Research Plans

A01 Establishment of the Ultra Slow Muon Microscope and Micro-scale muSR.
Ultra slow muons are generated by resonant ionization of thermal Mu atoms generated from the surface of a hot tungsten foil, placed at the intense surface muon beam line. In order to efficiently ionize the Mu near the W surface, we adopted a resonant ionization scheme via the {1S-2P-unbound} transition. In order to induce 1s-2p transitions, Lyman-α light of 122.088 nm is needed. To generate such Lyman-α (VUV;Vacuum Ultra Violet) laser, we have been adopting the resonant four-wave frequency mixing (ωVUV = 2ωr - ωt) where two 212.5 nm photons(ωr) are used for two-photon resonant excitation of the 4P55P[5/2] state in Kr, subtracted by a photon with a tunable difference wavelength (ωt ; 820 nm). In our new laser system developed by A04 group (Wada et. al.), ωr (100 mJ/p, 2ns) is generated as the 5th harmonics of 1062 nm (1 J/p), which consists of all solid state laser system, such as diode laser, fiber amplifier, regeneration amplifier, OPA, OPG etc., generating expectedly more than 71μJ/cm2 Lyman-α light of the saturation intensity for the Doppler broadened Mu at 2000 K.
A02 Spin transport and catalytic reactions in the boundary regions.

Microscopic features of transport phenomena of spins, ions, electrons and holes are essential to understand fundamental function in materials and life science. Local structure containing interfaces and boundaries is of great importance to promote or suppress transport properties. High sensitivities of spin polarized muons to various aspects of transport properties have been verified by pioneering works of researchers in this team utilizing muon as an active probe which interact with electrons and nucleus in host materials: i.e. spin current in semiconductors, ion diffusion in battery materials, electron transfer in biomolecules, and electric potential of electrons in defects of semiconductors and oxides compounds.

A muon takes either a diamagnetic (μ +) state, a hydrogen-like state with a bound electron so called muonium(μ +e-), and/or a negative muonium state with two bound electrons(μ +e- e-) depending on electron density of state(DOS) of materials. Therefore, electron binding and ionization processes of muons have potential to provide abundant information associated with spatial distribution and dynamical fluctuation of DOS which is key parameters to understand mechanism of chemical and biological reactions.

Ultra slow muon microscope, USMM, realized by ultra slow muon and muon micro beam enables us to explore local information of materials with spatial resolutions of either nanometer in depth profiling in the vicinity of surface or micrometer in three-dimensional mapping inside materials. In this team, local and microscopic transport properties as well as behavior of hydrogen in the following typical model systems will be investigated from the aspect of physics, chemistry and biology for the first time;

(1) Spin current across interfaces (Torikai, Yoshino, Nagamine, Shimomura, Higemoto, Ito, Maekawa, Kasai, Nakanishi)
In development of spintronics materials, monitoring of spin current in semiconductors is the central subjects to be studied. Conduction electron polarization excited by circularly polarized photons in n-type GaAs is clearly detected, verifying the high sensitivity of the muonium spin exchange method on the basis of Pauli’s exclusion principle. Ultra slow muon combined with this method becomes powerful tool to reveal local as well as microscopic features of spin current with detailed information on spin life time, spin diffusion length and spin scattering cross section across interfaces regardless of method of spin injection. Another important subject is to study spin density of states at interface which affects efficiency of spin injection significantly. It is also planned to observe electron spin tunneling in a thin insulating layer.

(2) Catalytic reactions on surfaces (Asakura, Ariga, Shimomura, Tsuneyuki, Nakanishi, Fukutani, Torikai)
Hydrogenation catalysts are commonly used catalysts in many fields. Hydrogen dynamics not only on surface but also near surface region is playing important role for the reaction. As hydrogen is one of the most difficult elements to be detected, chemistry of hydrogenation catalytic reactions has not been fully understood.
Photocatalytic reaction is activated by electron-hole pair generation by photo-irradiation on an oxide surface. Understanding and controlling oxygen vacancy defects are on one of the key issues for developing highly efficient photocatalysts. Because oxygen vacancy defects become a charge trapping site, which promote the charge transfer to reactants and enhance reaction when it exists near the surface, while they suppress the reaction as a recombination center of electron and hole when it exists far from the surface.

(3) Ion-diffusion in solid state batteries (Sugiyama, Nozaki, Harada, Kanno)
The interface between electrode and electrolyte is the key to control ion-diffusion in solid state batteries. μSR provides information on diffusion of Li and Na ions in battery materials. Ultra slow muon and micro beam, thus, clarify the depth profile of the diffusive nature, particularly at the hidden interface between cathode and electrolyte. USMM gives crucial information how to make a suitable interface between electrode and electrolyte, leading to an advanced solid state battery.

(4) Electron transfer in biological system (Nagamine, Sugawara, Shimomura, Higemoto, Torikai)
Electron transfer plays an important role in the biological system, such as photosynthesis and respiration in proteins and self-repairing from damage of DNA.
A labeled electron method using positive muons has revealed, in the case of cytochrome c and DNA, predominant one-dimensional nature of electron transfer along intra-molecular chain, and subsequent inter-molecular diffusion which strongly depend on hydration water contents.
Ultra slow muon provides a unique tool to study depth-resolved electron transfer phenomena at cell membrane and monitor distribution of hydration water in organisms.
Furthermore, by applying micro beam to examine anisotropic effects of ordered states, new aspects of the inter-molecular nature such as geometrical path of electron transfer will be explored.

A03 Heterogeneous electron correlation in the surface-bulk boundary and across thin layered structures.
PI: Ryosuke Kadono(KEK-IMSS, J-PARC)

A04 Extreme cooling and sharpening of the beam towards the “new physics” frontier beyond the standard model.
PI: Masahiko Iwasaki(RIKEN Nishina Center for Accelerator-Based Science)
 We plan to study the physics frontier beyond the standard model by utilizing muons to test fundamental properties in the lepton sector. For example, by trapping muons in a super precision magnetic field, we can precisely measure the muon magnetic moment and EDM (electric dipole moment) (see the figure). For efficient trapping, we need an ultra-cold muon beam with small transverse emittance, which will be obtained by accelerating ultra-slow muons. The quality of the measurement is enhanced

by further cooling and sharpening ultra-slow muons, so our main purpose here is to develop a strong tool towards these studies.

 We plan to achieve a beam with better emittance and time resolution by the following methods;
1) Reduction of the initial phase space by using a cold muonium source,
2) Reduction of the transverse emittance by using phase rotation coupled with a chirped laser, and
3) Short bunching of the muon beam by using phase rotation coupled with rf field.

 As a basis for these developments, we need a high-intensity ultra-slow muon beam by efficiently ionizing the thermal Mu emitted from materials surface. So our first goal is to realize the world highest intensity pulsed Lyman-α laser based on the design and technology (such as the optimized laser crystal) developed by the RIKEN group.
 The projected Lyman-α laser intensity is 100 microJoule/pulse so that the laser can cover wide spread Mu and still saturate 1s to 2p transition followed by rapid ionization by another laser. In the first two years of this project, we successfully constructed the laser system (see the photo) and produced the Lyman-α light. Laser optimization is in progress. The laser light will be soon introduced to the ultra-slow muon beam line developed by the A01 group.