Coupling Of Electronic States Research Articles

Coupled Electronic and Magnonic Topological States in Two-Dimensional Ferromagnets.

Magnetic materials offer a fertile playground for fundamental physics discovery, with not only electronic but also magnonic topological states intensively explored. However, one natural material with both electronic and magnonic nontrivial topologies is still unknown. Here, we demonstrate the coexistence of first-order topological magnon insulators (TMIs) and electronic second-order topological insulators (SOTIs) in 2D honeycomb ferromagnets, giving rise to the nontrivial corner states being connected by the charge-free magnonic edge states. We show that, with symmetry, the phase factor ± ϕ caused by the next nearest-neighbor Dzyaloshinskii-Moriya interaction breaks the pseudo-spin time-reversal symmetry , which leads to the split of magnon bands, i.e., the emergence of TMIs with a nonzero Chern number of , in experimentally feasible candidates of MoI3, CrSiTe3, and CrGeTe3 monolayers. Moreover, protected by the symmetry, the electronic SOTIs characterized by nontrivial corner states are obtained, bridging the topological aspect of fermions and bosons with a high possibility of innovative applications in spintronics devices.

Electronic state coupling induced by topological Co patterns at Pd metallene boosts CO2 electroreduction to HCOOH

The electrochemical conversion of CO2 to HCOOH has been extensively regarded as a promising strategy to achieve carbon utilization. However, the direct synthesis of value-added HCOOH still encounters the difficulty of low energy conversion and selectivity. The performance of catalysts is determined by the electronic occupation at their half-filled 3d orbitals and their structural symmetry. Herein, we intentionally decorate some Co atoms onto Pd metallene to form a series of topological patterns to reveal their reaction kinetics, in an effort to achieve a higher catalytic activity. This work provides a new insight into boosting CO2 electroreduction by designing topological patterns.

Nonadiabatic Field on Quantum Phase Space: A Century after Ehrenfest.

Nonadiabatic transition dynamics lies at the core of many electron/hole transfer, photoactivated, and vacuum field-coupled processes. About a century after Ehrenfest proposed "Phasenraum" and the Ehrenfest theorem, we report a conceptually novel trajectory-based nonadiabatic dynamics approach, nonadiabatic field (NAF), based on a generalized exact coordinate-momentum phase space formulation of quantum mechanics. It does not employ the conventional Born-Oppenheimer or Ehrenfest trajectory in the nonadiabatic coupling region. Instead, in NAF the equations of motion of the independent trajectory involve a nonadiabatic nuclear force term in addition to an adiabatic nuclear force term of a single electronic state. A few benchmark tests for gas phase and condensed phase systems indicate that NAF offers a practical tool to capture the correct correlation of electronic and nuclear dynamics for processes where the states remain coupled all the time as well as for the asymptotic region where the coupling of electronic states vanishes.

Spin-related electronic state coupling between surface strain and magnetic dopants in free-standing IrO2 monolayers

The acidic oxygen evolution reaction (OER) strongly depends on the spin-related electronic configuration of metal sites at catalyst surfaces, which can directly affect the structural stability and catalytic activity. Recently, layered two-dimensional IrO2 nanostructures with typical metallic characteristic have been successfully realized. However, the electronic state coupling between magnetic dopants and structural strain still need be understood as they can effectively enhance the catalytic activity. Herein, a series of magnetic dopants are intentionally implanted into the IrO2 monolayers with different structural deformations in order to trigger the spin-related electronic reconfigurations for regulating the orbital interactions and charge transfer with reactants. Accordingly, the acidic OER performance is obviously enhanced because doped magnetic dopants with half-filled 3d orbitals can donate some valence electrons to the intermediates more easily, leading to a lower activation energy. This work provides a new insight into designing catalysts for acidic OER by particular electronic state coupling.

