実験報告書様式（一般利用課題・成果公開利用）（ ※本報告書は英語で記述してください。ただし、産業利用課題として採択されている方は日本語で記述していただいても結構です。 ）MLF Experimental Report

提出日 Date of Report
June, 15 2016

課題番号 Project No.

装置責任者 Name of responsible person

2015A0161

Kenji Nakajima

実験課題名 Title of experiment
Magnetic excitations in the highly frustrated dimer magnet
Ba2CoSi2O6Cl2
実験責任者名 Name of principal investigator

装置名 Name of Instrument/（BL No.）AMATERAS (BL14)
実施日 Date of Experiment
March 23 to 26, 2016

Hidekazu Tanaka
所属 Affiliation
Tokyo Institute of technology

試料、実験方法、利用の結果得られた主なデータ、考察、結論等を、記述して下さい。（適宜、図表添付のこと）
Please report your samples, experimental method and results, discussion and conclusions. Please add figures andtables for better explanation.
1. 試料 Name of sample(s) and chemical formula, or compositions including physical form.
Ba2CoSi2O6Cl2

2. 実験方法及び結果 （実験がうまくいかなかった場合、その理由を記述してください。）
Experimental method and results.

If you failed to conduct experiment as planned, please describe reasons.

Ba2CoSi2O6Cl2 is magnetically described as an S=1/2 dimerized quantum magnet [1]. It is deduced from thecrystal structure that the exchange network in the dimer layer parallel to the ab plane is closely related to thatshown in Fig. 1(a), and the exchange interaction between layers is negligible. There are two kinds of interdimer
exchange interactions, as illustrated by thin solid and dashed lines in Fig. 1(a). These two interdimer exchangeinteractions act to interfere each other’s transfer of triplet. This situation is called “frustration” in the dimerizedmagnet. In the case of strong frustration, J=ʹ ≈ J×ʹ , the movement of triplet is much suppressed. In this situation,
magnetic excitations become dispersionless. When such a strongly frustrated dimerized magnet is subjected tomagnetic field, the ground state is determined by the Zeeman energy and the repulsive interaction between
triplets owing to the longitudinal€ component of the interdimer exchange interaction. With increasing magnetic
field, triplets are created on dimers at the critical field. However, the simultaneous occupation of triplets onneighboring dimers is unfavorable, because it causes a rapid increase in the interaction energy. Therefore, triplets
form a periodic array regarded as a “crystallization” to avoid the repulsive force between triplets [2]. With afurther increasing magnetic field, triplets occupy all the dimer sites, which leads to the saturation ofmagnetization. Consequently, magnetization varies stepwise. The magnetization curves in Ba2CoSi2O6Cl2
measured for two different field directions are sharply stepwise with a plateau at half of the saturation
magnetization [1]. This magnetization process is typical of the fully frustrated case, J=ʹ = J×ʹ .
In the present experiment, we measured magnetic excitations in Ba2CoSi2O6Cl2 using a cold-neutrondiskchopper spectrometer AMATERAS (BL14) installed at J-PARC, Japan. The wave vector ki of incident
neutron was first set to be parallel to the c* axis. As shown in Fig. €1(b), completely dispersionless three
excitations were observed at E=4-7 meV. The widths of these excitations are resolution-limited. Thus, all theexcitation peaks are single peaks that are not the superposition of two or more excitation peaks. These excitations
are considered to be related to Sz=±1 excitations, because the Sz=0 level is expected to be at E≈10 meV owing to1

2. 実験方法及び結果（つづき） Experimental method and results (continued)
strong XY anisotropy. Unexpected observation is that the intensities of these excitations have different Q
dependence, as shown in Figs. 1(c)-(e). The intensity of most intense middle excitation is independent of Q,
which is assigned as single singlet-triplet excitations to Sz=±1 states. On the other hand, the lower and higher
excitations are intense when both h and k are integers and half-integers, respectively. Because the intensities ofthese two excitations have the lattice periodicity, and the energy deference between the higher and middle
excitations and between the middle and lower excitations are order of the interdimer interaction [1], we candeduce that the interdimer interactions are responsible for these excitations. This verifies that interdimer
interactions in Ba2CoSi2O6Cl2 are fully frustrated, i.e., J=ʹ = J×ʹ . To the best of our knowledge, there is no reporton such dispersionless excitations with Q-dependent intensities. Although the origins of these excitations havenot been clearly identified yet, it is certain that the excitations of neighboring two dimers are strongly correlated.
One possible origin is follows: a single singlet-triplet
transition to Sz=0 state occurs. Before the triplet excitation€is relaxed to the singlet state, the neighboring dimer is excited to Sz=±1 states to form a bound state with the helpof the interdimer interactions.

Fig. 1: (a) Illustration of exchange network in Ba2CoSi2O6Cl2. Thick bonds are strong intradimer exchangeinteractions J, while thin solid and dashed bonds, J=ʹ and J×ʹ respectively, are weak interdimer exchangeinteractions. (b) Energy-momentum map of the scattering intensity along Q=(h, 0, 0) in Ba2CoSi2O6Cl2. Intensity
maps on Q=(h, k, 0) for energies between (c) 6.3 and 6.95 meV, (d) 5.3 and 6.3 meV and (d) 4.45 and 5.3 meV.

€€
References[1] H. Tanaka, N. Kurita, M. Okada, E. Kunihiro, Y. Shirata, K. Fujii, H. Uekusa, A. Matsuo, K. Kindo, and H.
Nojiri, J. Phys. Soc. Jpn. 83, 103701 (2014).
[2] T. M. Rice, Science 298, 760 (2002).

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