemented by the integrated circuits of micromagnetophoretic patterns with current wires, the complex fabrication process and multiple current operation steps restrict its practical application for biomolecule arrays. Here, a convenient approach using multifarious transit gates is proposed, for digital manipulation of biofunctionalized microrobotic particles that can pass through the local energy barriers by a time-dependent pulsed magnetic field instead of multiple current wires. The multifarious transit gates including return, delay, and resistance linear gates, as well as dividing, reversed, and rectifying T-junction gates, are investigated theoretically and experimentally for the programmable manipulation of microrobotic particles. The results demonstrate that, a suitable angle of the gating field at a suitable time zone is crucial to implement digital operations at integrated multifarious transit gates along bifurcation paths to trap microrobotic particles in specific apartments, paving the way for flexible on-chip arrays of biomolecules and cells.

Radiofrequency-Triggered Drug Release from Nanoliposomes with Millimeter-Scale Resolution Using a Superimposed Static Gating Field.

Drug delivery to a specific site in the body typically relies on the use of targeting agents that recognize a unique biomarker. Unfortunately, it is often difficult to identify unique molecular signatures that exist only at the site of interest. An alternative strategy is to deliver energy (e.g., light) to locally trigger release from a drug carrier; however, the use of this approach is limited because energy delivery to deep tissues is often impractical or invasive. In this work, radiofrequency-responsive superparamagnetic iron oxide nanoparticles (SPIONs) are used to trigger drug release from nanoscale vesicles. Because the body is inherently nonmagnetic, this approach allows for deep tissue targeting. To overcome the unfavorable meter-scale diffraction limit of SPION-compatible radiofrequency (RF) fields, a strong static gating field containing a sharp zero point is superimposed on the RF field. Only drug carriers that are at or near the zero point are susceptible to RF-triggered drug release, thereby localizing drug delivery with millimeter-scale resolution. This approach induces >40% drug release from thermally responsive doxorubicin-loaded liposomes within a 3.2 mm radius of the zero point with <10% release in the surrounding area, leading to a >2.5 therapeutic index in Huh 7 hepatocellular carcinoma cells.

Creation and modulation of conductivity in laser-irradiated SrTiO3 single crystals

We realize a controllable metallic behavior on stoichiometric (100) SrTiO3 single crystals via KrF laser irradiation, which strongly depends on vacuum and laser energy density. Moreover, this metallic conduction is caused by oxygen deficiency and can be manipulated by light illumination and electrostatic gating field. The laser-irradiated SrTiO3 shows sensitive persistent photoresponse with a relative change in the resistance of about 45% at 20 K for a 360 nm light. In addition, the conduction can also be modulated to a lower (higher) state by a positive (negative) backgate. Our results highlight the possibility of tuning the electrical properties of oxides.

基于回音壁腔光力系统的光学三极管

回音壁腔系统由于其高品质因子、小的模体积、以及简单的制造工艺而在光子器件方面有着潜在的应用。本文基于耦合的回音壁腔光力系统，通过光学泵浦-探测方案提出一种全光学三极管器件。该系统中透射信号由控制光场的功率所操控，而且信号场的放大和消逝可以通过控制栅极场的功率来实现，该器件表现出单光子光学三极管的行为。回音壁腔光力系统将在芯片尺度上的量子信息处理器件上有着潜在的应用。 Whispering gallery mode (WGM) cavities, due to their high-quality factors, small mode volumes, and simple fabrications, have potential applications in photonic devices. In this paper, we present an all-optical transistor based on a coupled realistic WGM cavity optomechanical system with an optical pump-probe scheme. In the system, the transmission of the signal field is manipulated by the optical control power, and the signal field can be efficiently attenuated or amplified depending on the power of a second gating (control) field, which indicates a promising candidate for single-photon transistors. This WGM cavity optomechanical system may have potential chip scale applications in on-chip quantum information processing devices.

