Strong Electron-phonon Interaction Research Articles

Giant two-phonon Raman scattering from nanoscale NbC precipitates in Nb

High-purity niobium (Nb), subjected to the processing methods used in the fabrication of superconducting rf cavities, displays micrometer-sized surface patches containing excess carbon. High-resolution transmission electron microscopy and electron energy-loss spectroscopy measurements are presented which reveal the presence of nanoscale NbC coherent precipitates in such regions. Raman backscatter spectroscopy on similar surface regions exhibit spectra consistent with the literature results on bulk NbC but with significantly enhanced two-phonon scattering. The unprecedented strength and sharpness of the two-phonon signal has prompted a theoretical analysis, using density functional theory (DFT), of phonon modes in NbC for two different interface models of the coherent precipitate. One model leads to overall compressive strain and a comparison to ab initio calculations of phonon dispersion curves under uniform compression of the NbC shows that the measured two-phonon peaks are linked directly to phonon anomalies arising from strong electron-phonon interaction. Another model of the extended interface between Nb and NbC, studied by DFT, gives insight into the frequency shifts of the acoustic and optical mode density of states measured by first-order Raman spectroscopy. The exact origin of the stronger two-phonon response is not known at present but it suggests the possibility of enhanced electron-phonon coupling in transition-metal carbides under strain found either in the bulk NbC inclusions or at their interfaces with Nb metal. Preliminary tunneling studies using a point contact method show some energy gaps larger than expected for bulk NbC.

Superconductivity in semimetallicBi3O2S3

Here we report further investigation on the thermodynamic, and transport properties, and the first assessment of theoretical calculations for the BiS2-layered Bi3O2S3 superconductor. The polycrystalline sample is synthesized with the superconducting transition temperature of Tc_onset = 5.75 K, that drops to 3.3 K by applying hydrostatic pressure of 6 kbar. Density-of-states calculations give substantial hybridization between Bi, O and S, with the largest Bi component at DOS, which supports the idea that BiS2 layer is relevant for producing electron-phonon coupling. The electron charge carrier concentration of only 1.5 x 10^19 cm-3 for Bi3O2S3 is additionally suggestive of strong electron-phonon interaction in Bi-O-S system. The analysis of Seebeck coefficient results strongly suggests that Bi3O2S3 is a semimetal. In fact, the semimetallic or narrow band gap behavior may be general in the BiS2-layered class of materials, including that of Bi4O4S3.

The Role of Transport Agents in MoS2 Single Crystals

We report resistivity, thermoelectric power, and thermal conductivity of MoS2 single crystals prepared by the chemical vapor transport (CVT) method using I2, Br2, and TeCl4 as transport agents. The material presents low-lying donor and acceptor levels, which dominate the in-plane charge transport. Intercalates into the van der Waals gap strongly influence the interplane resistivity. Thermoelectric power displays the characteristics of strong electron–phonon interaction. A detailed theoretical model of thermal conductivity reveals the presence of a high number of defects in the MoS2 structure. We show that these defects are inherent to CVT growth method, coming mostly from the transport agent molecules inclusion as identified by total reflection X-ray fluorescence analysis (TXRF) and in-beam activation analysis (IBAA).

Origin of electronic properties of PbGa2Se4 crystal: Experimental and theoretical investigations

The PbGa2Se4 crystal is a promising material for optoelectronic applications. It is caused by coexistence of the large polarized Pb cations and a huge contribution of anharmonic phonon subsystem caused by chalcogenide anions. In the present work the electronic and optical properties of the mentioned material were studied theoretically as well as experimentally by optical and X-ray photoelectron spectroscopy methods. The DFT approach has been used for the quantum chemical electronic properties calculations. Urbach rule and steepness parameters of the PbGa2Se4 crystal have been evaluated for the first time. These parameters and Urbach energies increase with increasing temperature of the samples that is typical for the semiconducting materials. The XPS measurements of the investigated crystal reveal that all the spectral features are originated from core-level states of the constituent elements. Simultaneously these results also confirm non-reactivity of the PbGa2Se4 surface. However, the titled single crystal possesses a number of intrinsic structural defects and vacancies thereby affecting its electronic properties. The electronic properties of PbGa2Se4 are also affected by the existence of a strong electron–phonon interaction due to chalcogenide anions causing the decrease of an energy gap with temperature increasing.

