Iron-based superconductors, particularly the 1111-type GdFeAsO system, represent a crucial platform for investigating unconventional superconductivity, yet optimizing critical parameters through advanced doping strategies remains challenging. This study innovatively introduces uranium doping at Gd sites to modulate electronic properties of GdFeAsO system. Through vacuum-assisted solid-state synthesis and comprehensive characterization, we demonstrate successful U substitution, as confirmed by the systematic lattice contractions along both a- and c-axes. The parent compound exhibits a pronounced resistivity anomaly around 128 K, associated with a spin-density-wave (SDW) transition. Notably, U doping potently suppresses this SDW instability, with superconductivity emerging in the doping range 0.05 <= x <= 0.2 and reaching a maximum critical temperature (Tc) of 48 K at optimal doping (x = 0.15). Magnetic studies corroborate the bulk superconductivity in this U-doped GdFeAsO system. For the x = 0.15 sample, the applied external field suppresses the superconducting onset temperature extremely slowly, indicating a considerably high upper critical field. Furthermore, the tetravalent state of U and modified charge environment reveal that U effectively introduces electron doping into this material. These results demonstrate the critical role of U-5f electrons in enhancing electron correlations and mediating the electron pairing interactions, collectively contributing to the emergence and optimization of Tc in this system. The layered Gd1-xUxFeAsO materials, with tunable carrier density, high Tc, and robust bulk superconducting properties, is a promising new platform for studying the fundamental physics of hightemperature superconductivity and developing high-performance superconducting devices.

NdFeAsO, a typical 1111-type iron-based superconductor (IBS), has a layered crystal structure consisting of alternating charge-reservoir Nd2O2 layers and conducting Fe2As2 layers, offering substantial potential for carrier doping. In this study, uranium (U) was incorporated into the Nd site, and a series of Nd1-xUxFeAsO samples were successfully synthesized. Rietveld refinement of X-ray diffraction (XRD) patterns revealed a systematic lattice contraction with increasing U content. Consistently, X-ray photoelectron spectroscopy (XPS) directly confirmed the substitution of U4+ for Nd3+ and the electron-doping nature of this substitution. Electrical transport measurements demonstrated that U doping effectively suppresses the spin-density wave (SDW) transition of the parent compound at ∼150 K and induces bulk superconductivity in the doping range 0.15 ≤ x ≤ 0.25. The optimally doped sample (x = 0.25) exhibits a high superconducting critical temperature (Tc) of 47.5 K, and its superconducting onset transition is markedly insensitive to the applied magnetic field, suggesting the presence of a high upper critical field (Hc2). This work demonstrates that U doping offers an alternative route for achieving superconductivity in the 1111-family IBS.

Uranium,the heaviest natural element,exhibits rich and complex physical behavior,including three types of charge density wave transitions and superconductivity at low temperatures.It is widely believed that these phenomena are closely linked to the properties of 5f electrons,which are highly susceptible to external perturbations.To elucidate the detailed electronic structure,particularly the 5f electron signatures,we fabricated high-quality single-crystal uranium films on W(110)substrates using molecular beam epitaxy and investigated their fine electronic properties and temperature-dependent evolution by angle-resolved photoemission spectroscopy(ARPES).Our experiments reveal three electron pockets around the Γ point and direct hybridization between 5f electrons and conduction electrons at regions distant from Γ.The Kondo temperature extracted from ARPES and electrical resistance measurements is approximately 131 K,indicating that the 5f electrons transition from a high-temperature localized state to a low-temperature itinerant state in the thin film system.Additionally,we observe another flat band with an energy scale of 148 meV.The detailed electronic structure and direct evidence of the localized-to-itinerant transition of 5f electrons provided in this study advance the understanding of strong electronic correlations in uranium-based materials.

