Izumi Mochizuki

Total reflection high-energy positron diffraction: An ideal diffraction technique for surface structure analysis

It is shown that the reflection high-energy positron diffraction (RHEPD) pattern from a Si(111)-(7 × 7) reconstructed surface for the total reflection condition, that is, the total reflection high-energy positron diffraction (TRHEPD) pattern, does not contain contributions from atoms in the bulk. Now, a method of observing the diffraction pattern formed only by the atoms on the topmost surface by a straightforward measurement of a bulk sample is available.

Observation of a shape resonance of the positronium negative ion

When an electron binds to its anti-matter counterpart, the positron, it forms the exotic atom positronium (Ps). Ps can further bind to another electron to form the positronium negative ion, Ps− (e−e+e−). Since its constituents are solely point-like particles with the same mass, this system provides an excellent testing ground for the three-body problem in quantum mechanics. While theoretical works on its energy level and dynamics have been performed extensively, experimental investigations of its characteristics have been hampered by the weak ion yield and short annihilation lifetime. Here we report on the laser spectroscopy study of Ps−, using a source of efficiently produced ions, generated from the bombardment of slow positrons onto a Na-coated W surface. A strong shape resonance of 1Po symmetry has been observed near the Ps (n=2) formation threshold. The resonance energy and width measured are in good agreement with the result of three-body calculations.

One-dimensional metallic surface states of Pt-induced atomic nanowires on Ge(0 0 1).

Surface states of platinum-induced atomic nanowires on a germanium (0 0 1) surface, which shows a structural phase transition at 80 K, were studied by angle-resolved photoelectron spectroscopy (ARPES). We observed four one-dimensional metallic surface states, among which, two bands were reported in our previous study (Yaji et al 2013 Phys. Rev. B 87 241413). One of the newly-found two bands is a quasi-one-dimensional state and is split into two due to the Rashba effect. Photoelectron intensity from one of the spin-polarized branches is reduced at a boundary of the surface Brillouin zone below the phase transition temperature. The reduction of the photoelectron intensity in the low temperature phase is interpreted as the interference of photoelectrons, not as the Peierls instability. We also discuss the low energy properties of the metallic surface states and their spin splitting using high-resolution ARPES with a vacuum ultraviolet laser.

Positronium and positronium negative ion emission from alkali-metal coated tungsten surfaces

The emission efficiencies of positronium atoms and positronium negative ions from tungsten surfaces increase by alkali-metal coating. We have studied the positronium emission from alkali-metal coated surfaces using the positronium time-of-flight method. We have also performed the observation of the photodetachment of Ps− emitted from a Na-coated surface and the production of an energy-tunable Ps beam employing the photodetachment technique. This paper describes our recent studies using alkali-metal coated surfaces.

Observation of a resonance in the photodetachment of positronium negative ions

The positronium negative ion, e−e+e−, is one of the simplest three body bound-states. As its constituents have a unique mass ratio, the ion is an attractive candidate for the investigation of the genuine three-body problem. In the present work, laser spectroscopy of this ion has been demonstrated in the UV region. A shape resonance near the positronium (n=2) formation threshold was clearly observed and the resonance energy was estimated.

Total reflection high-energy positron diffraction (TRHEPD)

Total reflection high-energy positron diffraction (TRHEPD) promises to be an ideal technique for determining the atomic arrangement of solid surfaces. TRHEPD makes full use of the total reflection of the positron beam from a surface in reflection high-energy positron diffraction (RHEPD), the positron counterpart of reflection high-energy electron diffraction (RHEED). Owing to the development and use of a brightness-enhanced intense positron beam at KEK, it is now possible to obtain clear TRHEPD patterns. It is shown that the TRHEPD pattern from a Si(111)-7×7 reconstructed surface for the total reflection condition does not contain contributions from atoms in the bulk.

Reflection high-energy positron diffraction study on the first surface layer

Reflection high-energy positron diffraction (RHEPD) is a powerful tool for studying surface structure. In particular, the topmost surface layer can be observed, facilitated by the characteristic of total reflection for positrons. A previous RHEPD study on a Ag surface, using a 22Na-based beam, is revisited and the analysis detailed.

Slow positron applications at slow positron facility of institute of materials structure science, KEK

Slow Positron Facility at High Energy Accelerator Research Organization (KEK) is a user dedicated facility with an energy-tunable (0.1 - 35 keV) slow positron beam created by a dedicated ∼ 50 MeV linac. It operates in a short pulse (width 1-12 ns, variable, 5×106 e+/s) and a long pulse (width 1.2 µs, 5×107 e+/s) modes of 50 Hz. High energy positrons from pair creation are moderated by reemission after thermalization in W foils. The reemitted positrons are then electrostatically accelerated to a desired energy up to 35 keV and magnetically transported. A pulse-stretching section (pulse stretcher) is installed in the middle of the beamline. It stretches the slow positron pulse for the experiments where too many positrons annihilating in the sample at the same time has to be avoided. Four experiment stations for TRHEPD (total-reflection high-energy positron diffraction), LEPD (low-energy positron diffraction), Ps− (positronium negative ion), and Ps-TOF (positronium time-of-flight) experiments are connected to the beamline branches, SPF-A3, SPF-A4, SPF-B1 and SPF-B2, respectively. Recent results of these stations are briefly described.Slow Positron Facility at High Energy Accelerator Research Organization (KEK) is a user dedicated facility with an energy-tunable (0.1 - 35 keV) slow positron beam created by a dedicated ∼ 50 MeV linac. It operates in a short pulse (width 1-12 ns, variable, 5×106 e+/s) and a long pulse (width 1.2 µs, 5×107 e+/s) modes of 50 Hz. High energy positrons from pair creation are moderated by reemission after thermalization in W foils. The reemitted positrons are then electrostatically accelerated to a desired energy up to 35 keV and magnetically transported. A pulse-stretching section (pulse stretcher) is installed in the middle of the beamline. It stretches the slow positron pulse for the experiments where too many positrons annihilating in the sample at the same time has to be avoided. Four experiment stations for TRHEPD (total-reflection high-energy positron diffraction), LEPD (low-energy positron diffraction), Ps− (positronium negative ion), and Ps-TOF (positronium time-of-flight) experiments are connected t...

Research progress at the Slow Positron Facility in the Institute of Materials Structure Science, KEK

Recent results at the Slow Positron Facility (SPF), Institute of Materials Structure Science (IMSS), KEK are reported. Studies using the total-reflection high-energy positron diffraction (TRHEPD) station revealed the structures of rutile-TiO2(110) (1×2), graphene on Cu (111) and Co (0001), and germanene on Al (111). First observations of the shape resonance in the Ps− photodetachment process were made using the positronium negative ion (Ps−) station. Experiments using the positronium time-of-flight (Ps-TOF) station showed significant enhancement of the Ps formation efficiency and the energy loss in the Ps formation-emission process. A pulse-stretching section has been implemented, which stretches the positron pulse width from 1.2 μs up to almost 20 ms.

Observation of a Low Energy Component of Positronium Emitted from Alkali-Metal Coated Polycrystalline Tungsten Surfaces

We have studied the emission of ortho-positronium from alkali-metal coated polycrystalline tungsten surfaces. The positronium time-of-flight spectra show that the yield of the 5 eV positronium component increases by alkali-metal coating. In addition, a low energy positronium component appears by Cs or K coating. We suggest that this component is due to positron energy loss by inter-band transition or surface plasmon excitation.