External 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.

Bloch oscillations in moiré flat band systems

Bloch oscillations of wave packets in moiré flat band systems are investigated. We find that the real space trajectories and group velocity of wave packet undergo Bloch oscillations in the moiré flat band. It is shown that the Bloch dynamics is affected significantly by the magnitude and orientation of an applied in-plane electric field. In the presence of both external perpendicular magnetic field and in-plane electric field, confinement and de-confinement of cyclotron orbits occur which strongly depend on the relative strength of the applied fields.

Geometry-modulated dipole polarizability of the two-dimensional Mott-Wannier excitons in gate-defined anisotropic quantum dot

A theoretical investigation on neutral excitons confined to a mono-layer (ML) semiconductor transition metal dichalcogenide (TMDC) materials under the influence of elliptically deformed gate induced confining potential is presented. It has been shown that the anisotropy of the confinement induces the anisotropy of linear response of the system on in-plane external electric field. The linear response is expressed in terms of principal moments of the static dipole polarizability tensor. In this manner the direction-dependent polarizability of the system can be fully controlled by tuning the parameters of gate-induced confining potential. The components of the polarizability tensor are determined using finite-field method based on the exact diagonalization of the electron-hole Hamiltonian including confining potential, Coulomb electron-hole interaction and an external electric field, within effective mass approximation, close to the K-points of the first Brillouin zone of a single-layer MX_2 material. The useful scaling relations for energies and dipole polarizabilities as functions of material parameters have been found. The influence of the anisotropy of the confining potential on correlated behavior of charge distribution inside the neutral system has also been demonstrated.

Half-metallic transition for ZGNRs adsorbing porphine molecules under an in-plane external electric field

The controlling of magnetic transition behavior by changing the coupling between molecule and nanoribbon has attracted a new topic. Here we study a zigzag graphene nanoribbon (ZGNR) absorbing the porphine molecules with different manners under an in-plane external electric field. These structureless systems may exhibit sensitive magnetic response. By employing the first-principles calculations, we find that the sensitivity of magnetic response for the asymmetry structure model is superior to the symmetry one. In general a series of the magnetic phase transitions for the asymmetry models can be induced by an in-plane external electric filed. Particularly, asymmetrical model of ZGNR absorbing porphine molecule at one side can transfer the half-metallic state from spin-up sub-band to spin-down one. Our analysis demonstrate that the phase transition is manipulated by the direction changing of the interior electric field. Interestingly, the models of structural asymmetry can exhibit spin-polarized electron band structures with the spin-polarization rate reaching up to 100%. The findings may provide an approach for the design of new devices to realize logic operation within spintronics.

Spin State Switching in Heptauthrene Nanostructure by Electric Field: Computational Study.

Recent experimental studies proved the presence of the triplet spin state in atomically precise heptauthrene nanostructure of nanographene type (composed of two interconnected triangles with zigzag edge). In the paper, we report the computational study predicting the possibility of controlling this spin state with an external in-plane electric field by causing the spin switching. We construct and discuss the ground state magnetic phase diagram involving (triplet) state, antiferromagnetic state and non-magnetic state and predict the switching possibility with the critical electric field of the order of 0.1 V/Å. We discuss the spin distribution across the nanostructure, finding its concentration along the longest zigzag edge. To model our system of interest, we use the mean-field Hubbard Hamiltonian, taking into account the in-plane external electric field as well as the in-plane magnetic field (in a form of the exchange field from the substrate). We also assess the effect of uniaxial strain on the magnetic phase diagram.

Anisotropic expansion of drifting spin helices in GaAs quantum wells

The drift of electron spin helices in an external in-plane electric field in GaAs quantum wells is studied by means of time-resolved magneto-optical Kerr microscopy. The evolution of the spin distribution measured for different excitation powers reveals that, for short delay times and higher excitation powers, the spin helix drift slows down while its envelope becomes anisotropic. The effect is understood as a local decrease of the electron gas mobility due to electron collisions with nonequilibrium holes within the excitation spot and is reproduced well in the kinetic theory framework. For larger delay times, when the electrons constituting the spin helix and nonequilibrium holes are separated by an electric field, the spin helix drift accelerates and the mobility reaches its unperturbed value again.

