⇦Chinese Physics Letters, 2024, Vol. 41, No. 3, Article code 037303 Opportunities of Advanced Physical Studies at the Hefei Advanced Light Facility Zhe Sun (孙喆) and Donglai Feng (封东来)* Affiliations National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China Received 29 January 2024; accepted manuscript online 26 February 2024; published online 14 March 2024 *Corresponding author. Email: dlfeng@ustc.edu.cn Citation Text: Sun Z and Feng D L 2024 Chin. Phys. Lett. 41 037303    Abstract
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 DOI:10.1088/0256-307X/41/3/037303 © 2024 Chinese Physics Society Article Text Synchrotron radiation has transformed the role of x-rays as a mainstream tool for probing the atomic and electronic structure of materials. Synchrotron-based x-ray sciences have been widely used to study the microscopic structure, electronic states, chemical composition, and other properties of materials in fields such as quantum materials, soft matter, energy storage, catalysis, biology, and electronics. Fourth-generation storage ring light sources based on multi-bend achromat lattices[1-3] can further enhance the brightness and coherence of x-rays[4,5] on top of what are attained using present third-generation light sources. A formidable challenge in the design and construction of fourth-generation light sources is to develop a very accurate magnet lattice with sufficient dynamic aperture to accommodate the low-emittance electron beams. Typically, the beam emittances are pushed down to the order of 100 or even 10 pm$\cdot$rad, so that diffraction-limited x-rays at short wavelengths can be generated, which are 1–2 orders of magnitude brighter than its predecessor. Revolutionary advances in x-ray science will be brought about owing to the highly coherent x-ray and small beam spots on samples. Hefei Advanced Light Facility (HALF) is a fourth-generation storage ring light source with a focus on the energy range from infrared to tender x-ray.[6-8] Figure 1 shows the layout of HALF and 10 beamlines in the phase-one construction. The storage ring has an electron energy of 2.2 GeV, a circumference of around 480 m, a natural beam emittance of about 86 pm$\cdot$rad, and an operating beam current of 350 mA. Its beamlines will supply advanced spectroscopic, imaging, and scattering techniques to users. With high coherence and enhanced spatial, temporal, and energy resolutions, versatile x-ray tools will boost the capabilities of exploring the dynamic properties and complex non-uniform systems under non-equilibrium or operando conditions. This advancement promises to significantly elevate the supporting scheme for both fundamental science and industrial research and development. Condensed matter physics studies the behavior of large ensembles of atoms and electrons, which collectively manifest emergent cooperative phenomena characterized by length scales and time scales governed by their underlying complex and dynamic many-body interactions. With the advantages of HALF, it becomes possible to explore small or heterogeneous sample regions, to apply complex environments, and to fetch spatiotemporal characteristics of intertwined phases at an unprecedented level of detail. In this perspective, we will provide a brief overview of key features and advantages of the experimental techniques at HALF that are particularly relevant to condensed matter physics. Angle-Resolved Photoemission Spectroscopy. Two angle-resolved photoemission spectroscopy (ARPES) beamlines will be built in the VUV (vacuum ultraviolet) and soft x-ray regime, respectively. Due to the high brightness of the advanced light source, these spectroscopic techniques can effectively utilize the highly focused x-ray beam, resulting in significant improvements in spatial resolution. In the VUV beamline, ARPES will achieve an energy resolution of 1 meV and a spatial resolution of 3 µm. In the soft x-ray beamline, a spatial resolution of 100 nm will be realized for the Nano-ARPES endstation, and a 3 µm spatial resolution will be achieved for the soft x-ray ARPES endstation. In comparison to similar facilities worldwide,[9-13] these technical specifications bring several advantages. Firstly, the high energy resolution of 1 meV allows for detailed investigations of electronic structures and energy bands in quantum materials and devices,[14] and enables researchers to discern subtle electronic properties and uncover new insights into material behavior and device functionalities.[15,16] Secondly, the spatial resolutions of 3 µm and 100 nm for the two beamlines provide excellent imaging capabilities of electronic properties in inhomogeneous materials and devices,[17-19] which may access the length scales and local properties that determine the dynamic behaviors of quantum states. Thirdly, soft x-ray will enable the capability of detecting interfacial electronic states which is crucial for devices. This ARPES toolbox not only opens up avenues for the precise determination of electronic states in quantum materials, but also aids in the design and optimization of novel materials and devices with tailored electronic properties.[20] Magnetic Spectromicroscopy. Magnetic properties encompass hierarchical magnetic and electronic textures that span from the atomic level to a micron or larger, requiring high spectral contrast and spatial sensitivity to probe them in detail. A toolbox for magnetic spectromicroscopy will be deployed, including XMCD (x-ray magnetic circular dichroism),[21,22] FMR (ferromagnetic resonance),[23,24] and PEEM (photoemission electron microscopy).[25] XMCD utilizes two EPUs (elliptically polarizing undulator) with a fast polarity switching rate in combination with a lock-in amplifier, which would enhance the detection sensitivity for weak magnetism to the 0.0001 $\mu_{\scriptscriptstyle{\rm B}}$ range. The superior sensitivity for weak magnetism allows for the study of previously undetectable weak magnetic phenomena and opens new possibilities for understanding the intricate spin behaviors in materials and devices.[26-29] The spatial resolution of 3 µm will provide researchers with the ability to investigate magnetic interactions and phenomena at the microscopic level. FMR and PEEM techniques will be incorporated into the toolbox and enable the study of spin dynamics or spin current torque devices.[30-35] This set of tools enables researchers to directly observe and manipulate the behavior of spins, which are fundamental to the operation of such devices. The knowledge can contribute to the advancement of spintronics and other related fields.
