Correlated electron materials and scattering spectroscopy
关联电子材料是凝聚态物理的基础前沿领域, 深刻体现了凝聚态物质中“多者异也 (more is different)”的特征. 由于材料内部自旋、轨道、电荷等多种自由度的相互耦合, 关联电子材料中蕴含了极其丰富的相互作用机制, 进而层展出一系列关联物态, 例如高温超导电性、重费米子、庞磁阻、量子自旋液体、量子相变、奇异金属、磁性关联拓扑等, 具有重大的材料学应用价值. 但是, 强关联特性意味着电子-电子间的相互作用已不可忽略, 传统的基于密度泛函的计算难以准确理解其机制或预言其关联物态特性, 因此关联电子材料的机理研究往往是由实验技术发展和实验观测结果所驱动的. 理解关联电子物态的关键在于明确材料中各个自由度的能量尺度、耦合方式和竞争关系. 散射谱学是关联电子材料非常重要的定量研究技术, 包括基于光子、电子、中子、缪子等为探测媒介的各类非弹性散射实验技术及方法学.
近年来, 我国学者在关联电子材料探索与谱学基础研究领域作出了重要的前沿性工作, 一批具有国际影响力的中青年科学家得以迅速成长. 一方面, 随着镍氧化物超导体、准二维铁基超导体、笼目结构材料、磁性拓扑材料、量子自旋液体材料、磁卡制冷材料等多个关联电子新材料家族不断涌现, 关联电子材料的物性和机理的谱学研究结果极大丰富了我们对其微观机理的认识. 另一方面,我国在中子源、同步辐射光源、自由电子激光、缪子源等大科学装置平台上的散射谱仪建设也发展迅速.
为此, 受《物理学报》编辑部委托, 我们邀请了十余位国内外优秀的青年科学家, 组织了 “关联电子材料与散射谱学”专题. 专题涵盖了多种新型关联电子材料的散射谱学最新研究结果, 亦涉及热电材料、压卡材料、磁卡材料等多种实用化的材料谱学研究. 同时, 还介绍了我国多个大科学装置上散射谱仪平台建设进展及其应用领域, 并以铁基超导体为例展示了单轴应变调控在不同散射谱学中的具体应用. 专题文章形式包括综述、观点评述等, 希望通过介绍关联电子材料与散射谱学领域的最新发展动态与学术思考, 推动关联电子物态相关研究的协同创新及多平台、多技术、多学科的交叉合作, 在物质科学基础前沿领域抢占国际制高点.
2025, 74 (8): 087402.
doi: 10.7498/aps.74.20241402
Abstract +
Unconventional superconductivity often competes or coexists with a variety of complex material states. In cuprate superconductors, there exist states including spin order, charge order, the pseudogap state, and the strange metal phase. A comprehensive understanding of their relationship is fundamental to establishing the mechanism of high-temperature superconductivity. Spin dynamics in cuprates has been extensively investigated using inelastic neutron scattering, but charge correlations remain far less understood. The latest development of resonant X-ray scattering (RXS) has been able to detect charge correlations with unprecedented sensitivity. A series of RXS studies have revealed that there universally exist the charge correlations in cuprate materials, which covers a wide range of the phase diagram. Resonant inelastic X-ray scattering (RIXS) experiments further show the dynamical behaviors of charge order. These findings highlight the important influence of charge correlations on the properties of cuprates. In this paper, we review the latest research progress in the charge order in cuprates by using RXS, with a particular emphasis on RIXS experiments. Our focus is placed on recent works on dynamical charge correlations at high temperatures as well as uniaxial strain tuning of charge order. We discuss topics including the underlying interactions, microscopic structure and symmetries, and the possible influence of charge order on both the superconducting and normal states.
2025, 74 (1): 012501.
doi: 10.7498/aps.74.20241412
Abstract +
Inelastic neutron scattering is a pivotal technique in materials science and physics research, revealing the microscopic dynamic properties of materials by observing the changes in energy and momentum of neutrons interacting with matter. This technique provides important information for quantitatively describing the phonon dispersion and magnetic excitation of materials. Inelastic neutron scattering spectrometers can be divided into triple-axis spectrometers and time-of-flight spectrometers, according to the method of selecting monochromatic neutrons. The former has high signal-to-noise ratio, flexibility, and precise tracking capabilities for specific measurement points, while the latter significantly improves experimental efficiency through various measures. The application of inelastic neutron scattering spectrometers is quite extensive, playing an indispensable role in advancing frontier scientific research in the study of mechanisms in various materials such as magnetism, superconductivity, thermoelectrics, and catalysis. The high-energy inelastic spectrometer at the China Spallation Neutron Source is the first time-of-flight neutron inelastic spectrometer in China, achieving high resolution and multi-energy coexistence with its innovative Fermi chopper design. Additionally, the number of available single neutron beams in the experiment of this facility has reached the international leading level.