Attosecond source generation using polarized gating two-color field combined with unipolar pulse

High-order harmonic generation and attosecond extreme-ultraviolet source generation have been theoretically investigated by controlling the two-color polarized gating field combined with the unipolar pulse. The results show that by properly optimizing the polarized two-color field as well as the unipolar pulse, not only is the harmonic cutoff remarkably extended, but the single short quantum path has also been selected to contribute to the harmonic spectrum, resulting in a 313 eV less modulated supercontinuum. Classical and quantum analyses are shown to explain the harmonic emission process. Finally, the proper superposition of harmonics, a series of isolated sub-50 as pulses, can be obtained.

Designing field-controllable graphene-dot-graphene single molecule switches: A quantum-theoretical proof-of-concept under realistic operating conditions.

A theoretical proof of the concept that a particularly designed graphene-based moletronics device, constituted by two semi-infinite graphene subunits, acting as source and drain electrodes, and a central benzenoid ring rotator (a "quantum dot"), could act as a field-controllable molecular switch is outlined and analyzed with the density functional theory approach. Besides the ideal (0 K) case, we also consider the operation of such a device under realistic operating (i.e., finite-temperature) conditions. An in-depth insight into the physics behind device controllability by an external field was gained by thorough analyses of the torsional potential of the dot under various conditions (absence or presence of an external gating field with varying strength), computing the torsional correlation time and transition probabilities within the Bloembergen-Purcell-Pound formalism. Both classical and quantum mechanical tunneling contributions to the intramolecular rotation were considered in the model. The main idea that we put forward in the present study is that intramolecular rotors can be controlled by the gating field even in cases when these groups do not possess a permanent dipole moment (as in cases considered previously by us [I. Petreska et al., J. Chem. Phys. 134, 014708-1-014708-12 (2011)] and also by other groups [P. E. Kornilovitch et al., Phys. Rev. B 66, 245413-1-245413-7 (2002)]). Consequently, one can control the molecular switching properties by an external electrostatic field utilizing even nonpolar intramolecular rotors (i.e., in a more general case than those considered so far). Molecular admittance of the currently considered graphene-based molecular switch under various conditions is analyzed employing non-equilibrium Green's function formalism, as well as by analysis of frontier molecular orbitals' behavior.

Characterization of intense laser-produced fast electrons using hard x-rays via bremsstrahlung

Energy distribution of high-power, short-pulse laser produced fast electrons was experimentally and numerically studied using high-energy bremsstrahlung x-rays. The hard x-ray photons and escaping electrons from various metal foils, irradiated by the 50 TW Leopard laser at Nevada Terawatt Facility, were recorded with a differential filter stack spectrometer that is sensitive to photons produced by mainly 0.5–2 MeV electrons and an electron spectrometer measuring >2 MeV electrons. The experimental bremsstrahlung and the slope of the measured escaped electrons were compared with an analytic calculation using an input electron spectrum estimated with the ponderomotive scaling. The result shows that the electron spectrum entering a Cu foil could be continuous single slope with the slope temperature of ∼1.5 MeV in the detector range. The experiment and analytic calculation were then compared with a 2D particle-in-cell code, PICLS, including a newly developed radiation transport module. The simulation shows that a two-temperature electron distribution is generated at the laser interaction region, but only the hot component of the fast electrons flow into the target during the interaction because the low energy electron component is trapped by self-generated magnetic field in the preformed plasma. A significant amount of the photons less than 100 keV observed in the experiment could be attributed to the low energy electrons entering the foil a few picoseconds later after the gating field disappears.

On-Chip Photonic Transistor Device and Biomolecule Mass Sensor Based on a Whispering Gallery Mode Cavity Optomechanical System

Whispering gallery mode (WGM) cavities, due to their high-quality factors, small mode volumes, and simple fabrications, have potential applications in photonic devices and sensing. In this paper, we present an all-optical transistor based on a coupled realistic WGM cavity optomechanical system. In the system, the transmission of the signal field is manipulated by the optical control power, and the signal field can be efficiently attenuated or amplified depending on the power of a second gating (control) field, which indicates a promising candidate for single-photon transistors. In addition, we theoretically propose a mass sensor based on this WGM cavity optomechanical system. The mass of external biomolecules (such as smallpox virus) deposited onto WGM cavity can be measured conveniently by tracking the mechanical resonance frequency shifts due to mass changes in the probe transmission spectrum. Through detecting the mass of viruses, the kinds of different viruses can be identified. This WGM cavity optomechanical system may have potential chip scale applications in quantum information processing, quantum measurement, and on-chip sensing devices.

