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Varelogiannis, G.; Perali, Andrea; Cappelluti, E.; Pietronero, L.

doi:10.1103/physrevb.54.r6877

Cited by 38 publications

(54 citation statements)

References 27 publications

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“…46 It is important to notice that the d-wave state we obtain is anisotropic and its anisotropies are driven by the ARDOS anisotropies in the same way as for the s-wave state discussed in the previous section. 48 This is clear in Figs. 9 and 10, where the d-wave solution away from the (/2,/2) direction has exactly the form of the s-wave solution.…”

Section: A Marginality Of the Superconducting Gap Symmetrymentioning

confidence: 74%

“…We will give an alternative interpretation of this behavior in the framework of MD. 48 Our basic observation is that the system behaves as if the superconductivity at ⌫ϪM direction is not influenced by the superconductivity at the ⌫ϪX direction, and this is precisely the implication of MD.…”

Section: B Temperature Dependence Of the Shape Of The Anisotropymentioning

confidence: 95%

“…47͒ indicate an anisotropic s-wave gap having T-dependent anisotropy that will be shown later to be evidence in support of MD. 48 However, various phase sensitive 49 and node sensitive 50 experiments on YBa 2 Cu 3 O 7 report evidence of a sign reversal of the order parameter supporting d waves, 51 even though experimental contradictions still persist for Y-Ba-Cu-O ͑Refs. 52, 53͒ that will be shown later to be understood within MD as resulting from the orthorhombic distortion of the CuO 2 planes.…”

Section: A Marginality Of the Superconducting Gap Symmetrymentioning

confidence: 99%

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Phonon-mediated unconventional superconductivity in strongly correlated systems

Varelogiannis

1998

Phys. Rev. B

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We propose an adiabatic approach for Hubbard models in the Fermi-liquid regime coupled to phonons. The Hubbard parameters are associated with the bandwidth W via an interpolation formula between the trivial for W strong-and weak-coupling limits of Hubbard models. Phonons are introduced via an adiabatic randomphase approximation scheme. We obtain simple conditions for phonon-driven instabilities in a Fermi liquid with short-ranged interactions. We report phonon-driven instabilities without nesting and describe the elimination of the Peierls instability in a nested system by Coulomb correlations. We also report the possibility for a phonon-driven phase separation ͑PS͒ instability as well as the strong enhancement of forward processes in the effective electron-phonon scattering near the phase separation instability. We show that the proximity to PS induces momentum decoupling ͑MD͒ in superconductivity which implies a tendency for decorrelation between the physics in the different regions of the Fermi surface. MD could induce anisotropic superconductivity with unconventional gap symmetry such as d wave. Whether anisotropy in the high-T c oxides is driven by MD or by anisotropic scattering ͑for example with spin fluctuations͒ becomes a crucial question. We discuss some qualitative implications of MD that explain puzzling qualitative aspects of superconductivity in the oxides and could advocate that MD is at the origin of anisotropies. Such effects are the marginality of the superconducting gap symmetry for the condensation free energy and the resulting possibility of gap symmetry transitions with the doping, the temperature dependence of the shape of the anisotropy, and the behavior of the anomalous dip above the gap in the density of states. We also show that in the MD regime the orthorhombic distortion of the CuO 2 planes in YBa 2 Cu 3 O 7 could be sufficient to explain the mixing of s-gap components in the dominantly d-wave gap. On the other hand, if spin fluctuations mediate the pairing in YBa 2 Cu 3 O 7 , at least 25% of the condensate must be located in the chains. Our analysis could rehabilitate phonons as potential mediators of the pairing in all ''unconventional'' superconductors including heavy-fermion and organic compounds.

