Quasi-One-Dimensional Spin Excitations in the Iron Pnictide <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>NaFe</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.53</mml:mn> </mm

Spectroscopic measurements in model 1D correlated systems offer insights for understanding their two-dimensional counterparts, which include the cuprate and iron pnictide/chalcogenide superconductors. A major challenge is the identification of such correlated systems with dominantly 1D physics. In this Letter, inelastic neutron scattering measurements on NaFe_{0.53}Cu_{0.47}As single crystal directly reveal quasi-1D spin excitations, resulting from atomic order that leads to magnetic Fe and nonmagnetic Cu chains. The dominant exchange interaction is antiferromagnetic along the chain [SJ_{∥}≈90.1(3) meV], whereas the inter-chain couplings are much weaker [SJ_{⊥}≈-2.4(1) meV and SJ_{c}≈0.15(5) meV]. The quasi-1D spin excitations in NaFe_{0.53}Cu_{0.47}As stem from both the Néel and stripe vectors, with Néel excitations sensitive to Fe impurities on the Cu site. The spin excitations in quasi-1D NaFe_{0.53}Cu_{0.47}As and quasi-2D FeSe exhibit a striking resemblance, suggesting a common origin for their coexistent stripe and Néel excitations. Our findings demonstrate magnetic dilution in NaFeAs leads to dimension reduction of its magnetic degree of freedom, presenting a strategy for discovering low-dimensional quantum materials.