28 August 2022 to 2 September 2022
Science and Technology Campus, University of Parma
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Mott-insulating state of alkali-metal clusters in sodalite studied by $\mu$SR

P-THU-17
1 Sept 2022, 17:20
1h 40m
Science and Technology Campus, University of Parma

Science and Technology Campus, University of Parma

University of Parma, Italy
Poster Strongly correlated electron systems Posters

Speaker

Prof. Takehito Nakano (Ibaraki University)

Description

In Mott insulators, band electrons are localized due to strong electron-electron interactions. Although the s-electrons of alkali metals are very delocalized, by confining them in the periodic nanospace of zeolite crystals and making them moderately localized, such a strongly correlated electron system can be created.$^1$ In sodalite, $\beta$-cages with an inner diameter of 0.7 nm are arranged in a bcc structure. By loading alkali atoms, an $A_4^{3+}$ cluster ($A$: alkali atom) is formed in the cage. The cluster has one unpaired s-electron. Antiferromagnetic order of Mott insulating state has been identified in $A =$ Na, K, and K-Rb alloy clusters.$^2$ $T_N$ systematically increases from 50 K (Na) to 90 K (K-Rb alloy). In ZF-$\mu^+$SR, a uniform local field is observed below $T_N$, and its value is higher for clusters with heavier chemical compositions.$^2$
To clarify the mechanism of the systematic change in the local field and its relation with the Mott-insulating state of this system, we investigate the muon Knight shift by high TF-$\mu^+$SR using NuTime at TRIUMF. We successfully obtained the hyperfine coupling constants between $\mu^+$ and the s-electron above $T_N$ from the $K-\chi$ plot. By combining the ZF-$\mu^+$SR local field,$^2$ we determined the size of the ordered moments, which systematically decreases from $\simeq$ 0.5 $\mu_B$ (Na) to $\simeq$ 0.3 $\mu_B$ (K-Rb alloy). It correlates perfectly with the increase in $T_N$, namely, the decrease in the electron correlation $U/t$ in the Mott-Hubbard model. From DFT calculations, we found that $\mu^+$ is in a hydride (Mu−) state at the cage center. This also explains that the systematic increase in the local field corresponds to the decrease in $U/t$ due to the shallower potential of the heavier alkali atoms.
$^1$T. Nakano and Y. Nozue, Adv. Phys.: X 2, 254-280 (2017).
$^2$T. Nakano et al., J. Phys. Soc. Jpn. 79, 073707-1-4 (2010).

Primary author

Prof. Takehito Nakano (Ibaraki University)

Co-authors

Mr Hiroyuki Baba (Ibaraki University) Dr Kazuki Ohishi (CROSS Neutron Science and Technology Center) Yipeng Cai (UBC&TRIUMF) Sungwon Yoon (TRIUMF/Chung-Ang University) Kenji M. Kojima (Centre for Molecular and Materials Science, TRIUMF, and Stewart Blusson Quantum Matter Institute, the University of British Columbia)

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