Magnetic properties of a quantum spin ladder in proximity to the isotropic limit

We report on the synthesis, crystal structure, magnetic, thermodynamic, and electron-spin-resonance properties of the coordination complex [Cu-2(pz)(3)(4-HOpy)(4)](ClO4)(4) (pz=pyrazine; 4-HOpy=4-hydroxypyridine). This material is identified as a spin-1/2 Heisenberg ladder system with exchange-coupling parameters J(rung)/k(B) = 12.1(1) K and J(leg)/k(B) = 10.5(3) K [J(rung)/J(leg) = 1.15(4)]. For single crystals our measurements revealed two critical fields, mu H-0(c1) = 4.63(5) T and mu H-0(c2) = 22.78(5) T (for H parallel to a*), separating the gapped spin-liquid, gapless Tomonaga-Luttinger-liquid, and fully spin-polarized phase. No signature of a field-induced transition into a magnetically ordered phase was found at temperatures down to 450 mK. The material bridges an important gap by providing an excellent physical realization of an almost isotropic spin-1/2 strong-rung Heisenberg ladder system with modest exchange-coupling energy and critical-field scales.