Low B field magneto-phonon resonances in single-layer and bilayer graphene.

Many-body effects resulting from strong electron-electron and electron-phonon interactions play a significant role in graphene physics. We report on their manifestation in low B field magneto-phonon resonances in high-quality exfoliated single-layer and bilayer graphene encapsulated in hexagonal boron nitride. These resonances allow us to extract characteristic effective Fermi velocities, as high as 1.20 × 10(6) m/s, for the observed "dressed" Landau level transitions, as well as the broadening of the resonances, which increases with the Landau level index.

Theoretical description of structural and electronic properties of organic photovoltaic materials.

We review recent progress in the modeling of organic solar cells and photovoltaic materials, as well as discuss the underlying theoretical methods with an emphasis on dynamical electronic processes occurring in organic semiconductors. The key feature of the latter is a strong electron-phonon interaction, making the evolution of electronic and structural degrees of freedom inseparable. We discuss commonly used approaches for first-principles modeling of this evolution, focusing on a multiscale framework based on the Holstein-Peierls Hamiltonian solved via polaron transformation. A challenge for both theoretical and experimental investigations of organic solar cells is the complex multiscale morphology of these devices. Nevertheless, predictive modeling of photovoltaic materials and devices is attainable and is rapidly developing, as reviewed here.

Effects of Spin-orbit Interaction on Persistent Current in a Nano Ring

The persistent current for polaronic transport in a one dimensional Holstein-Hubbard ring is studied in the presence of Rashba spin orbit (RSO) interaction. The energy is calculated within the Hartree-Fock approximation, using Lang-Firsov transformation followed by block diagonalization. We have found that the strong electron-phonon interaction will reduce the persistent current while the spin-orbit interaction enhances the amplitude of the persistent current. The Drude weight is also calculated.

Study of conduction phenomena in indium substituted Mn–Zn nano-ferrites

Nanocrystalline Indium substituted Mn–Zn ferrites were prepared by oxalate coprecipitation technique in combination with microwave heating. X-ray diffraction and scanning electron microscopy was used to study nano-structural parameters such as crystallite size, lattice constant, X-ray density, observed density and morphology of the synthesized nanopowders. The lattice constant, crystallite size and X-ray density increased with increase in Indium concentration. Thermal variation of dc resistivity was studied by two probe method. The resistivity decreased exponentially with increasing Indium concentration and the change of slope of dc resistivity plot was observed at Curie temperature . Small polaron hopping between Fe 2+ and Fe 3+ ions at octahedral sites was established. Substitution of Indium ions increased the dc resistivity. Differential method was used to study thermoelectric power. Thermo-emf remained constant with increasing temperature; the n-type conduction mechanism and localization of charge carriers at lattice sites due to strong electron–phonon interaction was established. • Preparation of single phase Indium substituted Mn–Zn ferrites by oxalate coprecipitation technique. • Study of thermal variation of dc resistivity and the change in activation energy at curie temperature. Curie temperature determined by Loria–Sinha technique. • Study of thermal variation of thermoelectric power, n-type of charge carriers and carrier concentration.

