S=1/2 quantum critical spin ladders produced by orbital ordering in Ba₂CuTeO₆

The ordered hexagonal perovskite Ba2CuTeO6 hosts weakly coupled S = 1/2 spin ladders produced by an orbital ordering of Cu2+. The magnetic susceptibility chi(T) of Ba2CuTeO6 is well described by that expected for isolated spin ladders with exchange coupling of J approximate to 86 K but shows a deviation from the expected thermally activated behavior at low temperatures below T* approximate to 25 K. An anomaly in chi(T), indicative of magnetic ordering, is observed at T-mag = 16 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 Tmag. The marginal magnetic transition, indicative of strong quantum fluctuations, is evidence that Ba2CuTeO6 is in very close proximity to a quantum critical point between magnetically ordered and spin-gapped phases controlled by interladder couplings