In-plane Electric Field Research Articles

Stark Effects of Rydberg Excitons in a Monolayer WSe2 P-N Junction.

The enhanced Coulomb interaction in two-dimensional semiconductors leads to tightly bound electron-hole pairs known as excitons. The large binding energy of excitons enables the formation of Rydberg excitons with high principal quantum numbers (n), analogous to Rydberg atoms. Rydberg excitons possess strong interactions among themselves as well as sensitive responses to external stimuli. Here, we probe Rydberg exciton resonances through photocurrent spectroscopy in a monolayer WSe2 p-n junction formed by a split-gate geometry. We show that an external in-plane electric field not only induces a large Stark shift of Rydberg excitons up to quantum principal number 3 but also mixes different orbitals and brightens otherwise dark states such as 3p and 3d. Our study provides an exciting platform for engineering Rydberg excitons for new quantum states and quantum sensing.

Orbital Magneto-Nonlinear Anomalous Hall Effect in Kagome Magnet Fe_{3}Sn_{2}.

It has been theoretically predicted that perturbation of the Berry curvature by electromagnetic fields gives rise to intrinsic nonlinear anomalous Hall effects that are independent of scattering. Two types of nonlinear anomalous Hall effects are expected. The electric nonlinear Hall effect has recently begun to receive attention, while very few studies are concerned with the magneto-nonlinear Hall effect. Here, we combine experiment and first-principles calculations to show that the kagome ferromagnet Fe_{3}Sn_{2} displays such a magneto-nonlinear Hall effect. By systematic field angular and temperature-dependent transport measurements, we unambiguously identify a large anomalous Hall current that is linear in both applied in-plane electric and magnetic fields, utilizing a unique in-plane configuration. We clarify its dominant orbital origin and connect it to the magneto-nonlinear Hall effect. The effect is governed by the intrinsic quantum geometric properties of Bloch electrons. Our results demonstrate the significance of the quantum geometry of electron wave functions from the orbital degree of freedom and open up a new direction in Hall transport effects.

Analysis on scattering characteristics of SH guided wave due to V-notch in a piezoelectric bi-material strip

In this paper, the problem of a V-notch with complex boundary conditions in a piezoelectric bi-material strip is studied. First, SH guided wave is considered as an external load acting on the piezoelectric bi-material strip; on the basis of repeated image superposition, the analytical expression of scattering wave is conducted, which satisfies the stress-free and electric insulation conditions on the upper and lower horizontal boundaries of the strip. Then, the analytical expression of standing wave is established, which satisfies the stress-free and electric insulation conditions on the boundaries of V-notch by the fractional Bessel function expansion method and Graf addition theorem. Finally, Green’s function method is applied, the bi-material strip is divided into two parts along the vertical interface, a pair of in-plane electric field and out-plane forces is applied on the vertical interface, and the first kind of Fredholm integral equations is set up and solved by applying the orthogonal function expansion technique and effective truncation. Results clarified the influence on the dynamic stress concentration factor and electric field intensity concentration factor under proper conditions. Besides, the analytical solutions are compared with the finite element solutions to verify the accuracy of the conclusions in this article.

Magnetospectroscopy of shallow donors in two dimensions in the presence of fluctuations of the electrostatic potential

Abstract Spectroscopy of shallow donors is a tool to test theoretical models and to reveal properties of semiconductors. In this work we consider intra-shallow impurity transitions by studying a CdTe/(Cd, Mg)Te structure grown by a molecular beam epitaxy in which both a CdTe quantum well and (Cd, Mg)Te barries are uniformly doped with iodine donors. Measurements of a photocurrent (PC) at the far-infrared were carried out at 4.2 K and magnetic fields B up to 7 T with the energy of photons originated from a molecular laser in the range 2.2 meV–12.8 meV. Spectra (a PC signal vs. B, at a constant energy of photons) show lines which position does not depend on the photon energy but shifts with the in-plane electric field. These dependencies, which do not follow a well-established picture of shallow donor magnetospectroscopy in quantum wells, are explained within a model which unifies the role of fluctuations of the electrostatic potential and a magnetic-field induced electron localization.