Driving force and nonequilibrium vibronic dynamics in charge separation of strongly bound electron–hole pairs

Electron-hole pairs in organic photovoltaics efficiently dissociate although their Coulomb-binding energy exceeds thermal energy at room temperature. The vibronic coupling of electronic states to structured vibrational environments containing multiple underdamped modes is thought to assist charge separation. However, non-perturbative simulations of such large, spatially extended, electronic-vibrational (vibronic) systems remain an unmet challenge which current methods bypass by considering effective one-dimensional Coulomb potentials or unstructured environments where the effect of underdamped modes is ignored. Here we address this challenge with a non-perturbative simulation tool and investigate the charge separation dynamics in one, two and three-dimensional donor-acceptor networks to identify under what conditions underdamped vibrational motion induces efficient long-range charge separation. The resulting comprehensive picture of ultrafast charge separation differentiates electronic or vibronic couplings mechanisms for a wide range of driving forces and identifies the role of entropic effects in extended systems. This provides a toolbox for the design of efficient charge separation pathways in artificial nanostructures.

Detection of HCl molecules by resonantly enhanced sum-frequency mixing of mid- and near-infrared laser pulses

We perform experimental studies of resonantly enhanced sum-frequency mixing (SFM), driven by tunable, spectrally narrowband mid-infrared and fixed-frequency nanosecond laser pulses, aiming at applications in molecular gas detection. The mid-infrared pulses are tuned in the vicinity of two-photon rovibrational transitions in the electronic ground state to provide strong resonance enhancements of the nonlinear susceptibility, while a probe laser at shorter wavelength uses an off-resonant single-photon coupling to excited electronic states. This SFM approach benefits from the advantageous combination of typically small detunings among the mid-infrared, vibrational transitions and the typically large transition dipole moment for couplings of electronic states. Moreover, compared to resonantly enhanced third harmonic generation (THG), a signal wave at much shorter wavelength permits simple and efficient detection. We demonstrate resonantly enhanced SFM via rovibrational states in gaseous hydrogen chloride molecules and compare its features to THG. The SFM spectra offer a large signal-to-noise ratio of 4 orders of magnitude and a detection limit down to a pressure of 0.1 mbar, corresponding to a particle density of $0.35\ifmmode\times\else\texttimes\fi{}{10}^{15}$ per ${\mathrm{cm}}^{3}$.

Probing the Strong Nonadiabatic Interactions in the Triazolyl Radical Using Photodetachment Spectroscopy and Resonant Photoelectron Imaging of Cryogenically Cooled Anions.

Although the adiabatic potential energy surfaces defined by the Born-Oppenheimer approximation are the cornerstones for understanding the electronic structure and spectroscopy of molecular systems, nonadiabatic effects due to the coupling of electronic states by nuclear motions are common in complex molecular systems. The nonadiabatic effects were so strong in the 1,2,3-triazolyl radical (C2H2N3) that the photoelectron spectrum of the triazolide anion was rendered unassignable and could only be understood using nonadiabatic calculations, involving the four low-lying electronic states of triazolyl. Using photodetachment spectroscopy and resonant photoelectron imaging of cryogenically cooled anions, we are able to completely unravel the complex vibronic levels of the triazolyl radical. Photodetachment spectroscopy reveals a dipole-bound state for the triazolide anion at 172 cm-1 below the detachment threshold and 32 vibrational Feshbach resonances. Resonant photoelectron imaging is conducted by tuning the detachment laser to each of the Feshbach resonances. Combining the photodetachment spectrum and the resonant photoelectron spectra, we are able to assign all 28 vibronic peaks resolved for the triazolyl radical. Fundamental frequencies for 12 vibrational modes of the ground state of the triazolyl radical are measured experimentally. The current study provides unprecedented experimental vibronic information, which will be valuable to verify theoretical models to treat nonadiabatic effects involving multiple electronic states.

Electronic State Coupling Between Structural Deformation and Surficial Defects in Co‐Layered LaCoSi for Hydrogen Evolution Reaction

The exploitation of the earth‐abundant catalysts with high reactive activity for hydrogen evolution reaction (HER) remains a grand challenge, which is strongly related with their superficial electronic structure. Herein, the Co‐layered LaCoSi material is proposed to disclose the electronic state coupling between structural strain and defect distribution, where the bonding interaction between reactants and catalysts can be affected by the electronic reconfiguration at reactive sites. Comprehensive first‐principle calculations disclose that the catalytic activity of Co site can be obviously enhanced by tensile strain, and the antibonding interaction between Co and adsorbed hydrogen is also enforced. However, the introduced Si defect can make the neighboring Si atoms gain more 3d electrons from Co site, leading to a compressive‐strain‐response HER performance. The analysis about electronic structure demonstrates that the coupled electronic states play a critical role in enhancing HER performance. This work provides a new insight into designing new‐type catalysts by considering the synergistic effect of surface strain and defect engineering.

