What is new for spin-glass in a quasi-2D system*

In conventional spin glasses, magnetic interaction is not strongly anisotropic and the entire spin system is believed to be frozen below the spin-glass transition temperature. Along {\em any} direction, spin correlations are highly disordered. In La$_2$Cu$_{0.94}$Li$_{0.06}$O$_4$, for which the in-plane exchange interaction dominates the interplane one, only a fraction of spins with antiferromagnetic correlations extending to neighboring planes become spin-glass. The remaining spins with only in-plane antiferromagnetic correlations remain spin-liquid at low temperature. Spin correlations are highly disordered only along the interlayer direction, but highly ordered in-plane. Such a novel partial spin freezing out of a two-dimensional spin-liquid observed in this cold neutron scattering study is likely due to a delicate balance between disorder and quantum fluctuations in the quasi-two dimensional $S$=1/2 Heisenberg system.