One-dimensional quantum antiferromagnetism in the p-orbital CsO₂ compound revealed by electron paramagnetic resonance

Recently, it was proposed that the orbital ordering of pi(x,y)* molecular orbitals in the superoxide CsO2 compound leads to the formation of spin-1/2 chains below the structural phase transition occurring at T-s1 = 61 K on cooling. Here we report a detailed X-band electron paramagnetic resonance (EPR) study of this phase in CsO2 powder. The EPR signal appears as a broad line below T-s1, which is replaced by the antiferromagnetic resonance below the Neel temperature T-N = 8.3 K. The temperature dependence of the EPR linewidth between T-s1 and T-N agrees with the predictions for the one-dimensional Heisenberg antiferromagnetic chain of S = 1/2 spins in the presence of symmetric anisotropic exchange interaction. Complementary analysis of the EPR line shape, linewidth, and the signal intensity within the Tomonaga-Luttinger liquid (TLL) framework allows for a determination of the TLL exponent K = 0.48. Present EPR data thus fully comply with the quantum antiferromagnetic state of spin-1/2 chains in the orbitally ordered phase of CsO2, which is therefore a unique p-orbital system where such a state could be studied