Distribution of charge carriers at strong electron–phonon interaction and “vertical dispersion” in ARPES spectra of cuprates

Abstract Strong electron–phonon interaction (EPI) generating spontaneous breaking translational symmetry (polarons formation) is acknowledged in lightly doped cuprates due to interpretation of broad bands in their ARPES and optical conductivity spectra. To study systems with strong EPI at significant carrier density we construct distribution suitable for carriers with momentum uncertainty. Obtained concentrations of carriers in different states are used to predict ARPES spectra of such systems. Doping evolution of spectra calculated and “vertical dispersion” in them demonstrate the excellent qualitative and quantitative agreement with those observed in cuprates. “Vertical dispersion” in undoped and essentially doped cuprates is understood from a single viewpoint.

Interplay of charge, spin, and lattice degrees of freedom in the spectral properties of the one-dimensional Hubbard-Holstein model

We calculate the spectral function of the one dimensional Hubbard-Holstein model using the time dependent Density Matrix Renormalization Group (tDMRG), focusing on the regime of large local Coulomb repulsion, and away from electronic half-filling. We argue that, from weak to intermediate electron-phonon coupling, phonons interact only with the electronic charge, and not with the spin degrees of freedom. For strong electron-phonon interaction, spinon and holon bands are not discernible anymore and the system is well described by a spinless polaronic liquid. In this regime, we observe multiple peaks in the spectrum with an energy separation corresponding to the energy of the lattice vibrations (i.e., phonons). We support the numerical results by introducing a well controlled analytical approach based on Ogata-Shiba's factorized wave-function, showing that the spectrum can be understood as a convolution of three contributions, originating from charge, spin, and lattice sectors. We recognize and interpret these signatures in the spectral properties and discuss the experimental implications.

Diverse photoinduced dynamics in an organic charge-transfer complex having strong electron-phonon interactions.

CONSPECTUS: Phenomena that occur in nonequilibrium states created by photoexcitation differ qualitatively from those that occur at thermal equilibrium, and various physical theories developed for thermal equilibrium states can hardly be applied to such phenomena. Recently it has been realized that understanding phenomena in nonequilibrium states in solids is important for photoenergy usage and ultrafast computing. Consequently, much effort has been devoted to revealing such phenomena by developing various ultrafast observation techniques and theories applicable to nonequilibrium states. This Account describes our recent studies of diverse photoinduced dynamics in a strongly correlated organic solid using various ultrafast techniques. Solids in which the electronic behavior is affected by Coulomb interactions between electrons are designated as strongly correlated materials and are known to exhibit unique physical properties even at thermal equilibrium. Among them, many organic charge-transfer (CT) complexes have low dimensionality and flexibility in addition to strong correlations; thus, their physical properties change sensitively in response to changes in pressure or electric field. Photoexcitation is also expected to drastically change their physical properties and would be useful for ultrafast photoswitching devices. However, in nonequilibrium states, the complicated dynamics due to these characteristics prevents us from understanding and using these materials for photonic devices. The CT complex (EDO-TTF)2PF6 (EDO-TTF = 4,5-ethylenedioxytetrathiafulvalene) exhibits unique photoinduced dynamics due to strong electron-electron and electron-phonon interactions. We have performed detailed studies of the dynamics of this complex using transient electronic spectroscopy at the 10 and 100 fs time scales. These studies include transient vibrational spectroscopy, which is sensitive to the charges and structures of constituent molecules, and transient electron diffraction, which provides direct information on the crystal structure. Photoexcitation of the charge-ordered low-temperature phase of (EDO-TTF)2PF6 creates a new photoinduced phase over 40 fs via the Franck-Condon state, in which electrons and vibrations are coherently and strongly coupled. This new photoinduced phase is assigned to an insulator-like state in which the charge order differs from that of the initial state. In the photoinduced phase, translations of component molecules proceed before the rearrangements of intramolecular conformations. Subsequently, the charge order and structure gradually approach those of the high-temperature phase over 100 ps. This unusual two-step photoinduced phase transition presumably originates from steric effects due to the bent EDO-TTF as well as strong electron-lattice interactions.