17. Gene Therapy Strategies to Treat Fragile X Syndrome

Fragile X syndrome (FXS) is a severe debilitating neurodevelopmental disorder of the autism spectrum that results from an aberrant trinucleotide repeat extension in the 5’ region of the FMR1 gene. This extension pathologically reduces or eliminates the expression of the fragile X mental retardation protein (FMRP). FMRP is known to be a scrupulous translational modulator at the synapse and is also known to stabilize and traffic mRNAs important for proper neurological functions. We utilized C57/BL6 mice with a knock-out of the Fmr1 gene (FMRP-KO) in this study because this mouse model reproduces many of the behavioral phenotypes seen in human Fragile X patients. We used adeno-associated viral vectors (AAV) serotype 2/9, which contains the inverted terminal repeats of serotype 2, the human neuron specific synapsin promoter and the largest isoform of FMRP (isoform 1), as well as downstream woodchuck hepatitis virus post-transcriptional regulatory element and a poly A site to elevate mRNA stability, in order to re-introduce FMRP in the brain of neonatal FMRP KO mice. This vector, as well as a cognate null vector containing no transgene used as a control, were administered via bilateral intra-cerebroventricular injections at different early postnatal time points in wild type (WT) and FMRP-KO mice. AAVs were produced and purified by the University of Pennsylvania Vector Core facility. We also investigated whether FMRP overexpression in WT mice could induce behavioral changes. We tested the treated mice in an array of behavioral tests to monitor changes in autism-associated behaviors such as repetitive stereotypical motions, hyperactivity, deficits in social interaction, and general anxiety. Long term FMRP transgene expression was confirmed by immunocytochemistry and quantitative western immunoblotting where strong forebrain and neuron-specific expression was observed. Overall, we observed partial to full amelioration of certain fragile X phenotypes such as motor activity, sensorimotor gating and open field latency in FMRP-KO mice injected with the AAV-FMRP vectors. Also other observations points towards possible pathological effects due to FMRP overexpression in the forebrain of WT mice; an important consideration for future gene therapy of human FXS. These results show that the time of neonatal injection and the total number of AAV particles administered during intra-cerebroventricular injections can affect the efficacy, distribution of transgene expression, and ultimately, the successful reversal of abnormal behaviors.

Visible-light-enhanced gating effect at the LaAlO₃/SrTiO₃ interface.

Electrostatic gating field and light illumination are two widely used stimuli for semiconductor devices. Via capacitive effect, a gate field modifies the carrier density of the devices, while illumination generates extra carriers by exciting trapped electrons. Here we report an unusual illumination-enhanced gating effect in a two-dimensional electron gas at the LaAlO3/SrTiO3 interface, which has been the focus of emergent phenomena exploration. We find that light illumination decreases, rather than increases, the carrier density of the gas when the interface is negatively gated through the SrTiO3 layer, and the density drop can be 20 times as large as that caused by the conventional capacitive effect. This effect is further found to stem from an illumination-accelerated interface polarization, an originally extremely slow process. This unusual effect provides a promising controlling of the correlated oxide electronics in which a much larger gating capacity is demanding due to their intrinsic larger carrier density.

Photodetection Based on Ionic Liquid Gated Plasmonic Ag Nanoparticle/Graphene Nanohybrid Field Effect Transistors

A novel photodetection scheme using ionic liquid gated graphene field-effect transistors (FET) with plasmonic Ag nanoparticles (Ag-NPs) is reported. The incident light is transduced into graphene FET channel conductance change by coupling the light induced plasmonic field on Ag-NPs to the FET gating field via the electrical double layer of the ionic liquid. Significantly enhanced photoresponsivity peaked at the localized surface plasmonic resonance wavelength is demonstrated.