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Register a new userEffects of electron-phonon coupling in angle-resolved photoemission spectra of SrTiO3
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We have studied the O 2p valence-band structure of Nb-doped SrTiO3, in which a dilute concentration of electrons are doped into the d0 band insulator, by angle-resolved photoemission spectroscopy (ARPES) measurements. We found that ARPES spectra at the valence band maxima at the M [k = (pi/a, pi/a, 0)]and R [k = (pi/a, pi/a, pi/a)] points start from ~ 3.3 eV below the Fermi level (EF), consistent with the indirect band gap of 3.3 eV and the EF position at the bottom of the conduction band. The peak position of the ARPES spectra were, however, shifted toward higher binding energies by ~ 500 meV from the 3.3 eV threshold. Because the bands at M and R have pure O 2p character, we attribute this ~ 500 meV shift to strong coupling of the oxygen p hole with optical phonons in analogy with the peak shifts observed for d-electron photoemission spectra in various transition-metal oxides.
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Lattice contribution to the electronic self-energy in complex correlated oxides is a fascinating subject that has lately stimulated lively discussions. Expectations of electron-phonon self-energy effects for simpler materials, such as Pd and Al, have
resulted in several misconceptions in strongly correlated oxides. Here we analyze a number of arguments claiming that phonons cannot be the origin of certain self-energy effects seen in high-$T_c$ cuprate superconductors via angle resolved photoemission experiments (ARPES), including the temperature dependence, doping dependence of the renormalization effects, the inter-band scattering in the bilayer systems, and impurity substitution. We show that in light of experimental evidences and detailed simulations, these arguments are not well founded.
We present a detailed study on the influence of strong electron-phonon coupling to the photoemission spectra of lead. Representing the strong-coupling regime of superconductivity, the spectra of lead show characteristic features that demonstrate the correspondence of physical properties in the normal and the superconducting state, as predicted by the Eliashberg theory. These features appear on an energy scale of a few meV and are accessible for photoemission only by using modern spectrometers with high resolution in energy and angle.
Numerous angle resolved photoemission spectroscopy (ARPES) studies of a wide class of low-density metallic systems, ranging from doped transition metal oxides to quasi two-dimensional interfaces between insulators, exhibit phonon sidebands below the quasi-particle peak as a unique hallmark of polaronic correlations. Here, we single out properties of ARPES spectra that can provide a robust estimate of the effective range (screening length) of the electron-phonon interaction, regardless of the limited experimental resolution, dimensionality and particular features of the electronic structure, facilitating a general methodology for an analysis of a whole class of materials.
Laser-based angle-resolved photoemission measurements with super-high resolution have been carried out on an optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_8$ high temperature superconductor. New high energy features at $sim$115 meV and $sim$150 meV, besides the prominent $sim$70 meV one, are found to develop in the nodal electron self-energy in the superconducting state. These high energy features, which can not be attributed to electron coupling with single phonon or magnetic resonance mode, point to the existence of a new form of electron coupling in high temperature superconductors.
We employed femtosecond time- and angle-resolved photoelectron spectroscopy to analyze the response of the electronic structure of the 122 Fe-pnictide parent compounds Ba/EuFe_2As_2 and optimally doped BaFe_{1.85}Co_{0.15}As_2 near the Gamma point to femtosecond optical excitation. We identify pronounced changes of the electron population within several 100 meV above and below the Fermi level, which we explain as combination of (i) coherent lattice vibrations, (ii) a hot electron and hole distribution, and (iii) transient modifications of the chemical potential. The response of the three different materials is very similar. In the Fourier transformation of the time-dependent photoemission intensity we identify three modes at 5.6, 3.3, and 2.6 THz. While the highest frequency mode is safely assigned to the A_{1g} mode, the other two modes require a discussion in comparison to literature. The time-dependent evolution of the hot electron distribution follows a simplified description of a transient three temperature model which considers two heat baths of lattice vibrations, which are more weakly and strongly coupled to transiently excited electron population. Still the energy transfer from electrons to the strongly coupled phonons results in a rather weak, momentum-averaged electron-phonon coupling quantified by values for lambda<omega^2> between 30 and 70 meV^2. The chemical potential is found to present a transient modulation induced by the coherent phonons. This change in the chemical potential is particularly strong in a two band system like in the 122 Fe-pnictide compounds investigated here due to the pronounced variation of the electrons density of states close to the equilibrium chemical potential.
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