On the sign of the linear magnetoelectric coefficient in Cr2O3

We establish the sign of the linear magnetoelectric (ME) coefficient, α, in chromia, Cr2O3. Cr2O3 is the prototypical linear ME material, in which an electric (magnetic) field induces a linearly proportional magnetization (polarization), and a single magnetic domain can be selected by annealing in combined magnetic (H) and electric (E) fields. Opposite antiferromagnetic (AFM) domains have opposite ME responses, and which AFM domain corresponds to which sign of response has previously been unclear. We use density functional theory (DFT) to calculate the magnetic response of a single AFM domain of Cr2O3 to an applied in-plane electric field at zero kelvin. We find that the domain with nearest neighbor magnetic moments oriented away from (towards) each other has a negative (positive) in-plane ME coefficient, α⊥ , at zero kelvin. We show that this sign is consistent with all other DFT calculations in the literature that specified the domain orientation, independent of the choice of DFT code or functional, the method used to apply the field, and whether the direct (magnetic field) or inverse (electric field) ME response was calculated. Next, we reanalyze our previously published spherical neutron polarimetry data to determine the AFM domain produced by annealing in combined E and H fields oriented along the crystallographic symmetry axis at room temperature. We find that the AFM domain with nearest-neighbor magnetic moments oriented away from (towards) each other is produced by annealing in (anti-)parallel E and H fields, corresponding to a positive (negative) axial ME coefficient, α∥ , at room temperature. Since α⊥ at zero kelvin and α∥ at room temperature are known to be of opposite sign, our computational and experimental results are consistent.

Physical temperature sensing of targets with shelters under indoor scenes by a revised Ku band microwave radiometer

This work is aimed to solve technical difficulties for microwave radiometers in Ku band and above under indoor scenes to detect physical temperatures of targets with shelters. Firstly, a revised microwave radiometer in Ku-band is developed with a novel calibration scheme. Secondly, in order to the designed radiometer pocess a high temperature accuracy for targets with shelters, crucial technical difficulties are investigated: 1) The long-neglected impact on temperature sensitivity of the single channel radiometer by the frequency stability of LO is analyzed and a LO with high frequency stability and small volume is designed. 2) The calibration accuracy deterioration caused by the receiver's nonideality is analyzed, and a novel calibration algorithm is proposed. 3) the method of plane wave spectrum (PWS) is proposed to derive the planar electric field in near field of the horn antenna for temperature inversion. 4) A more complete microwave thermal radiation transmission model of clothed human body is proposed. Thirdly, several experiments are set up and show that the designed radiometer can track physical temperatures of targets sheltered by cotton cloth relatively accurately, so that methods proposed for indoor near field applications are confirmed effective. Noteworthily, it is the first time in the microwave passive sensing domain that our work systematically proposes the temperature inversion theory for physical temperatures of targets with shelters accurately in noncontact under indoor scenes, and the design scheme for the microwave radiometer optimized portable along with a high temperature sensitivity applied for microwave security and medical monitoring.

Response of helielectric nematics under an in-plane electric field.

In traditional chiral nematic liquid crystals, the apolar cholesterics, the dielectric effect is the main driving force for responding to an electric field. The emerging polar chiral nematics, dubbed helielectric nematics, are the polar counterparts of the cholesterics. The head-to-tail symmetry breaking of the new matter state enables it to respond sensitively to the polarity of an electric field. Here, we report on the observation of a sequential polar winding/unwinding process of polarization helices under an electric field applied perpendicular to the helical axes, which behaves distinctly from the unwinding of the apolar cholesteric helices. Understanding the helix-unwinding behaviors provides insights for developing switchable devices based on helielectric nematics.

Nanotube ferroelectric tunnel junctions with an ultrahigh tunneling electroresistance ratio.

