Dispersion of incoherent spectral features in systems with strong electron-phonon coupling

We study (inverse) photoemission from systems with strong coupling of doped carriers to phonons. Using an adiabatic approximation, we develop a method for calculating spectra. This method is particularly simple for systems where the electron-phonon coupling can be neglected in the initial state, e.g., the undoped t-J model. The theory then naturally explains why the electron-phonon coupling just leads to a broadening of spectra calculated without electron-phonon coupling, without changing the dispersion. This is in agreement with recent angle-resolved photoemission spectroscopy (ARPES) on undoped cuprates, and it supports the interpretation in terms of strong electron-phonon interaction. The theory also shows that for systems with strong electron-phonon coupling in the initial state, the result cannot in general be related to the spectrum obtained without electron-phonon coupling.     

Quantum dephasing in carbon nanotubes due to electron-phonon coupling

We report on the effect of electron-phonon coupling on quantum transport in carbon nanotubes. The vibrational atomic displacements as well as the electron-phonon coupling strength are introduced through a time-dependent perturbation of the pi-electron Hamiltonian. The effect of dephasing on the Kubo conductance is studied for metallic and semiconducting nanotubes, and from a phenomenological law, coherence length (time) scales are found to fluctuate within the range 10 to 150 nm (0.01 to 4 ps) depending on the energy of charge carriers and phonon amplitude.     

Renormalization of phonon frequencies by conduction electrons in layer systems with application to YBa2Cu3O7

It is generally assumed that there are free electronic carriers associated with the CuO₂ planes in high-temperature superconductors. Regarding these carriers as confined to these planes their influence on phonon frequencies is studied within the framework of a shell model of lattice dynamics taking into account the long-range part of the electron-phonon coupling. We find a complete suppression of the LO-TO splitting for modes polarized along the planes by metallic screening and a strong coupling of electronic plasmon and particle-hole excitations especially to some To-phonon branches polarized perpendicular to the conducting planes, particularly pronounced in systems with at least two CuO₂ planes per unit cell. Numerical results are presented for YBa₂Cu₃O₇.     

Electron Raman scattering in metals with strong electron-phonon coupling

Inelastic light scattering by the carriers interacting with phonons in the anisotropic metals with large penetration depth is theoretically studied. It is shown that the strong temperature dependence of the Raman scattering intensity in the region of phonon frequencies is the main characteristic feature of these processes. The effects of anisotropy, impurities and the strength of electron-phonon interaction on the frequency and temperature dependences of the polarization operator are analysed. Taking into account the anisotropy vertex corrections which obey a system of the Boltzman-type integral equations should leads to the considerable changes of the frequency behavior of scattering cross section for low frequencies. However, the changes of the temperature dependence are not so drastic. Increasing the electron-phonon coupling constant affects the particle-hole polarization operator in two possible ways to weaken temperature dependence and to make flatter frequency curves. The same effects are also from impurities. Some theoretical consequences which concern the role of electron-phonon interaction for electron Raman scattering in high-T _c superconductors aboveT _c are proposed.     

Polaronic behavior of undoped high-T(c) cuprate superconductors from angle-resolved photoemission spectra

We present angle-resolved photoemission spectroscopy (ARPES) data on undoped La2CuO4, indicating polaronic coupling between bosons and charge carriers. Using a shell model, we calculate the electron-phonon coupling and find that it is strong enough to give self-trapped polarons. We develop an efficient method for calculating ARPES spectra in undoped systems. Using the calculated couplings, we find the width of the phonon sideband in good agreement with experiment. We analyze reasons for the observed dependence of the width on the binding energy.     

The polaron state of a crystal

The quantum mechanics of an electron-nuclear system with strong electron-phonon coupling is considered. First, a two-site model is treated in the adiabatic approximation. As the coupling constant increases, electron transfer undergoes qualitative changes; more specifically, a potential barrier forms in the adiabatic potential, the electron transfer becomes associated with the tunneling of nuclei through the barrier, and the level splitting in the system falls off exponentially. The properties of a similar crystal model are discussed. It is shown that electron transfer in a crystal in the case of strong coupling is likewise associated with the tunneling of nuclei through barriers in the deformation space. Strong coupling modifies the electron-electron interaction terms. The Hamiltonian (exchange) terms, which are not associated with electron transfer, are only weakly modified. At the same time, the terms involving transfer (the band terms) undergo exponential reduction and vanish in the limit as M → ∞ (M is the ion mass) and the carriers become small polarons. This reduction provides a basis for the natural mechanism of enhancement of the isotope effect.     