Dopant-driven Interlayer Electronic State Coupling in Layered Bi2SeO2 Surface for Accelerating CO2 Reduction to HCOOH

Dopant-driven Interlayer Electronic State Coupling in Layered Bi2SeO2 Surface for Accelerating CO2 Reduction to HCOOH

Strain-Driven Bimetallic-Interface Orbital Hybridization for Hydrogen Evolution Reaction.

Enforcing the bimetallic-interface orbital hybridization in single-atom catalysts (SACs) plays a critical role in determining their catalytic activity. However, the electronic state coupling among interacting sites can be affected by surficial strain, but the relative physical mechanism still needs to be understood. Herein, we propose a series of bimetallic-hybridized SACs with structural strain to disclose their interfacial charge transfer and orbital interaction, in which asymmetric superexchange interaction between adjacent Fe and Ni sites can enforce their electronic state coupling by a structural deformation. As a result, the spin-resolved electronic structure, d-band center, and Gibbs free energy can be changed by external strain, leading to a higher reactive activity. Our findings provide a new insight into understanding the contribution of surface strain to enhancing their catalytic activity.

Moiré-induced band-gap opening in one-dimensional superlattices of carbon nanotubes on hexagonal boron nitride

Recently, two-dimensional (2D) moir\'e superlattices have been extensively studied, and many interesting physical phenomena have been observed. However, their one-dimensional (1D) counterpart---1D moir\'e superlattices---have been rarely explored yet. Here, we performed theoretical calculations of low-energy bands of 1D moir\'e superlattices of single-walled carbon nanotubes (CNTs) on hexagonal boron nitride (hBN) using a newly developed low-energy effective continuum model for 1D superlattices. We observed moir\'e-induced opening of small band gaps ranging from a few meV to a few tens of meV, which depends sensitively on the CNT chirality. The observed band-gap opening can be well understood by considering the coupling of electronic states between CNTs and the hBN using the effective continuum model. The results have been confirmed by the density functional theory calculations.

Investigation of the global second-derivative non-adiabatic contributions: Rovibrational energies of [formula omitted], [formula omitted], and prospects for [formula omitted] (Part II)

The influence of the non-adiabaticity on the rovibrational bound states of H2+, H2, and H3+ is investigated. For this purpose a full configuration interaction (FCI) treatment using Gaussian basis functions is applied to calculate the energies of the electronic states as well as all couplings between them caused by the nuclear motion. These ‘derivative couplings’ were evaluated up to second order by means of a perturbation treatment. While this has been possible already for H2+ and H2, nothing equivalent has been available for H3+. The present work is an extension of the investigation of earlier non-adiabatic investigations based on first derivative couplings of electronic states that led to the concept of geometry-dependent effective nuclear masses needed for only one single potential energy surface. Our new implementation allowed to include for the first time also for H3+ all non-adiabatic effects up to the order of O(μ-2), μ being the reduced nuclear mass. These new inclusions of all nonadiabatic effects could reduce also for H3+ the deviations to experimental data for most rovibrational levels to less than 0.1 cm−1 without any empirically adjustable parameters. For H3+, the accuracy is slightly improved when also relativistic and QED effects are taken into account. For two questionable assignments of observed transitions in H3+ we propose a new labeling.

Ultrafast Photon-Induced Tunneling Microscopy.

Unification of the techniques of ultrafast science and scanning tunneling microscopy (STM) has the potential of tracking electronic motion in molecules simultaneously in real space and real time. Laser pulses can couple to an STM junction either in the weak-field or in the strong-field interaction regime. The strong-field regime entails significant modification (dressing) of the tunneling barrier of the STM junction, whereas the weak-field or the photon-driven regime entails perturbative interaction. Here, we describe how photons carried in an ultrashort pulse interact with an STM junction, defining the basic fundamental framework of ultrafast photon-induced tunneling microscopy. Selective dipole coupling of electronic states by photons is shown to be controllable by adjusting the DC bias at the STM junction. An ultrafast tunneling microscopy involving photons is established. Consolidation of the technique calls for innovative approaches to detect photon-induced tunneling currents at the STM junction. We introduce and characterize here three techniques involving dispersion, polarization, and frequency modulation of the laser pulses to lock-in detect the laser-induced tunneling current. We show that photon-induced tunneling currents can simultaneously achieve angstrom scale spatial resolution and sub-femtosecond temporal resolution. Ultrafast photon-induced tunneling microscopy will be able to directly probe electron dynamics in complex molecular systems, without the need of reconstruction techniques.