Low-dimensional ferroelectric tunnel junctions are appealing for the realization of nanoscale nonvolatile memory devices due to their inherent advantages of device miniaturization. Those based on current mechanisms have limitations, including low tunneling electroresistance (TER) effects and complex heterostructures. Here, we introduce an entirely new TER mechanism to construct a nanotube ferroelectric tunnel junction with ferroelectric nanotubes as the tunneling region. When rolling a ferroelectric monolayer into a nanotube, due to the coexistence of its intrinsic ferroelectric polarization with the flexoelectric polarization induced by bending, a metal-insulator transition occurs depending on the radiative polarization states. For the pristine monolayer, its out-of-plane polarization is tunable by an in-plane electric field, and the conducting states of the ferroelectric nanotube can thus be tuned between metallic and insulating states via axial electric means. Using α-In2Se3 as an example, our first-principles density functional theory calculations and nonequilibrium Green's function formalism confirm the feasibility of the TER mechanism and indicate an ultrahigh TER ratio that exceeds 9.9 × 1010% of the proposed nanotube ferroelectric tunnel junctions. Our findings provide a promising approach based on simple homogeneous structures for high density ferroelectric microelectric devices with excellent ON/OFF performance.

Large valley polarization and the valley-dependent Hall effect in a Janus TiTeBr monolayer.

Ferrovalley materials hold great promise for implementation of logic and memory devices in valleytronics. However, there have so far been limited ferrovalley materials exhibiting significant valley polarization and high Curie temperature (TC). Using first-principles calculations, we predict that the TiTeBr monolayer is a promising ferrovalley candidate. It exhibits intrinsic ferromagnetism with TC as high as 220 K. It is indicated that an out-of-plane alignment of magnetization demonstrates a valley polarization up to 113 meV in the topmost valence band, as further verified by perturbation theory considering both the spin polarization and spin-orbit coupling. Under an in-plane electric field, the valley-dependent Berry curvature results in the anomalous valley Hall effect (AVHE). Moreover, under a suitable in-plane biaxial strain, the TiTeBr monolayer transforms into a Chern insulator with a nonzero Chern number, yet retains its ferrovalley characters and thus the emergent quantum anomalous valley Hall effect (QAVHE). Our study indicates that the TiTeBr monolayer is a promising ferrovalley material, and it provides a platform for investigating the valley-dependent Hall effect.

Realizing altermagnetism in two-dimensional metal-organic framework semiconductors with electric-field-controlled anisotropic spin current.

Altermagnets exhibit momentum-dependent spin-splitting in a collinear antiferromagnetic order due to their peculiar crystallographic and magnetic symmetry, resulting in the creation of spin currents with light elements. Here, we report two two-dimensional (2D) metal-organic framework (MOF) semiconductors, M(pyz)2 (M = Ca and Sr, pyz = pyrazine), which exhibit both altermagnetism and topological nodal point and line by using first-principles calculations and group theory. The altermagnetic 2D MOFs exhibit unconventional spin-splitting and macroscopic zero magnetization caused by 4-fold rotation in crystalline real space and 2-fold rotation in spin space, leading to the generation and control of anisotropic spin currents when an in-plane electric field ( E ) is applied. In particular, pure spin current with the spin Hall effect occurs when E is applied along the angular bisector of the two spin arrangements. Our work indicates the existence of altermagnetic MOF systems and a universal approach to generate electric-field-controlled spin currents for potential applications in antiferromagnetic spintronics.

Prediction of the two-dimensional ferromagnetic semiconductor Janus 2H-ZrTeI monolayer with large valley and piezoelectric polarizations.

Two-dimensional room-temperature Janus ferrovalley semiconductors with valley polarization and piezoelectric polarization offer new perspectives for designing multifunctional nanodevices. Herein, using first-principles calculations, we predict that the Janus 2H-ZrTeI monolayer is an intrinsic ferromagnetic semiconductor with in-plane magnetic anisotropy and a Curie temperature of 111 K. The Janus ZrTeI monolayer possesses a significant valley polarization of 141 meV due to time-reversal and inversion symmetry breaking. Based on the valley-contrasting Berry curvature, the anomalous valley Hall effect can be observed under an in-plane electric field. Meanwhile, the breaking of the inversion symmetry and mirror symmetry results in large longitudinal and transverse piezoelectric coefficients. By applying biaxial strain, the Janus 2H-ZrTeI monolayer can also be transformed into a Weyl nodal line semimetal. Furthermore, bilayers of ZrTeI with AB and BA stacking configurations allow the coexistence of valley polarization and ferroelectricity, enabling the manipulation of magnetism, ferroelectric polarization, and valley polarization through interlayer sliding. Our work provides a platform for studying valley polarization, piezoelectricity, and multiferroic coupling, which is significant for the application of multifunctional devices.

