Spin-pairing and penetration depth measurements from nuclear magnetic resonance in NaFe$_{0.975}$Co$_{0.025}$As

We have performed $^{75}$As nuclear magnetic resonance (NMR) Knight shift measurements on single crystals of NaFe$_{0.975}$Co$_{0.025}$As to show that its superconductivity is a spin-paired, singlet state consistent with predictions of the weak-coupling BCS theory. We use a spectator nucleus, $^{23}$Na, uncoupled from the superconducting condensate, to determine the diamagnetic magnetization and to correct for its effect on the $^{75}$As NMR spectra. The resulting temperature dependence of the spin susceptibility follows the Yosida function as predicted by BCS for an isotropic, single-valued energy gap. Additionally, we have analyzed the $^{23}$Na spectra that become significantly broadened by vortices to obtain the superconducting penetration depth as a function of temperature with $lambda_{ab}(0) = 5,327 pm$ 78$,AA$.

Magnetic field dependence of spin-lattice relaxation in the s$pm$ state of Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$

The spatially averaged density of states, <N(0)>, of an unconventional d-wave superconductor is magnetic field dependent, proportional to $H^{1/2}$, owing to the Doppler shift of quasiparticle excitations in a background of vortex supercurrents[1,2]. This phenomenon, called the Volovik effect, has been predicted to exist for a sign changing $spm$ state [3], although it is absent in a single band s-wave superconductor. Consequently, we expect there to be Doppler contributions to the NMR spin-lattice relaxation rate, $1/T_1 propto <N(0)^2>$, for an $spm$ state which will depend on magnetic field. We have measured the $^{75}$As $1/T_1$ in a high-quality, single crystal of Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$ over a wide range of field up to 28 T. Our spatially resolved measurements show that indeed there are Doppler contributions to $1/T_1$ which increase closer to the vortex core, with a spatial average proportional to $H^2$, inconsistent with recent theory [4]

$^{75}$As NMR of Ba(Fe$_{0.93}$Co$_{0.07}$)$_{2}$As$_{2}$ in High Magnetic Field

The superconducting state of an optimally doped single crystal of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ was investigated by $^{75}$As NMR in high magnetic fields from 6.4 T to 28 T. It was found that the Knight shift is least affected by vortex supercurrents in high magnetic fields, $H>11$ T, revealing slow, possibly higher order than linear, increase with temperature at $T lesssim 0.5 , T_c$, with $T_c approx 23 , K$. This is consistent with the extended s-wave state with $A_{1g}$ symmetry but the precise details of the gap structure are harder to resolve. Measurements of the NMR spin-spin relaxation time, $T_2$, indicate a strong indirect exchange interaction at all temperatures. Below the superconducting transition temperature vortex dynamics lead to an anomalous dip in $T_2$ at the vortex freezing transition from which we obtain the vortex phase diagram up to $H = 28$ T.