Ultrafast transient response and electron-phonon coupling in the iron-pnictide superconductorBa(Fe1−xCox)

Abstract

The transient response of Ba(Fe1−xCox)2As2, x=0.08 was studied by pump-probe optical reflectivity. After ultrafast photoexcitation, hot electrons were found to relax with two different characteristic times, indicating the presence of two distinct decay channels: a faster one, of less than 1 ps in the considered pump fluence range, and a slower one, corresponding to lattice thermalization and lasting ∼ = 6ps. Our analysis indicates that the fast relaxation should be attributed to preferential scattering of the electrons with only a subset of the lattice vibration modes, with a second moment of the Eliashberg function λ ω 2 ∼ = 64 meV 2 . The simultaneous excitation of a strong fully symmetric A1g optical phonon corroborates this conclusion and makes it possible to deduce the value of λ ∼ = 0.12. This small value for the electron-phonon coupling confirms that a phonon mediated process cannot be the only mechanism leading to the formation of superconducting pairs in this family of pnictides. PACS numbers: 74.70.Xa; 78.47.J-; 74.25.KcThe discovery of high temperature superconductivity in iron pnictide compounds in 2008(Refs. 1 and 2 ) has raised a lot of questions about the nature of this phenomenon. One of these questions concerns the role of electron-phonon (e-ph) coupling, which is at the heart of conventional Bardeen-Cooper-Shrieffer (BCS) superconductivity 3 .For unconventional superconductors, electron-lattice interaction mechanisms have been extensively studied in cuprates; these materials present some similarities with pnictides, such as the bidimensionality of the crystallographic structure with Cu-O planes instead of Fe-As ones and the presence of a magnetic phase in the underdoped part of the phase diagram 4,5 . While the electron pairing mechanism is still controversial in superconducting cuprates, some elements are today accepted: first, their high critical temperatures are not compatible with a BCS scheme; second, the e-ph interaction is anisotropic, as theoretically predicted 6 and verified by means of time-resolved experiments 7-11 ; indeed, the e-ph coupling constant is strongly mode-selective, and ranges from λ ∼ = 0.13 to λ ∼ = 0.55. This selectivity is linked to the marked bidimensional layered structure of cuprates, leading to a preferential coupling between electrons coming from a specific k-direction of the Fermi surface and one particular phonon mode 6,8 .Much less information is available on e-ph coupling in pnictides. Theoretical works on the 1111 family (LaFeAsO 1−x F x ), employing Density Functionnal perturbation theory, predicted an isotropic coupling, equally distributed for the whole phonon population, and too small to be responsible for superconductivity through a BCS type mechanism 5,12 . On the other hand, a very strong coupling (λ ∼ = 1) between electrons and the A 1g mode (consisting of a breathing movement of As atoms) was predicted by a model in which the electronic polarization of As atoms involve e-ph interaction 13 . For 122 compounds (doped AF e 2 As 2 , A=Ba, Sr, Ca) spin f...

Superconductivity in iron compounds

“…Many have pointed out that the electron-phonon coupling is too weak (by about a factor of 5, Osborn et al, 2009) in these materials to account for the >20 K T c s. Boeri, Dolgov, and Golubov (2008) Rettig et al (2010) found in the 122 parent compound EuFe 2 As 2 , using time resolved ARPES, that el-ph < 0:5 while Mansart et al (2010) (2010a), following the same analysis as used in their FeSe 1Àx isotope effect work, argued that, when adjusted for structural changes, the results of Liu et al and Shirage et al are also consistent with a conventional % 0:35-0:4. Obviously, the possible partial role of the phonons in superconductivity in these materials is still not entirely decided but the evidence from the isotope measurements to date, with the possible exception of the low T c FeSe 1Àx , argues against electron-phonon coupling as the primary pairing mechanism.…”

The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides

“…IV G, the electron-phonon mechanism appears to be too weak to give rise to the high T c values observed in the Fe-based superconductors. 274,461,[485][486][487][488] The proximity of the superconducting compositions to those with long-range antiferromagnetic/spin density wave order suggests that an electronic mechanism might be responsible that involves the exchange of antiferromagnetic spin fluctuations. Indeed, the experimental evidence is strong that the highest-T c Fe-based superconductors are spin singlet, s ± -wave superconductors with this superconducting mechanism.…”

Ultrafast probes of coherent oscillations in Fe-based superconductors

“…Therefore, understanding of couplings among those instabilities is essential to unveil the mechanism of HTSC. Various ultrafast techniques have revealed coherent lattice vibrations that strongly modulate the properties of Fe-based superconductors [1,2,[7][8][9][10][11][12]. Although the electron-phonon coupling (EPC) on its own fails to account for the large gap and the high superconducting transition temperature, it is still considered as one of the important ingredients for the paring possibly in conjunction with the magnetic interaction [13][14][15][16].…”