External Gate Voltage Research Articles

Selective Rashba spin–orbit coupling manipulation in triple quantum wells

We explore the Rashba (α) spin–orbit coupling (SOC) manipulation in AlInSb/InSb triple quantum wells (QWs) with three subbands occupied in the presence of the external gate voltage V b . Diverse behaviors of α can be demonstrated by adjusting the structural parameters of the wells, which is greatly desirable for the design of multifunctional QW-based spintronic devices. Specifically, the first (lowest) subband always has larger Rashba coupling against the variations in the structural parameters and V b than the other subbands, conducive to spin detection. α ν s (ν = 1, 2, 3 is the subband index) of the second and third subbands take on dual (magnitude and sign) change owing to the regulation of the inner barrier height δ, may facilitating the realization of stretchable persistent spin helix in multiple QWs. Strikingly, α 3 can keep essentially zero in a wide gate range corresponding to asymmetrical QW structural configurations, by adjusting δ. However, when widening the well to a certain extent the δ-dependence of α become fixed, i.e. ∣α 1∣ > ∣α 2∣ > ∣α 3∣ maintains in wide triple-well systems. All these findings may shed light on selective and precise SOC control in QW systems for experiments.

Polarized Photodetectors Based on 2D 2H‐MoTe2/1T'‐MoTe2/MoSe2 Van Der Waals Heterojunction

AbstractThe van der Waals heterojunction based on transition metal dichalcogenides has broad research value and application prospects as the basic material for advanced near‐infrared polarized photodetectors. a type II heterojunction based on the 2H‐MoTe2/1T′‐MoTe2/MoSe2 structure is reported for photovoltaic photodetectors. Notably, this device generates a self‐powered photocurrent, eliminating the need for an external bias or gate voltage. This device has the ability to detect polarized light, and its photocurrent anisotropy ratio is 55. The device performance is significantly amplified due to the proficient facilitation of electron‐hole separation through the type II band structure of MoSe2 at the bottom and 2H‐MoTe2 at the top, as well as enhanced exciton splitting by 1T′‐MoTe2 situated in the middle. Consequently, the device demonstrates exceptional proficiency, presenting a noteworthy response rate of 0.76 A W−1, a high detection rate of 3 × 109 Jones, an elevated EQE of 71%, and rapid rising and falling response speeds of 13 ms/10 ms respectively. Moreover, the device's anisotropy ratio of photocurrent highlights its sensitivity to polarized light, making it a promising candidate for applications in polarized photodetection. Overall, this innovative heterostructure opens new avenues for exploring and developing advanced optoelectronic devices based on 2D van der Waals materials.

Topological minibands and interaction driven quantum anomalous Hall state in topological insulator based moiré heterostructures

The presence of topological flat minibands in moiré materials provides an opportunity to explore the interplay between topology and correlation. In this work, we study moiré minibands in topological insulator films with two hybridized surface states under a moiré superlattice potential created by two-dimensional insulating materials. We show the lowest conduction (highest valence) Kramers’ pair of minibands can be Z2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\\mathbb{Z}}}_{2}$$\\end{document} non-trivial when the minima (maxima) of moiré potential approximately form a hexagonal lattice with six-fold rotation symmetry. Coulomb interaction can drive the non-trivial Kramers’ minibands into the quantum anomalous Hall state when they are half-filled, which is further stabilized by applying external gate voltages to break inversion. We propose the monolayer Sb2 on top of Sb2Te3 films as a candidate based on first principles calculations. Our work demonstrates the topological insulator based moiré heterostructure as a potential platform for studying interacting topological phases.

Bipolar thermoelectric superconducting single-electron transistor

Thermoelectric effects in normal metals and superconductors are usually very small due to the presence of electron-hole symmetry. Here, we show that superconducting junctions brought out of equilibrium manifest a sizable bipolar thermoelectric effect that stems from a violation of the detailed balance determined by the crucial role of the interactions at the mean-field level. To fully control the effect, we consider a thermally biased junction where the capacitance of the central S′ region is small enough to establish a Coulomb blockade regime. By exploiting charging effects we are able to tune the Seebeck voltage, the thermocurrent, and thereby the power output of this structure, via an external gate voltage. We then analyze the main figures of merit of bipolar thermoelectricity and we prospect for possible applications. Published by the American Physical Society 2024