Effects of electric field and light on resistivity switching of Eu0.7Sr0.3MnO3 thin films.

Based on the excellent piezoelectric properties of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) single crystals, a hole-doped manganite film/PMN-PT heterostructure has been constructed to achieve electric-field and light co-control of physical properties. Here, we report the resistivity switching behavior of Eu0.7Sr0.3MnO3/PMN-PT(111) multiferroic heterostructures under different in-plane reading currents, temperatures, light stimuli and electric fields, and discuss the underlying coupling mechanisms of resistivity change. The transition from the electric-field induced lattice strain effect to polarization current effect can be controlled effectively by decreasing the in-plane reading current at room temperature. With the decrease of temperature, the interfacial charge effect dominates over the lattice strain effect due to the reduced charge carrier density. In addition, light stimulus can lead to the delocalization of eg carriers, and thus enhance the lattice strain effect and suppress the interfacial charge effect. This work helps to understand essential physics of magnetoelectric coupling and also provides a potential method to realize energy-efficient multi-field control of manganite thin films.

Stark effect and orbital hybridization of moiré interlayer excitons in the MoSe2/WSe2 heterobilayer.

In this paper, we undertake a theoretical investigation into the effects of both in-plane and out-of-plane static electric fields on moiré interlayer excitons (IXs) within a WSe2/MoSe2 heterobilayer. We thoroughly analyze a wide range of properties pertaining to the IXs, including the binding energy, Stark shift, orbital hybridization, photoluminescence (PL) spectra, and radiative lifetime. Various factors influencing IX behavior, such as the dielectric environment, spacing separation, and moiré trap effects, are examined in detail. Our results demonstrate that the in-plane electric field leads to energy splitting between states with non-zero angular momentum, such as the 2p± dark states. Consequently, we analyze IX orbital hybridization, including hybrid Rydberg states like 1s, 2p±, and 2s. In contrast, we show that an out-of-plane electric field induced by a double-gate setup causes a quadratic Stark effect on the center of mass (COM) eigenenergies, leading to energy splitting of degenerate states and resulting in orbital hybridization of COM eigenvectors. Additionally, we demonstrate that a parallel electric field brightens the 2p± dark state through a one-photon PL process, due to the hybridization phenomena between s- and p-type Rydberg states. In short, our investigation is in great agreement with previous research and can assist experimenters in designing novel optoelectronic applications, such as on-chip electro-optic modulators and TeraHertz devices.

On the grain boundary charge transport in p-type polycrystalline nanoribbon transistors.

Grain boundaries (GB) profoundly influence charge transport, and their localized potential barrier with a high density of defect states plays a crucial role in polycrystalline materials. There are a couple of models to estimate the density of states (DoS) of nanostructured materials in field-effect transistors (FETs) that probe interface traps between the semiconductor and dielectric but not at the grain boundaries. Here, we report on utilizing Levinson's and Seto's models of grain boundary transport and correlate them with the temperature-dependent hopping transport in copper iodide (CuI) polycrystalline nanoribbon (PNR) FETs. Experimentally, PNRs are obtained by e-beam lithography and thermal evaporation of CuI. To investigate the impact of GB, the devices are fabricated with different channel aspect ratios by varying widths (80, 260, and 570 nm) and lengths (20 to 90 μm). Owing to the high hole concentration, PNR FETs operate in depletion mode at 300 K. At various low temperatures (80-300 K), the figures-of-merits of FETs are estimated to understand device performance. We determine GB barrier heights, activation energy, and density of GB trap states and find equivalence between the two models. Furthermore, we calculate temperature-dependent hopping and trap-limited transport parameters to obtain DoS at the Fermi energy, trapped and free charge carrier density, localization length, hopping distance, hopping energy, etc. at various channel lengths. Based on this quantitative analysis, we propose a channel length-dependent GB barrier height variation due to the in-plane electric field and elucidate CuI energy band levels.

