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Register a new userRaman scattering as a probe of charge nematic fluctuations in iron based superconductors
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We report Raman scattering measurement of charge nematic fluctuations in the tetragonal phase of BaFe$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$ (x=0.04) single crystals. In both systems, the observed nematic fluctuations are found to exhibit divergent Curie-Weiss like behavior with very similar characteristic temperature scales, indicating a universal tendency towards charge nematic order in 122 iron-based superconductors.
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Iron-based superconducting layered compounds have the second highest transition temperature after cuprate superconductors. Their discovery is a milestone in the history of high-temperature superconductivity and will have profound implications for high-temperature superconducting mechanism as well as industrial applications. Raman scattering has been extensively applied to correlated electron systems including the new superconductors due to its unique ability to probe multiple primary excitations and their coupling. In this review, we will give a brief summary of the existing Raman experiments in the iron-based materials and their implication for pairing mechanism in particular. And we will also address some open issues from the experiments.
Nematicity is ubiquitous in electronic phases of high-$T_c$ superconductors, particularly in the Fe-based systems. We used inelastic x-ray scattering to extract the temperature-dependent nematic correlation length $xi$ from the anomalous softening of acoustic phonon modes in FeSe, underdoped Ba(Fe$_{0.97}$Co$_{0.03}$)$_2$As$_2$ and optimally doped Ba(Fe$_{0.94}$Co$_{0.06}$)$_2$As$_2$. In all cases, we find that $xi$ is well described by a power law $(T-T_0)^{-1/2}$ extending over a wide temperature range. We attributed this mean-field behavior and the extended fluctuation regime to a sizable nemato-elastic coupling, which may be detrimental to superconductivity.
Electronic nematicity is often found in unconventional superconductors, suggesting its relevance for electronic pairing. In the strongly hole-doped iron-based superconductors, the symmetry channel and strength of the nematic fluctuations, as well as the possible presence of long-range nematic order, remain controversial. Here, we address these questions using transport measurements under elastic strain. By decomposing the strain response into the appropriate symmetry channels, we demonstrate the emergence of a giant in-plane symmetric contribution, associated with the growth of both strong electronic correlations and the sensitivity of these correlations to strain. We find weakened remnants of the nematic fluctuations that are present at optimal doping, but no change in the symmetry channel of nematic fluctuations with hole doping. Furthermore, we find no evidence for a nematic-ordered state in the AFe$_2$As$_2$(A = K, Rb, Cs) superconductors. These results revise the current understanding of nematicity in hole-doped iron-based superconductors.
Based on a two-band model, we study the electronic Raman scattering intensity in both normal and superconducting states of iron-pnictide superconductors. For the normal state, due to the match or mismatch of the symmetries between band hybridization and Raman vertex, it is predicted that overall $B_{1g}$ Raman intensity should be much weaker than that of the $B_{2g}$ channel. Moreover, in the non-resonant regime, there should exhibit a interband excitation peak at frequency $omegasimeq 7.3 t_1 (6.8t_1)$ in the $B_{1g}$ ($B_{2g}$) channel. For the superconducting state, it is shown that $beta$-band contributes most to the $B_{2g}$ Raman intensity as a result of multiple effects of Raman vertex, gap symmetry, and Fermi surface topology. Both extended $s$- and $d_{xy}$-wave pairings in the unfolded BZ can give a good description to the reported $B_{2g}$ Raman data [Muschler {em et al.}, Phys. Rev. B. {bf 80}, 180510 (2009).], while $d_{x^2-y^2}$-wave pairing in the unfolded BZ seems to be ruled out.
Using a variational Monte Carlo method, we investigate the nematic state in iron-base superconductors based on a three-band Hubbard model. Our results demonstrate that the nematic state, formed by introducing an anisotropic hopping order into the projected wave function, can arise in the underdoped regime when a realistic off-site Coulomb interaction $V$ is considered. {color {red} We demonstrate that the off-site Coulomb interaction $V$, which is neglected so far in the analysis of iron-base superconductors, make a dominant contribution to the stabilization of nematic state. We calculate the doping dependencies of the anisotropic properties such as the unequal occupation of $d_{xz}$ and $d_{yz}$ orbitals, anisotropies of kinetic energy and spin correlations, and show that they are all suppressed upon electron doping, which are consistent with the intrinsic anisotropies observed by optical spectrum measurement and ARPES experiments.
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