Fig1： Statistics showing beamtime demand (bargraph) and availability (line) for D1/D2 during each running period. ［enlarged view (131KB)］

　J-PARC MUSE began normal operations in April 2013 with the intention of hosting 24 user experiments (General Use) under the Inter-University Research Program, to be conducted on the muon D-line by the end of the 2013A term. Unfortunately, this plan was disrupted by the accident at the Hadron Experimental Facility, which occurred on May 23, 2013 and led to the immediate shutdown of the entire J-PARC facility for an indefinite period. This resulted in the cancellation of 14 experiments and some components of a Project Use experiment, and resulted in 48 out of 67 days in the 2013A term being unavailable for user runs. Although the effects of the accident were slightly mitigated by the fact that a long-term shutdown (six months) had originally been scheduled for the upgrading of LINAC in the summer of 2013, this delay continued until the belated resumption of operations of the Materials and Life Science Facility (MLF) in mid-February, 2014. As a result, only 20.5 days were allocated for General Use in the 2013B term and, to accommodate 10 user experiments within the available time, usage was reduced to only 2 days per user experiment, making the competition for beam time among users extremely intense (see Fig.1).

１. D-line: New μSR spectrometer with "Kalliope"

Fig2： The new μSR spectrometer located in the D1 area. ［enlarged view (49KB)］

　In the D1 area, towards the end of the extended shutdown period, the existing DΩ1 μSR spectrometer was finally removed to be replaced with a new full-fledged spectrometer (see Fig. 2). The new spectrometer is furnished with 20 "Kalliope" module detectors assembled around a bore of magnet (max. field of 0.4 tesla, provided by JAEA-ASRC). Each detector module consists of 32 segmented scintillator telescopes and avalanche photodiode sensors, the signals of which are digitized by on-board electronic circuits so that they are directly accessible to data acquisition computers via Ethernet. The spectrometer was commissioned using a muon beam at the beginning of the 2013B term and demonstrated a large acceptance of positron events, at up to 2 × 10⁸/hour at a proton power of 300 kW, which is nearly five times as large as that of the previously used DΩ1 spectrometer. Such a high rate necessitates a drastic change in the approach to conducting μSR experiments and associated data analysis in the foreseeable future.

２. U-line: Ultra slow muon beamline taking shape

Fig3： The experimental area of the Ultra Slow Muon beamline, where the silver cage in the U1A area covers the μSR spectrometer high-voltage stage. ［enlarged view (82KB)］

　Following the completion of the U-line in FY2012, which consists of a superconducting curved solenoid and axial-focusing solenoid systems, ultra slow muon (USM) beamlines at the U1A area (supported by the Grant-in-Aid for Innovative Research Areas, MEXT) have made steady progress. Although the schedule has been delayed, partly because of the above-mentioned Hadron accident and problems with the resonant muonium ionization laser system, the USM device was almost complete by the end of FY2013 (see Fig. 3). Commissioning for the first USM beam is scheduled for FY2014.

３. S-line: Beamline construction

　Construction of the S-line, supported by a major supplemental budget near the end of FY2012, made considerable progress. Despite the delay caused by the Hadron accident (which rendered the MLF building inaccessible to construction work until the end of September, 2013), major tasks including the installation of a magnet power supply gallery, user cabins, and radiation shields have been completed (see Fig. 4). Magnets for the beamline were delivered towards the end of FY2013, and their installation is scheduled for the summer shutdown period of FY2014.
　A new μSR spectrometer identical to that installed in the D1 area is almost complete, to be ready for installation in the S1 area in the summer of FY2014.

