$^{31}$P NMR studies of an iron-based superconductor Ba$_{0.5}$Sr$_{0.5}$Fe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ with $T_\mathrm{c}$ = 29 K

We report $^{31}$ P NMR studies of an oriented polycrystalline superconductor of Ba $_{0.5}$ Sr $_{0.5}$ Fe $_{2}$ (As $_{1 - x}$ P $_{x}$ ) $_{2}$ with $x \sim$ 0.4 ( $T_{c}$ = 29 K) at $T$ = 14 $-$ 325 K and $B_{0}$ = 1 and 5 T. The $^{31}$ P Knight shift $K_{a b}$ at $B_{0} ⊥ c$ shows a nearly $T$ -independent uniform spin susceptibility above $T_{c}$ and a spin singlet decrease below $T_{c}$ . The $^{31}$ P nuclear spin-lattice relaxation rate 1/ $T_{1}$ shows an asymptotic behavior of $a + b T$ ( $a$ and $b$ are constants) at $T >$ 100 K and the minimum at 40 K with an upturn toward $T_{c}$ . The $a$ term in 1/ $T_{1}$ indicates the presence of two-dimensional antiferromagnetic spin fluctuations. The negative $θ$ = $-$ 15 K of the Curie-Weiss-type antiferromagnetic spin susceptibility $χ$ ( $Q$ ) $\propto$ 1/( $T$ + $θ$ ) in the analysis of 1/ $T_{1} T$ suggests antiferromagnetic instability in the superconducting state. Discussions are made from the self-consistent renormalization (SCR) theory for the spin fluctuations with interlayer correlation.

$^{31}$ P NMR studies of an iron-based superconductor Ba $_{0.5}$ Sr $_{0.5}$ Fe $_{2}$ (As $_{1 - x}$ P $_{x}$ ) $_{2}$ with $T_{c}$ = 29 K

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$^{31}$ P NMR studies of an iron-based superconductor Ba $_{0.5}$ Sr $_{0.5}$ Fe $_{2}$ (As $_{1 - x}$ P $_{x}$ ) $_{2}$ with $T_{c}$ = 29 K

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3131P NMR studies of an iron-based superconductor Ba0.50.5​Sr0.50.5​Fe22​(As1−x1−x​Pxx​)22​ with TcTc​ = 29 K

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3131P NMR studies of an iron-based superconductor Ba0.50.5​Sr0.50.5​Fe22​(As1−x1−x​Pxx​)22​ with TcTc​ = 29 K

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$^{31}$ P NMR studies of an iron-based superconductor Ba $_{0.5}$ Sr $_{0.5}$ Fe $_{2}$ (As $_{1 - x}$ P $_{x}$ ) $_{2}$ with $T_{c}$ = 29 K

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$^{31}$ P NMR studies of an iron-based superconductor Ba $_{0.5}$ Sr $_{0.5}$ Fe $_{2}$ (As $_{1 - x}$ P $_{x}$ ) $_{2}$ with $T_{c}$ = 29 K

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