2025, 74 (1): 012801.
doi: 10.7498/aps.74.20241178
Abstract +
2025, 74 (1): 017301.
doi: 10.7498/aps.74.20241163
Abstract +
Due to the unique crystal structures and excellent transport properties, the Zintl phase thermoelectric materials have aroused extensive interest in energy storage and conversion. To explore the origins of those excellent performances, a series of experimental and theoretical techniques have been applied, such as neutron scattering, thermal conductivity, and molecular dynamics simulations with machine learning. In this paper, the progress of neutron scattering research on the structure and dynamics of Zintl phase is summarized, for example A14MPn11 compounds with zero-dimensional (0D) substructures, 1D chains-based compounds, 2D layered A2BX2 compounds (including the binary Mg3Sb2) and their structural variants, as well as AB4X3, and ZrBeSi-type compounds. The underlying mechanisms of intrinsically low lattice thermal conductivity in those Zintl phase are discussed in detail. These compounds generally exhibit the following characteristics: 1) strong anharmonicity, which is characterized by strong atomic vibrations and anharmonic phonon-phonon scattering; 2) weak chemical bonding, which usually leads to low sound velocity and interatomic force constants, and corresponding to low-energy phonon branches; 3) intrinsic vacancy defect, which weakens the bond strengths, softens the lattice, and enhances anharmonic phonon-phonon scattering. Neutron diffraction is applied to studying crystal structures, lattice parameters, atomic occupancies, and atomic displacement parameters. Inelastic neutron scattering measures the lattice dynamics, and density of states, which are related to lattice thermal conductivity. Hence, the physical mechanisms of Zintl compounds are analyzed for optimizing material properties and designing new functional materials.
2025, 74 (1): 017401.
doi: 10.7498/aps.74.20241534
Abstract +
2024, 73 (19): 190703.
doi: 10.7498/aps.73.20240943
Abstract +
The Hefei Advanced Light Facility is the fourth-generation diffraction-limited storage ring light source, scheduled to begin operation in 2028. With its high-brightness and highly coherent X-rays, it will break through the current spatiotemporal resolution bottlenecks of X-ray techniques in studying correlated electron systems, providing crucial information for understanding the nature and microscopic origins of novel physical properties in these materials. This article introduces the main scientific goals and technical advantages of the Hefei Advanced Light Facility, focusing on the application perspectives of advanced technologies such as angle-resolved photoemission spectroscopy, magnetic circular dichroism, coherent X-ray scattering, and coherent X-ray imaging in researches of quantum materials and correlated electron systems. These techniques will enable the detailed analysis of the distribution and dynamics of electronic/spin/orbital states, reveal various novel quantum phenomena, and elucidate the fluctuations of order parameters in correlated electron systems. The completion of the Hefei Advanced Light Facility will provide advanced technical supports for decoding complex quantum states and non-equilibrium properties, ultimately promoting the application of quantum materials and correlated electron systems in frontier fields such as energy and information.
2024, 73 (19): 194101.
doi: 10.7498/aps.73.20240930
Abstract +
2024, 73 (19): 197103.
doi: 10.7498/aps.73.20241080
Abstract +
In the study of quantum materials, introducing pressure and strain that can change lattice parameters and symmetry is an effective experimental method for manipulating the electronic properties of the system. In measurements under hydrostatic pressure or in-plane epitaxial strain, the changes in lattice parameters will lead to significant changes in the electronic structure, thereby triggering off novel quantum phenomena and phase transitions. By comparison, the in-plane uniaxial strain, which has been widely employed in recent years, not only changes lattice parameters, but also directly destroys and controls the symmetry of the system, thereby affecting the electronic ordering state and even collective excitation of the system. This article provides a comprehensive overview of the basic concepts of uniaxial strain, the development of experimental methods, and some research progress in using these methods to regulate superconductivity and electronic nematicity in iron-based superconductors. This review contains six sections. Section 1 focuses on a genetral introduction for the uniaxial strain techque and the arrangement of this paper. Section 2 is devoted to the basic concepts and formulas related to elastic moduli and the decomposition of uniaxial strain into irreducible symmetric channels under D4h point group. Section 3 gives iron-based superconductors (FeSCs) and discusses the uniaxial-pressure detwinning method and related research progress. Section 4 introduces the establishment of the elastoresistance as a probe of the nematic susceptibility and discusses the key researches in this direction. Section 5 describes the research progress of the effects of uniaxial strain on superconductivity and nematicity. In sections 4 and 5, key experimental techniques, such as elastoresistance, are discussed in detail. Section 6 extends the discussion to several types of quantum materials suitable for uniaxial-strain tuning method beyond the FeSCs. Finally, we provide a brief summary and outlook on the uniaxial strain tuning technique. Overall, this review article provides valuable resources for the beginners in the field of FeSC and those who are interested in using uniaxial strain to modulate the electronic properties of quantum materials. By summarizing recent advancements and experimental techniques, this review hopes to inspire further research and innovation in studying electronic materials under uniaxial strain.