Electro-optical control of spin-orbit coupling in AlInAs/GaInAs single and double quantum wells

The electro-optical manipulation of the spin-orbit couplings in single and double quantum wells has been explored with the help of the external laser field and gate voltage. We focus on the situation of two-subband occupation of the wells, and obtain the Rashba (αν, ν=1,2 is the subband index) and Dresselhaus coefficients βν by solving the coupled Poisson and Schrödinger equations. Firstly, the laser dressing effect is demonstrated in that the laser field alters the confining potential and quantized energy levels of electrons localized in the well, in favor of flexible spin-orbit control. In combination with the gate voltage, we have realized selective control of the Rashba coupling, i.e., separately tuning αν of one subband and keeping the other unaffected. Moreover, the electro-optical manipulation can even alter the relative signs of αν, making it feasible to realize skyrmion-lattice density excitation. Further, the Dresselhaus coupling, which is insensitive to the electrical manipulation, can be controlled by the laser field, greatly improving the possibility of persistent-skyrmion-lattice formation by reaching the condition of α1=β1 and α2=−β2. These diverse SO behaviors under the electro-optical manipulation should simulate experiments for exploring promising applications in novel spintronic devices.

Giant quantum oscillations of acoustoelectric current in narrow graphene nanoribbons

A theory is presented for the acoustoelectric (AE) effect in narrow graphene nanoribbons (GNRs) deposited on a piezoelectric substrate when a surface acoustic wave (SAW) is launched onto the surface of the piezoelectric. It is assumed that the electron density in such GNRs can be controlled by applying an external gate voltage, and the acoustic wavelength is much smaller than the mean free path of electrons, so that the quantum mode of interaction of those electrons with the SAW in realized. Using the kinetic theory approach, we calculate the AE current flowing through the GNR sample and arising as a result of momentum transfer from coherent SAW phonons to conduction electrons. It is shown that size quantization of the electron energy spectrum in narrow GNRs leads to giant periodic oscillations of the AE current with a change in the gate voltage. AE current surges occur whenever the Fermi level, rising with increasing gate voltage, crosses in turn the bottom of each of the successive size-quantized electron energy subbands. The oscillations predicted are giant in the sense that the maximum values of the current exceed its minimum values by at least an order of magnitude, and they can be observed in narrow GNRs ∼10 nm wide even at room temperature.

Gate-Tunable Spin Seebeck Effect and Pure Spin Current Generation in Molecular Junctions Based on Bipolar Magnetic Molecules.

Generating pure spin currents is very desirable in spintronics, as it provides a promising way to substantially reduce Joule heating and achieve ultrahigh integration density. However, to date, most spintronic devices exhibit spin currents that are accompanied by charge currents. The generation of pure spin currents on the nanoscale, particularly at the single-molecule level, remains challenging. Here, we propose that by exploiting our recently reported bipolar magnetic molecules (BMMs) as the core component of single-molecule devices, where the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) come from different spin channels, the generation of pure spin currents can be easily realized via the spin Seebeck effect (SSE) with applied temperature gradient. Moreover, the spin Seebeck coefficient can be modulated over a wide range by applying an external gate voltage. The proposal is verified through first-principles calculations on two BMM-based molecular junctions.

Atomistic designing of 2D quantum materials heterostructures CdF/CrI3 for Berry curvature driven tunable intrinsic anomalous Hall state

The coupling between topology and magnetism can explore rich physics with fundamental interest. Passing through the phase of Bismuth-based topological insulators magnetized by the 3d/4f transition metal doping, currently the fabrication of quantum heterostructures by suitable new-generation 2D materials, has emerged as a prospective alternative. Following the current trends, the present investigation deals with the atomistic designing and investigation of the quantum heterostructures of the newly predicted massive Dirac semimetal CdF and well-known layered ferromagnetic insulator CrI3 using the first-principles density functional theory calculations supplemented by the low energy tight-binding model Hamiltonian. The designed strategy ensures the lattice mismatch should be within the permissible range. We have addressed the physical characteristics of heterostructures in terms of the non-trivial topological band inversion between Cd-5s and I-2p orbitals. Proximity effect induces magnetic interactions, breaks the time-reversal symmetry at the interface, and leads to Berry curvature-driven tunable intrinsic anomalous Hall conductance (AHC) at the Fermi energy. Our analysis reveals the electrons with high Fermi velocity (106 m s−1) in the heterostructures and the band topology at the Fermi level can be tuned effectively using very small external gate voltage or homogeneous electric field. Our investigation can open up new avenues for designing new topological phases in the heterostructure community and possible tailoring routes of the intrinsic AHC in moderate temperature.