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Fig. 1. Schematic layout of the Hefei Advanced Light Facility and beamlines in the phase-I construction.
Coherent X-ray Scattering. Coherent x-ray scattering experiments record interference patterns a.k.a. “speckle” from samples illuminated by a partially coherent beam. The fluctuations of speckle patterns are related to equilibrium and out-of-equilibrium dynamics with heterogeneities on a wide range of time and length scales.[36,37] Such capabilities are beyond most scattering techniques that measure averaged or static structures, and enable researchers to study elemental composition, chemical bonding, and magnetic properties in a non-destructive manner.[38-40] A coherent x-ray scattering beamline will be constructed at HALF. This facility provides 5 T magnetic field for x-ray scattering and the ability to perform coherent scattering across a wide energy range. The highly coherent nature of the x-ray beam allows for studies spanning over a broad range of length scales and time scales.[41,42] This also opens new avenues for understanding the structural and dynamical behaviors of soft materials like colloidal glasses, complex liquids, and polymers, and their responses to mechanical stress. Coherent X-ray Diffraction Imaging (Ptychography). Coherent x-ray diffraction imaging does not produce an image directly, instead, it collects the far-field scattering dataset from the sample and obtains the image by solving an inverse problem. The image resolution does not depend on the size of the focused beam, which overcomes the resolution limit of x-ray lenses. The construction of a coherent x-ray diffraction imaging experimental station utilizing the ptychography technique is underway at HALF. This setup aims to achieve a spatial resolution of 3 nm for high-efficiency imaging of the spatial distribution of the structural, electronic, magnetic, and chemical compositions of materials and devices.[43,44] The high energy resolution of the x-ray beam enables the combination of spectroscopy and microscopy, and it becomes convenient to monitor the valence states and subtle charge/orbital/spin variations of electronic states.[45] The high-resolution imaging and chemical state mapping can help uncover new phenomena, understand complex material behaviors, and provide insights into the mechanisms underlying the properties of quantum materials and devices,[46-48] which can facilitate the design and optimization of new functional materials with desired properties for energy storage, information processing, and other applications. Achieving these advanced specifications poses a great deal of technical challenges. Maintaining a high energy resolution requires mechanical stability and the precise control of various instruments.[49-52] Beamlines demand the development and implementation of advanced diagnosis techniques, precise focusing systems, and high-quality mirrors, which are crucial to achieve high spatial resolution and high coherence.[53,54] To enhance experimental efficiency, developing new algorithms and software tools for data analysis and visualization are required.[55,56] Convenient remote access should also be improved by developing new tools, such as virtual beamlines, and by providing training and support for users. Overcoming these challenges will be essential for harnessing the full potential of the HALF beamlines and realizing its promised benefits for valuable research outcomes with significant impact. The construction and development of synchrotron radiation facilities are of great significance in meeting cutting-edge scientific demands, and require collaboration and input from experts in multiple fields. Currently, the beamline scientists are working closely with expert users to ensure that the design and functionality of the facilities can meet the urgent demands from scientific frontiers. Only through the concerted efforts of users from diverse disciplines can we fully utilize the potential of HALF. 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