４. Muon Target: Final review for actual installation of rotating target

Fig4： A radiation shield for the S-line (left), the S1 area (center), and a user cabin (right) in Experimental Hall No.1. ［enlarged view (188KB)］

Fig5： Members of the external review committee observing the rotating muon production target in the MLF building. ［enlarged view (262KB)］

　The development of the rotating muon production target is mostly complete, and the new target was originally scheduled for installation by the end of FY2013. This plan was postponed in the wake of the Hadron accident, following calls for more stringent safety reviews prior to actual installation. A review was conducted in the fall of 2013 by an external committee (chaired by Prof. H. Kurisita, Tohoku Univ., see Fig. 5) to assess the technical feasibility and safety of the current target design, and a report was then submitted to the J-PARC Center fundamentally endorsing the target design.

１. Muonium as a shallow donor in barium titanate

Fig6：(a) A Fourier transform of the μSR spectrum for BaTiO₃. (b) Temperature dependence of the fractional yield for μ⁺ and Mu0. ［enlarged view (209KB)］

　One of the scientifi c topics that gained prominence in FY2013 through the Inter-University Research Program at MUSE was the electronic properties of barium titanate (BaTiO₃). The perovskite oxide BaTiO₃ is one of the most important ferroelectric materials and is widely used in electronic devices. It exhibits ferroelectricity at ambient temperature, where the associated high dielectric constant is indispensable for downsizing multilayer ceramic capacitors.
　It has been inferred from infrared absorption spectroscopy that hydrogen impurity in BaTiO₃ may form an O-H bond, suggesting that hydrogen is stabilized as interstitial H⁺ to comprise OH⁻ ions. It is further suggested by firstprinciple calculations that the electronic levels associated with the OH⁻ state may not be formed in the band gap, remaining instead near the bottom of the conduction band, which may therefore serve as a shallow electron donor. This may cause a serious problem in that the performance of BaTiO₃ as an insulating material for capacitors could be degraded by hydrogen impurities that abound in the environment.
　A research team comprised of T. U. Ito and coworkers conducted experiments to test this possibility using muons as sensitive simulators for the electronic state of interstitial hydrogen in matter. Because of the negligibly small difference between hydrogen and muonium (Mu⁰, a muonic analogue of a neutral H⁰ atom) in the reduced electron mass (~ 0.5%), the electronic state of the implanted Mu⁰ is mostly equivalent to that of interstitial H⁰. The team observed a pair of satellite signals around the center line (corresponding to the μ⁺ state) in the muon spin rotation (μSR) spectra measured in BaTiO₃ at lower temperatures, which is a typical sign that a Mu⁰ state with an extremely small hyperfi ne parameter has formed (see Fig. 6(a)). They also found that the Mu⁰ state disappears upon heating above temperatures as low as 100 K, which is interpreted as "ionization" of Mu⁰ taking place with a small activation energy of ~ 10 meV (see Fig. 6(b)). Thus, it has been shown that muonium acts as a shallow donor in BaTiO₃, strongly suggesting that interstitial hydrogen should exhibit similar behavior [1].

２. Magnetic frustration in an iridium thiospinel CuIr₂S₄

Fig7：(a) Time-dependent μSR spectra observed at various temperatures in a CuIr2S4 powder sample under zero external field. (b) μSR spectra at 2 K under various longitudinal fi elds. (c) μSR spectra in Cu_1−xZn_xIr₂S₄ under zero external field with Zn content x = 0.01 and (d) x = 0.1. (e) Octamer confi guration associated with charge order in CuIr₂S₄. The exchange interaction S^γ_i S^γ_j for each Ir⁴⁺ pair is represented by lines along the respective γγ bond (γ = x, y, z). The octupolar manifold at each Ir⁴⁺ site represents the spin density profi le in an isospin-up state hole (an eigenstate of the J_eff = 1/2 multiplet under strong spin-orbit interaction). The bond length of the (1)-(4) and (2)-(3) Ir⁴⁺ pairs is reported to shrink by ~ 15% following charge ordering and the associated structural transition. ［enlarged view (315KB)］

