Local Electron-phonon Interaction Research Articles

Suppression of local electron-phonon interaction in pi-conjugated oligomers and its monitoring using Raman scattering

For efficient operation of many organic electronic devices, organic semiconductors with high charge carrier mobility are required. However, in most of the known organic semiconductors, the charge mobility is low, since it is limited by the strong local electron-phonon interaction. In the present work, using the example of thiophene-phenylene co-oligomers, a class of organic semiconductors that combine a sufficiently high charge mobility with light emission and therefore promising for light-emitting transistors and electrically pumped lasers, the mechanism of suppression of the electron-phonon interaction by introducing electronegative atoms or an additional thiophene ring is studied. It was found that such structural changes alter the contribution of various vibrational modes to the local electron-phonon interaction, in particular, to the suppression of the contribution of the low-frequency torsion mode. In addition, it is shown that for the two modes that make the largest contribution to the local electron-phonon interaction in an unsubstituted oligomer, this change correlates with their intensity of Raman scattering, and this confirms the promise of studying the electron-phonon interaction using Raman spectroscopy. The results obtained improve the understanding of the relationship between the local electron-phonon interaction and the molecular structure of organic semiconductors, which is extremely important for the directed design of such materials with high charge mobility.

Suppression of Local Electron–Phonon Interaction in $${\pi}$$-Conjugated Oligomers and Its Monitoring Using Raman Scattering

Suppression of Local Electron–Phonon Interaction in $${\pi}$$-Conjugated Oligomers and Its Monitoring Using Raman Scattering

Full configuration interaction quantum Monte Carlo for coupled electron-boson systems and infinite spaces

We extend the scope of full configuration interaction quantum Monte Carlo (FCIQMC) to be applied to coupled fermion-boson hamiltonians, alleviating the a priori truncation in boson occupation which is necessary for many other wave function based approaches to be tractable. Detailing the required algorithmic changes for efficient excitation generation, we apply FCIQMC in two contrasting settings. The first is a sign-problem-free Hubbard--Holstein model of local electron-phonon interactions, where we show that with care to control for population bias via importance sampling and/or reweighting, the method can achieve unbiased energies extrapolated to the thermodynamic limit, without suffering additional computational overheads from relaxing boson occupation constraints. Secondly, we apply the method as a `solver' within a quantum embedding scheme which maps electronic systems to local electron-boson auxiliary models, with the bosons representing coupling to long-range plasmonic-like fluctuations. We are able to sample these general electron-boson hamiltonians with ease despite a formal sign problem, including a faithful reconstruction of converged reduced density matrices of the system.

Спектры поглощения пробного сигнала и резонансной флуоресценции для излучателей при их взаимодействии с локальным окружением в прозрачных средах

The current paper demonstrates theoretical analysis of two types of spectral curves for several configurations of system of two-level light emitters, considering the influence of local field and close environment inside a transparent medium. Probe field absorption spectra and resonant fluorescence spectra are calculated under excitation of a strong monochromatic cw laser. The sensitivity of absorption and emission optical spectroscopy method is compared for revealing the effects of the medium on individual emitters and their ensembles. Spectral curves were calculated for model emitters considering local field influence of a transparent dielectric medium and local electron-phonon interactions, which determined the response of the emitters to an external laser field and effective relaxation mechanisms. The calculation formalism is based on a semiclassical approach, while the relaxation processes associated with the phonon contribution are introduced phenomenologically with references to other studies.

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The current paper demonstrates theoretical analysis of two types of spectral curves for several configurations of system of two-level light emitters, considering the influence of local field and close environment inside a transparent medium. Probe field absorption spectra and resonant fluorescence spectra are calculated under excitation of a strong monochromatic cw laser. The sensitivity of absorption and emission optical spectroscopy method is compared for revealing the effects of the medium on individual emitters and their ensembles. Spectral curves were calculated for model emitters considering local field influence of a transparent dielectric medium and local electron-phonon interactions, which determined the response of the emitters to an external laser field and effective relaxation mechanisms. The calculation formalism is based on a semiclassical approach, while the relaxation processes associated with the phonon contribution are introduced phenomenologically with references to other studies.