共振拉曼效应和电子-声子耦合对线性多烯分子共振拉曼光谱的影响

线性多烯分子具有高强度且信息丰富的共振拉曼光谱,在生物学、光电材料和医学等方面都有一定应用.而含有共轭双键的短链β胡萝卜素分子是多烯分子中极具有代表性的分子.在激发光作用下π电子与CC键振动相互作用影响着吸收光谱和拉曼光谱,而共振拉曼效应和电子-声子耦合影响着共振拉曼光谱的强度、频率和线型.测量了β胡罗卜素分子在二氯乙...     

Diagrammatic weak-coupling expansion for the magneto-polaron energy

The problem of a Pekar-Fröhlich polaron in a magnetic field is considered for weak electron-phonon interaction. A diagrammatic technique for zero temperature is developed. Previously obtained results for two dimensions are reproduced. For three dimensions the polaron ground-state energy and the longitudinal effective mass are derived in the first orders of the electron-phonon coupling constant for non-zero values of a magnetic field. In a strong magnetic field the bulk polaron is shown to be equivalent to a one-dimensional polaron with a renormalized electron-phonon coupling constant. This leads to exact results in the strong-coupling limit.     

Effects of Thermal Lattice Vibration on the Effective Potential of Weak-Coupling Bipolaron in a Quantum Dot

Based on the Huybrechts' linear-combination operator,effects of thermal lattice vibration on the effective potential of weak-coupling bipolaron in semiconductor quantum dots are studied by using the LLP variational method and quantum statistical theory.The results show that the absolute value of the induced potential of the bipolaron increases with increasing the electron-phonon coupling strength,but decreases with increasing the temperature and the distance of electrons,respectively;the absolute value of the effective potential increases with increasing the radius of the quantum dot,electron-phonon coupling strength and the distance of electrons,respectively,but decreases with increasing the temperature;the temperature and electron-phonon interaction have the important influence on the formation and state properties of the bipolaron:the bipolarons in the bound state are closer and more stable when the electron-phonon coupling strength is larger or the temperature is lower;the confinement potential and coulomb repulsive potential between electrons are unfavorable to the formation of bipolarons in the bound state.     

Synergistic polaron formation in the Hubbard-Holstein model at small doping

We study the effect of dynamical Holstein phonons on the physics of the Hubbard model at small doping using the dynamical cluster approximation on a 2x2 cluster. Nonlocal antiferromagnetic correlations are found to significantly enhance the electron-phonon coupling, resulting in polaron formation for moderate coupling strengths. At finite doping, the electron-phonon coupling is found to strongly enhance the nonlocal spin correlations, indicating a synergistic interplay between the electron-phonon coupling and antiferromagnetic correlations. Although it enhances the pairing interaction, the electron-phonon coupling is found to decrease the superconducting transition temperature, due to the reduction in the quasiparticle fraction.     

Electric field effect tuning of electron-phonon coupling in graphene

Gate-modulated low-temperature Raman spectra reveal that the electric field effect (EFE), pervasive in contemporary electronics, has marked impacts on long-wavelength optical phonons of graphene. The EFE in this two-dimensional honeycomb lattice of carbon atoms creates large density modulations of carriers with linear dispersion (known as Dirac fermions). Our EFE Raman spectra display the interactions of lattice vibrations with these unusual carriers. The changes of phonon frequency and linewidth demonstrate optically the particle-hole symmetry about the charge-neutral Dirac point. The linear dependence of the phonon frequency on the EFE-modulated Fermi energy is explained as the electron-phonon coupling of massless Dirac fermions.     

Electron-phonon interaction in a free standing beryllium monolayer

We report ab initio study of the electron-phonon coupling in a free standing Beryllium monolayer. The calculations were carried out using a linear-response approach in the plane-wave pseudopotential representation. The Eliashberg spectral function α²F(ω) and the electron-phonon coupling parameter λ are evaluated at the Fermi level. The obtained results show a large contribution to the electron-phonon coupling from the low-energy transverse mode scattering.     

Structure and superconductivity in the ternary silicide CaAlSi

Using the linear response-linearized Muffin-tin orbital (LR-LMTO) method, we study the electronic band structure, phonon spectra, electron-phonon coupling and superconductivity for c-axis ferromagnetic-like (F-like) and antiferromagnetic-like (AF-like) structures in ternary silicide CaAlSi. The following conclusions are drawn from our calculations. If Al and Si atoms are assumed to arrange along the c axis in an F-like long-range ordering (-Al-Al-Al-and-Si-Si-Si-), one could obtain the ultrasoft B_1g phonon mode and thus very strong electron-phonon coupling in CaAlSi. However, the appearance of imaginary frequency phonon modes indicates the instability of such a structure. For Al and Si atoms arranging along the c axis in an AF-like long-range ordering (-Al-Si-Al-), the calculated electron-phonon coupling constant is equal to 0.8 and the logarithmically averaged frequency is 146.8 K. This calculated result can correctly yield the superconducting transition temperature of CaAlSi by the standard BCS theory in the moderate electron-phonon coupling strength. We propose that an AF-like superlattice model for Al (or Si) atoms along the c direction may mediate the inconsistency estimated from theory and experiment, and explain the anomalous superconductivity in CaAlSi.      

