Takashi Imazono

Varied-line-spacing laminar-type holographic grating for the standard soft x-ray flat-field spectrograph

An aspheric wave-front recording system was designed to produce a holographic grating for use in a standard soft X- ray flat field spectrograph interchangeable with a mechanically ruled varied-line-spacing (VLS) grating. The grating grooves recorded with the designed aspheric wave- front recording system were processed to form a laminar groove profile by means of reactive ion etching. Measurements done with synchrotron radiation and a laboratory X-ray source are reported for this laminar-type grating and a commercial grating replicated from a mechanically ruled VLS grating that was specifically designed and fabricated for the standard soft X-ray flat- field spectrography. The laminar-type holographic grating is found to have an absolute first-order efficiency of approximately 10% for wavelengths of approximately 4.5 - 12 nm. It is also shown that the holographic grating is effective in suppressing the higher orders and stray-light level for soft X-ray of 4.36 nm (C-K) and has a comparable spectral resolution to the replica VLS grating.

Studies of multilayer structure in depth direction by soft X-ray spectroscopy

Abstract It is demonstrated that two kinds of soft X-ray spectroscopy are useful as nondestructive methods to investigate multilayer structures modified by interdiffusion or by chemical reaction of adjoining layers in depth direction. One is the total electron yield (TEY) spectroscopy involving angular dependence measurement. Using this method, it was found that in LiF/Si/LiF trilayers, the Si layers exhibited a characteristic similar to porous Si, and in CaF2/Si/CaF2 trilayers, it was found that CaF2 segregated through the Si layer. Moreover, it has been shown that the thickness of the top layer of a Mo/Si X-ray multilayer can be determined by analyzing TEY signals generated by the standing wave. The other is the soft X-ray emission spectroscopy involving spectral shape analysis. Using this method, it was found that in Mo/Si X-ray multilayers, the interdiffusion or chemical reaction giving rise to deterioration of reflectance character occurs in as-deposited samples as well as in heated samples. In antiferromagnetic Fe/Si multilayers, it was confirmed that there was no existence of pure Si layers, but insulating FeSi2 layers were present. This result suggests that the source of antiferromagnetic coupling is not conduction electrons but quantum wave interference.

Efficiency and polarization performance of a multilayer-coated laminar grating in the 6.5–6.9-nm wavelength region

We applied a Mo/B4C multilayer coating to a laminar holographic grating with 2400 grooves/mm and a 1-m radius of curvature. By use of synchrotron soft x rays the multilayer-coated grating was evaluated to have diffraction efficiencies of 3.1% and 0.017% for s- and p-polarized radiation, respectively, at a 6.7-nm wavelength at a 45.35 degrees grazing angle of incidence in the +1 (inside) grating order. Thus the polarizance was estimated to be 98.9% at least. The zero-order peak was suppressed by the destructive interference caused by the groove profile.

BURIED INTERFACES OF HEAT-LOADED Mo/Si MULTILAYERS STUDIED BY SOFT-X-RAY EMISSION SPECTROSCOPY

We measured Si L2,3 soft-X-ray emission spectra from Mo/Si multilayers annealed at 400°C. We showed that MoSi2 was formed at the interface, and the thickness of the MoSi2 interlayer increased with the annealing time. By an analysis of the soft-X-ray emission spectra and the patterns of small-angle X-ray diffraction, we estimated the thickness of the MoSi2 interlayer and a diffusion coefficient between Mo and Si at 400°C as 1.1 (± 0.2) × 10-18cm2/s. We showed that soft-X-ray emission spectroscopy is a useful tool for studying the thermal change of the buried interface nondestructively.

