Isolated dimers in a 2D-like quantum spin system Li$_{x}$Cu$_{2-z}$Zn$_{z}$O$_{2}$

The existence of isolated dimers induced by the non-magnetic dopant Zn in a 2D-like quantum spin system Li$_{x}$Cu$_{2-z}$Zn$_{z}$O$_{2}$ is found. Low Zn substitution ($<$ 5 {\%} substituted CuO$_{2}$ chain) reduces spiral ordering temperature and a finite size effect is found to support a picture of phase separated domains with aggregated dimers near Zn dopant centers at the domain boundaries. Higher Zn substitution ($>$ 5{\%} substituted CuO2 chain) introduces a novel phase transition to show a cusp shape anomaly on the magnetic susceptibility. The isotropic magnetic susceptibility reduction for high Zn level at low temperature suggests an incomplete spin gap opening and the magnetic-field dependence of the new transition temperature shows a character of weak spin-phonon coupling. Moreover, susceptibility data fitting using isolated dimer plus frustrated spin chain model describes the magnetic behaviors of samples of high Zn level between 2 - 400 K well.