NMR and NQR studies on transition-metal arsenide superconductors LaRu<sub>2</sub>As<sub>2</sub>, KCa<sub>2</sub>Fe<sub>4</sub>As<sub>4</sub>F<sub>2</sub>, and A<sub>2</sub>Cr<sub>3</sub>As<sub>3</sub>*
Jun Luo, Chunguang Wang, Zhicheng Wang, Qi Guo, Ji Yang, Rui Zhou, K Matano, T Oguchi, Zhian Ren, Guanghan Cao, Guo-Qing Zheng
Abstract
We report 75 As-nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on transition-metal arsenides LaRu 2 As 2 , KCa 2 Fe 4 As 4 F 2 , and A 2 Cr 3 As 3 . In the superconducting state of LaRu 2 As 2 , a Hebel–Slichter coherence peak is found in the temperature dependence of the spin-lattice relaxation rate 1/ T 1 just below T c , which indicates that LaRu 2 As 2 is a full-gap superperconducor. For KCa 2 Fe 4 As 4 F 2 , antiferromagnetic spin fluctuations are observed in the normal state. We further find that the anisotropy rate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>R</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">AF</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mi>T</mml:mi> <mml:mn>1</mml:mn> <mml:mi>c</mml:mi> </mml:msubsup> <mml:mo>/</mml:mo> <mml:msubsup> <mml:mi>T</mml:mi> <mml:mn>1</mml:mn> <mml:mrow> <mml:mi>a</mml:mi> <mml:mi>b</mml:mi> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> is small and temperature independent, implying that the low energy spin fluctuations are isotropic in spin space. Our results indicate that KCa 2 Fe 4 As 4 F 2 is a moderately overdoped iron-arsenide high-temperature superconductor with a stoichiometric composition. For A 2 Cr 3 As 3 ( A = Na, K, Rb, Cs), we calculate the electric field gradient by first-principle method and assign the 75 As-NQR peaks to two crystallographically different As sites, paving the way for further NMR investigation.