[Paper Review] Development of a versatile micro-focused angle-resolved photoemission spectroscopy system with Kirkpatrick-Baez mirror optics

Miho Kitamura, S. Souma+12 more|arXiv (Cornell University)|Mar 14, 2022

Topological Materials and Phenomena39 references53 citations

TL;DR

This paper presents a versatile micro-focused angle-resolved photoemission spectroscopy (micro-ARPES) system using Kirkpatrick-Baez (K-B) mirror optics at the Photon Factory, KEK. By monolithically integrating two high-precision elliptical mirrors with a five-axis motion stage and minimizing the sample-mirror distance to 400 mm, the system achieves a beam spot size of 10 µm (horizontal) × 12 µm (vertical), enabling high-resolution spatial mapping of electronic band structures. The key contribution is the detection of a subtle band anomaly linked to tiny crystal bending near the edge of a Bi2Se3 topological insulator, demonstrating the system’s capability for resolving local electronic inhomogeneities.

ABSTRACT

Angle-resolved photoemission spectroscopy using a micro-focused beam spot (micro-ARPES) is becoming a powerful tool to elucidate key electronic states of exotic quantum materials. We have developed a versatile micro-ARPES system based on synchrotron radiation beam focused with a Kirkpatrick-Baez mirror optics. The mirrors are monolithically installed on a stage, which is driven with five-axes motion, and are vibrationally separated from the ARPES measurement system. Spatial mapping of the Auphotolithography pattern on Si signifies the beam spot size of 10 $\mu$m (horizontal) x 12 $\mu$m (vertical) at the sample position, which is well suited to resolve the fine structure in local electronic states. Utilization of the micro beam and the high precision sample motion system enables the accurate spatially resolved band-structure mapping, as demonstrated by the observation of a small band anomaly associated with tiny sample bending near the edge of a cleaved topological insulator single crystal.

Motivation & Objective

• To develop a high-spatial-resolution micro-ARPES system capable of mapping local electronic structures in quantum materials.
• To overcome limitations of conventional micro-ARPES systems with larger beam spots (typically 50×50 µm²) and long sample-mirror distances.
• To enable detection of subtle electronic modulations, such as those induced by mechanical strain or surface curvature, which are undetectable with standard ARPES.
• To integrate a compact, vibrationally isolated K-B mirror optics system directly adjacent to the ARPES chamber for improved beam stability and spot size control.

Proposed method

• The system employs a Kirkpatrick-Baez (K-B) mirror optics configuration with two monolithic elliptical mirrors for independent horizontal and vertical beam focusing.
• The mirrors are fabricated with sub-1 µrad RMS slope error and 0.2 nm RMS surface roughness to ensure high optical precision.
• The K-B mirror stage is mounted on a five-axis motion system (x, y, z, θz, θx) for precise beam alignment and sample positioning.
• The beamline uses a variable-angle Monk-Gillieson monochromator and a 15.5 m to 32.0 m path to deliver tunable photon energy from 60 to 200 eV.
• The sample-mirror distance is kept short at 400 mm to minimize beam divergence and achieve a small spot size.
• The system maintains high photon flux and spatial resolution across a wide range of photon energies, as confirmed by spot size measurements at 60 eV and 200 eV.

Experimental results

Research questions

• RQ1 Can a compact K-B mirror optics system achieve sub-20 µm beam focusing at the sample position while maintaining high photon flux and stability?
• RQ2 How does the beam spot size vary with photon energy, and does chromatic aberration affect spatial resolution in the typical ARPES energy range?
• RQ3 Can the system resolve local electronic band structure modulations induced by nanoscale sample distortions, such as bending near crystal edges?
• RQ4 To what extent does the high-precision sample motion system enhance spatial mapping accuracy of electronic states?
• RQ5 Can the system detect subtle band anomalies in topological insulators that are invisible with conventional micro-ARPES due to larger beam spots?

Key findings

• The micro-ARPES system achieves a beam spot size of 10 µm (horizontal) × 12 µm (vertical) at the sample position, with a sample-mirror distance of only 400 mm.
• The beam spot size remains stable across different photon energies, with no measurable chromatic aberration, as confirmed by identical spot sizes at 60 eV and 200 eV.
• The system successfully mapped the spatial variation of the Au 4f7/2 peak on a photolithography pattern, demonstrating sub-20 µm spatial resolution.
• The system detected a systematic shift in the top of the valence band toward higher momentum (positive q) from point A to point D near the edge of a Bi2Se3 crystal, indicating local band structure modulation.
• This band shift is attributed to a small crystal bending of a few degrees near the cleaved edge, which is resolved due to the high spatial resolution and is invisible to conventional ARPES.
• The system enables accurate, high-statistics spatially resolved band-structure mapping of exotic quantum materials with local electronic inhomogeneities.