Bloch oscillations probed quantum phases in HgTe quantum wells

The semiconductor quantum well based on mercury telluride is characterized by two distinct phases: conventional insulating phase and topological insulating phase with helical edge states. The system undergoes a topological quantum phase transition from one phase to the other, tuned by the critical geometric parameters of the quantum well. It is shown that the quantum states in each phase exhibit distinct flavors of Bloch oscillations, depending strongly on the geometric parameters and crystal momentum of the system. In particular, the group and Berry velocities and the real-space trajectories exhibit pronounced Bloch oscillations. Interestingly, the x- and y-components of the group velocity are interchanged by interchanging their corresponding components of the crystal momentum. In addition, a Gaussian wave packet undergoes distinct time evolution in each quantum phase of the HgTe quantum well. Moreover, the effects of applied in-plane electric and transverse magnetic fields are determined within the framework of Newtonian mechanics, leading to the geometric visualization of such an oscillatory motion. We find that in the presence of both applied in-plane electric and transverse magnetic fields simultaneously, the system undergoes a dynamic phase transition between confined and de-confined states, tuned by the relative strength of the fields. It is argued that the distinct Bloch oscillations originate from the peculiar band structure of HgTe quantum wells in each quantum phase. Furthermore, we find that the direct-current drift velocity in each quantum phase exhibits negative differential conductivity, a hallmark of the Bloch oscillation regime.

Revealing defect-bound excitons in WS2 monolayer at room temperature by exploiting the transverse electric polarized wave supported by a Si3N4/Ag heterostructure

Abstract Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers are promising materials for light-emitting devices due to their excellent electric and optical properties. However, defects are inevitably introduced in the fabrication of TMDC monolayers, significantly influencing their emission properties. Although photoluminescence (PL) is considered as an effective tool for investigating the defects in TMDC monolayers. However, the PL from the defect-bound excitons is revealed only at low temperatures. Here, we show that the PL from the defect-bound excitons in a WS2 monolayer can be effectively revealed at room temperature by exploiting the transverse electric polarized wave supported by a Si3N4/Ag heterostructure. It is revealed that the defect-bound excitons in all possible positions of the WS2 monolayer can be effectively excited by the TE wave with significantly enhanced in-plane electric field localized on the surface of the Si3N4 layer. In addition, the emission from defect-bound excitons can propagate to the collection point with small attenuation. More importantly, the exciton dynamics in the WS2 monolayer can be modified by the Si3N4/Ag heterostructure, allowing the simultaneous excitation of neutral excitons, charge excitons (trions), and defect-bound excitons in the WS2 monolayer attached on the Si3N4/Ag heterostructure. We inspect the PL spectra obtained at different positions and find that the relative intensity of defect-bound excitons depends on the collection position. We also examine the dependences of the PL intensity and bandwidth on the excitation power for the three types of excitons. It is found that they exhibit different behaviors from those observed in the optical measurements by using the traditional excitation method. Our findings suggest a new way for exciting and studying the dynamics of multi-excitons at room temperature and indicate the potential applications of the TE wave in probing the defects in TMDC monolayers.