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Section: A Marginality Of the Superconducting Gap Symmetrymentioning

confidence: 74%

Section: B Temperature Dependence Of the Shape Of The Anisotropymentioning

confidence: 95%

Section: A Marginality Of the Superconducting Gap Symmetrymentioning

confidence: 99%

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Phonon-mediated unconventional superconductivity in strongly correlated systems

Varelogiannis

1998

Phys. Rev. B

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“…The resulting anisotropy of the gap is a consequence of the small-q form of the interfacial EPI (cf. [36]). Taking the maximum required excitation energy at the gap edge to calculate the gap over T c ratio, we obtain the strong coupling (non-BCS) value ∆/k B T c =2.1 (in contrast to the BCS value ∆/k B T c =1.76).…”

mentioning

confidence: 99%

Multiband full-bandwidth anisotropic Eliashberg theory of interfacial electron-phonon coupling and high−Tc superconductivity in FeSe
Aperis
¹
,
Oppeneer
²

2018
Phys. Rev. B
41
10
77
1
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We examine the impact of interfacial phonons on the superconducting state of FeSe/SrTiO3 developing a materials' specific multiband, full bandwidth, anisotropic Eliashberg theory for this system. Our selfconsistent calculations highlight the importance of the interfacial electron-phonon interaction, which is hidden behind the seemingly weak coupling constant λm=0.4, in mediating the high-Tc, and explain other puzzling experimental observations like the s-wave symmetry and replica bands. We discover that the formation of replica bands has a Tc decreasing effect that is nevertheless compensated by deep Fermi-sea Cooper pairing which has a Tc enhancing effect. We predict a strong coupling dip-hump signature in the tunneling spectra due to the interfacial coupling.Superconductivity in monolayer-thick FeSe on SrTiO 3 reaches amazingly high transition temperatures of typically T c =50-70 K [1-6] and up to 100 K [7], much higher than the 8-K value of bulk FeSe [8]. A coupling between SrTiO 3 phonons and FeSe electrons occurs at the FeSe/SrTiO 3 interface, which manifests itself as electron replica bands [6]. The value of this coupling is estimated by experiments to be around 0.4, thus it is commonly believed to moderately enhance T c but not be enough to explain it [6,9,10].The superconducting state in iron-based superconductors is customarily associated with residual spin fluctuations due to the remnant quasi-nesting between electron and hole Fermi sheets that give rise to a sign alternating gap [11]. However, for FeSe/STO the situation is markedly different. Charge transfer at the interface induces electron doping in FeSe [3,4], leading to a distinct Fermi surface consisting of only two electron sheets around the corners of the tetragonal Brillouin zone (M point) [12]. The observed anisotropic superconducting gap has a more conventional form with plain s-wave symmetry [13] and is thus nodeless in the entire Brillouin zone. Furthermore, angular resolved photoemission spectroscopy (ARPES) measurements [6] reveal an interfaceinduced electron-phonon interaction (EPI) between FeSe electrons and polar STO phonons that is strongly peaked at the q=0 phonon wavevector [6,[14][15][16][17][18]. There is growing experimental evidence for the pivotal role of such interfacial phonons in engineering high-T c heterostructures that involve FeSe [5,9,19] or even FeAs [20] monolayers.Although ab initio calculations confirm the existence of small-q phonons as a strictly interfacial phenomenon in FeSe/STO [12,[21][22][23] and indicate the importance of the coupling between substrate phonons and FeSe electrons [24], the estimated low value of the electron-phonon coupling constant (λ ≤ 0.4) has been widely considered insufficient to explain the impressive T c enhancement unless another, dominant pairing mechanism is at play [6]. On the other hand, Eliashberg calculations within a single band model suggest that interfacial phonons may lead to the high T c with a coupling of merely half of that estimated by experiments [25]. However, a mater...

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“…This assumption, although it has not been based on solid physical arguments or computations, nevertheless has been succesfull in interpreting many puzzling features of the high-T c superconductors. Some features are the peak in the microwave conductivity [9], the momentum dependence of the anomalous dip above the gap and the enhancement of the anisotropy close to T c reported by ARPES on BiSr 2 CaCu 2 O 8 [10], as well as the presence of different gap symmetries in different oxides and even variations of the gap symmetry with doping [11]. An effectively similar hypothesis of small-q scattering in the oxides is actually investigated by many authors in different contexts [12].…”

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confidence: 71%

Small- q electron-phonon scattering and linear dc resistivity in high- T _c oxides

Varelogiannis

Economou

1998

Europhys. Lett.

