S=12quantum critical spin ladders produced by orbital ordering inBa2CuTeO6

The ordered hexagonal perovskite ${\mathrm{Ba}}₂{\mathrm{CuTeO}}₆$ hosts weakly coupled $S=\frac{1}{2}$ spin ladders produced by an orbital ordering of ${\mathrm{Cu}}²⁺$. The magnetic susceptibility $\ensuremath{\chi}(T)$ of ${\mathrm{Ba}}₂{\mathrm{CuTeO}}₆$ is well described by that expected for isolated spin ladders with exchange coupling of $J\ensuremath{\approx}86\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ but shows a deviation from the expected thermally activated behavior at low temperatures below ${T}^*\ensuremath{\approx}25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. An anomaly in $\ensuremath{\chi}(T)$, indicative of magnetic ordering, is observed at ${T}_{\mathrm{mag}}=16\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. No clear signature of long-range ordering, however, is captured so far in NMR $1/{T}₁$, specific heat or neutron diffraction measurements at and below ${T}_{\mathrm{mag}}$. The marginal magnetic transition, indicative of strong quantum fluctuations, is evidence that ${\mathrm{Ba}}₂{\mathrm{CuTeO}}₆$ is in very close proximity to a quantum critical point between magnetically ordered and spin-gapped phases controlled by interladder couplings.