BL5U at UVSOR-II for Three-dimensional Angle-resolved
Photoemission Spectroscopy
Takahiro Ito*^, Shin-ichi Kimura*^, Hojun Im^ Eiken Nakamura*, Masahiro
Sakai*, Toshio Horigome*, Kazuo Soda** and Tsunehiro Takeuchi**UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan^ School of Physical Sciences, The Graduate University for Advanced Studies, Okazaki 444-8585, Japan** Department of Materials, Physics and Energy Engineering, Nagoya University, Nagoya 464-8603, Japan^EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan
Abstract. BL5U at UVSOR-II has been reconstructed in 2004's for three-dimensional angle-resolved photoemission (3DARPES) study on solids and surfaces. The beamline is equipped with a helically/linearly polarized undulator, an old-type
monochromator named as SGM-TRAIN designed in 1995's, and a new photoemission end-station. Since their performances
are insufficient to study anomalous electronic/magnetic properties on materials in recent years, we reconstructed the beamline
for a high-resolution study as follows; (1) The manipulation of the pre-focusing mirror was updated to a high-precisionsystem controlled by pulse-motors. (2) The entrance slit with water cooling system was adopted for avoiding the heat loadfrom undulator radiation. (3) The SGM-TRAIN was re-arranged to the optimum condition to the undulator light. (4) The
free tuning program of the undulator gap and monochromator-control systems was introduced. Due to the reconstructions,
we successfully improved the throughput intensity and energy resolution of the beamline to be able to investigate threedimensional electronic-structures using 3D-ARPES.
Keywords: angle-resolved photoemission, beamline reconstruction, UVSOR-II, strongly correlated electron systems
PACS: 41.60.Ap,71.18.+y,71.20.-b

INTRODUCTION
Functionalities (superconductivity, magnetism, etc.) of materials originate from the electronic structure near theFermi level (E?), especially the Fermi surface. To pursue the origin of the functionalities on materials, we have constructed a new high-energy-resolution photoemission apparatus at the beamline BL5U of UVSOR-II, Institute for
Molecular Science, Okazaki, Japan. The beamline is designed for high-energy-resolution three-dimensional angleresolved photoemission (3D-ARPES) study on solids and surfaces with the horizontally and circularly polarized synchrotron radiation from the variably polarized undulator. The beamline consists of a variable polarized undulator (circular, planar polarization), a Spherical Grating Monochromator with Translational and Rotational Assembly Includinga Normal incidence mount (SGM-TRAIN), and a high-resolution angle-resolved photoemission spectrometer.
After the upgrading of UVSOR, in which the electron-beam emittance was reduced from 190 to 27 nm-rad andthe name was changed to UVSOR-II, BL5U has been reconstructed in FY2004 for a high-resolution photoemission(HRPES) study on solids and surfaces. The energy and angular resolutions of the photoemission apparatus of theBL5U end-station have been improved to A£ ^ 1 . 2 meV and A0 ~ ±0.1 degree, respectively, estimated by utilizing aHe-la (/zv=21.216 eV) discharge lamp. Thus, at least for the endstation, the performance is available for studying theanomalous physical properties such as metal-insulator transition, superconductivity, magnetic phase transition, etc. On
the other hand, there is a problem with the beamline. For example, the SGM-TRAIN has been aligned to the bending
magnet radiation (B5) because the bending magnet radiation was mainly used as the light source of the beamline
before the upgrading of UVSOR. So, it was necessary on the SGM-TRAIN to be set on the line of the undulator (U5),and on some parts of beamline to be upgraded. In this paper, the present performance of the SGM-TRAIN and theend-station after the improvements are reported.

CP879, Synchrotron Radiation Instrumentation: Ninth International Conference,edited by Jae-Young Choi and Seungyu Rah© 2007 American Institute of Physics 978-0-7354-0373-4/07/$23.00587
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FIGURE 1. High-energy-resolution angle-resolved photoemission apparatus at UVSOR-II BL5U.

(a)

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12KINETJC ENERGY / eV125**i*<*<_He la,7:V(hv=21.216eV)\ T~5K
AE~1.2mev\~*

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*+•EF•*^-5.0Binding Energy (meV)

FIGURE 2. Energy-resolution test of the photoelectron analyzer (MBS-TOYAMA 'Peter' A-l) at UVSOR-II BL5U. (a) PESspectrum at around E^ of gold film obtained by using He l a photons at 5 K (circles). The results of the fitting using the Fermidistribution-function is superimposed (line), (b) Si 2p photoemission peaks obtained with the previous (gray) and present (black)apparatus installed at UVSOR-II BL5U.