First-principles prediction of intrinsic piezoelectricity, spin-valley splitting and magneto-crystal anisotropy in 2H-VS2 magnetic semiconductor

The piezoelectricity, valley character and magnetic properties of intrinsic ferrovalley 2H-VS2 monolayer are studied using density functional theory. The results suggest that 2H-VS2 monolayer exhibits obvious bipolar magnetic semiconductor characters with Curie temperature (TC) of 278 K, large piezoelectric coefficient d11 of 3.925 pm/V, in-plane magneto-crystal anisotropy (MA) and large valley splitting (ΔKK′) of 75.1 meV. Applying in-plane strain (-6% to 6 %) can well adjust the ΔKK′ and TC from 72 meV to 76.7 meV and from 98 K to 397 K, respectively, resulting an obvious change of electronic structures from bipolar magnetic semiconductor to spin gapless semiconductor phase. The magnetic anisotropy energy (MAE) of 2H-VS2 monolayer firstly increases and then decreases as the strain increases from −6% to 6 %, which mainly originates from the contribution of V dx2-y2 and dxy orbitals. In addition, 2H-VS2 monolayer retains a large valley-contrasting Berry curvature and non-zero anomalous Hall conductivity, indicating that the transverse velocity of carriers is opposite to the effect of in-plane longitudinal electric field. These results suggest that the 2H-VS2 monolayer can become good candidates for the spintronic and valleytronic applications.

Breaking symmetry of carbon nitride skeleton by embedding pyridine ring to promote photocatalytic performance

Breaking symmetry of polymeric carbon nitride (CN) skeleton by heteroatomic substitution can induce in-plane polar electric field, thus promoting the migration and separation of photoexcited charge carriers. Herein, a carbon-doped CN (pyCN) by embedding pyridine ring was prepared through a skeleton modification strategy. Owing to the n → π* electron transition and the formed in-plane polar electric field, the obtained pyCN displayed obvious extended optical absorption and photoinduced charge migration efficiency in comparison with pure CN. The optimized photocatalytic H2 evolution rate over pyCN reached 242.9 μmol·h−1, which is 6 times of that over pure CN with TEOA as a sacrifice. Additionally, the wavelength for H2 evolution over pyCN extends to 550 nm while that is 450 nm over pure CN, demonstrating an improved efficiency of sunlight utilization after skeleton modification. In addition, pyCN showed synergistic oxidation of bisphenol A (BPA) during hydrogen generation under visible light. The conversion efficiency of BPA reached 18.7 % in 24 h according to the external standard analysis. This work provides an efficient approach for enhancing optical absorption and inhibiting photoexcited charge recombination for higher photocatalytic performance and wilder applications.

The Resistive Cylindrical Chamber

A new generation of gaseous particle detectors named Resistive Cylindrical Chamber (RCC) (Cardarelli, 2021; Rocchi, 2022) [1,2] has been developed to overcome the limitations of Resistive Plate Chambers (Santonico and Cardarelli, 1981) [3] and broaden their application range. The principle behind this new technology consists in the transition from a planar to a cylindrical geometry while maintaining an almost planar electric field. The cylindrical structure of the electrodes allows to reach the following goals: increase the gas pressure to improve the intrinsic efficiency of the detector even for thin gas gaps or light gas mixtures; design the detector in order to produce an electric field gradient possibly useful to contain the development of the avalanche discharge. These features could lead to design detectors of simple mechanical realization with time resolution comparable with that of MRPCs maintaining a high efficiency of detection on a single thin gas-gap. The device pressurization could also allows to use new gases in view of the transition to eco-friendly gas.

Crystalline carbon nitride with in-plane built-in electric field accelerates carrier separation for excellent photocatalytic hydrogen evolution

Achieving a high carrier migration efficiency by constructing built-in electric field is one of the promising approaches for promoting photocatalytic activity. Herein, we have designed a donor-acceptor (D-A) crystalline carbon nitride (APMCN) with 4-amino-2,6-dihydroxypyrimidine (AP) as electron donor, in which the pyrimidine ring was well embedded in the heptazine ring via hydrogen-bonding effect during hydrothermal process. The APMCN shows superior charge-transfer due to giant built-in electric field (5.94 times higher than pristine carbon nitride), thereby exhibiting excellent photocatalytic H2 evolution rate (1350 µmol/h) with a high AQY (62.8%) at 400 nm. Mechanistic analysis based on detailed experimental investigation together with theoretical analysis reveals that the excellent photocatalytic activity is attributed to the promoted charge separation by the giant internal electric field originated from the D–A structure.