Role Of Electron-phonon Interaction Research Articles

Excitonic ordering in strongly correlated spin crossover systems: Induced magnetism and excitonic excitation spectrum

The effects associated with interatomic hoppings of excitons and the excitonic Bose condensate formation in strongly correlated spin crossover systems are considered in the framework of the effective Hamiltonian for the two-band Kanamori model. The appearance of antiferromagnetic ordering due to the exciton order is found even in the absence of interatomic exchange interaction. The spectrum of excitonic excitations is calculated at various points of the temperature vs crystal field phase diagram. Outside the region of exciton ordering, the spectrum has a gap, which vanishes at the boundary of the exciton condensate phase. The nonuniform spectral weight distribution over the Brillouin zone is found. The role of electron-phonon interaction is discussed as well.

Theory of phonon instabilities in Weyl semimetals at high magnetic fields

The behavior of three-dimensional (3D) semimetals under strong magnetic fields is a topic of recurring interest in condensed matter physics. Recently, the advent of Weyl and Dirac semimetals has brought about an interesting platform for potentially uncovering phases of matter that combine nontrivial band topology and interactions. While electronic instabilities of such semimetals at strong magnetic fields have been explored theoretically and experimentally, the role of electron-phonon interactions therein has been largely neglected. In this paper, we study the interplay of electron-electron and electron-phonon interactions in a minimal two-node model of Weyl semimetal. Using a Kadanoff-Wilson renormalization group approach, we analyze lattice (Peierls) instabilities emerging from chiral and nonchiral Landau levels as a function of the magnetic field. We consider both the adiabatic and the nonadiabatic phonon regimes, in the presence or in the absence of improper symmetries that relate Weyl nodes of opposite chirality. We find that (i) the Cooper channel, often neglected in recent studies, can prevent purely electronic instabilities while enabling lattice instabilities that are not Bardeen-Cooper-Schrieffer-like; (ii) breaking the improper symmetry that relates the two Weyl nodes suppresses the Cooper channel, thereby increasing the critical temperature for the lattice instability; (iii) in the adiabatic phonon regime, lattice instabilities can preempt purely electronic instabilities; (iv) pseudoscalar phonons are more prone to undergo a Peierls instability than scalar phonons. In short, our study emphasizes the importance of taking electron-phonon interactions into account for a complete understanding of interacting phases of matter in Dirac and Weyl semimetals at high magnetic fields.

Chemically driven energetic molecular ferroelectrics

Chemically driven thermal wave triggers high energy release rate in covalently-bonded molecular energetic materials. Molecular ferroelectrics bridge thermal wave and electrical energy by pyroelectric associated with heating frequency, thermal mass and heat transfer. Herein we design energetic molecular ferroelectrics consisting of imidazolium cations (energetic ion) and perchlorate anions (oxidizer), and describe its thermal wave energy conversion with a specific power of 1.8 kW kg−1. Such a molecular ferroelectric crystal shows an estimated detonation velocity of 7.20 ± 0.27 km s−1 comparable to trinitrotoluene and hexanitrostilbene. A polarization-dependent heat transfer and specific power suggests the role of electron-phonon interaction in tuning energy density of energetic molecular ferroelectrics. These findings represent a class of molecular ferroelectric energetic compounds for emerging energy applications demanding high power density.

Cascade of replica bands in flat-band systems: Predictions for twisted bilayer graphene

We investigate the effect of electron-phonon interactions (EPI) in systems exhibiting one or more flat electron bands close to the Fermi level and a comparatively large phonon energy scale. After solving the self-consistent full-bandwidth Eliashberg equations, we compute Angular Resolved Photo Emission Spectroscopy and Scanning Tunneling Spectroscopy/Microscopy spectra. We obtain a sequence of quasiparticle replica bands in both the normal and superconducting states that originate from frequency dependent features of the electron mass renormalization function. We show that these replica bands can be used to extract the relevant phonon energy scale from experiments. Focusing in particular on twisted bilayer graphene, we predict replica-band formation which, when observed, will shed light on the role of EPI in this archetypal flat-band system.