Dynamic behaviors of ferroelectric domain structures in Eu-doped PMN-PT transparent ceramics studied by piezoresponse force microscopy

In this paper, we study ferroelectric domain structures, optical properties, as well as domain dynamic performances induced by external in-plane electric field of Eu-doped lead magnesium niobate-lead titanate [Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT)] transparent ceramics mainly via piezoresponse force microscopy (PFM). It is found that Eu doping play a significant role on the domain structures and domain switching behavior in PMN-PT transparent ferroelectrics. For some compositions the switching of domain structures exhibit a normal evolutionary trend, while for the compositions with low Eu doping content, unusual anisotropic polarization switching behaviors were obtained in macrodomain regions under in-plane bias in the Eu-doped PMN-PT transparent ceramics.

Controllable inversion symmetry breaking in single layer graphene induced by sub-lattice contrasted charge polarization

It has been revealed that inversion symmetry breaking in graphene could induce non-trivial Berry curvature, which leads to topologically protected valleytronic applications. Here, we propose an active controllable way of realizing the inversion symmetry breaking of single layer graphene, which is induced by the contrasted AB sub-lattice charge polarization. Our theory model indicates the split of in-plane optical phonons (iTO/iLO) at Γ point is the direct result of inversion symmetry breaking in single layer graphene. Density functional theory calculation was used to quantify the sub-lattice charge polarization and the split of iTO/iLO phonons under the external in-plane electric field, which was further confirmed by Raman spectroscopy. We believe that the method of symmetry manipulation by external in-plane electric field advances fundamental comprehension for the inner mechanism of topological phase transition of graphene.

High on/off Current Ratio in Dual in-Plane Gate Spin Transistor Based on the VB Defect Hexagonal BN

With the size of devices based on Si technology approaching its physical limit, the electronics industry is looking for technologies and materials for next-generation electronic devices. In this work, the two-dimensional B vacancy (VB) hexagonal (h)-BNs have been systematically investigated and a dual in-plane gate spin transistor based on the VB defect h-BN is proposed. The results show that the VB defect h-BN has a dynamically stable half-metal property with a large spin band gap 4.66 eV. Interestingly, we also find the VB defect h-BN nanoribbon can transfer from a spin semiconductor to a half-metal due to the π electrons transferring of N located near VB defect under the in-plane external electric field. Moreover, the on/off current ratio of the spin transistor based on the VB defect h-BN can reach 4.18 × 104. Our results may provide a promising approach to build the dual in-plane-gate spin transistor and have potential applications in spin devices.

Electric field tuning of spin splitting in topological insulator quantum dots doped with a single magnetic ion

We investigate theoretically the electron spin states in a disk-shaped topological insulator quantum dot (TIQD) containing a single magnetic Mn2+ ion. We demonstrate that the energy spectra and the density distributions of the symmetry-protected edge states in a HgTe TIQD can be modulated effectively by a single magnetic impurity Mn2+. Additionally, when an in-plane external electric field is applied., it not only tunes the spin splittings of edge states via the s(p)-d exchange interaction between the electron (hole) and the magnetic Mn2+ ion respectively, but also gives rise to the bright-to-dark transitions in the optical transition spectrum. Such spin properties of TIQDs with single Mn2+ ion as illustrated in this work could offer a new platform for topological electro-optical devices.

