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

RIKEN Accel. Prog. Rep. 50 (2017)

Magnetic ordering and spin dynamics driven by p-orbital in RbO2
F. Astuti,䠆1,䠆3 D. P. Sari䠆2,䠆3 I. Watanabe,䠆1,*3 M. Miyajima,䠆4 and T. Kambe䠆4Magnetism in the ʌ-electron system has attracted
attention for the possibility of the new kinds of the magnetic
informative materials. Alkali-metal superoxides AO2 (A=Na, K, Rb, Cs) present an interesting example of magnetic
materials on the basis of p-elements. These systems have adumbbell-type bonding state of O atoms forming theand resulting in one unpaired
valence state ofdumbbell. They further show theʌ-electron on thechanging of crystal structure introducing the splitting of thep-orbital degeneracy, similar to the Jahn-Taller effect.
Beside the crystallographic phase transition due to, the magnetic
molecular ordering of the disordered
order in alkali metal superoxide is interesting to study. In
the case of superoxides, the number of unpaired electrons isdumbbell, and magnetic
only one on thesuperexchange interaction is expected between those
unpaired spins through the A metal. Accordingly, a different
magnetically ordered state from that observed in the solid
oxygen molecule1) is expected in superoxides, but detailed
information on magnetic properties is still missing. The
magnetic ordering of KO2, RbO2, and CsO2 have been
observed at temperatures of 7 K, 15 K, and 9.6 K,
respectively, using specific heat measurement.2) The
Tomonaga Luttinger Liquid (TLL) model suggests that afield-induced magnetic order should appear in the CsO2 thatis related to the TLL state.3)

Fig. 1. ZF-PSR time spectra for CsO2 for the firstmicrosecond from 10 K down to base temperature.
Therefore, a detailed investigation on the magneticproperties near or in the zero-field (ZF) condition isstrongly required to describe the magnetically ordered statethat appears in the CsO2 and other alkali metal superoxides.

Fig. 2. Temperature dependence of the initial asymmetry
and relaxation rate Ȝ of the ZF-ȝ65WLPHVSHFWUD measured
at the RIKEN-RAL Muon Facility. The anomaly in the ȝ65
measurement is observed in between 10 and 15 K aroundthe suggested TN.
WeWeFDUULHG
carriedRXW
outȝ65μSR PHDVXUHPHQWV
measurementsLQ
in &V2CsO2 2 at
at thethe PSI
Switzerland using the continuous muon beam. Clear
spontaneous muon-spin precession behavior indicates theappearance of a long-range magnetic ordered state. This isevidence of the coherent static magnetically ordered state ofʌ-electrons in oxygen molecules. Another alkali-metal
superoxide, RbO2, which has a crystal structure similar torested at the RIKEN-RAL. In
that of CsO2 (CaC2-like), was tested
this system, only a type of anomaly in the magnetic
susceptibility of RbO2 is reported at TN ~ 15 K, asindicated.4) Unfortunately, we could not observe clear
muon-spin precession as shown in Fig.1, although thedecrease in the initial asymmetry around the suggested TN
was observed, as displayed in Fig. 2. The decrease in theinitial asymmetry DQG LQFUHDVH LQ WKH UHOD[DWLRQ UDWH Ȝ possibly means the magnetically ordered state appears,
causing a depolarization behavior that is faster than the time
limitation of the pulsed muon facility. This ordered statemight accommodate the fast muon-spin precession as wellas the case of CsO2. Therefore, it is necessary to test RbO2
at PSI using the continuous muon beam in order to detect
clear evidence of the appearance of magnetically orderedstates.
References
1) C. Uyeda et al., JPSJ, 54 (1985)
2) A. Zumsteg et al., Phys. Cond. Matter, 267-291 (1974)

1

Department of Physics, Hokkaido University
2
Department of Physics, Osaka University
3
RIKEN Nishina Center
4
Department of Physics, Okayama University3) M. Klansjek et al., PRL 115, 057205 (2015)4) M. Labhart et al., PRB 20, 53 (1979)- 239 -