Spin Gap at the Phase Transition between Spin Liquid and Long-Range Ordered State in La_1-xSr_xCuO_{2. 5}

抄録

Two-leg spin S=1/2 antiferromagnetic (AF) spin ladder compound LaCuO_{2. 5} resides near the quantum critical point. The spin gap at the quantum phase transition from the spin liquid state to the pseudo AF long-range ordered state at about 120 K was investigated by Raman scattering. As temperature increases from 20 K, a dip appears at about 300cm_-1 in the magnetic excitation spectrum and develops toward low energy keeping the upper bound at 320 cm_-1 and reaches ω∼0 at about 120 K. The in-gap states decrease with further increase of temperature and the gap opens at about 200 K. The low temperature phase below 120 K is not a simple Neel state.