Surface multiferroics in silicon enabled by hole-carrier doping

We predict a coexistence of magnetic and electric orders on clean Si(0 0 1) surfaces by first-principles calculations. Upon hole-carrier doping, the Si surfaces can be ferromagnetic, with polarized spins concentrated in an atom-thick space near the surface, due to an exchange splitting of localized s-like surface states on surface Si dimers. The surface magnetization can be controlled by reorienting the electric polarization of Si dimers, manifested as a transition from the magnetic antiferroelectric ground state to ferroelectric p(2 × 1) reconstruction that can be driven by an in-plane external electric field. The coupling between magnetic and electric orders can be further enhanced by strain silicon technology, rendering the Si surfaces as the first metal-free material displaying a multiferroic behavior.

Stark shift of excitons and trions in two-dimensional materials

The effect of an external in-plane electric field on neutral and charged exciton states in two-dimensional (2D) materials is theoretically investigated. These states are argued to be strongly bound, so that electron-hole dissociation is not observed up to high electric field intensities. Trions in the anisotropic case of monolayer phosphorene are demonstrated to be especially robust under electric fields, so that fields as high as 100 kV/cm yield no significant effect on the trion binding energy or probability density distribution. Polarizabilities of excitons are obtained from the parabolicity of numerically calculated Stark shifts. For trions, a fourth order Stark shift is observed, which enables the experimental verification of hyperpolarizability in 2D materials, as observed in the highly excited states of the Rydberg series of atoms and ions.

Zitterbewegung and bulk-edge Landau-Zener tunneling in topological insulators

We investigate the ballistic zitterbewegung dynamics and the Landau-Zener tunneling between edge and bulk states of wave packets in two-dimensional topological insulators. In bulk, we use the Ehrenfest theorem to show that an external in-plane electric field not only drifts the packet longitudinally, but also induces a transverse finite side-jump for both trivial and topological regimes. For finite ribbons of width $W$, we show that the Landau-Zener tunneling between bulk and edge states vanishes for large $W$ as their electric field-induced coupling decays with $W^{-3/2}$. This is demonstrated by expanding the time-dependent Schr\"odinger equation in terms of Houston states. Hence we cannot picture the quantum spin Hall states as arising from the zitterbewegung bulk trajectories `leaking' into the edge states, as proposed in Phys. Rev. B 87, 161115 (2013).

Electric field manipulation of multiple nonequivalent Dirac cones in the electronic structures of hexagonal CrB4 sheet**Project supported by the National Natural Sciences Foundation of China (Grant Nos. 11704294 and 11504281), the Natural Science Foundation of Hubei Province, China (Grant No. 2016CFB586), and the Fundamental Research Funds for the Central Universities, China (Grant Nos. 2017IVA078, 2018IVB017, 2017IB013, 2018IB009, and 2018IB011).

Two-dimensional materials with Dirac cones have significant applications in photoelectric technology. The origin and manipulation of multiple Dirac cones need to be better understood. By first-principle calculations, we study the influence of external fields on the electronic structure of the hexagonal CrB4 sheet with double nonequivalent Dirac cones. Our results show that the two cones are not sensitive to tensile strain and out-of-plane electric field, but present obviously different behaviors under the in-plane external electric field (along the B–B direction), i.e., one cone holds while the other vanishes with a gap opening. More interestingly, a new nonequivalent cone emerges under a proper in-plane electric field. We also discuss the origin of the cones in CrB4 sheet. Our study provides a new method on how to obtain Dirac cones by the external field manipulation, which may motivate potential applications in nanoelectronics.

Half metal phase in the zigzag phosphorene nanoribbon

Exploring half-metallic nanostructures is a crucial solution for developing high-performance spintronic devices. Black phosphorene is an emerging two-dimensional material possessing strong anisotropic band structure and high mobility. Based on the first principles calculations, we investigated the electronic and magnetic properties of zigzag phosphorene nanoribbons (ZPNRs) with three different functionalization groups (OH/CN, OH/NO2, NH2/NO2) at the edges. We find that the interplay between edge functionalization and edge oxidation can induce the half metal phase in the ZPNRs, and the half metal phase can be controlled by the external transverse in-plane electric field and the proportion of the functional groups and edge oxidation. The results may pave a new way to construst nanoscale spintronic devices based on black phosphorene nanoribbons.

