Spin-gap formation due to spin-Peierls instability in $\ddot{I}€$-orbital-ordered NaO${}_2$

Abstract

We have investigated the low-temperature magnetism of sodium superoxide (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 NaO${}_2$ is a spin-singlet state. To understand this ground state in 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 the singlet-ground state of NaO${}_2$ due to the spin-Peierls instability.

6 pages, 3 figures