Anomalous isotope effect in iron-based superconductors

The role of electron-phonon interactions in iron-based superconductor is currently under debate with conflicting experimental reports on the isotope effect. To address this important issue, we employ the renormalization-group method to investigate the competition between electron-electron and electron-phonon interactions in these materials. The renormalization-group analysis shows that the ground state is a phonon-dressed unconventional superconductor: the dominant electronic interactions account for pairing mechanism while electron-phonon interactions are subdominant. Because of the phonon dressing, the isotope effect of the critical temperature can be normal or reversed, depending on whether the retarded intra- or inter-band interactions are altered upon isotope substitutions. The connection between the anomalous isotope effect and the unconventional pairing symmetry is discussed at the end.

The role of electron-phonon interactions on the coherence lifetime of monolayer transition metal dichalcogenides

We investigate the excitonic dephasing of transition metal dichalcogenides, namely MoS2, MoSe2 and WSe2 atomic monolayer thick and bulk crystals, in order to understand the factors that determine the optical coherence in these materials. Coherent nonlinear optical spectroscopy, temperature dependent absorption combined with theoretical calculations of the phonon spectra, reveal the important role electron-phonon interactions plat in dephasing process. The temperature dependence of the electronic band gap and the excitonic linewidth combined with ‘ab initio’ calculations of the phonon energies and the phonon density of state reveal strong interaction with the E′ and E″ phonon modes.

The role of electron-phonon interactions on the coherence lifetime of monolayer transition metal dichalcogenides

We investigate the excitonic dephasing of transition metal dichalcogenides, namely MoS2, MoSe2 and WSe2 atomic monolayer thick and bulk crystals, in order to understand the factors that determine the optical coherence in these materials. Coherent nonlinear optical spectroscopy, temperature dependent absorption combined with theoretical calculations of the phonon spectra, reveal the important role electron-phonon interactions plat in dephasing process. The temperature dependence of the electronic band gap and the excitonic linewidth combined with ‘ab initio’ calculations of the phonon energies and the phonon density of state reveal strong interaction with the E’ and E” phonon modes.

Phonon Properties of Americium Sulphide

We have reported the phonon properties of AmS by using breathing shell models (BSM) which includes breathing motion of electrons of the Am atoms due to f-d hybridization. The phonon dispersion curves, density of states and specific heat calculated from present model. The calculated phonon dispersion curves of AmS are presented follow the same trend as observed in uranium sulphide. We have discussed the significance of this approach in predicting the phonon dispersion curves of this compound and examine the role of electron-phonon interaction.

Role of electron-phonon interaction in a magnetically driven mechanism for superconductivity

We use the renormalization group method to examine the effect of phonon mediated interaction on d-wave superconductivity, as driven by spin fluctuations in a quasi-one-dimensional electron system. The influence of a tight-binding electron-phonon interaction on the spin-density-wave and d-wave superconducting instability lines is calculated for arbitrary temperature, phonon frequency and antinesting of the Fermi surface.The domain of electron-phonon coupling strength where spin-density-wave order becomes unstable against the formation of a bond-order-wave or Peierls state is determined at weak antinesting. We show the existence of a positive isotope effect for spin-density-wave and d-wave superconducting critical temperatures which scales with the antinesting distance from quantum critical point where the two instabilities merge. We single out a low phonon frequency zone where the bond-oder-wave ordering gives rise to triplet f-wave superconductivity under nesting alteration, with both orderings displaying a negative isotope effect. We also study the electron-phonon strengthening of spin fluctuations at the origin of extended quantum criticality in the metallic phase above superconductivity. The impact of our results on quasi-one-dimensional organic conductors like the Bechgaard salts where a Peierls distortion is absent and superconductivity emerges near a spin-density-wave state under pressure is emphasized.

Long-wavelength optical phonon behavior in uniaxial strained graphene: Role of electron-phonon interaction

We derive the frequency shifts and the broadening of $\ensuremath{\Gamma}$-point longitudinal optical (LO) and transverse optical (TO) phonon modes, due to electron-phonon interaction, in graphene under uniaxial strain as a function of the electron density and the disorder amount. We show that, in the absence of a shear strain component, such interaction gives rise to a lifting of the degeneracy of the LO and TO modes which contributes to the splitting of the G Raman band. The anisotropy of the electronic spectrum, induced by the strain, results in a polarization dependence of the LO and TO modes. This dependence is in agreement with the experimental results showing a periodic modulation of the Raman intensity of the split G peak. Moreover, the anomalous behavior of the frequency shift reported in undeformed graphene is found to be robust under strain.