Graphene and NTCDA adsorbed on Ag(111): Temperature-dependent binding distance and phonon coupling to the interface state

Flat lying physisorbates on surfaces can be expected to be lifted by heating. The details of this lattice expansion depend on the competition between two mechanisms. On the one hand, the interaction potential of each atom with the substrate is anharmonic and constitutes the reason for lifting. On the other hand, the internal bending stiffness of the planar adsorbate reduces its tendency to lift with temperature. Here we show that this competition can be investigated in a simplified molecular-dynamics simulation which considers only the vertical motion of the atoms of the adsorbate in respective model potentials. For two prototypical systems, i.e., graphene and 1,4,5,8-naphthalene-tetracarboxylic acid dianhydride (NTCDA) on Ag(111), we observe lifting by 0.04 \AA{} (graphene) and 0.06 \AA{} (NTCDA) between zero and room temperature. For the latter system, the impact of the temperature-dependent geometrical and vibrational properties on the occurring interface state (IS) is discussed. By introducing a local electron-phonon interaction model, we combine the indirect dependence of the IS on the average binding distance with direct coupling to the vibrational modes and obtain good agreement with the experimental results.

Toward probing of the local electron-phonon interaction in small-molecule organic semiconductors with Raman spectroscopy.

Electron-phonon interaction strongly affects and often limits charge transport in organic semiconductors (OSs). However, approaches to its experimental probing are still in their infancy. In this study, we probe the local electron-phonon interaction (quantified by the charge-transfer reorganization energy) in small-molecule OSs by means of Raman spectroscopy. Applying density functional theory calculations to four series of oligomeric OSs-polyenes, oligofurans, oligoacenes, and heteroacenes-we extend the previous evidence that the intense Raman vibrational modes considerably contribute to the reorganization energy in several molecules and molecular charge-transfer complexes, to a broader scope of OSs. The correlation between the contribution of the vibrational mode to the reorganization energy and its Raman intensity is especially prominent for the resonance conditions. The experimental Raman spectra obtained with various excitation wavelengths are in good agreement with the theoretical ones, indicating the reliability of our calculations. We also establish for the first time relations between the spectrally integrated Raman intensity, the reorganization energy, and the molecular polarizability for the resonance and off-resonance conditions. The results obtained are expected to facilitate the experimental studies of the electron-phonon interaction in OSs for an improved understanding of charge transport in these materials.

Influence of high-energy local orbitals and electron-phonon interactions on the band gaps and optical absorption spectra of hexagonal boron nitride

We report ab initio band diagram and optical absorption spectra of hexagonal boron nitride ($h$-BN), focusing on unravelling how the completeness of the basis set for $GW$ calculations and electron-phonon interactions (EPIs) impact on them. The completeness of the basis set, an issue which was seldom discussed in previous optical spectra calculations of $h$-BN, is found crucial in providing converged quasiparticle band gaps. In the comparison among three different codes, we demonstrate that by including high-energy local orbitals in the all-electron linearized augmented plane waves based $GW$ calculations, the quasiparticle direct and fundamental indirect band gaps are widened by $\ensuremath{\sim}0.2$ eV, giving values of 6.81 eV and 6.25 eV, respectively at the $G{W}_{0}$ level. EPIs, on the other hand, reduce them to 6.62 eV and 6.03 eV respectively at 0 K, and 6.60 eV and 5.98 eV respectively at 300 K. With clamped crystal structure, the first peak of the absorption spectrum is at 6.07 eV, originating from the direct exciton contributed by electron transitions around $K$ in the Brillouin zone. After including the EPIs-renormalized quasiparticles in the Bethe-Salpeter equation, the exciton-phonon coupling shifts the first peak to 5.83 eV at 300 K, lower than the experimental value of $\ensuremath{\sim}6.00$ eV. This accuracy is acceptable to an ab initio description of excited states with no fitting parameter.

Electron-electron versus electron-phonon interactions in lattice models: Screening effects described by a density functional theory approach

We present a density functional theory (DFT) for lattice models with local electron-electron (e-e) and electron-phonon (e-ph) interactions. Exchange-correlation potentials are derived via dynamical mean field theory for the infinite-dimensional Bethe lattice, and analytically for an isolated Hubbard-Holstein site. These potentials exhibit discontinuities as a function of the density, which depend on the relative strength of the e-e and e-ph interactions. By comparing to exact benchmarks, we show that the DFT formalism gives a good description of the linear conductance and real-time dynamics.