2024, 73 (19): 197104.
doi: 10.7498/aps.73.20240898
Abstract +
Ever since the discovery, nickelate superconductors have attracted great attention, declaring a “nickel age” of superconductivity. Currently, there are two types of nickelate superconductors: low-valence nickelate superconductors REn+1NinO2n+2 (RE, rare earth; n, number of adjacent NiO2 layers) and high-pressure nickelate superconductors La3Ni2O7 and La4Ni3O10. Charge order plays a crucial role in studying the strongly correlated systems, especially the cuprate superconductors, in which potential correlation between charge order and superconductivity has been indicated. Thus, great efforts have been made to explore the charge order in nickelate superconductors. In the infinite-layer nickelate RENiO2, the evidence of charge order with in-plane wavevector of Q // ≈ (1/3, 0) has been found in the undoped and underdoped regime but not in the superconducting samples. However, subsequent studies have indicated that this is not the true charge order inherent in the NiO2 plane,which carries unconventional superconductivity, but rather originates from the ordered excess apical oxygen in the partially reduced impurity phases. On the other hand, the overdoped low-valence nickelate La4Ni3O8 shows well-defined intertwined charge and magnetic order, with an in-plane wavevector of Q // = (1/3, 1/3). Resonant X-ray scattering study has found that nickel orbitals play the most important role in the multi-orbital contribution of charge order formation in this material, which is significantly different from the cuprates with oxygen orbitals dominating the charge modulation. Although the spin order in La3Ni2O7 has been well established, there is still controversy over its spin structure and the existence of coexisting charge order. In La4Ni3O10, intertwined charge and spin density waves have been reported, the origin and characteristics of which remain unknown. Owing to the research on the nickelate superconductors just starting, many questions have not yet been answered, and the exploration of charge order in nickelate superconductors will still be the center of superconductor research.
2024, 73 (19): 197301.
doi: 10.7498/aps.73.20241009
Abstract +
The essence of quantum materials lies in the intricate coupling among charge, spin, orbital and lattice degrees of freedom. Although X-ray photoemission spectroscopy and inelastic neutron scattering have advantages in detecting fermionic single-particle spectral function and bosonic spin excitations in quantum materials, respectively, probing other bosonic collective excitations especially their coupling is not possible until the establishment of the advanced resonant inelastic X-ray scattering (RIXS). In the past decades, RIXS has flourished with continuously improved energy resolution which made a paradigm shift from measuring crystal-field splitting and the charge-transfer excitation, to probing collective excitations and the order parameters of all degrees of freedom. This review paper summarises the latest research progress of quantum materials studied by the soft X-ray RIXS. For instance, three-dimensional collective charge excitations, plasmons, were discovered experimentally by RIXS in both electron and hole doped cuprate superconductors. The collective orbital excitations and excitons were found in copper and nickel based quantum materials. For the newly discovered nickelate superconductors, RIXS has made substantial contributions to characterising their electronic and magnetic excitations and the related ordering phenomena critical for an in-depth understanding of the underlying superconducting mechanicsm. The RIXS is a unique tool in probing the higher-order spin excitations in quantum materials due to the strong spin-orbit coupling and the core-valence exchange interaction. The RIXS is also found to be superior in probing the Stoner magnetic excitations in magnetic metals and topological magnetic materials. Finally, the development of RIXS technology in Chinese large-scale research facilities is briefly prospected.
2024, 73 (19): 197401.
doi: 10.7498/aps.73.20240983
Abstract +
In the 38 years since the discovery of cuprate superconductors, the theoretical mechanism of high-temperature superconductivity remains unresolved. Recent experimental progress has focused on exploring microscopic mechanisms by using novel characterization techniques. The development of synchrotron radiation has driven significant progress in spectroscopic methods. Resonant inelastic X-ray scattering (RIXS), based on synchrotron radiation, has been widely used to study cuprate superconductors due to its ability to perform bulk measurements, provide energy-momentum resolution, and directly probe various elemental excitations. The RIXS can measure phonons, which bind Cooper pairs in the BCS theory, and magnetic fluctuations and competing orders predicted by the Hubbard model in strongly correlated systems, allowing for the study of their interrelationships. This paper reviews the progress in using RIXS to measure charge density waves and related low-energy excitations, including phonon anomalies, in cuprate superconductors. It also examines the relationship between magnetic excitation and the highest superconducting transition temperature, and provides prospects for future research directions and challenges.
2024, 73 (19): 197601.
doi: 10.7498/aps.73.20240940
Abstract +
Muon spin relaxation/rotation (μSR) is a highly sensitive technique for investigating magnetic properties on an atomic scale. With the continuous development of this technique, the researches in condensed matter physics have been significantly promoted. Firstly, this article introduces the advantages and uniqueness of μSR technique, followed by several recent progress contributed by μSR in the field of condensed matter physics, including revealing the magnetic ground state of superconducting nickelates La3Ni2O7 and (R, Sr)NiO2, the investigation into the charge density wave in kagome lattice superconductor AV3Sb5 (A = K, Rb), identifying the magnetic droplets immersed in a sea of quantum spin liquid ground state in NaYbSe2, and the exploration of magnetic monopole near a magnetoelectric surface of Cr2O3. Finally, this article summarizes the current construction status and upgrade plans of muon facilities in the world.
2024, 73 (19): 197602.
doi: 10.7498/aps.73.20240926
Abstract +
