Effect of spin-orbit coupling on the electron-phonon interaction of the superconductors Pb and Tl

6 páginas, 4 figuras, 1 tabla.-- PACS number(s): 63.20.kd, 71.15.Rf, 63.20.dk, 71.15.Mb.-- et al.The influence of spin-orbit coupling (SOC) on the electron-phonon interaction and on the pairing strength is investigated for Pb and Tl within density-functional theory. For Pb we find that SOC increases the coupling constant λ by 44%, solving the longstanding puzzle of too small λ values obtained consistently in previous first-principles calculations. The origin of the SOC-induced enhancement of λ lies both in a softening of the phonon spectrum and in an increase in the electron-phonon coupling matrix elements. In contrast, for the neighboring element Tl only weak effects of SOC on the phonon spectrum and the electron-phonon interaction is obtained, leading to an overall reduction in λ by 12%. In both cases, the inclusion of SOC improves the agreement with experiment.Peer reviewe

Electron-phonon coupling in topological surface states: The role of polar optical modes

The use of topological edge states for spintronic applications could be severely hampered by limited lifetimes due to intrinsic many-body interactions, in particular electron-phonon coupling. Previous works to determine the intrinsic coupling strength did not provide a coherent answer. Here, the electron-phonon interaction in the metallic surface state of 3D topological insulators is revised within a first principles framework. For the archetypical cases of Bi2Se3 and Bi2Te3, we find an overall weak coupling constant of less than 0.15, but with a characteristic energy dependence. Derived electronic self-energies compare favorably with previous angle-resolved photoemission spectroscopy results. The prevailing coupling is carried by optical modes of polar character, which is weakly screened by the metallic surface state and can be reduced by doping into bulk bands. We do not find any indication of a strong coupling to the A1g mode or the presence of a Kohn anomaly in the surface phonon spectrum. The weak intrinsic electron-phonon coupling guarantees long-lived quasiparticles at elevated temperatures

Proximity Eliashberg theory of electrostatic field-effect-doping in superconducting films

We calculate the effect of a static electric field on the critical temperature of a s-wave one band superconductor in the framework of proximity effect Eliashberg theory. In the weak electrostatic field limit the theory has no free parameters while, in general, the only free parameter is the thickness of the surface layer where the electric field acts. We conclude that the best situation for increasing the critical temperature is to have a very thin film of a superconducting material with a strong increase of electron-phonon (boson) constant upon charging.Comment: 9 pages, 5 figure

Electron–phonon coupling and superconductivity in a 2D Tl–Pb compound on Si(111)

[EN] Electron-phonon interaction in a single-layer Tl-Pb compound on Si(111) is investigated within the density-functional theory and linear-response approach in the mixed-basis pseudopotential representation. It is found that phonon-induced scattering of electrons at the Fermi level is primarily determined by surface electronic states responsible for bonding at the interface and by low-energy, predominantly shear-vertical vibrations of adatoms. The contribution of substrate-localized vibrations involved in the electron-phonon scattering turns out to be small. We have also estimated the superconducting transition temperature T-c by solving the linearized gap equation of the Eliashberg theory. An analysis of phonon-mediated transitions for a number of electronic states in the Tl-Pb surface bands showed that the strength of the coupling varies with the binding energy, increasing as it approaches the Fermi level, and significantly depends on the surface band to which the state belongs.This work was supported by the University of the Basque Country (Grants no. GIC07-IT-366-07 and No. IT-756-13) and the Spanish Ministry of Science and Innovation (Grant no. FIS2016-75862-P). The authors acknowledge support by the state of Baden-Wurttemberg through bwHPC

Ab initio lattice dynamics and electron-phonon coupling of Bi(111)

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).We present a comprehensive ab initio study of structural, electronic, lattice dynamical, and electron-phonon coupling properties of the Bi(111) surface within density functional perturbation theory. Relativistic corrections due to spin-orbit coupling are consistently taken into account. Changes of interatomic couplings are confined mostly to the first two bilayers, resulting in superbulk modes with frequencies higher than the optic bulk spectrum, and in an enhanced density of states at lower frequencies for atoms in the first bilayer. We give results for the momentum-dependent electron-phonon coupling of electronic states belonging to the two surface electronic bands along ΓM¯ which cross the Fermi energy. For larger momenta, the lower surface band exhibits a moderate electron-phonon coupling of about 0.45, which is larger than the coupling constant of bulk Bi. For momenta close to Γ¯, states of both surface bands show even stronger couplings because of interband transitions to bulk states near Γ¯ around the Fermi level. For these cases, the state-dependent Eliashberg functions exhibit pronounced peaks at low energy and strongly deviate in shape from a Debye-type spectrum, indicating that an extraction of the coupling strength from measured electronic self-energies based on this simple model is likely to fail.The work of M.A.O. and T.S.R. was partially funded by US Department of Energy, DE-FG02-07ER46354.Peer Reviewe

Temperature studies of Raman spectra in MnBi2Te4 and MnSb2Te4 magnetic topological insulators