In aheavy-fermion system, the competition between the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and the Kondo effect determines the ground-state properties, including superconductivity, magnetic orderings, and non-Fermi-liquid states. However, the temperature-dependent development of this competition remains unclear, particularly within the framework of ferromagnetic ground states. Here, the electronic structure and heavy quasiparticle band properties are exhaustively studied in CeNiSb2, a prototypical ferromagnetic Kondo lattice, using high-resolution angle-resolved photoemission spectroscopy. Our measurements reveal the threedimensional electronic structure and Fermi surface topologies in CeNiSb2. The discrepancy of the quasiparticle bands along the P-M direction indicates band-dependent hybridization between the f electrons and conduction electrons (c-f hybridization), which may arise from differences in the conduction-band properties. The intensity of the quasiparticle band increases monotonically with decreasing temperature and deviates from the Kondo-like behavior at low temperature. This evolution provides a comprehensive picture of how the RKKY interaction and the Kondo effect develop and influence the behavior of f electrons across the temperature range. Our experimental results offer a spectroscopic study of the quasiparticle band and its temperature-dependent evolution in CeNiSb2, which may be crucial for understanding the underlying physics in heavy fermion compounds.

The anomalous metal state (AMS), observed in failed superconductors, provides insights into superconductivity and quantum criticality, with studies revealing unconventional quantum phases like the Bose metal. Recently, layered transition metal dichalcogenide (TMD) superconductors approaching the two-dimensional limit have garnered significant attention for the enhanced phase fluctuations and electronic correlations. Investigating AMS in these systems, particularly in the absence of an external magnetic field, could offer valuable insights into the dimensionality-driven emergence of exotic quantum phenomena, including triplet Cooper pairing, phase fluctuation dynamics, and especially the recently discovered field-free superconducting diode effects. However, the field-free AMS has yet to be observed in TMD superconductors. Here, we report the dimensionality-tunable AMS near the superconducting quantum phase transitions in a layered TMD superconductor 2H-Ta2S3Se. In samples with thicknesses below 10 nm, we demonstrate magnetic field-driven AMS under external magnetic field, characterized by the vanishing of the Hall resistance and the presence of finite longitudinal resistance. Remarkably, an unexpected zero-field AMS emerges as the sample thickness is reduced to 3 nm. This AMS aligns well with the quantum vortex creep model and exhibits non-reciprocal transport behaviors, suggesting the onset of spontaneous time-reversal symmetry breaking accompanied by vortex motion as the system approaches the two-dimensional limit. Our findings open new avenues for exploring dimensionality-driven exotic superconducting quantum critical phases, and pave the way for a deeper understanding of zero-field superconducting diode effects.

Epitaxial growth of uranium films is essential for exploring the exotic properties of U 5f electrons. However, the growth mechanism during the initial stages of U film formation remains unclear. In this study, the adsorption behavior of uranium (U) atoms on a tungsten (W)(110) substrate was investigated using scanning tunneling microscopy (STM) and first-principles calculations. It was found that an isolated U atom is more stable on the hollow site of the W(110) surface compared to the top and bridge sites. Additionally, two U atoms tend to adsorb on neighboring edge-sharing rhomboid hollow sites of W(110). As the concentration of U atoms increases, a directional two-dimensional (2D) growth trend along two high-symmetry directions of 0 degrees and approximately 105 degrees is observed. When the film thickness reaches 4 monolayers (ML), the close-packed U atoms exhibit a pseudo-hexagonal arrangement, accompanied by an enhanced tunneling signal near the Fermi level. Furthermore, thickness-dependent dI/dV spectra were obtained, showing strong consistency with the calculated results. These findings provide a clearer understanding of the initial growth mechanism of ultra-thin U films on the W (110) surface and open new scientific avenues for exploring the remarkable properties of uranium.

Uranium compounds have long been of significant interest in condensed matter physics due to their diverse and often exotic properties, including heavy-fermion behavior, non-Fermi-liquid characteristics, quantum criticality, and unconventional superconductivity. These phenomena arise from the complex interplay between crystal structure, hybridization of localized 5f electrons with conduction electrons, and electron-electron correlations. Here, we present a comprehensive study of the electronic structure of UFeGa5, a layered uranium intermetallic compound, using high-resolution angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations. Three-dimensional electronic structure of UFeGa5 was systematically studied and fine structures of the U5 f bands near the Fermi energy were clearly resolved. Our results reveal that UFeGa5 exhibits a predominantly itinerant nature of 5f electrons, with strong band dispersion and significant contributions to the Fermi surface. Theoretical calculations assuming itinerant 5 f electrons are in excellent agreement with the experimental ARPES data, confirming the weak electron correlation effects in this compound. Temperature-dependent ARPES measurements further demonstrate that the f electrons retain itinerant across the studied temperature range (7.5 to 100 K).