Multifarious Impact of Rhodanine Acceptor Group on the Optical Properties of Some Semiconductor Probes

AbstractSteady state optical properties such as photo‐absorption and photo‐luminescence characteristics of organic molecules in its bulk states viz, film or crystalline forms are highly sensitive to subtle alteration of chemical substitution, molecular geometry in bulk and the electronic state coupling between a pair of molecules. Here we highlight a distinct interference of rhodanine substitution on such optical properties of a group of similar molecules (2TR, 23TR). For comparison an anthracene conjugate model molecule was also chosen (AR). In the former case, despite of having similar molecular structure except the presence of a methyl group at the C3 position (which ensures the identical electronic states in both the case) and similar molecular packing in single crystals, 2TR and 23TR exhibited diagonally opposite solid‐state fluorescence behavior. 2TR was non‐fluorescent whereas, 23TR was highly emissive in its crystalline solid state. A minute substitutional change in chemical structure was found to alter the solid‐state fluorescence behavior immensely that was attributed to the non‐radiative loss in the former case due to the solid‐state C−C bond flip which was further proven by temperature dependent fluorescence experiments. At a sample temperature of <120 K, 20‐fold enhancement in the fluorescence intensity was observed.

Structural and electronic properties of double-walled [formula omitted]-graphyne nanotubes

The structural and electronic properties of double-walled α-graphyne nanotubes (α-DWGNTs) are investigated using a combination of computation and theoretical methods, including density functional theory. DWGNTs differ from conventional double-walled carbon nanotubes by the presence of carbon atoms featuring both sp and sp2 hybridizations, endowing DWGNTs with a set of desirable properties that could lead to a number of applications. Classical molecular dynamics (MD) and Born–Oppenheimer molecular dynamics (BOMD) simulations were performed to reveal the structural stability of these nanotubes at room temperature. Their dynamical stability was confirmed using tight-binding based phonons calculations. The high flexibility of the graphyne sheet and van der Waals interactions between the shells lead to strong deformations of the outer tube. Consequently, the electronic structure of the component single-walled tubes undergoes significant changes once cast into the double-walled geometries due to the coupling of electronic states from the inner wall with the outer tube. Such strong inter-wall hybridization can not only modulate the electronic band gap of semiconducting tubes, in comparison to their isolated single-walled counterparts, but can also affect the electronic properties of the double-walled systems. These behaviors open up possibilities of using such nanostructures in the design of electronic device applications.

Permanent Dipole Moments Enhance Electronic Coupling and Singlet Fission in Pentacene.

Singlet fission (SF), the photophysical process in which one singlet exciton is transformed into two triplets, depends inter alia on the coupling of electronic states. Here, we use fluorination and the resulting changes in partial charge distribution across the chromophore backbone as a particularly powerful tool to control this parameter in pentacene. We find that the introduction of a permanent dipole moment leads to an enhanced coupling of Frenkel exciton and charge transfer states and to an increased SF rate which we probed using ultrafast transient absorption spectroscopy. These findings are contrasted with H-aggregate formation and a significantly reduced triplet-pair state lifetime in a fluorinated pentacene for which the different partial charge distribution leads to a negligible dipole moment.

