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

RIKEN Accel. Prog. Rep. 47 (2014)

SR remeasurement of La2CuO4 to reinvestigate muon sites
B. Adiperdana,*1,*2 T. Kawamata,*3 T. Adachi,*4 P. Kumar,*5 A. Amato, *5 I. A. Dharmawan,*2 R. E. Siregar,*2 S. Sulaiman,*6
M. I. Mohamed-Ibrahim,*6 I. Watanabe,*1,*2,*6 and Y. Koike*3
In a previous report,1) we have predicted a muon site in
La2CuO4 (LCO) with the tetragonal structure (the structureabove 550 K) by calculating the minimum potential energy
for muons, according to the density functional theory (DFT),and the hyperfine field contribution from Cu spins. We
predicted two possible muon sites in LCO, although a single
muon site with an internal field between 410-430 G wasexperimentally achieved2). Because of this disagreement,
we remeasured the internal field at the muon site using theSR method with higher statistics. In addition to thisexperimental effort, we also continued a similar estimation
to find all possible minimum potentials in the orthorhombicstructure (the structure that occurs below 550 K).
Zero-field (ZF) SR measurements were performed atthe RIKEN-RAL Muon Facility and PSI using a single
crystal of LCO in the magnetically ordered state at 10 K
and 1.7 K, respectively. The DFT calculation wasperformed by using the RIKEN integrated cluster of clusters(RICC) system using the Vienna ab-initio simulation
package (VASP). A supercell structure that contains 27 unit
cells (3×3×3 unit cells) was adopted for calculation taking
into account the effect of relaxations of the local lattice andmuon positions. The minimum potential in the static casewas used to determine the initial positions of lattice points
prior to final calculations with relaxation effects. The dipole
calculation was performed on the basis of theantiferromagnetic ordered state, which has been
experimentally determined by Vaknin et al.3) The magnetic
moment was traced from 0.10 to 0.70 B/Cu until thedipolar fields fit the internal fields experimentally
determined by ZF-SR.

B2 has the largest amplitude while B1 and B3 have muchsmaller amplitudes that are less than 1/30 of that of B2. Theinternal field of B2 corresponds to that experimentallyobserved.2) In terms of DFT calculations, three minimum
potentials were found in LCO from our current study, as
shown in Fig. 2. This result is qualitatively explained bythree muon positions in LCO.
We attempted to explain the observed internal fields on
the basis of the dipole-dipole interaction by tuning themagnitude of the magnetic moment of the Cu spin. As aresult, we found that the three observed components of theinternal field can be explained if the magnetic moment ofthe Cu spin is reduced to be around 0.23 B, as shown in
Fig. 3. Such a local reduction of the Cu spin is currently
being argued.4)
(a)

(b)

Fig. 2 (a) Minimum potential positions estimated fromDFT and (b) shape of the potential energy between minima.

Fig. 3 Comparison between calculated dipolar field
spectra and the internal field obtained from SR
experiments.

_Fig. 1 Fourier spectra of La2CuO4 ZF-SR obtained withhigh statistics in the RIKEN-RAL and PSI.
Fourier spectra of ZF-SR are shown in Fig. 1. Threecomponents of internal fields were found. We marked themas B1, B2, and B3 from the lower field component.
＊1

RIKEN Nishina Center＊2
Department of Physics, Universitas Padjadjaran＊3
Department of Applied Physics, Tohoku University＊4
Department of Engineering and Applied Physics, Sophia University＊5
Lab. For Muon Spin Spectroscopy, Paul Scherrer Institut＊6
Computational Chemistry and Physics Lab., Universiti Sains Malaysia

In conclusion, we found new additional muon sites inLCO from the ZF-SR experiment with higher statistics.
Our DFT calculation supported those three muon sites.
Assuming only the dipole-dipole interaction, we suggest thepossible local reduction of magnetic moment of the Cu spin
to be approximately 50%. The reason for this reduction iscurrently being discussed.
References
1)

2)
3)
4)

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B. Adiperdana et al.: RIKEN Accel. Prog. Rep. 46, 224(2012).
B. Hitti et al.: Hyp. Int. 63, 287 (1990).
Vaknin et al : Phys. Rev. Lett. 59, 2808 (1987).
R. De Renzi et al. : private communications.