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 $[{\mathrm{Cu}}_{2}{(\mathrm{pz})}_{3}{(4\text{\ensuremath{-}}\mathrm{HOpy})}_{4}]{({\mathrm{ClO}}_{4})}_{4}$ (pz=pyrazine; 4-HOpy=4-hydroxypyridine). This material is identified as a spin-1/2 Heisenberg ladder system with exchange-coupling parameters ${J}_{rung}/{k}_{B}=12.1(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and ${J}_{leg}/{k}_{B}=10.5(3)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ $[{J}_{rung}/{J}_{leg}=1.15(4)]$. For single crystals our measurements revealed two critical fields, ${\ensuremath{\mu}}_{0}{H}_{c1}=4.63(5)\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ and ${\ensuremath{\mu}}_{0}{H}_{c2}=22.78(5)\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ (for $H\ensuremath{\parallel}{a}^{*}$), 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.