Local vibrational modes of Si vacancy spin qubits in SiC

Silicon carbide is a very promising platform for quantum applications because of extraordinary spin and optical properties of point defects in this technologically-friendly material. These properties are strongly influenced by crystal vibrations, but the exact relationship between them and the behavior of spin qubits is not fully investigated. We uncover the local vibrational modes of the Si vacancy spin qubits in as-grown 4H-SiC. We apply the resonant microwave field to isolate the contribution from one particular type of defects, the so-called V2 center, and observe the zero-phonon line together with seven equally-separated phonon replicas. Furthermore, we present first-principles calculations of the photoluminescence lineshape, which are in excellent agreement with our experimental data. To boost up the calculation accuracy and decrease the computation time, we extract the force constants using machine learning algorithms. This allows us to identify dominant modes in the lattice vibrations coupled to an excited electron during optical emission in the Si vacancy. The resonance phonon energy of 36 meV and the Debye-Waller factor of about 6% are obtained. We establish experimentally that the activation energy of the optically-induced spin polarization is given by the local vibrational energy. Our findings give insight into the coupling of electronic states to vibrational modes in SiC spin qubits, which is essential to predict their spin, optical, mechanical and thermal properties. The approach described can be applied to a large variety of spin defects with spectrally overlapped contributions in SiC as well as in other 3D and 2D materials.

Control of the orbital character of indirect excitons in MoS2/WS2 heterobilayers

Valley selective hybridization and residual coupling of electronic states in commensurate van der Waals heterobilayers enable the control of the orbital character of interlayer excitons. We demonstrate electric field control of layer index, orbital character, lifetime and emission energy of indirect excitons in MoS2/WS2 heterobilayers embedded in an vdW field effect structure. Different excitonic dipoles normal to the layers are found to stem from bound electrons and holes located in different valleys of MoS2/WS2 with a valley selective degree of hybridization. For the energetically lowest emission lines, coupling of electronic states causes a field-dependent level anticrossing that goes along with a change of the IX lifetime from 400 ns to 100 ns. In the hybridized regime the exiton is delocalized between the two constituent layers, whereas for large positive or negative electric fields, the layer index of the bound hole is field-dependent. Our results demonstrate the design of novel van der Waals solids with the possibility to in-situ control their physical properties via external stimuli such as electric fields.

Wearable strain sensor based on highly conductive carbon nanotube/polyurethane composite fibers

Highly conductive and stretchable fibers have recently attracted increasing attention owing to their potential for application in flexible wearable electronics. Carboxylated carbon nanotubes (c-CNTs) are coated onto flexible fibers as a convenient way of fabricating wearable strain sensors. However, the conductivity of a c-CNT is reduced due to the destruction of the graphitized structure of the CNT during carboxylation. It still remains a significant challenge to endow c-CNT composite fibers with high conductivity. In this study, highly conductive fibers were prepared by coating metal ion-linked c-CNTs onto polyurethane (PU) fibers in order to improve the electron transport rate between the c-CNTs. The metal-coordination junctions formed by Fe2+ ions and carboxyl significantly enhanced the conductivity of the PU/CNT@Fe2+ fibers (up to 72 S m−1). The high conductivity is the result of coordination junctions with strong electronic state coupling facilitating electron transport, which was proved by density functional theory calculations. The resulting coordination effect enhanced the interaction between the c-CNTs, which made the conductive network more flexible. The strain sensor based on PU/CNT@Fe2+ fibers exhibited high sensitivity (gauge factor = 36 at 50% strain), a large strain range, inconspicuous drift and durability. The fibrous strain sensor was successfully used to monitor joint movement and facial expression.

Coupled electronic states in CdTe quantum dot assemblies fabricated by utilizing chemical bonding between ligands.

Semiconductor quantum dot superlattices (QDSLs) have attracted much attention as key materials for realizing new optoelectronic devices such as solar cells with high conversion efficiency and thermoelectric elements with high electrical conductivity. To improve the charge transport properties of QDSL-based optoelectronic devices, it is important that the QD structures form minibands, which are the coupled electronic states between QDs. A shorter inter-QD distance and a periodic arrangement of QDs are the essential conditions for the formation of minibands. In this study, we use CdTe QDs capped with short ligands of N-acetyl-l cysteine (NAC) to fabricate three-dimensional QD assemblies by utilizing chemical bonding between NACs. Absorption spectra clearly display the quantum resonance phenomenon originating from the coupling of the wave functions between the adjacent QDs in CdTe QD assemblies. Furthermore, we demonstrate the formation of minibands in CdTe QD assemblies by examining both, the excitation energy dependence of photoluminescence (PL) spectra and the detection energy dependence of PL excitation spectra. The fabrication method of QD assemblies utilizing chemical bonding between NACs can be applied to all QDs capped with NAC as a ligand.