Enhancing graphene's resistance to oxidative corrosion by positive charge doping: A DFT view

Although graphene has been widely used as a protective coating for many applications, the failure mechanism and the strategy to improve its resistance to the chemical etching is largely unknown. We herein have comparatively studied the generally oxidative corrosion process of neutral and positive-charge-doping (PCD) graphene via DFT (density functional theory) simulations. It is found that the positive-charge-doping (PCD) generally enhances the protecting ability of graphene for substrates from reactive environments. The C-network breakdown of graphene due to oxidative etching goes through three stages: the formation of oxidative corrosion crystal nucleus (OCCN), subsequent formation and expansion of oxidative lined defect (OLD), then the generation and extension of oxidative corrosion crack (OCC). Comparing to the neutral or negative charge doping, PCD decreases the electronic reactivity and thus suppresses the oxygen atoms diffusing on graphene of the rate-limiting step, especially in the stage of OCCN nucleation. Our work indicates a possible strategy to improve the resistance to oxidative corrosion (ROC) of graphene via positive charge doping. This protection enhancement of graphene can be achieved taking advantages of the electron capture of the neighboring substrates, reactive environments or external gate voltage in applications.

High Performance Broadband Full Linear Polarimeter based on ReS2 Nanobelts

AbstractPolarization‐sensitive photodetectors can simultaneously sense the polarization states and intensity of light. Traditional polarimeters integrate photodetectors with complex and bulky optical systems such as polarizers and waveplates, which can not meet the current demand of on‐chip polarization photonic devices. This work has demonstrated a full linear polarization polarimeter based on two independent ReS2 nanobelt devices vertically stacked with a designed twist angle. The device achieves a high responsivity (959 A W−1) without applying an external gate voltage. In addition, the ReS2 nanobelt photodetector displays a strong polarization‐sensitive photoresponse with a linear dichroic ratio of 1.72 at 665 nm. By utilizing those two vertically stacked nanobelt devices, the linear polarization states of the incident light can be distinguished in a wide range of wavelengths from 590 to 800 nm. This study provides a simple route for future polarization‐sensitive optoelectronic devices.

A controllable conduction-type photovoltaic effect in MoTe2

In this study, we unveiled that a conduction type can be controlled from the 2H-MoTe2 ambipolar semiconducting phase to the selective opposite n-type or p-type FETs. The polarity inversion has been achieved through the trapped interface states that exist between h-BN and SiO2 upon illumination with an external gate voltage. For the n-type doping effect, we found that it has a positive coefficient with the thickness of h-BN but the p-type doping was not achieved with a thickness of 50 nm or beyond. For the n-type MoTe2 FET, the carrier concentration n(n−MoTe2) and mobility μ(n−MoTe2) were calculated to be ∼2.07 × 1012 cm−2 and ∼20.17 cm2/V/s, respectively, while for the p-type FET n(p−MoTe2) and μ(p−MoTe2) are ∼2.16 × 1012 cm−2 and ∼24.04 cm2/V/s, respectively. Moreover, we also develop a lateral PIN diode with an intrinsic region that was in-between n- and p-type FETs. An electrical performance was also monitored, and an ideal rectifying behavior was reached with a rectification ratio (IfIr) of almost >106. The PIN diode also exhibits a self-biased photovoltaic behavior with an open-circuit voltage (Voc = 440.9 mV). This research work may pave the way for fabricating photodiodes with low power consumption using transition metal dichalcogenides materials.

Spin filtering and negative differential resistance in PAQR-ZGNR junctions

The spin transport of a Polyacene quinone radical (PAQR) molecule sandwiched between two ferromagnetic ZGNR electrodes with an even width number of 6 is studied employing the nonequilibrium Green's function method combined with the density functional theory (NEGF-DFT). Lateral symmetry mismatch of wave functions between PAQR and ZGNR may suppress all the transport channels except only one spin-down channel near the chemical potential in the linear regime. A half-metallic behavior with perfect spin filtering efficiency and strong negative differential resistance is expected. Under high bias voltage, the spin filtering efficiency changes from −100% to 90% when other transport channel is involved. Applying an external gate voltage, we can modulate greatly the conductance of the junction. These unique features are insensitive to the length of PAQR chain and suggest great application prospect for PAQR-ZGNR molecule devices.