Charge carrier mobility in systems with local electron-phonon interaction

Charge carrier mobility in systems with local electron-phonon interaction

Effect of Short-Range Correlations on Spectral Properties of Doped Mott Insulators

In the framework of cluster perturbation theory for the 2D Hubbard and Hubbard-Holstein models at low hole doping we have studied the effect of local and short-range correlations in strongly correlated systems on the anomalous features in the electronic spectrum by investigating the fine structure of quasiparticle bands. Different anomalous features of spectrum are obtained as the result of intrinsic properties of strongly correlated electron and polaron bands in the presence of short-range correlations. Particularly, features similar to the electron-like Fermi-pockets of cuprates at hole doping $p\sim0.1$ are obtained without ad hoc introducing a charge density wave order parameter within the Hubbard model in a unified manner with other known peculiarities of the pseudogap phase like Fermi-arcs, pockets, waterfalls, and kink-like features. The Fermi surface is mainly formed by dispersive quasiparticle bands with large spectral weight, formed by coherent low-energy exications. Within the Hubbard-Holstein model at moderate phonon frequencies we show that modest values of local electron-phonon interaction are capable of introducing low-energy kink-like features and affecting the Fermi surface by hybridization of the fermionic quasiparticle bands with the Franck-Condon resonances.

Properties of a localized magnetic impurity in a superconducting host: The Anderson–Holstein–BCS model

A symmetric Anderson–Holstein model with a BCS interaction term is considered to investigate the effect of local electron–phonon interaction on a magnetic impurity in a superconductor. The Kikuchi–Morita Cluster variation (CV) method is used to calculate the local impurity magnetic moment and the binding energy between the impurity and the conduction electrons in the superconductor. The effect of electron–phonon interaction and the order parameter on the bound state is discussed.

Resolving voltage-time dilemma using an atomic-scale lever of subpicosecond electron-phonon interaction.

Nanoelectronic memory based on trapped charge need to be small and fast, but fundamentally it faces a voltage-time dilemma because the requirement of a high-energy barrier for data retention under zero/low electrical stimuli is incompatible with the demand of a low-energy barrier for fast switching under a modest programming voltage. One solution is to embed an atomic-level lever of localized electron-phonon interaction to autonomously reconfigure trap-site's barrier in accordance to the electron-occupancy of the site. Here we demonstrate an atomically levered resistance-switching memory built on locally flexible amorphous nanometallic thin films: charge detrapping can be triggered by a mechanical force, the fastest one being a plasmonic Lorentz force induced by a nearby electron or positron bunch passing in 10(-13) s. The observation provided the first real-time evidence of an electron-phonon interaction in action, which enables nanometallic memory to turn on at a subpicosecond speed yet retain long-term memory, thus suitable for universal memory and other nanoelectron applications.

Tuning the in-plane electron behavior in high-Tccuprate superconductors via apical atoms: A first-principles Wannier-states analysis

Using a recently developed first-principles Wannier-states approach that takes into account large on-site Coulomb repulsion, we derive the low-energy effective one-band Hamiltonians for several prototypical cuprate superconductors. The material dependence is found to originate primarily from the different energy of the apical atom ${p}_{z}$ state. Specifically, the general properties of the low-energy hole state, namely, the Zhang-Rice singlet, are significantly modified, via additional intrasublattice hoppings, nearest-neighbor ``super repulsion,'' and other microscopic many-body processes. Implications on modulations of local pairing gaps, charge distribution, hole-hopping range, electron-phonon interaction, and multilayer effects in cuprate superconductors are discussed.

Electron-phonon interactions in perovskites containing Fe and Cr studied by Raman scattering using oxygen-isotope and cation substitution

We use temperature-dependent inelastic light scattering to study the origin of the strong multiphonon scattering of a local oxygen breathing mode present in the mixed $B$-site orthorhombic (space group $Pnma$) perovskite ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$ but absent in isostructural ${\text{LaFeO}}_{3}$ and ${\text{LaCrO}}_{3}$. It is seen that the multiphonon scattering is critically sensitive to the presence of both Fe and Cr ions on the $B$ site. These results support our interpretation that the multiphonon scattering is activated by local electron-phonon interactions according to the Franck-Condon picture following an Fe-Cr charge transfer. Further, $^{18}\text{O}$ substitution is performed on the $x=0$, 0.04, and 0.5 compounds and clearly shows that all modes appearing above the first-order phonon-scattering region in these compounds originate from higher-order oxygen stretching vibrations. In particular this is the case for the strong second-order scattering dominating the scattering response in ${\text{LaFeO}}_{3}$. Accordingly we propose that these modes are generated by infrared-active longitudinal optical (IR LO) two-phonon and combination scattering activated by Fr\ohlich interaction. For $x=0.02$ and 0.04 the characteristic IR LO two-phonon and Franck-Condon multiphonon-scattering profiles mix. We also study the influence of isovalent cation substitution and Sr doping in $A{\text{Fe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$ ($A=\text{La}$, Nd, and Gd) and ${\text{La}}_{1\ensuremath{-}y}{\text{Sr}}_{y}{\text{Fe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ ($y=0$, 0.16, and 0.5) on the strong electron-phonon coupling present in ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$. The Franck-Condon effect in ${\text{LaFe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$, is not significantly affected by isovalent $A$-site substitution, despite the increasing orthorhombic distortion associated with decreasing $A$-site ionic radii. On the contrary, aliovalent Sr doping causes a rapid decrease in the Franck-Condon scattering. This shows that the strong electron-phonon coupling in these compounds is highly sensitive to local lattice and electronic decoherence but insensitive to global lattice distortions. Finally, a preliminary assignment of the ${A}_{g}$ and ${B}_{2g}$ phonon modes in $A{\text{Fe}}_{0.5}{\text{Cr}}_{0.5}{\text{O}}_{3}$ ($A=\text{La}$, Nd, and Gd) is made based on the present observations and published results for ${\text{LaCrO}}_{3}$ and $A{\text{MnO}}_{3}$. The modes associated with oxygen octahedral tilt and bending vibrations are heavily influenced by the magnitude of the orthorhombic distortion.

