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

RIKEN Accel. Prog. Rep. 51 (2018)

Study of Magnetic Ordering by p-orbital in RbO2 using µSR
F. Astuti,∗1,∗2 M. Miyajima,∗3 R. Asih,∗2,∗4 I. Watanabe,∗1,∗4 and T. Kambe∗3
Alkali metal superoxides AO2 (A= Na, K, Rb, Cs)present an interesting example of magnetic materials based on p-elements. This become the first example of an inorganic quantum spin system with unpaired π-electrons.1) Alkalimetal superoxides adopt
the rocksalt-type crystal structure and two oxygenatoms form a dumbbell shaped structure sharing one
excess electron, O2 − , which is known as the “superoxide” anion. This leads to one unpaired spin (S=1/2)
in a pair of degenerate π ∗ (antibonding) molecular orbitals. The magnetic ordering of KO2 , RbO2 , andCsO2 have been observed at temperatures of 7 K, 15 K,
and 9.6 K, respectively by using specific heat measurement.2) The Tomonaga Luttinger Liquid (TLL) model
suggested for CsO2 is supposed to present a fieldinduced magnetic order related to the TLL state.3)
Therefore, detailed investigation on the magnetic
properties near or in the zero-field (ZF) condition isstrongly required to describe the magnetically orderedstate appearing in CsO2 and other alkali metal superoxides. Last year, we have carried out µSR measurements in RbO2 at the RIKEN-RAL muon facility byusing the pulsed muon beam. At that time, we feltthat the sample quality was not so good. For that reason, in the next beam time, we improved the sample
quality and measured the new batch of RbO2 samples.
No clear muon-spin precession was seen at any temperature (Fig. 1), however the decrease in the initialasymmetry around the suggested TN was clearly observed. The anomaly was also observed at ∼15 K as
shown in Fig. 2(a).
The asymmetry parameter can represent the magnetic volume fraction. By comparing the asymmetry
parameters in Fig. 2(b), it seems that the new sample
has bigger magnetic volume fraction than the old sample. It means that we succesfully improved the sample
quality.
The decrease in the initial asymmetry possiblymeans that the magnetically ordered state appears
causing the depolarization behavior faster than the observable limit of the pulsed muon facility. This orderedstate might accomodate the fast muon-spin precession.
Therefore, it is indispensable to test RbO2 by using
the continuous muon beam in order to detect clear evidence of the appearance of magnetically ordered states.
As we expected, the result of µSR measurement at PSI,
Switzerland, by using continuous muon beam showed
clear-muon spin precession at the temperature ∼15 K,
∗1
∗2
∗3
∗4

Department of Condensed Matter Physics, Hokkaido University
RIKEN Nishina CenterDepartment of Physics, Okayama University
Department of Physics, Osaka University- 204 -Fig. 1. ZF-µSR time spectrum for RbO2 for the first 10
microsecond from 17 K down to base temperature.

((aa()a) )(b)Fig. 2. (a) Temperature dependence of the initial asymmetry and relaxation rate of the ZF-µSR time spectrum measured at the RIKEN-RAL Muon Facility. The
anomaly in the µSR measurement is observed at 15 K
around the suggested TN . (b) Comparison between thenew and old RbO2 samples. The result of the old sample has been reported in RIKEN APR 2016 Vol.50.

indicating long-range magnetic ordered state (the result is not shown in this report).
References
1) S. Riyadi et al., Phys. Rev. Lett. 108, 217206 (2012).
2) A. Zumsteg et al., Phys. Cond. Matter, 267–291 (1974).
3) M. Klanjsek et al., Phys. Rev. Lett. 115, 057205 (2015).