Inelastic x-ray scattering study of superconductingSmFeAsO1−xFysingle crystals: Evidence for strong momentum-dependent doping-induced renormalizations of optical phonons

Abstract

We report inelastic x-ray scattering experiments on the lattice dynamics in SmFeAsO and superconducting SmFeAsO 0.60 F 0.35 single crystals. Particular attention was paid to the dispersions along the ͓100͔ direction of three optical modes close to 23 meV, polarized out of the FeAs planes. Remarkably, two of these modes are strongly renormalized upon fluorine doping. These results provide significant insight into the energy and momentum dependence of the coupling of the lattice to the electron system and underline the importance of spin-phonon coupling in the superconducting iron pnictides. DOI: 10.1103/PhysRevB.80.220504 PACS number͑s͒: 74.25.Kc, 63.20.Ϫe, 74.25.Gz, 78.70.Ck The recent discovery of superconductivity in FeAs-based compounds has stimulated considerable interest. 1 To date three distinct families of FeAs-based superconductors with respective formulas XFeAs ͑X =Li or Na͒, MFe 2 As 2 ͑M =Ca,Ba,Sr͒, and REFeAsO ͑where RE is a rare-earth metal͒-the so-called "111's," "122's," and "1111's"-have been identified. Among them, the last family has the highest values of the superconducting transition temperature T c : depending on the lanthanide, T c can be as high as 55 K. 2 The high value of T c and early DFT calculations 3 initially appeared to support the notion of an exotic mechanism for superconductivity in these materials. However, recent experiments have established that the role of phonons in the electron coupling cannot be totally ignored. In particular, the observation of large Fe isotope effects 4 and the fact that the Fe-As configuration is intimately related to T c ͑Ref. 5͒ and to the magnetic state of the iron sublattice 6,7 show that the lattice is actively involved in this problem. In addition, substantial deviations have been found between the experimentally observed phonon density of states ͑PDOS͒ and ab initio calculations, 8 suggesting that these calculations do not capture all the relevant physics. A spin-polarized calculation is, for example, required to obtain good agreement with the experimental phonon dispersion in the case of undoped CaFe 2 As 2 , even above the magnetic ordering temperature T N . 9 Furthermore, it was recently demonstrated that zonecenter Raman active phonons are strongly renormalized at T N in Ba 1−x K x Fe 2 As 2 ͑Ref. 10͒ and Ba͑Fe 1−x Co x ͒ 2 As 2 . 11 These results all highlight the role of spin-phonon coupling in iron pnictides.In previous experiments on NdFeAsO 1−x F x powders, an unexpected signature of the doping in the PDOS was discovered. 12 In particular, it was found that even at room temperature, at least one of the three phonon branches located around 23 meV softens on doping. All the three modes are polarized out of the ͑Fe,As͒ planes, parallel to the c axis, and involve motions of either the As or Fe ions. At least one of the modes couples directly to the magnetic iron sublattice. 7,13 Here, we utilize inelastic x-ray scattering ͑IXS͒ to investigate the phonon dispersions of SmFeAsO and superconducting SmFeAsO 0.60 F 0.35 single crystals, with...

Effect of pressure, temperature, fluorine doping, and rare earth elements on the phonon density of states ofLFeAsO studied by nuclear inelastic scattering

“…On the other hand, the presence of this effect also in the Nd compound, where impurities were not observed, suggests the universality of such an effect. Similar increase of the DPS at ∼20 meV upon doping was also observed by inelastic x-ray scattering 19,21 in Nd and Sm compounds. The effect was explained by the shift of the peak position due to the doping.…”

Coupling of the AsA1gphonon to magnetism in iron pnictides

“…Free electron part. The band structure of the system is taken into account via the free electron term H 0 , (2) where c † kμσ creates an electron on the Fe d-orbital μ with spin σ and wave vector k, and c † iμσ represents the same operator in real space. F r μν (k) is the electronic k-dispersion relation 24 and μ is the crystal field.…”

“…Several phononic spectroscopic signatures show unconventional behavior in the magnetic state. [1][2][3][4][5][6][7][8] At the theoretical level, ab initio calculations show that the lattice constants and the phonon frequencies depend sensitively on the presence of magnetism, 9,10 and the comparison with experimental measurements is improved when magnetism is included in the calculations. 9,[11][12][13][14][15] Furthermore, the electron-phonon coupling has been shown to be enhanced by magnetism.…”

Anomalous dependence ofc-axis polarized FeB1gphonon mode with Fe and Se concentrations in Fe1Um1, Subedi2, Toulemonde3 et al. 2012Phys. Rev. BSelf Cite326402View full textAdd to dashboardCite

“…For example, Raman spectroscopy has proved useful in the understanding of the structural, magnetic and electronic properties of superconducting pnictides. [23][24][25][26][27][28][29][30][31][32][33][34][35][36] Available Raman data on single crystals of Fe 1+y Te 1−x Se x is to date limited. In refs.…”