Investigating the oxidation kinetics of uranium is of significant importance due to its critical influence on the performance of nuclear materials. Here, we characterized the oxidation kinetics of single-crystal alpha-U films under controlled oxygen exposures (1-6 x 1014 Langmuir) at room temperature by in situ ultrahigh vacuum (UHV) spectroscopic ellipsometry (SE), a technique that effectively eliminates atmospheric interference. An ellipsometric optical model incorporating an interfacial transition layer between UO2 and metallic uranium was developed. The transition layer, identified as a graded mixture of UO2 and U, was modeled by an arc-tangent distribution based on the X-ray photoelectron spectroscopy (XPS) depth profile. The thicknesses of the UO2 layer and transition layer were determined through an overall ellipsometric data fitting methodology, ensuring the uniqueness and accuracy of the fitting results. Three oxidation kinetics stages, including linear, parabolic and inverse logarithmic regimes, were identified. The transition layer emerged during the inverse logarithmic oxidation stage. Based on the obtained optical constants of UO2 and U, the optical properties of UOx in the transition layer were determined for the first time by the Bruggeman model, establishing a foundational dataset for future ellipsometric studies of oxidation kinetics in bulk uranium system.

The interplay among topology, crystal symmetry, magnetic order, and strong electron correlation can give rise to a plethora of exotic physical phenomena. The ZrSiS family is known as typical topological Dirac semimetals, among them LnSbTe (Ln denotes lanthanide) compounds exhibit intriguing characteristics due to the presence of Ln 4 f electrons, resulting in quantum states and unique properties. In this paper, the topological electronic structure of PrSbTe is systematically studied by angle -resolved photoemission spectroscopy (ARPES), combined with magnetic, specific heat measurements, and band structure calculations. The detailed three-dimensional electronic structure of PrSbTe has been obtained, and a diamond -shaped Fermi surface and multiple Dirac nodal lines have been observed, which are in remarkable agreement with theoretical calculations. Moreover, the 4 f electrons in PrSbTe are rather localized, which can be revealed by on -resonant ARPES data and further confirmed by the rather small Sommerfeld coefficient of gamma = 2.6231 mJ/mol K2. Our results provide more detailed information about the LnSbTe family, which gives a deeper understanding of the interaction between Ln 4 f electrons and the topological states.

Uranium exhibits high chemical activity. The preparation of single crystalline uranium films via molecular beam epitaxy, followed by in situ characterization under ultrahigh vacuum condition, effectively avoids sample contamination caused by the environmental atmosphere. In this work, single crystalline uranium films were grown on the W(1 1 0) surface by a deflected electron beam evaporator. The optical constants of the W(1 1 0) substrate and the uranium films were obtained through in situ ultrahigh vacuum spectroscopic ellipsometry, while the electronic structure of uranium films was measured by angle-resolved photoemission spectroscopy (ARPES). Notably, the optical constants of uranium film differ from those previously reported measured by the ex situ experiments. The band structure and optical constants of the uranium film were calculated by the ab initio method, and theoretical calculations were compared with the experimental results. The calculated band structure of alpha-U is consistent with the bulk electronic structure of uranium films measured by ARPES, except for a surface- state-like feature. The parameters of the Drude oscillator derived from the ellipsometric fitting were used to revise the calculated optical constants, which obviously improves the accuracy of the calculations. Our study provides the most strictly fundamental data of the uranium up to now.

Uranium-based compounds display rich and exotic physical properties, but remain less studied compared with 4f-electron rare-earth compounds. The lack of high quality single crystal samples hinders the use of many specific techniques to study the properties of uranium-based compounds. In the present study, we successfully obtain high quality single crystalline UCu5 (111) films by depositing uranium atoms on the Cu (111) substrate after annealing at 800 K. Surface structures with the increase of uranium coverage have been systematically studied, and moir & eacute; patterns are observed in the UCu5 (111) films, which gradually become weaker with the increase of film thickness of UCu5. dI/dV spectra of UCu5 films with the thickness from 1 to 5 unit cells show similar electronic properties, which exhibit a strong asymmetric dip-peak structure near the Fermi energy. Temperature-dependent dI/dV measurements further confirm that this asymmetric feature is related to Kondo physics and due to the hybridization between U 5 f electrons and surrounding conduction electrons. Different terminated surfaces can be obtained by sputtering UCu5 thick films with Ar+ ions and subsequent annealing, which show drastically different electronic structure. Our results provide a path for the preparation of single crystalline uranium-based compounds and related materials.

