Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 49 (2016)

Spindynamics
dynamics for p electronsSpinelectronsin
inCsO
CsO22andandNaO
NaO22
D. P. Sari,䠆1,䠆2 R. Asih,䠆1,䠆2 I. Watanabe,䠆2 A. Kumar,䠆3 and G. Blake䠆3Alkali metal superoxides (AO2) such as NaO2, KO2,RbO2, and CsO2 forming ionic crystals that adopt crystal
structures related to rocksalt structures are ideal model
system for the study of the p-electron magnetism. The
magnetic properties of KO2 and RbO2 systems have alreadybeen well investigated, unlike those of NaO2 and CsO2,
because of the ease of fabricating the sample. In a
superoxide anion (O2)-, there is one unpaired spin (S = ½)
because three electrons occupy a pair of degenerate
antibonding S* molecular orbitals. AO2 compounds undergo
structural distortion splitting the S* energy levels and lifting
the orbital degeneracy similar to the Jahn-Teller effect fordegenerate orbitals. Orbital ordering similar to that found intransition metal systems may be expected, but has scarcely
been studied experimentally.
The neutron diffraction, EPR, and AFMR measurements
revealed the crystal structure of KO2, RbO2, and CsO2 to be
FM in the crystallographic ab plane and AFM along thecrystallographic c axis.
From the NMR study on CsO2 by Klanjsek et al., a huge,nonmonotonic temperature dependence of the exchange
coupling originating from thermal liberations of O2molecules above ~70 K was observed. TomonagaLutingger liquid behavior (~15K – 40K) was observed, inwhich antiferromagnetic (AFM) spin chains are formed as aresult of p-orbital ordering, and then magnetic phasetransition occurs again jumping to the ordered Neel state at

Fig. 1. ZF-PSR time spectrum for CsO2 for the firstmicrosecond from 10 K down to base temperature.

Fig. 2. Internal Field from at least 2 muon sites in CsO 2.
around ~10 K [1]. The latter supports the prediction ofsusceptibility measurement in which an anomaly in themagnetic susceptibility was observed around 10 K,
indicating a possible crossover from 1D chain character
which was fit with the original Boner-Fisher formulation(15 K – 70 K) to 3D AFM ordering [2].
We performed PSR measurement at the Continuous
Muon Facility PSI, Switzerland. We obtained time spectra
show a non-ordered state still until 10 K and then theinternal field appeared at around 8 K from at least 2 muonsites in CsO2 and tends to increase with decreasing
temperature [Figs. 1 and 2]. There is no clear enhancementin the depolarization rate due to the spin dynamics observed
around the transition temperature. We confirm that thissample is of good quality based on our susceptibility
measurement with a small contribution of the paramagneticstate in the low temperature regime.
The sample quality of NaO2 revealed by oursusceptibility measurement was not very good. We onlyobserved a paramagnetic state from the M-H curve.
Unfortunately, we could not obtain PSR data from the PSI
for NaO2 owing to a cryostat problem. We only have
ZF-PSR data obtained using the Pulsed Muon at theRIKEN-RAL Muon Facility in the UK. The initialasymmetry from room temperature to 6 K shows agradually decreasing trend, indicating fast depolarization
behavior with a small volume fraction. Furthermore, in thenext measurement at the RIKEN-RAL, we plan todetermine whether NaO2 is magnetically ordered in theground state, like CsO2.
References

1) M. Klanjsek et al., Phys. Rev. Lett. 115, 057205 (2015).
2) S. Riyadi et al., Phys. Rev. Lett. 108, 217206 (2012).

RIKEN Nishina Center
䠆2
Department of Physics, Osaka University
䠆3
Zernike Institute for Advanced Materials, University of Groningen
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