Interplay between local electron correlation and Janh-Teller electron-phonon interaction

The infinite-$U$ Anderson model coupled to a Jahn-Teller phonon is studied using the slave boson method on the basis of the large degeneracy expansion (1/N) scheme. The model Hamiltonian acts on the orbital degrees of freedom. The main focus is on the interplay between strong local electron correlation and weak Jahn-Teller electron-phonon interaction. The Kondo temperature is found to decrease by Jahn-Teller interaction. The influence of the Jahn-Teller interaction on dynamical correlation functions is very significant in sharp contrast with the case of the Holstein-type phonon which couples to charge degrees of freedom.

Effects of electron-phonon interaction on nonequilibrium transport through a single-molecule transistor

On the basis of the nonequilibrium Green's function and nonperturbative canonical transformation for the local electron-phonon interaction (EPI), the quantum transport through a single-molecule transistor (SMT) has been investigated with particular attention paid to the joint effect of the EPI and SMT-lead coupling on the spectral function and conductance. In addition to the usual EPI-induced renormalized effects (such as the redshift, sharpening, and phonon sidebands of the SMT level), owing to improved disentangling the electron-phonon system it has been found that the profile of the spectral function of the SMT electron is sensitive to lead chemical potentials, thus can readily be manipulated by tuning the bias as well as the SMT-gate voltage. As a consequence, the broken particle-hole symmetry in this system can be clearly recognized through the phonon sidebands in the spectral function. These EPI effects also manifest themselves in the nonequilibrium transport properties of the SMT, particularly at low temperature.

Intersubband transition rates of localized electron–phonon interaction in structural defect superlattices

We investigate the intersubband-transition rates of in-gaps electrons assisted by localized-interface-optical-phonons (LIOPs) in AlAs/GaAs SL with a defect layer for different widths and different materials of defect layer and constituent layers. The rates are calculated under Fermi's golden rule and by using transfer-matrix method and on the basis of the dielectric continuum theory. The dependence of the maximum transition rate on the energy spacing between the initial and final states of in-gaps electrons, involving different LIOP modes is revealed. The results show that the intersubband-transition rates between the in-gaps states of electrons significantly depend not only on the structural configurations of the defect layer but also on physical and geometrical parameters of the systems as well as nature of the LIOP modes involved in the transition processes.

Anisotropy effects in phonon-assisted charge-carrier transport in organic molecular crystals

We present a theoretical description of polaron dc conductivities in organic molecular crystals. Our approach is based on a rigorous evaluation of the Kubo formula for electrical conductivity within a mixed HolsteinPeierls model. It generalizes the result of Holstein’s local-coupling theory by treating both local and nonlocal electron-phonon interactions nonperturbatively. The general theory is supplemented by an application to a simplified model crystal in order to emphasize the essential physics. Accompanied by an illustrative numerical example, special emphasis is put on the emergence of anisotropy effects in the temperature dependence of the conductivity tensor. These anisotropy effects are shown to originate from phonon-assisted currents due to the nonlocal electron-lattice interaction which demonstrates the importance to go beyond local-coupling theories in order to describe the experimental observations.

On the possibility of the direct study of local electron-phonon interaction in semiconductors

Local electron-phonon interaction in deep-level states of defects in semiconductors was studied by induced absorption spectroscopy. Using ZnS:Cu single crystals as an example, it was shown that the laser modulation of two-step impurity absorption is an efficient technique for direct investigations of phonon relaxation effects in deep-level states. It was shown that the localized states in ZnS are prone to extremely strong electron-phonon coupling.