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Register a new userThe electron-phonon theory of superconductors: Tc map and vertex correction
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W. Fan
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The strong coupling Eliashberg theory plus vertex correction is used to calculate maps of transition temperature (Tc) in parameter-space characterizing superconductivity. Based on these Tc maps, crossover behaviors are found when electron-phonon interaction increases from weak-coupling region to strong coupling region. Especially, the combined interaction of vertex correction and Coulomb interaction can efficiently depress Tc from extremely high values in standard strong-coupling theory to reasonable values found in experiments and successfully explain the doing-dependent Tc of cuprate superconductors. The strong non-adiabatic effect is the barrier for high-Tc in compounds with compositions of light atoms and with high phonon frequencies.
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The phonon-mode decomposition of the electron-phonon coupling in the MgB2-like system Li_{1-x}BC is explored using first principles calculations. It is found that the high temperature superconductivity of such systems results from extremely strong coupling to only ~2% of the phonon modes. Novel characteristics of E_2g branches include (1) ``mode lambda values of 25 and greater compared to a mean of $sim 0.4$ for other modes, (2) a precipitous Kohn anomaly, and (3) E_2g phonon linewidths within a factor of ~2 of the frequency itself, indicating impending breakdown of linear electron-phonon theory. This behavior in borne out by recent inelastic x-ray scattering studies of MgB2 by Shukla et al.
Electron-phonon coupling (EPC) is one of the most common and fundamental interactions in solids. It not only dominates many basic dynamic processes like resistivity, thermal conductivity etc, but also provides the pairing glue in conventional superconductors. But in high-temperature superconductors (HTSC), it is still controversial whether or not EPC is in favor of paring. Despite the controversies, many experiments have provided clear evidence for EPC in HTSC. In this paper, we briefly review EPC in cuprate and iron-based superconducting systems revealed by Raman scattering. We introduce how to extract the coupling information through phonon lineshape. Then we discuss the strength of EPC in different HTSC systems and possible factors affecting the strength. The comparative study between Raman phonon theories and experiments allows us to gain insight into some crucial electronic properties, especially superconductivity. Finally we summarize and compare EPC in the two existing HTSC systems, and discuss what role it may play in HTSC.
In this paper we discuss the normal and superconducting state properties of two pnictide superconductors, LaOFeAs and LaONiAs, using Migdal-Eliashberg theory and density functional perturbation theory. For pure LaOFeAs, the calculated electron-phonon coupling constant $lambda=0.21$ and logarithmic-averaged frequency $omega_{ln}=206 K$, give a maximum $T_c$ of 0.8 K, using the standard Migdal-Eliashberg theory. Inclusion of multiband effects increases the Tc only marginally. To reproduce the experimental $T_c$, a 5-6 times larger coupling constant would be needed. Our results indicate that standard electron-phonon coupling is not sufficient to explain superconductivity in the whole family of Fe-As based superconductors. At the same time, the electron-phonon coupling in Ni-As based compounds is much stronger and its normal and superconducting state properties can be well described by standard Migdal-Eliashberg theory.
The electronic structure, optical and x-ray absorption spectra, angle dependence of the cyclotron masses and extremal cross sections of the Fermi surface, phonon spectra, electron-phonon Eliashberg and transport spectral functions, temperature dependence of electrical resistivity of the MB2 (M=Ti and Zr) diborides were investigated from first principles using the full potential linear muffin-tin orbital method. The calculations of the dynamic matrix were carried out within the framework of the linear response theory. A good agreement with experimental data of optical and x-ray absorption spectra, phonon spectra, electron-phonon spectral functions, electrical resistivity, cyclotron masses and extremal cross sections of the Fermi surface was achieved.
A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature ($T_{rm c}$). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving nonmagnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe overlap with each other, the effects of two orders on superconductivity remain perplexing. Here we construct the three-dimensional electronic phase diagram, temperature ($T$) against pressure ($P$) and isovalent S-substitution ($x$), for FeSe$_{1-x}$S$_{x}$, in which we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended nonmagnetic tetragonal phase emerges, where we find a striking enhancement of $T_{rm c}$. The completed phase diagram uncovers two superconducting domes with similarly high $T_{rm c}$ on both ends of the dome-shaped antiferromagnetic phase. The $T_{rm c}(P,x)$ variation implies that nematic fluctuations unless accompanying magnetism are not relevant for high-$T_{rm c}$ superconductivity in this system.
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