Carrier dynamics and related electronic band properties of InN films

In the present article, we focus our discussion on the carrier dynamics of the scattering and recombination processes of InN films and the related band-edge energy structure. Various reports on this matter are summarized and some issues are reexamined. The analysis result based on the apparent band–band transition matrix element is consistent with a reported effective heavy hole mass of 0.59 (±0.06) m0. An Ep value related to the transition matrix element of 10–14 eV is thought to be plausible. The ambiguity of the band-edge structure is evaluated by the uncertainty of electron density. The distortion of the conduction band bottom and the ambiguity of the estimation of the many-body effect are discussed. The enhancement of the anisotropic electron-scattering nature with the decrease in residual electron density reveals that the residual electron source has isotropic potentials: point defects or small complexes. Infrared reflectance spectrum analysis reveals the high electron mobility inside grains in spite of the scattering by edge-type dislocations which cause the anisotropic carrier scattering. The recombination processes at low temperatures are dominated by nonradiative processes related to edge-type dislocations, while the thermally activated nonradiative recombination process is independent of the dislocation density. The activation processes and energies of the recombination related to phonon localization are characterized. InN is a peculiar material that has high carrier mobility and a strong electron–phonon interaction, which possibly induces the high nonradiative carrier recombination rate. The control of phonon localization is thus required.

Analysis of thet2−Vmodel

We study a model (i.e., the ${t}_{2}\text{\ensuremath{-}}V$ model involving next-nearest-neighbor hopping and nearest-neighbor repulsion) in one dimension that generically depicts the dominant transport mechanism in cooperative strong electron-phonon interaction systems. Using analytic and numerical approaches, hard-core bosons are shown to typically undergo a striking discontinuous transition from a superfluid to a supersolid. Topological inequivalence of rings with even and odd number of sites is manifested through observable differences (in structure factor peaks) at the transition. Connections are also identified between the ${t}_{2}\text{\ensuremath{-}}V$ model and other topologically interesting models.

Direct measurement of coherent phonon dynamics in solution-processed stibnite thin films

We report observations of coherent phonon oscillations in solution-processed polycrystalline stibnite $({\mathrm{Sb}}_{2}$${\mathrm{S}}_{3})$ photovoltaic thin films using transient absorption spectroscopy. Detailed optical spectroscopy correlated by extensive first-principles lattice dynamics calculations indicates that the coherent ${B}_{3g}$ longitudinal optical phonon mode with a frequency of 63.74 \ifmmode\pm\else\textpm\fi{} 0.05 ${\mathrm{cm}}^{\ensuremath{-}1}$ (or 1.911 \ifmmode\pm\else\textpm\fi{} 0.002 THz) at room temperature is generated via the impulsive stimulated-Raman-scattering mechanism. These strong electron-phonon interactions indicate a dominant energy-loss channel in these materials that could impose a fundamental limit on their solar energy conversion efficiency.

Infrared phonon spectrum of the tetragonal helimagnetBa2CuGe2O7

The lattice dynamics of Ba$_2$CuGe$_2$O$_7$, a compound which develops Dzyaloshinsky-Moriya (DM) helical magnetism below $T_N$ = 3.2 K, has been studied by measuring the infrared reflectivity of a single crystal with the radiation polarized both in the $ab$ plane and along the $c$ axis of its tetragonal cell, from 7 K to 300 K. In this compound, where the unit cell has no inversion symmetry, fourteen $E$ phonon modes of the $ab$ plane, out of the eighteen predicted, and all the ten $B_2$ modes of the $c$ axis, have been observed. They have been assigned to the atomic motions by a comparison with shell-model calculations, which provided vibrational frequencies in good agreement with the experiment, while most calculated intensities turned to be much lower than the experimental values. This discrepancy has been tentatively explained by assuming strong electron-phonon interactions, a hypothesis supported by the failure of the $f$- sum rule if restricted to the phonon region. Indeed, we observe a remarkable increase in the oscillator strengths at $T$'s low but higher than $T_N$, which suggests that the dielectric constant of Ba$_2$CuGe$_2$O$_7$ may increase at those temperatures.

