Disorder-induced Spin Gap in the Zigzag Spin-1/2 Chain Cuprate Sr_{0.9}Ca_{0.1}CuO_2

TL;DR

This study reveals that doping SrCuO_2 with calcium induces a spin gap in the zigzag spin-1/2 chain, evidenced by exponential decay in NMR relaxation rates, likely caused by impurity-driven exchange coupling alternation.

Contribution

It demonstrates that disorder from calcium doping can induce a spin gap in a zigzag spin chain, supported by NMR measurements and theoretical modeling.

Findings

Doped Sr_{0.9}Ca_{0.1}CuO_2 shows an exponential decrease in T_1^{-1} below 90 K.

Pure SrCuO_2 exhibits temperature-independent T_1^{-1}.

The spin gap is attributed to impurity-driven exchange coupling alternation.

Abstract

We report a comparative study of 63Cu Nuclear Magnetic Resonance spin lattice relaxation rates, T_1^{-1}, on undoped SrCuO_2 and Ca doped Sr_{0.9}Ca_{0.1}CuO_2 spin chain compounds. A temperature independent T_1^{-1} is observed for SrCuO_2 as expected for an S=1/2 Heisenberg chain. Surprisingly, we observe an exponential decrease of T_1^{-1} for T < 90,K in the Ca-doped sample evidencing the opening of a spin gap. The data analysis within the J_1-J_2 Heisenberg model employing density-matrix renormalization group calculations suggests an impurity driven small alternation of the J_2-exchange coupling as a possible cause of the spin gap.