Inelastic effects in Aharonov-Bohm molecular interferometers

Inelastic effects arising from electron-phonon coupling in molecular Aharonov-Bohm (AB) interferometers are studied using the nonequilibrium Green's function method. Results for the magnetoconductance are compared for different values of the electron-phonon coupling strength. At low-bias voltages, the coupling to the phonons does not change the lifetime and leads mainly to scattering phase shifts of the conducting electrons. As a result of these dephasing processes, the magnetoconductance of the molecular AB interferometer becomes more sensitive to the threading magnetic flux as the electron-phonon coupling is increased, opposite to the behavior of an electric gate.     

Optical determination of electron-phonon coupling in carbon nanotubes

We report on an optical method to directly measure electron-phonon coupling in carbon nanotubes by correlating the first and second harmonic of the resonant Raman excitation profile. The method is applicable to 1D and 0D systems and is not limited to materials that exhibit photoluminescence. Experimental results for electron-phonon coupling with the radial breathing mode in 5 different nanotubes show coupling strengths from 3-11 meV. The results are in good agreement with the chirality and diameter dependence of the e-ph coupling calculated by Goupalov et al.     

电-声子耦合强度对量子点系统噪声的影响

利用Lang-Firsov正则变换和Keldysh非平衡格林函数方法研究了低温下具有电子-声子相互作用的量子点系统的噪声。我们特别注意了电-声子耦合强度的变化对量子点系统噪声的影响。数值结果表明：随着电-声子耦合强度的增大,系统的噪声增大,同时微分噪声谱中会出现一系列的声子伴带峰,峰的高度和数目对电-声子耦合强度的变化非常敏感。我们也研究了系统的Fano因子,它显示系统噪声对肖特基(Schottky)公式的偏离。在高偏压区,Fano因子随着电-声子耦合强度的增大而增大。     

电-声子耦合强度对量子点系统噪声的影响

利用Lang-Firsov正则变换和Keldysh非平衡格林函数方法研究了低温下具有电子-声子相互作用的量子点系统的噪声.我们特别注意了电-声子耦合强度的变化对量子点系统噪声的影响.数值结果表明:随着电-声子耦合强度的增大,系统的噪声增大,同时微分噪声谱中会出现一系列的声子伴带峰,峰的高度和数目对电-声子耦合强度的变化非常敏感.我们也研究了系统的Fano因子,它显示系统噪声对肖特基(Schottky)公式的偏离.在高偏压区,Fano因子随着电-声子耦合强度的增大而增大.     

Phonon-assisted thermoelectric effects in strongly interacting quantum dot

The joint effects of the electron-phonon interaction and Kondo effect on the charge and heat transport through a single molecule transistor are investigated by applying the improved canonical transformation and noncrossing approximation technique. We find that the electron-phonon interaction decreases the conductance, thermopower and the Wiedemann-Franz law in the Kondo regime due to the splitting and the decreasing of the main Kondo peak. However, the thermoelectric figure of merit achieves enhancement with the electron-phonon coupling strength increasing. In addition, the dip value of the thermopower at the Kondo temperature for the different electron-phonon coupling strength can give a straightforward reliable estimate of the electron-phonon coupling strength.     

Thermally activated charge carriers and mid-infrared optical excitations in quarter-filled CDW systems

The optical properties of the quarter-filled single-band CDW systems have been reexamined in the model with the electron-phonon coupling related to the variations of electron site energies. It appears that the indirect, electron-mediated coupling between phase phonons and external electromagnetic fields vanishes for symmetry reasons, at variance with the infrared selection rules used in the generally accepted microscopic theory. It is shown that the phase phonon modes and the electric fields couple directly, with the coupling constant proportional to the magnitude of the charge-density wave. The single-particle contributions to the optical conductivity tensor are determined for the ordered CDW state and the related weakly doped metallic state by means of the Bethe-Salpeter equations for elementary electron-hole excitations. It turns out that this gauge-invariant approach establishes a clear connection between the effective numbers of residual, thermally activated and bound charge carriers. Finally, the relation between these numbers and the activation energy of dc conductivity and the optical CDW gap scale is explained in the way consistent with the conductivity sum rules.     

Pre-resonant enhancement of the Raman scattering from KI:Tl+

E_g and T_2g spectra at room and nitrogen temperature at several laser wavelengths are reported for KI:Tl⁺. As resonance with Tl⁺ -absorption bands is approached the optic phonon parts of the spectra are enhanced relative to the acoustic phonon parts, and distortions occur within the acoustic and optic parts of the spectra. A model for the scattering is employed which involves electron-phonon coupling to nearest and nn neighbors. Exchange and crystal field contributions to the electron-phonon coupling are distinguished.