Robustness of edge states in topological quantum dots against global electric field

The topological insulator has attracted increasing attention as a new state of quantum matter featured by the symmetry-protected edge states. Although the qualitative robustness of the edge states against local perturbations has been well established, it is not clear how these topological edge states respond quantitatively to a global perturbation. Here, we study the response of topological edge states in a HgTe quantum dot to an external in-plane electric field—a paradigmatic global perturbation in solid-state environments. We find that the stability of the topological edge state could be larger than that of the ground bulk state by several orders of magnitudes. This robustness may be verified by standard transport measurements in the Coulomb blockage regime. Our work may pave the way towards utilizing these topological edge states as stable memory devices for charge and/or spin information and stable emitter of single terahertz photons or entangled terahertz photon pairs for quantum communication.

Edge Chemistry Effects on the Structural, Electronic, and Electric Response Properties of Boron Nitride Quantum Dots

The effects of edge hydrogenation and hydroxylation on the relative stability and electronic properties of hexagonal boron nitride quantum dots (h-BNQDs) are investigated. Zigzag edge hydroxylation is found to result in considerable energetic stabilization of h-BNQDs as well as a reduction of their electronic gap with respect to their hydrogenated counterparts. The application of an external in-plane electric field leads to a monotonous decrease of the gap. When compared to their edge-hydrogenated counterparts, significantly lower field intensities are required to achieve full gap closure of the zigzag edge hydroxylated h-BNQDs. These results indicate that edge chemistry may provide a viable route for the design of stable and robust electronic devices based on nanoscale hexagonal boron-nitride systems.

Electric field control of the indirect magnetic coupling through a short graphene nanoribbon

In the paper we consider the system composed of two magnetic planes attached to zigzag terminations of the graphene nanostructure being an ultrashort fragment of the armchair nanoribbon. We investigate theoretically an indirect coupling between these magnetic planes mediated by charge carriers as a function of external in-plane electric field. The calculations are based on a tight-binding model supplemented with Hubbard term to account for coulombic interactions. For selected sizes of the graphene nanostructure, particularly high sensitivity of the coupling to the electric field is found. This leads to the possibility of control over coupling magnitude and continuous switching of its sign between antiferromagnetic and ferromagnetic one. Such a phenomenon is demonstrated in the numerical results and its origin is analysed. The robustness of this effect against armchair edge deformation and variation of exchange energy between magnetic planes and spins of charge carriers is discussed in detailed way.

Domain dynamics of La-doped PMN-PT transparent ceramics studied by piezoresponse force microscope

Ferroelectric domain structures and their dynamic behavior of La-doped lead magnesium niobate-lead titanate [Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT)] transparent ceramics under an external in-plane electric field are investigated by piezoresponse force microscopy (PFM). A remarkable effect of La doping on the domain configurations and domain kinetic behavior in La-doped PMN-PT transparent ferroelectric ceramics were found by PFM. For most compositions the switching behavior of domain structures shows a normal evolutionary trend, while for some certain composition anisotropic polarization switching dynamics were observed in macrodomain regions under in-plane bias in the La-doped PMN-PT transparent ceramics.

Effects of Edge Oxidation on the Structural, Electronic, and Magnetic Properties of Zigzag Boron Nitride Nanoribbons.

The effects of edge chemistry on the relative stability and electronic properties of zigzag boron nitride nanoribbons (ZBNNRs) are investigated. Among all functional groups considered, fully hydroxylated ZBNNRs are found to be the most energetically stable. When an in-plane external electric field is applied perpendicular to the axis of both hydrogenated and hydroxylated ZBNNRs, a spin-polarized half-metallic state is induced, whose character is different than that predicted for zigzag graphene nanoribbons. The onset field for achieving the half-metallic state is found to mainly depend on the width of the ribbon. Our results indicate that edge functionalization of ZBNNRs may open the way for the design of new nanoelectronic and nanospintronic devices.