Dynamical spin-orbital correlations versus random singlets in Ba3CuSb2O9 investigated by magnetization and electron spin resonance

High-field magnetization and high-frequency electron spin resonance (ESR) are employed to differentiate magnetism between an orthorhombic and a hexagonal majority phase of Ba3CuSb2O9. For the orthorhombic sample, an ESR signal changes its temperature dependence at T-S similar to 200 K, suggesting a static Jahn-Teller (JT) ordering. A magnetization curve follows a power-law behavior M similar to H-alpha m with the exponent alpha(m) = 0.72 +/- 0.06 for 8 < H < 26 T and alpha(m) = 1.06 +/- 0.04 for H > 26 T. The ESR linewidth exhibits a critical-like divergence, Delta H-pp(T) proportional to T-alpha with the exponents of alpha = 0.22 +/- 0.07 and 0.32 +/- 0.04. The sublinear magnetization and the critical ESR line broadening are taken as evidence of a random singlet state. For the hexagonal sample, both Delta H-pp(T) and g factor are described by the same thermally activated process with the energy barrier of 300 K. This evidences intrinsic coupling of spins to orbital degrees of freedom and thereby gives support for a dynamic spin-orbital entangled state. Our results demonstrate that magnetism in the spin-orbital coupled compound Ba3CuSb2O9 is dictated by a spatiotemporal structure of the JT distortions.