Workshop "NMR, µSR, Mössbauer spectroscopies in the study of Fe-based and other unconventional high-Tc superconductors"
Very Slow Spin fluctuations in Ba(Fe1-xRhx)2As2 iron-based superconductors, revealed by Nuclear Magnetic Resonance Spectroscopy
Lucia Bossoni, Universitá degli Studi di Pavia, Italy
Within the iron-based superconductors, the close proximity of superconductivity to spin density wave (SDW) order, together with Fermi surface reconstruction and the strange-metal behavior have suggested the insurgence of a quantum critical phase transition, prompted by chemical pressure. The study of spin fluctuations, at finite temperature, is then fundamental to unveil these non-trivial states. Nuclear Magnetic Resonance (NMR) spectroscopy happens to be an ideal local probe for spin dynamic.
In this study we show that in the Ba(Fe1−xRhx)2As2 family of iron-pnictides, very slow spin fluctuations persist up to the over-doped region. Indeed, the spin-lattice relaxation rate (1/T1), and the spin-echo decay rate (1/T2) provide insights onto such spin dynamic. 75As NMR T2 measurements in Ba(Fe1−xRhx)2As2 superconductors are performed for a large range of doping . We show that 1/T2 increases upon cooling, in the normal phase, suggesting the onset of an unconventional very low-frequency dynamic. The motion is favored at large Rh content, and it is influenced by the magnetic field intensity. A further dynamic, evidenced by the enhanced spin-lattice relaxation rate, and the narrowing of the spectral linewidth, seems to be in agreement with the T2 phenomenology.
We firstly attempt to derive the correlation times of the fluctuations and their energy barriers. Secondly we study the interaction of the dynamic with static fields up to 17 T.
These results are discussed in the light of the J1-J2 Heisenberg model, where frustration may give rise to antiferromagnetic nematic fluctuations, involving domain walls motion . A second interpretation, based on the occurrence of fluctuating charge stripes is discussed in the light of the analogies with the behavior observed in the cuprates .
Finally we build a new phase diagram, based on our recent observations.
 L. Bossoni et al., Phys. Rev. B 88, 100503(R) (2013)
 P. Carretta et al., Phys. Rev. Lett. 88, 047601 (2002)
 T. Wu et al., Nature 477, 191 (2011)