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Register a new userPersistence of slow fluctuations in the overdoped regime of Ba(Fe1-xRhx)2As2 superconductors
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We present nuclear magnetic resonance evidence that very slow ($leq 1$ MHz) spin fluctuations persist into the overdoped regime of Ba(Fe$_{1-x}$Rh$_{x}$)$_2$As$_2$ superconductors. Measurements of the $^{75}$As spin echo decay rate, obtained both with Hahn Echo and Carr Purcell Meiboom Gill pulse sequences, show that the slowing down of spin fluctuations can be described by short-range diffusive dynamics, likely involving domain walls motions separating $(pi/a,0)$ from $(0,pi/a)$ correlated regions. This slowing down of the fluctuations is weakly sensitive to the external magnetic field and, although fading away with doping, it extends deeply into the overdoped regime.
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Most iron-based superconductors are characterized by the s+- symmetry of their order parameter, and are expected to go through a transition to the s++ state if enough disorder is introduced. We previously reported the observation of this transition in Ba(Fe1-xRhx)2As2 through a study of the disorder dependence of the critical temperature and low-temperature London penetration depth. In this paper we report on the analysis of the electrodynamic response of the same sample across the transition and we identify peculiarities in the behaviour of the surface resistance and normal conductivity, that can be considered as traces of the transition itself.
The evolution of 75As NMR parameters with composition and temperature was probed in the Ba(Fe1-xRux)2As2 system where Fe is replaced by isovalent Ru. While the Ru-end member was found to be a conventional Fermi liquid, the composition (x=0.5) corresponding to the highest Tc (20K) in this system shows an upturn in 75As 1/T1T below about 80 K evidencing the presence of antiferromagnetic (AFM) fluctuations. These results are similar to those obtained in another system with isovalent substitution BaFe2(As1-xPx)2 [Y. Nakai, T. Iye, S. Kitagawa, K. Ishida, H. Ikeda, S. Kasahara, H. Shishido, T. Shibauchi, Y. Matsuda, and T. Terashima, Phys. Rev. Lett. 105, 107003 (2010)] and point to the possible role of AFM fluctuations in driving superconductivity.
This work presents 75As NMR spin echo decay rate (1/T2) measurements in Ba(Fe1-xRhx)2As2 superconductors, for 0.041 < x < 0.094. It is shown that 1/T2 increases upon cooling, in the normal phase, suggesting the onset of an unconventional very low-frequency activated dynamic. The correlation times of the fluctuations and their energy barriers are derived. The motion is favored at large Rh content, while it is hindered by the application of a magnetic field perpendicular to the FeAs layers. The same dynamic is observed in the spin-lattice relaxation rate, in a quantitatively consistent manner. These results are discussed in the light of nematic fluctuations involving domain wall motion. The analogies with the behaviour observed in the cuprates are also outlined.
The orbital symmetries of electron doped iron-arsenide superconductors Ba(Fe1-xCox)2As2 have been measured with x-ray absorption spectroscopy. The data reveal signatures of Fe d electron itinerancy, weak electronic correlations, and a high degree of Fe-As hybridization related to the bonding topology of the Fe dxz+yz states, which are found to contribute substantially at the Fermi level. The energies and detailed orbital character of Fe and As derived unoccupied s and d states are found to be in remarkably good agreement with the predictions of standard density functional theory.
This paper has been withdrawn by the author due to some experimental mistakes. In this paper, we reported that C66, C44 and (C11-C12)/2 show remarkable softening toward the structural transition temperature TS. The data reported in this paper were acquired using the ultrasonic frequency lower than 25 MHz. Recently, we performed high-frequency measurements for the same system. We found that the anomaly of C44 and (C11-C12)/2 tend to disappear rapidly with increasing the frequency. On the other hand, C66 anomaly is still there at high frequencies. Therefore, we concluded that the observed anomalies in C44 and (C11-C12)/2 are not true. They would be ascribed to certain influence by the large softening of C66. So, we have checked our data through careful measurements by using ultrasonic frequency higher than 60 MHz, so far. Then, it has been found that C66 shows still nice softening toward TS, but that its temperature dependence is slightly different from the results of this paper. We have accumulated reliable data now. They will be reported in near future.
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