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Register a new userGlassy Transition in the Vortex Lattice of Ba(Fe0.93Rh0.07)2As2 superconductor, probed by NMR and ac-susceptibility
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By using Nuclear Magnetic Resonance and ac-susceptibility, the characteristic correlation times for the vortex dynamics, in an iron-based superconductor, have been derived. Upon cooling, the vortex dynamics displays a crossover consistent with a vortex glass transition. The correlation times, in the fast motions regime, merge onto a universal curve which is fit by the Vogel-Fulcher law, rather than by an Arrhenius law. Moreover, the pinning barrier shows a weak dependence on the magnetic field which can be heuristically justified within a fragile glass scenario. In addition, the glass freezing temperatures obtained by the two techniques merge onto the de Almeida-Thouless line. Finally the phase diagram for the mixed phase has been derived.
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Beta-NMR has been used to study vortex lattice disorder near the surface of the high-Tc superconductor YBCO. The magnetic field distribution from the vortex lattice was detected by implanting a low energy beam of highly polarized 8Li into a thin overlayer of silver on optimally doped, twinned and detwinned YBCO samples. The resonance in Ag broadens significantly below the transition temperature Tc as expected from the emerging field lines of the vortex lattice in YBCO. However, the lineshape is more symmetric and the dependence on the applied magnetic field is much weaker than expected from an ideal vortex lattice, indicating that the vortex density varies across the face of the sample, likely due to pinning at twin boundaries. At low temperatures the broadening from such disorder does not scale with the superfluid density.
We demonstrate that the anisotropy R of the paramagnetic spin fluctuations grows toward Tc at 75As sites in the optimally electron-doped superconductor Ba[(Fe0.92Co0.08)2]2As2, with stronger spin fluctuations along the c-axis. Our finding is in remarkable contrast with the case of high T$_c$ cuprates, where R is independent of temperature above Tc.
We investigate the nature of the SDW (Spin Density Wave) transition in the underdoped regime of an iron-based high Tc superconductor Ba(Fe1-xCox)2As2 by 75As NMR, with primary focus on a composition with x = 0.02 (T_SDW = 99 K).We demonstrate that critical slowing down toward the three dimensional SDW transition sets in at the tetragonal to orthorhombic structural phase transition, Ts = 105 K, suggesting strong interplay between structural distortion and spin correlations. In the critical regime between Ts and T_SDW, the dynamical structure factor of electron spins S(q,Wn) measured with the longitudinal NMR relaxation rate 1/T1 exhibits a divergent behavior obeying a power law, 1/T1~S(q, Wn)~(T/T_SDW-1)^a with the critical exponent a ~ 0.33.
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.
Fundamental and higher harmonics of the AC magnetic susceptibility have been measured on a LaO_0.92F_0.08FeAs sample as a function of the temperature, at various amplitudes and frequencies of the AC magnetic field, with a small superimposed DC field parallel to the AC field. The granularity of the sample has been investigated and the inter-grain and intra-grain contributions have been clearly individuated looking at both the first and third harmonics. The vortex dynamics has been also analyzed, and a comparison with the magnetic behavior of both the MgB_2 and the cuprate superconductors has been performed. Some vortex dissipative phenomena, i.e. the thermally activated flux flow and the flux creep, have been detected in the presented measurements, similar to what obtained on YBCO. Nevertheless, although the general behavior is similar, several differences have been also evidenced between these different classes of superconductors, mainly in the third harmonics. We infer that different vortex dynamics have to be included into the analysis of the magnetic response in this iron-based new material.
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