Evolution of ferroelectricity in tetrathiafulvalene-p-chloranil as a function of pressure and temperature.

The neutral-to-ionic phase transition in the mixed-stack charge-transfer complex tetrathiafulvalene-p-chloranil (TTF-CA) has been studied by pressure-dependent infrared spectroscopy up to p = 11kbar and down to low temperatures, T = 10K. By tracking the C=O antisymmetric stretching mode of CA molecules, we accurately determine the ionicity of TTF-CA in the pressure-temperature phase diagram. At any point, the TTF-CA crystal bears only a single ionicity; there is no coexistence region or an exotic high-pressure phase. Our findings shed new light on the role of electron-phonon interaction in the neutral-ionic transition.

Optical characterization of electron-phonon interactions at the saddle point in graphene.

The role of many-body interactions is experimentally and theoretically investigated near the saddle point absorption peak of graphene. The time and energy-resolved differential optical transmission measurements reveal the dominant role played by electron-acoustic phonon coupling in band structure renormalization. Using a Born approximation for electron-phonon coupling and experimental estimates of the dynamic lattice temperature, we compute the differential transmission line shape. Comparing the numerical and experimental line shapes, we deduce the effective acoustic deformation potential to be Deff(ac)≃5 eV. This value is in accord with recent theoretical predictions but differs from those extracted using electrical transport measurements.

The origin of superconductivity

We have shown in the other article of ours, published in the same issue as this one and entitled “Superconductivity in a Fermi liquid: The role of electron-phonon interaction,” that the quasiparticle interaction is just the particle interaction with an opposite sign. In other words, the interaction between two quasielectrons in k- or momentum-space is attractive while the interaction between two electrons in real space is repulsive. Since the quasiparticles are responsible for all properties of a Fermi liquid, then investigations of behaviors of quasipartilces will be sufficient for one to understand the relevant properties of the system consisting of those quasiparticles (particles), Moreover, as shown in our earlier work [1,2], pairing of two quasiparticles in a spin singlet state due to the Coulomb interaction is well-reasoned without needing any boson like retarded mediation between them, and a quartet structure among paired four quasiparticles will be further formed, leading to the doubly lower biding energy than that from a single Cooper’s pair. Under a certain condition a superconducting phase transition, corresponding to the resonance of a many-electron system with repulsion in the spin singlet state, may occur naturally. This showcases the physical picture of our earlier assertion [3] that superconductivity takes place naturally due to the Coulombic repulsive interaction.

Superconductivity and antiferromagnetism in cuprates and pnictides: Evidence of the role of Coulomb correlation

We consider the Hubbard model in terms of the perturbative diagrammatic approach (UNF⩽1) where the interaction between two electrons with antiparallel spins in the lowest order of perturbation is described by the short-range repulsive contact (on-site) interaction (U>0). We argue that in layered 2D cuprates the long-range order antiferromagnetism is driven mainly by the Van Hove singularity, whereas in the case of pnictides the antiferromagnetism exists as a result of the nesting condition. We show that when the interaction is quite strong (UNF≈1) in the case of the Van Hove singularity the electron system undergoes the antiferromagnetic phase transition with the log-range order parameter and large insulating gap. The long-range antiferromagnetism quickly disappear, as shown, with the doping away from the Van Hove singularity, but the antiferromagnetic short-range correlation persists (UNF<1) due to Coulomb repulsive interaction which is the mechanism for superconductivity in cuprates. We argue that in the case of pnictides the antiferromagnetism appears when the nesting conditions for the Fermi surface are met. Since the doping steadily changes the nesting conditions, the antiferromagnetism and superconductivity may coexist as has been observed in pnictides. We show that the proximity of the antiferromagnetism and superconductivity implies the repulsive interaction between electrons, which turns into attractive between quasiparticles as shown by the authors in the article published on the same issue as this one and entitled “Superconductivity from a Fermi Liquid: The Role of Electron-Phonon Interaction”, and the superconductivity in the nearly antiferromagnetic Fermi liquid exists as a result of the repulsive interaction.