Quantum nonlinear planar Hall effect in bilayer graphene: An orbital effect of a steady in-plane magnetic field

We study the quantum nonlinear planar Hall effect in bilayer graphene under a steady in-plane magnetic field. When time-reversal symmetry is broken by the magnetic field, a charge current occurs in the second-order response to an external electric field as a result of the Berry curvature dipole in momentum space. We show that a nonlinear planar Hall effect originating from the anomalous velocity is caused by an orbital effect of an in-plane magnetic field on electrons in bilayer graphene in the complete absence of spin-orbit coupling. Taking into account the symmetry analysis, we derive the dominant dependence of the Berry curvature dipole moment on the magnetic field components. Moreover, we illustrate how to control and modulate the Berry curvature dipole with an external planar magnetic field, gate voltage, and Fermi energy.

Graphene hyperbolic metamaterials: Fundamentals and applications

Metamaterials have shown potential for next-generation optical materials since they have special electromagnetic responses which cannot be obtained in natural media. Among various metamaterials, hyperbolic metamaterials (HMMs) with highly anisotropic hyperbolic dispersion provide new ways to manipulate electromagnetic waves. Besides, graphene has attracted lots of attention since it possesses excellent optoelectronic properties. Graphene HMMs combine the extraordinary properties of graphene and the strong light modulation capability of HMMs. The experimental fabrication of graphene HMMs recently proved that graphene HMMs are a good platform for terahertz optical devices. The flexible tunability is a hallmark of graphene-based HMMs devices by external gate voltage, electrostatic biasing, or magnetic field, etc. This review provides an overview of up-to-now studies of graphene HMMs and an outlook for the future of this field.

Low-Threshold pentacene OTFT by using NdTaON gate dielectric and ITO gate electrode

Ta-doped Nd oxynitrides (NdxTa(1-x)ON) are adopted as gate dielectrics in pentacene organic thin-film transistors on ITO glass for transparent applications. The device with Nd0.76Ta0.24ON can achieve the optimal performance with a high carrier mobility of 2.32 cm2/V·s and a small threshold voltage of −0.21 V. The capacitance–voltage results indicate that besides producing a high permittivity (high-k) for the gate dielectric, Ta incorporation in NdON can generate negative fixed charges in the oxide to assist the external gate voltage to produce a small threshold voltage. The threshold voltage is further reduced by the ITO gate electrode because ITO with higher work function can decrease the energy barrier for the charge-carrier injection at the source/channel interface. XPS and AFM reveal that the moisture resistance of pure Nd oxynitride can be enhanced by Ta doping to improve the smoothness of the dielectric surface, which results in weaker surface-roughness and grain-boundary scatterings, and consequently higher carrier mobility. The carrier mobility is further enhanced by the ITO gate electrode because similar to metal-gate/high-k stack, metallic ITO gate with higher electron concentration than Si gate can offer stronger screening effect on the carrier scattering induced by the severe atomic oscillation in the high-k NdTaON gate dielectric.

Ab Initio Study of Graphene/hBN Van der Waals Heterostructures: Effect of Electric Field, Twist Angles and p-n Doping on the Electronic Properties.

In this work, we study the structural and electronic properties of boron nitride bilayers sandwiched between graphene sheets. Different stacking, twist angles, doping, as well as an applied external gate voltage, are reported to induce important changes in the electronic band structure near the Fermi level. Small electronic lateral gaps of the order of few meV can appear near the Dirac points K. We further discuss how the bandstructures change applying a perpendicular external electric field, showing how its application lifts the degeneracy of the Dirac cones and, in the twisted case, moves their crossing points away from the Fermi energy. Then, we consider the possibility of co-doping, in an asymmetric way, the two external graphene layers. This is a situation that could be realized in heterostructures deposited on a substrate. We show that the co-doping acts as an effective external electric field, breaking the Dirac cones degeneracy. Finally, our work demonstrates how, by playing with field strength and p-n co-doping, it is possible to tune the small lateral gaps, pointing towards a possible application of C/BN sandwich structures as nano-optical terahertz devices.

