SPIN GAP BEHAVIOR IN LADDER-TYPE OF QUASI-ONE-DIMENSIONAL SPIN (S=1/2) SYSTEM SRCU2O3

Low-energy spin excitation in a spin S=1/2 two-leg ladder system SrCu2O3 has been investigated by Cu NMR. From an activated decrease of the nuclear-spin lattice relaxation rate, 1/T-1, in a T-range of 100-300 K, a spin gap, Delta, is demonstrated to open with a value of 650 K, which is in quantitative agreement with the theoretical value of Delta similar to 0.5 J= 650 K, if the same value of 1300 K as in the square lattice system is used as the antiferromagnetic exchange coupling constant, J. It is furthermore deduced from the measurement of the spin-echo decay rate, 1/T-2G, that a magnetic coherence length, xi/a (a: lattice spacing between Cu atoms along the chain), remains finite with (xi/a) similar to 13, comparable to a value in La2CuO4 near T-N. This class of cuprate is thus a frustrated quntum antiferromagnet (AF) with a spin gap.