B K emission spectra for MgB2 and Al-doped MgB2

Recently, MgB2 has been shown to be a superconductor with Tc 1⁄4 39K by Nagamatsu et al. Since this discovery, the nature of the superconductivity in MgB2 has been studied using B isotope effect, neutron inelastic scattering and band calculations, among others. The results strongly suggest that MgB2 is a conventional BCS-type phononmediated superconductor. At the same time, a new superconductor from MgB2 as a starting material is being developed. Al-doped MgB2 (e.g. Mg1 xAlxB2) was one of the materials whose superconductivity was determined. Unfortunately, it could not attain a higher Tc, but it can contribute to the development of a new superconductor with a higher Tc. Band calculation is one of the methods used in such studies. Neaton and Perali showed that a decrease in the unit cell volume by Al doping decreases the density of states (DOS) at the Fermi level resulting in a decline in Tc. Suzuki et al. showed that holes disappear at about x 1⁄4 0:6, at which the Fermi level is equal to the top of the band. The B-NMR study by Kotegawa et al. showed that the decline in Tc by Al doping is due to the decrease in DOS at the Fermi level. To confirm these results, soft-X-ray emission (SXE) spectroscopy is a useful method. Since the SXE spectrum reflects the partial density of states (PDOS) of the valence band, B K emission for MgB2 shows the PDOS of the B 2p band. B 2p is a dominant component of the Fermi level, which is closely related with the Tc of superconductors. Thus, the B K emission spectra of MgB2 and related materials provide much information on the superconductivity of MgB2. B K emission spectra for MgB2, 13–15) AlB2 16) and other diborides have already been reported, but not yet for Al-doped MgB2. In this paper, we will show the B K emission spectra measured for MgB2 (x 1⁄4 0) and Mg0:8Al0:2B2 (x 1⁄4 0:2). We further discuss the spectra by comparing with those obtained by band calculations and soft-X-ray absorption (SXA), which show the PDOS of the conduction band. The samples were sintered polycrystals of MgB2 and Mg0:8Al0:2B2. Phase purity and the Tc of the samples were estimated before the SXE measurement. X-ray diffraction patterns showed that all the samples were of a hexagonal phase. The diffraction peaks for MgB2 were sharp, with the lattice constants a 1⁄4 0:3086 nm and c 1⁄4 0:3524 nm, and those for Mg0:8Al0:2B2 were broadened. Numerous reports showed that at this Al content region, Al-doped MgB2 tends to include another hexagonal phase with slightly different lattice constants, and its diffraction peaks are broadened or split. Broad diffraction peaks reflected these effects. The diffraction peaks shifted towards larger angles for Mg0:8Al0:2B2, denoting that lattice constants decreased in Mg0:8Al0:2B2. The temperature-dependent magnetization measurements showed that the Tc values of the samples were 39K for MgB2 and 29K for Mg0:8Al0:2B2, respectively. An SXE experiment was performed at BL-16B of the Photon Factory, KEK. The FWHM of the incident synchrotron radiation soft X-ray at an excitation energy of 191.1 eV was 0.5 eV. The SXE spectrometer utilized was a planefocusing type with a 1200-grooves/mm grating. The energy resolution of the spectrometer was about 0.8 eV around 200 eV near the B K emission. The sample preparation chamber was combined with the spectrometer. In this chamber, we can file the samples to remove surface contaminations without air exposure. Figure 1 shows the results of the SXE experiment. The solid and open circles show the B K SXE spectra for MgB2 and Mg0:8Al0:2B2, respectively, whereas the black and gray lines show the smoothed spectra respectively. These spectra were normalized using the height of the main peak of the smoothed spectra. Those for MgB2 (solid circles and black line) have a main peak at 183.5 eV and a slight shoulder at about 185 eV, and show a monotonic decrease towards the low-energy side of the main peak. The small peak at 191 eV is due to the elastic scattering of the incident soft X-rays (Rayleigh). The spectral feature almost resembles the PDOS obtained by band calculations for MgB2 and SXE spectra already reported. The spectrum for the Mg0:8Al0:2B2 is very similar to that of MgB2, but slightly shifted towards the lowenergy side. The inset in Fig. 1 shows the smoothed spectra used to estimate the peak shift, which was about 0.3 eV. The shift was also confirmed by the higher-order spectra we measured simultaneously. We ascertained that the spectra did not contain oxides after filing by comparing with the B K emission spectrum for B2O3. Thus, the shift is not related with the experimental error. We also measured the B K emission spectrum for Mg0:9Al0:1B2 (x 1⁄4 0:1). Although the

A beam intensity monitor for the evaluation beamline for soft x-ray optical elements

Evaluation Beamline for Soft X-Ray Optical Elements (BL-11) at the SR Center of Ritsumeikan University has been operated to measure the wavelength and angular characteristics of soft x-ray optical components in a wavelength range of 0.65-25 nm using a reflecto-diffractometer (RD). The beam intensity monitor that has been equipped in BL-11 has observed the signal of the zero-th order light. For the purpose of more accurate evaluation of the performance of optical components, a new beam intensity monitor to measure the intensity of the first order light from the monochromator in BL-11 has been developed and installed in just front of RD. The strong positive correlation between the signal of the beam monitor and a detector equipped in the RD is shown. It is successful that the beam intensity of the first order light can be monitored in real time.

Design of soft x-ray wide-band multilayer gratings for constant deviation monochromators

Constant deviation condition performing scanning energy by the rotation of grating around itself is the one of the most practical mounting for grating monochromators. Conventional multilayer coating can obtain high diffraction efficiency merely in a narrow band width. It is found that a new modified multilayer structure on the laminar-type grating intended to enhance the diffraction efficiency for constant deviation condition shows the diffraction efficiency over 5% in a wide band range of 1.5-2.5 keV in calculation.

Improvement of the soft x-ray polarimeter and ellipsometer (SXPE) for complete polarization analysis

A dedicated apparatus for polarization measurements in a soft x-ray region, which was previously developed and installed in a soft x-ray beamline (BL-11) at SR Center, Ritsumeikan University, is thoroughly improved. It allows us to adjust and set the angle of incidence and the height position of a polarization optical component more accurately even in vacuum. Using two Mo/Si multilayer mirrors fabricated as polarizers, the linear polarization degree of a 13.9-nm radiation from the BL-11 beamline is evaluated and determined to be 91%, as well as the polarization performance of the polarizers used.

X-RAY EMISSION AND ABSORPTION SPECTRA AND ELECTRONIC STRUCTURE OF MgB2

Measurements of X-ray emission and absorption of the constituents of MgB2 are presented. The results obtained are in good agreement with calculated X-ray spectra, with dipole matrix elements taken into account. The comparison of X-ray emission spectra of graphite, AlB2, and MgB2 in the binding energy scale supports the idea of charge transfer from σ to π bands, which creates holes at the top of the bonding σ bands and drives the high-Tc superconductivity in MgB2.