Observation of a Spin Gap in Sr${\mathrm{Cu}}_2$${\mathrm{O}}_3$ Comprising Spin-½ Quasi-1D Two-Leg Ladders

Magnetic properties of Sr${\mathrm{Cu}}_2$${\mathrm{O}}_3$ and ${\mathrm{Sr}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_5$, containing two-leg and three-leg $S=\frac{1}{2}$ ladders made of antiferromagnetic Cu-O-Cu linear bonds, were investigated. The susceptibility of the two-leg ladder material, Sr${\mathrm{Cu}}_2$${\mathrm{O}}_3$, is characteristic of thermal excitation from a nonmagnetic ground state with a spin gap of about 420 K, while the susceptibility of ${\mathrm{Sr}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_5$, containing three-leg ladders, reflects a gapless spin excitation spectrum. The temperature dependence of the nuclear-spin lattice relaxation rate, $\frac{1}{T_1}$, of ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ NMR also indicates the existence of a spin gap only for Sr${\mathrm{Cu}}_2$${\mathrm{O}}_3$. The spin gap we observed for Sr${\mathrm{Cu}}_2$${\mathrm{O}}_3$ confirms a recent theoretical prediction for the magnetic behavior of spin = ½ antiferromagnetically coupled Heisenberg chains.