Optical properties of BiTeBr and BiTeCl

We present a comparative study of the optical properties - reflectance, transmission and optical conductivity - and Raman spectra of two layered bismuth-tellurohalides BiTeBr and BiTeCl at 300 K and 5 K, for light polarized in the a-b planes. Despite different space groups, the optical properties of the two compounds are very similar. Both materials are doped semiconductors, with the absorption edge above the optical gap which is lower in BiTeBr (0.62 eV) than in BiTeCl (0.77 eV). The same Rashba splitting is observed in the two materials. A non-Drude free carrier contribution in the optical conductivity, as well as three Raman and two infrared phonon modes, are observed in each compound. There is a dramatic difference in the highest infrared phonon intensity for the two compounds, and a difference in the doping levels. Aspects of the strong electron-phonon interaction are identified. Several interband transitions are assigned, among them the low-lying absorption $\beta$ which has the same value 0.25 eV in both compounds, and is caused by the Rashba spin splitting of the conduction band. An additional weak transition is found in BiTeCl, caused by the lower crystal symmetry.

Dynamics of phonon clouds of correlated polarons

We investigate the Anderson-Holstein model with strong repulsion of impurity electrons using a strongcoupling approximation. The strong electron-phonon interaction determines the formation of polarons with heavy clouds of phonons surrounding the impurity electrons. We describe the collective mode of phonon clouds and the relation between electron propagators and correlation functions. We obtain the dependence of the collective mode energy on the hybridization parameter in the case T = 0 and prove that it is suppressed by increasing hybridization.

Optical properties of Be-doped GaAs nanowires on Si substrate grown by a catalyst-free molecular beam epitaxy vapor–liquid–solid method

The effect of Be doping on the crystal structure and the optical properties of catalyst-free Be-doped GaAs nanowires (NWs) grown on a (111)Si substrate were investigated by scanning transmission electron microscopy (STEM), X-ray diffraction (XRD) analysis, and photoreflectance (PR) and photoluminescence (PL) techniques. Be-doped NWs sample showed a striped pattern in STEM images. Owing to a high arsenic flux under the growth condition, the sample did not have a wurtzite but a zinc-blend (ZB) structure, and the observed striped pattern in STEM images suggests the presence of twin boundaries and stacking faults. The bandgap energy of the Be-doped NWs was lower than that of the nondoped NWs sample in the entire temperature range. In addition, the deviation from the conventional Varshni curve was found to be large in the low-temperature region. However, this is well explained by considering the strong electron–phonon interaction and high average phonon temperature. Therefore, we concluded that the induced strain increased the linear thermal expansion coefficient. This is because the Be atom has a smaller covalent radius than the Ga and As atoms.

Point-contact spectroscopy of electron-phonon interaction in superconductors

The possibility to reconstruct the electron-phonon interaction (EPI) function was demonstrated for S–c–N and S–c–S point contacts using the superconducting inelastic contribution to the excess current caused by Andreev reflection processes. Superconductors with both weak (Sn, Al) and strong (Pb, In) EPI were considered. It was shown that in the latter case it is necessary to account for the elastic component of current which is related to the frequency dependence of the superconducting energy gap arising due to electron-phonon renormalization of the energy spectrum of the superconductor.

Dressed tunneling approximation for electronic transport through molecular transistors

A theoretical approach for the nonequilibrium transport properties of nanoscale systems coupled to metallic electrodes with strong electron-phonon interactions is presented. It consists of a resummation of the dominant Feynman diagrams from the perturbative expansion in the coupling to the leads. We show that this scheme eliminates the main pathologies found in previous simple analytical approaches for the polaronic regime. The results for the spectral and transport properties are compared with those from several other approaches for a wide range of parameters. The method can be formulated in a simple way to obtain the full counting statistics. Results for the shot and thermal noise are presented.