Raman spectra of magnetic topological crystalline insulators in a wide temperature range including the magnetic ordering region are studied in detail. The anharmonicity parameters and Grüneisen mode parameters of Raman-active phonons in the studied crystals have been determined. It has been shown that the temperature dependence of the frequency of the (~48 cm–1) phonon in MnBi2Te4 coincides within ±0.1 cm–1 with the standard anharmonic model disregarding the spin–phonon coupling. The polarization dependences of Raman spectra in the MnSb2Te4 crystals indicate that Sb and Mn atoms are strongly mixed in them unlike the isostructural MnBi2Te4 crystals.This work was supported by the Azerbaijan Ministry of Science and Education (program “Development of the Preparation Technology of Multifunctional Convertors Based on Nanostructures”). E.V.C. acknowledges the s-upport of St. Petersburg State University (project no. 94031444).Peer reviewe

Electron-phonon coupling in surface electronic states on Be(101̅ 0)

8 páginas, 6 figuras, 3 tablas.-- PACS number(s): 63.20.kd, 68.47.De, 73.20.−r.-- et al.We present an ab initio study of the electron-phonon interaction in surface electronic states on Be(101̅ 0). The calculations based on density-functional theory were carried out using a linear-response approach and a mixed-basis pseudopotential method. It is shown that the strength of the electron-phonon coupling is sensitive to the energy and momentum position of an electronic state and varies from 0.16 to 0.54. The difference in the electron-phonon interaction of two Shockley-type surface states at Ā can be understood on the basis of their different energy positions in the projected band gap. Previous experiments addressing the electron-phonon coupling strength of these two surface states are discussed in light of our theoretical findings.We acknowledge financial support of the University of the Basque Country UPV/EHU (Grant No. GIC071T36607), the Departamento de Educación del Gobierno Vasco, and the Spanish Ministerio de Ciencia y Tecnología (MCyT) (Grant No. FIS 2007-66711-C01-01).Peer reviewe

Vibrational properties of Cu(100 ) − c(2×2) − Pd surface and subsurface alloys

Using interaction potentials from the embedded-atom method we investigated the structural and vibrational properties of a Cu(100)−c(2×2)−Pd surface alloy and an underlayer c(2×2) alloy with a mixed CuPd second layer. The calculated surface phonon frequencies are in agreement with the experimental values obtained by electron energy-loss spectroscopy. From the calculated local phonon densities of states we find that surface effects are most pronounced in the first two layers for both systems studied. The results also indicate a very strong Pd-Cu bonding accompanied by a weaker bonding of the Cu surface atoms to their nearest neighbors. This has considerable influence on the surface phonon frequencies.This work was supported by the Ministry of Science of Russia Grant No. 40.012.1.1.1153 and the University of Basque CountryPeer reviewe

Vibrations in binary and ternary topological insulators: First-principles calculations and Raman spectroscopy measurements

Density-functional perturbation theory and Raman spectroscopy are used in the study of vibrations in bulk V 2VI 3 binary and ternary topological insulators as well as in ultrathin films composed of one and two quintuple layers. Full dispersion curves along the Brillouin zone symmetry directions are calculated and analyzed and the zone-center Raman active modes are compared to the experimental results. The calculations of the two quintuple layer films reveal modes that correspond to displacements of entire quintuple layer blocks. These modes have low frequencies and are Raman active. The phonon dispersion curves of one and two quintuple layer Bi 2Se 3 films are compared with recent helium atom scattering experiments. © 2012 American Physical Society.K.A.K. and O.E.T. acknowledge financial support by the RSSF and RFBR (Grant No. 12-02-00226). We acknowledge partial support from the University of the Basque Country (Project No.GV-UPV/EHU and Grant No. IT-366-07) and Ministerio de Ciencia e Inovación (Grant No. FIS2010-19609-C02-00).Peer Reviewe

Electron-phonon coupling in topological surface states: The role of polar optical modes

The use of topological edge states for spintronic applications could be severely hampered by limited lifetimes due to intrinsic many-body interactions, in particular electron-phonon coupling. Previous works to determine the intrinsic coupling strength did not provide a coherent answer. Here, the electron-phonon interaction in the metallic surface state of 3D topological insulators is revised within a first principles framework. For the archetypical cases of Bi2Se3 and Bi2Te3, we find an overall weak coupling constant of less than 0.15, but with a characteristic energy dependence. Derived electronic self-energies compare favorably with previous angle-resolved photoemission spectroscopy results. The prevailing coupling is carried by optical modes of polar character, which is weakly screened by the metallic surface state and can be reduced by doping into bulk bands. We do not find any indication of a strong coupling to the A1g mode or the presence of a Kohn anomaly in the surface phonon spectrum. The weak intrinsic electron-phonon coupling guarantees long-lived quasiparticles at elevated temperatures.We acknowledge the support by the Basque Departamento de Education, UPV/EHU (Grant No. IT-756-13), Spanish Ministerio de Economia y Competitividad (MINECO Grant No. FIS2013-48286-C2-2-P), and Ministerio de Ciencia e Innovación de España (Grant No. FIS2016-75862-P). Partial support by the Saint Petersburg State University Project No. 15.61.202.2015 is also acknowledged. We further acknowledge support by Deutsche Forschungsgemeinschaft and Open Access Publishing Fund of Karlsruhe Institute of Technology.Peer Reviewe