Recently, it was proposed that the orbital ordering of ${\mathrm{<ruby><rb>\pi </rb><rt>ぱい</rt></ruby>}}_{x,y}^{*}$ molecular orbitals in the superoxide ${\mathrm{CsO}}_2$ compound leads to the formation of spin-1/2 chains below the structural phase transition occurring at $T_{\mathrm{s}1}=61 \mathrm{K}$ on cooling. Here we report a detailed $X$-band electron paramagnetic resonance (EPR) study of this phase in ${\mathrm{CsO}}_2$ powder. The EPR signal appears as a broad line below $T_{\mathrm{s}1}$, which is replaced by the antiferromagnetic resonance below the Néel temperature $T_{\mathrm{N}}=8.3 \mathrm{K}$. The temperature dependence of the EPR linewidth between $T_{\mathrm{s}1}$ and $T_{\mathrm{N}}$ 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 ${\mathrm{CsO}}_2$, which is therefore a unique $p$-orbital system where such a state could be studied.

• Received 10 March 2015
• Revised 28 April 2015

DOI:https://doi.org/10.1103/PhysRevB.91.174419