We have investigated the low-temperature magnetism of sodium superoxide (${\mathrm{NaO}}_2$), in which spin, orbital, and lattice degrees of freedom are closely entangled. The magnetic susceptibility shows anomalies at $T_1=220$ K and $T_2=190$ K, which correspond well to the structural phase transition temperatures, and a sudden decrease below $T_3=34$ K. At 4.2 K, the magnetization shows a clear stepwise anomaly around 30 T with a large hysteresis. In addition, the muon spin relaxation experiments indicate no magnetic phase transition down to $T=0.3$ K. The inelastic neutron scattering spectrum exhibits magnetic excitation with a finite energy gap. These results confirm that the ground state of ${\mathrm{NaO}}_2$ is a spin-singlet state. To understand this ground state in ${\mathrm{NaO}}_2$, we performed Raman scattering experiments. All the Raman-active libration modes expected for the marcasite phase below $T_2$ are observed. Furthermore, we find that several new peaks appear below $T_3$. This directly evidences the low crystal symmetry, namely, the presence of the phase transition at $T_3$. We conclude that the singlet ground state of ${\mathrm{NaO}}_2$ is due to the spin-Peierls instability.

Close

PrevNext

View figure in article

• Molecular magnetism
• 1-dimensional spin chains
• Inelastic neutron scattering
• Muon spin relaxation & rotation
• X-ray diffraction