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We examine the effect on the DC resistivity of small-q electron-phonon scattering, in a system with the electronic topology of the high-Tc oxides. Despite the fact that the scattering is dominantly forward, its contribution to the transport can be significant due to "ondulations" of the bands in the flat region and to the umpklapp process. When the extended van-Hove singularities are sufficiently close to EF the acoustic branch of the phonons contribute significantly to the transport. In that case one can obtain linear T dependent resistivity down to temperatures as low as 10 K, even if electrons are scattered also by optical phonons of about 500K as reported by Raman measurements.One of the most puzzling experimental facts in the optimally doped oxides is the linear temperature dependence of their DC resistivity for very low temperatures. In Bi 2 (Sr 0.97 P r 0.03 ) 2 CuO 6 the DC resistivity is linear from 10K up to several hundreds of Kelvins [1]. This has been considered as a sign of absence of electron phonon scattering. In fact, eventhough a linear T dependence of the DC resistivity is also possible in the case of electronphonon scattering, this happens for temperatures that are higher than about one fourth of the characteristic phonon energies. From this perspective, the data of Ref.[1] indicate a complete absence of electron-phonon scattering for frequencies above 50K, while the optical phonons in the cuprates extend up to about 1000K.To explain the linear T -dependence of the DC resistivity, several scenarios avoiding electron-phonon scattering have been proposed. A first approach was the so called Marginal Fermi Liquid [2], where the linearity was evidence of deviation from the classical Fermi liquid behavior in the case of some special collective excitation scattering. A second type of approach was that of gauge theory models where the linearity is a signature of a complete breakdown of Landau Fermi Liquid theory replaced by a quasi-one dimensional Luttinger liquid behavior [3]. A third type of approach was that of a nearly antiferomagnetic Fermi liquid [4], in which case there is singular electron-paramagnon scattering supposed to be also at the origin of d-wave superconductivity. Finally a linear T -dependent resistivity has also been associated with the presence of van Hove singularities in the vicinity of the Fermi surface and simple Coulomb scattering [5]. The relevance of the last two approaches has been questioned recently by Hlubina and Rice [6].Although in all the previous scenarios electron-phonon scattering is neglected, there is strong evidence from Raman experiments that there is significant coupling of the electrons with the optical oxygen vibrations [7], and the absence of signature of this coupling in the DC resistivity is an unresolved puzzle. It also appears rather unphysical that phonons could be irrelevant for the T -dependence of the DC resistivity at so high temperatures.We will propose here an alternative scenario that could reconcile the Raman and DC transport results. In this ...

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Density-of-states-driven anisotropies induced by momentum decoupling inBi2Sr2

Cited by 38 publications

References 27 publications

Phonon-mediated unconventional superconductivity in strongly correlated systems

Phonon-mediated unconventional superconductivity in strongly correlated systems

Multiband full-bandwidth anisotropic Eliashberg theory of interfacial electron-phonon coupling and high−Tc superconductivity in FeSe
Aperis
¹
,
Oppeneer
²

2018
Phys. Rev. B
41
10
77
1
View full text Add to dashboard Cite

Small- q electron-phonon scattering and linear dc resistivity in high- T _c oxides

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Density-of-states-driven anisotropies induced by momentum decoupling inBi2Sr2

Cited by 38 publications

References 27 publications

Phonon-mediated unconventional superconductivity in strongly correlated systems

Phonon-mediated unconventional superconductivity in strongly correlated systems

Multiband full-bandwidth anisotropic Eliashberg theory of interfacial electron-phonon coupling and high−Tc superconductivity in FeSeAperis1, Oppeneer2 2018Phys. Rev. B4110771View full textAdd to dashboardCite

Small- q electron-phonon scattering and linear dc resistivity in high- T c oxides

Contact Info

Product

Resources

About

Multiband full-bandwidth anisotropic Eliashberg theory of interfacial electron-phonon coupling and high−Tc superconductivity in FeSe
Aperis
¹
,
Oppeneer
²

2018
Phys. Rev. B
41
10
77
1
View full text Add to dashboard Cite

Small- q electron-phonon scattering and linear dc resistivity in high- T _c oxides