The role of electron-phonon interaction on the electronic property of metallic zigzag and armchair carbon nano tubes

We have investigated metal–insulator transition due to electron–phonon interaction for metallic carbon nanotubes in the context of Holstein model Hamiltonian. Self-consistent second order perturbation theory has been implemented to find the electronic self-energy. The results show that the increase of electron–phonon strength leads to the appearance band of gap in the electronic density of states. Furthermore, the effect of temperature on the interacting density of states has been studied. To be more explicit, quasi one particle weight and relaxation time have been investigated due to the increase of electron–phonon coupling strength.

Quantum approach to electronic noise calculations in the presence of electron-phonon interactions

A quantum-mechanical approach to the calculation of electronic noise for nanoscale devices is presented. This method is based on the nonequilibrium Green's-function formalism with electron-phonon scattering mechanisms and takes the effects of the Pauli exclusion principle and the long-range Coulomb interactions into account. As examples the drain current noise characteristics of silicon nanowire transistors at room temperature are simulated. The drain current noise in the saturation regime is primarily shot-noise dominant but is suppressed for higher gate biases due to the electron-electron correlation in the channel region. The role of electron-phonon interactions on noise, the transition from thermal to shot noise, and the physical origin of the shot-noise phenomenon are also investigated.

Weak itinerant magnetism, new developments: TiBe2

Stimulated by theoretical considerations on the role of electron-phonon interaction in the weak itinerant ferromagnet ZrZn2, analogous magnetism was discovered in the system TiBe2-xCux by Matthias et al. This system is ferromagnetic for x > 0.05, while for x < 0.05 there is evidence of antiferromagnetism. Based on the analysis of the measured specific heat anomaly and of the band structure of TiBe2, it is argued that this substance is indeed a spin density wave antiferromagnet.

Resistivity saturation in PrFeAsO1−xFy superconductor: evidence of strong electron–phonon coupling

We have measured the resistivity ofPrFeAsO1−xFy samples over a wide range of temperature in order to elucidate the role ofelectron–phonon interaction on normal- and superconducting-state properties. The linearT dependenceof ρ above 170 K followed by a saturation-like behavior at higher temperature is aclear signature of strong electron–phonon coupling. From the analysis of theT dependenceof ρ, we have estimated several normal-state parameters that are useful for understanding theorigin of superconductivity in this system. Our results suggest that Fe-basedoxypnictides are phonon-mediated BCS superconductors like Chevrel phases and A15compounds.

Role of electron-phonon interactions and external strain on the electronic properties of semiconducting carbon nanotubes

The electron-phonon interactions determine the temperature dependent photoluminescence of semiconducting carbon nanotubes. Both effects, the energy shifts and spectral narrowing of the transitions, can be attributed to the electron-phonon interaction. In this paper, we present an accurate measurement of the temperature induced broadening of the photoluminescence transitions of carbon nanotubes, and model this broadening in terms of the theory, previously used to model the thermal broadening of critical points in conventional semiconductors. Through this fitting procedure, parameters could be estimated which provide important insight into the strength of the electron-phonon interactions. Moreover, careful studies of the energy shifts induced by the external strain had revealed a $n\ensuremath{-}m$ family behavior. We further conclude that using a mathematical expression that combines the theory of semiconducting carbon nanotubes under hydrostatic pressure and strain, this family behavior observed experimentally could be theoretically reproduced, providing tools to model and predict the effect of strain on the electronic properties of carbon nanotubes.

Role of electron-phonon interaction on quasiparticle dispersion in the strongly correlated cuprate superconductors

The role of the electron-phonon (el-ph) interaction in high-${T}_{c}$ cuprates is investigated theoretically by considering correlated electrons interacting with Holstein phonons within the two-dimensional single-band Hubbard model in the limit of large Coulomb interaction. It is shown that the electron-phonon interaction gives rise to a kink in the dispersion along the nodal direction and to a peak/dip/hump structure in the one-electron spectral weight whose dependence on temperature, isotope substitution, and doping is discussed in relation to recent photoemission experiments on the nodal quasiparticles in Bi2201 and LSCO. A comparison with these experiments shows that electron correlations play a crucial role in the determination of the kink position, size, and doping dependence. We also show that the ``unusual'' oxygen isotope shift in the real part of the self-energy, experimentally observed in the optimally doped Bi2212 samples, can be qualitatively explained within the framework that incorporates both strong electron correlations and the adiabatic electron-phonon interaction.