We report $^{75}$As NMR measurements on the new quasi one-dimensional superconductor K$_{2}$Cr$_{3}$As$_{3}$ ($T_{c} sim 6.1$~K) [J. K. Bao et al., Phys. Rev. X {bf 5}, 011013 (2015)]. We found evidence for strong enhancement of Cr spin fluctuations
above $T_c$ in the [Cr$_{3}$As$_{3}$]$_{infty}$ double-walled subnano-tubes based on the nuclear spin-lattice relaxation rate $1/T_{1}$. The power law temperature dependence, $1/T_{1}T sim T^{-gamma}$ ($gamma sim 0.25$), is consistent with the Tomonaga-Luttinger liquid. Moreover, absence of the Hebel-Slichter coherence peak of $1/T_{1}$ just below $T_{c}$ suggests unconventional nature of superconductivity.
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 cr
itical 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.
Application of pressures or electron-doping through Co substitution into Fe sites transforms the itinerant antiferromagnet BaFe(2)As(2) into a superconductor with the Tc exceeding 20K. We carried out systematic transport measurements of BaFe(2-x)Co(x
)As(2) superconductors in pressures up to 2.5GPa, and elucidate the interplay between the effects of electron-doping and pressures. For the underdoped sample with nominal composition x = 0.08, application of pressure strongly suppresses a magnetic instability while enhancing Tc by nearly a factor of two from 11K to 21K. In contrast, the optimally doped x=0.20 sample shows very little enhancement of Tc=22K under applied pressure. Our results strongly suggest that the proximity to a magnetic instability is the key to the mechanism of superconductivity in iron-pnictides.
We report 19-F NMR investigation of the new high temperature superconductor LaFeAsO(0.89)F(0.11) (Tc ~ 28K). We demonstrate that low frequency spin fluctuations exhibit pseudo gap behavior above Tc. We also deduce the London penetration depth lambda from NMR line broadening below Tc.
The CoO$_{2}$ layers in sodium-cobaltates Na$_{x}$CoO$_{2}$ may be viewed as a spin $S=1/2$ triangular-lattice doped with charge carriers. The underlying physics of the cobaltates is very similar to that of the high $T_{c}$ cuprates. We will present
unequivocal $^{59}$Co NMR evidence that below $T_{CO}sim51 K$, the insulating ground state of the itinerant antiferromagnet Na$_{0.5}$CoO$_{2}$ ($T_{N}sim 86 K$) is induced by charge ordering.