　Geometrical frustration in electronic degrees of freedom such as spin, charge, and orbit, which is often realized on stages of highly symmetric crystals, has become a prominent topic in the fi eld of condensed matter physics. Inorganic compounds with an AB₂X₄ spinel structure have been serving as fascinating laboratories for the study of their unusual physical properties with respect to geometrical frustration. A thiospinel compound, CuIr₂S₄, is a recent example, where a charge order of mixedvalent Ir ions transitioning into isomorphic octamers of Ir³⁺₈ S₂₄ and Ir⁴⁺₈ S₂₄, with lattice dimerization of the Ir⁴⁺ pairs in the latter complex, is realized on a metal-insulator (MI) transition at 230 K. While it is proposed that a pair of Ir⁴⁺ atoms (5d⁵, total spin S = 1/2) form a non-magnetic spin-singlet dimer driven by orbital order and associated Peierls instability, the magnetic property of the ground state is yet to be clarifi ed microscopically.
　Recently, a research team composed of K.M. Kojima and coworkers has shown, using muon and Cu-NMR studies, that a spin glass-like magnetic ground state is realized in CuIr₂S₄ below ~ 100 K [2]. As shown in Fig. 7, they observed slow Gaussian damping at 200 K, which is expected for muons exposed to random local magnetic fields from nuclear magnetic moments (primarily from ⁶³Cu and ⁶⁵Cu in CuIr₂S₄), indicating that the compound is non-magnetic at this temperature. In contrast, fast exponential depolarization sets in below ~ 100 K, where the depolarization rate and the relative amplitude of the depolarizing component increase with decreasing temperature. This observation is contrary to expectation, as the currently accepted scenario suggests that Ir⁴⁺ pairs should form a non-magnetic spin-singlet state upon MI transition. The spin glass-like behavior suggests that competing interactions are acting on the Ir⁴⁺ atoms, leading to a magnetically frustrated state. They suggest that spinorbit interaction (which has gained rapid recognition in recent years as an important player in the physics of 5d electron systems, e.g., Sr₂IrO₄) might be the source of this geometrical frustration, along with unquenched local spins, where an Ir⁴⁺ atom is represented by an eigenstate of effective isospin J_eff = 1/2 multiplet.
　With regards to CuIr₂S₄, it is known that substitution of Zn for Cu suppresses the MI transition, eventually leading to superconductivity in Cu_1−xZn_xIr₂S₄ for x > 0.25. μSR studies of Zn-substituted samples indicate that the spin glass-like magnetism is strongly suppressed (see Figs. 5-2-3-7(c), (d)), which is in parallel with high-T_c cuprates and/or iron-pnictides. Thus, CuIr₂S₄ may serve as a new material for the study of superconductivity under the strong infl uence of spin-orbit interactions.

Fig8： A MuSAC/MAC meeting, where committee members are briefed on KEK-MSL activities. ［enlarged view (119KB)］

　The 12^th Muon Science Advisory Committee (MuSAC) and the Muon Advisory Committee (MAC) were jointly convened on February 27–28, 2014 at the Tokai campus to review: (i) the activities of the Muon Science Laboratory (MSL, as part of the Institute of Materials Structure Science, to be reviewed by MuSAC under the charge of the IMSS director), and (ii) the operation/maintenance of the MUSE facility and related technical developments (as a part of J-PARC, to be reviewed by MAC under the supervision of the J-PARC Center director) in the past year.
　The committee was charged with making recommendations on the following four issues: 1) user operation, concerning D1/D2 instrument and muon production target upgrades along with responses to emergencies such as major radiation hazards (in the wake of the Hadron accident), 2) construction of the ultra-slow muon beamline and S-line (in the S1 area) with regards to its roadmap and scientifi c goals, and 3) future plans and instrumentation, particularly for the remaining S-line branches and the H-line. The fi nal report will be delivered to both the KEK-IMSS and J-PARC Center directorates in May 2014 (Fig. 8).

References
[1] T. U. Ito et al., Appl. Phys. Lett. 103, 042905 (2013).
[2] K. M. Kojima et al., Phys. Rev. Lett. 112, 087203 (2014).