Electric field-induced modification of Dzyaloshinskii-Moriya interaction in Fe/Mg0(001) with heavy metal insertions

The effect of external electric field on the Dzyaloshinskii-Moriya interaction (DMI) in Fe/M/MgO(001) (M=Os, Ir, Pt, Au) is investigated by means of first principles full-potential linearized augmented plane wave (FLAPW)-based Density Functional Theory method. We find that, apart from the Au layer insertion, the sign of the DMI is altered by the introduction of the heavy metal, such that the magnetic chirality in Fe/M/MgO(001) is inverted from that of Fe/MgO(001). The magnitudes of DMI in the Fe/Os/MgO and Fe/Ir/MgO are enhanced by 5 - 7 times compared to that without the heavy metal insertion. The increase of the DMI can be attributed to the strength of spin-orbit coupling of the heavy metal layer, implied by the asymmetry of the orbital moments of the heavy metals within the spin spiral structures. When an external electric field is applied, no significant modification to the DMI can be observed. Our work suggests that additional heavy metal layer might be necessary for a fine control of DMI, and hence of the overall magnetic domain-related properties, by external gate voltages.

Multiple optical bistabilities in graphene arrays-bulk dielectric composites

We theoretically investigate the optical bistability in compound systems composed of one-dimensional graphene arrays and dielectrics. Graphene sheets are periodically assembled in the dielectric substrate to form the arrays of graphene and a bulk dielectric contacts on the right of the structure. The standing wave resonances can enhance the nonlinearity of graphene arrays and consequently multiple optical bistabilities are achieved with appropriate red-detunings in incident wavelengths. The realization of multiple bistabilities and the bistable thresholds depend on the thickness of the bulk dielectric. Otherwise, the multiple bistabilities could be modulated by impurity-doping in graphene or an external gate voltage on graphene arrays as well. This investigation has potentials applied in multivalued all-optical switches and optical memory.

Highly Tunable Carrier Tunneling in Vertical Graphene-WS2-Graphene van der Waals Heterostructures.

Owing to the fascinating properties, the emergence of two-dimensional (2D) materials brings various important applications of electronic and optoelectronic devices from field-effect transistors (FETs) to photodetectors. As a zero-band-gap material, graphene has excellent electric conductivity and ultrahigh carrier mobility, while the ON/OFF ratio of the graphene FET is severely low. Semiconducting 2D transition metal chalcogenides (TMDCs) exhibit an appropriate band gap, realizing FETs with high ON/OFF ratio and compensating for the disadvantages of graphene transistors. However, a Schottky barrier often forms at the interface between the TMDC and metallic contact, which limits the on-state current of the devices. Here, we lift the two limits of the 2D-FET by demonstrating highly tunable field-effect tunneling transistors based on vertical graphene-WS2-graphene van der Waals heterostructures. Our devices show a low off-state current below 1 pA and a high ON/OFF ratio exceeding 106 at room temperature. Moreover, the carrier transport polarity of the device can be effectively tuned from n-type under small bias voltage to bipolar under large bias by controlling the crossover from a direct tunneling region to the Fowler-Nordheim tunneling region. Further, we find that the effective barrier height can be controlled by an external gate voltage. The temperature dependence of carrier transport demonstrates that both tunneling and thermionic emission contribute to the operation current at elevated temperature, which significantly enhances the on-state current of the tunneling transistors.

Gate control of interlayer exchange coupling in ferromagnetic semiconductor trilayers with perpendicular magnetic anisotropy

Interlayer exchange coupling (IEC) has been intensively investigated in magnetic multilayers, owing to its potential for magnetic memory and logic device applications. Although IEC can be reliably obtained in metallic ferromagnetic multilayer systems by adjusting structural parameters, it is difficult to achieve gate control of IEC in metallic systems due to their large carrier densities. Here, we demonstrate that IEC can be reliably controlled in ferromagnetic semiconductor (FMS) trilayer structures by means of an external gate voltage. We show that, by designing a quantum-well-type trilayer structure based on (Ga,Mn)(As,P) FMSs and adapting the ionic liquid gating technique, the carrier density in the nonmagnetic spacer of the system can be modulated with gate voltages of only a few volts. Due to this capability, we are able to vary the strength of IEC by as much as 49% in the FMS trilayer. These results provide important insights into design of spintronic devices and their energy-efficient operation.