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Abstract:
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.