Abstract
Low-energy spin excitation in a spin S =1/2 two-leg ladder system SrCu 2 O 3 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, Δ, is demonstrated to open with a value of 680 K, which is in quantitative agreement with the theoretical value of Δ∼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, ξ/ a ( a : lattice spacing between Cu atoms along the chain), remains finite with (ξ/ a )∼13, comparable to a value in La 2 CuO 4 near T N . This class of cuprate is thus a frustrated quntum antiferromagnet (AF) with a spin gap.
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