STATUS OF THE END STATION AT UVSOR-II BL5U
Figure 1 shows a present status of the apparatus installed at BL5U. The most striking point is a new type hemispherical photoelectron analyzer, MBS-TOYAMA 'Peter' A-l. The photoelectron analyzer achieved the experimental energy-resolution of AE ~1.2 meV, which has been evaluated from the fitting of the PES spectrum near E? ofgold by temperature-dependent Fermi-distribution-function broadened by a Gaussian meaning the energy resolution(Fig. 2(a)) [1]. The achieved energy-resolution is almost comparable to the world best (sub-meV) one [2, 3].
To evaluate the improvement of the photoemission spectrometer for BL5U, the PES spectrum of Si(lll) 7x7
surface obtained by the present apparatus is compared with previous one (Fig. 2(b)). In Figure 2(b), the present
spectrum seems to have some advantages to the previous one, i.e., the better S/N ratio, the sharper double peakstructure originating from Si 2p spin orbit splitting, in spite of the compatible experimental conditions (photon energy,temperature, slit size, surface preparation). The observed differences indicate the improved efficiency as well as theenergy resolution of the present spectrometer.

588
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helical ccw mode~jSlrt O.Q5mm*Q.05mmH200|-

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150200111111L250Photon Energy (eV)FIGURE 3. Throughput spectra from the SGM-TRAIN at BL5U after the reconstruction. The spectra before the reconstructionin the G3 res;ion are also plotted [4].
1'•*uiT~P^'• **•»^ r-n* * * * * * hv=lleVGold\ E F = 7 . 0 eV

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Kinetic Energy (eV)FIGURE 4.

HRPES spectrum at the Fermi level of gold measured after the present reconstruction.

THE RECONSTRUCTION OF THE MONOCHROMATOR, "SGM-TRAIN"
To perform a high energy-resolution 3D-ARPES, both of the high energy-resolution and high photon-flux are necessary. The undulator provides the high photon-flux, while the high energy-resolution has been insufficient because ofthe problems with the monochromator. At the first stage of the beamline construction, the SGM-TRAIN, designed as
keeping the Rowland mount similar to the Dragon-type monochromator [4], was aligned to the bending-magnet radiation [5]. It should be noted here that the undulator source has been used only for the free-electron-laser experiments
before upgrading of the UVSOR [6]. After upgrading of the UVSOR, we planned to use the high-brilliant undulator
radiation. Accordingly, some problems became known; (1) Mechanism for optimizing the pre-focusing mirror was toorough for the undulator light. (2) Entrance slit must be cooled against the higher heat load than the bending magnet radiation. (3) The grating G3 with normal incident mount for the low energy-region (hv = 5^25 eV), which is important
for the high-energy resolution as well as a bulk-sensitive photoemission, had not been optimized [7]. These problems
came from the design-concept of the beamline BL5U, which has been used for the core-level and valence-band photoemission spectroscopy, where the required energy-range (> 30 eV) could be covered with the other two gratings Gland G2. Then we reconstructed and optimized BL5U as following to improve the efficiency in the low-energy range,
to perform 3D-ARPES experiment.
Figure 3 shows the improved throughput from the SGM-TRAIN after the present reconstruction in comparison
with the previous throughput spectra. We can clearly find that the mesh current increases more than 50 times in the G3region.The photon flux (Nph) is evaluated to be about 1012 photons/sec in the energy resolution of /ZV/A/ZV^1000. Notethat the sharp oscillatory structure observed in the throughput spectra originates from the interference between twoundulators locating at the upstream and downstream of one long straight-section of an optical klystron-type undulator(U5). Due to the high-throughput of the photocurrent at the low-energy region, the HRPES measurement at BL5U
becomes available. For example, HRPES spectrum at the Fermi-level of gold (Fig. 4) has been corrected within
1 hour with the resolution of hv/AE ~ 1000. Therefore the present reconstruction makes it possible to insight into the589
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origin of the anomalous physical properties by using the high-resolution bulk-sensitive angle-resolved photoemission
apparatus at UVSOR-II BL5U. Furthermore, we plan to construct a new 3D-ARPES beamline (BL7U) in the VUVregion at UVSOR-II [8], to pioneer a photoemission frontier research. The beamline is expected to cover the energy
region of 6 - 40 eV with 20,000< hv/AE <60,000 and with Nph > 1011 photons/sec in 10" 4 band width. After theconstruction of new 3D-ARPES beamline BL7U, two-types of 3D-ARPES endstations (BL5U and BL7U) will playcomplementary role each other; the reconstructed BL5U for general researches (ex. core-level, valence band PES,wide-energy range 3D-ARPES) with medium-resolution, while the new BL7U for special ones (ex. 3D-ARPES ataround £p) requireing extensively high-resolution.

CONCLUSION
In conclusion, we reconstructed the high-energy-resolution three-dimensional angle-resolved photoemission beamline. The beamline is equipped with a variable polarized undulator, the SGM-TRAIN monochromator and a 200-mm
radius hemispherical photoelectron analyzer. After improvement of the monochromator, the beamline covers the very
wide photon energy-range of 5 - 250 eV with the photoemission energy resolution of hv/AE > 1000.

ACKNOWLEDGMENTS
We thank the staff members of the UVSOR facility and the users at UVSOR-II BL5U for their supports and fruitfulcomments.

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