Recent studies on the electronic structures of the 5f-electron based antiferromagnetic compounds, i.e., uranium dipnictides, have aroused widespread interest in the complex interplay among different channels of interactions, such as Kondo entanglement and antiferromagnetic ordering. Here, we use scanning tunneling microscopy/spectroscopy to explore the complex low-energy excitations in the 5f-electron based antiferromagnet UAs2. The crystal-field excitations are revealed as peaks above the Fermi level in the dI/dV spectra. Temperature-dependent spectroscopic measurements find that Kondo resonance and antiferromagnetic order are manifested as two peaks below the Fermi level, demonstrating that Kondo coherence can be undisturbedly established in the antiferromagnetic phase. The crystal-field excitations, Kondo resonance, and antiferromagnetic state are all locally altered by the presence of As-atom vacancies at the atomic scale and these phenomena prove that the nonmagnetic atom vacancy has a strong influence on low-energy excitations in heavy fermion compounds.

Searching for single-atom systems with large magnetic anisotropy energies and tunable magnetic states is of vital importance for both fundamental research of magnetism at the atomic scale and realization of future spin-based quantum computation or information storage schemes. Single $5f$ electron based actinide atoms are potential candidates for inducing large magnetic anisotropy energies (MAEs), yet they have been much less studied as compared with $3d$ or $4f$ single-atom systems. Here we present the adsorptive, electronic, and magnetic properties of a single $5f$ electron based uranium atom on two-monolayer MgO/Ag(001) by combining scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory. Our results reveal that single U atoms spontaneously adsorb at the hollow sites of the MgO/Ag(001) surface and they can be controllably switched between the hollow and the O-top sites of MgO/Ag(001) via STM atom manipulation. Most importantly, single U atoms at the O-top sites reveal complex tunneling spectral features, including a symmetric dip at the Fermi energy, which is the manifestation of the existence of a relatively large $5f$-driven magnetic anisotropy energy, whereas single U atoms at the hollow sites exhibit a two-lobe subatomic structure stemming from the valence electron orbitals of U itself and show no signs related with magnetic anisotropy. This work proves that single $5f$ electron based U atoms can possess a considerable uniaxial magnetic anisotropy via adsorbing at the appropriate sites on the carefully chosen supporting surface, and their magnetic states can be tuned by atom manipulation techniques.

The hybridization between the localized 4f level (f) with conduction (c) electrons in γ-Ce upon cooling has been previously revealed in single crystalline thin films experimentally and theoretically, whereas its influence on the γ → α phase transition was not explicitly verified, due to the fact that the phase transition happened in the bulk-layer, leaving the surface in the γ phase. Here in our work, we circumvent this issue by investigating the effect of alloying addition of La on Ce, by means of crystal structure, electronic transport and angle resolved photoemission spectroscopy measurements, together with a phenomenological periodic Anderson model and a modified Anderson impurity model. Our current researches indicate that the weakening of f–c hybridization is the major factor in the suppression of γ → α phase transition by La doping. The consistency of our results with the effects of other rare earth and actinide alloying additions on the γ → α phase transition of Ce is also discussed. Our work demonstrates the importance of the interaction between f and c electrons in understanding the unconventional phase transition in Ce, which is intuitive for further researches on other rare earth and actinide metals and alloys with similar phase transition behaviors.

In the heavy fermion system, development of the Kondo coherence usually occurs as a crossover behavior. However, accurate definition of the onset temperature for complete formation of the heavy quasiparticles is to some extent controversial. Here, the electronic structure of the quasi-two-dimensional Kondo lattice CeSb2 has been studied by high resolution angle-resolved photoemission spectroscopy (ARPES) and dynamical meanfield theory approach combined with density functional theory. A heavy quasiparticle band, which originates from the hybridization between f electrons and conduction electrons, has been observed directly. Moreover, temperature-dependent electronic structure study reveals a transition from high-temperature local spins to lowtemperature itinerant electrons in CeSb2 and the onset temperature of the hybridized c-f spectral weight revealed by laser-ARPES is consistent with the crossover temperature from transport and magnetic measurements. Our findings are essential for a microscopic understanding of different energy scales in the heavy fermion system and demonstrate the probability of laser-ARPES in studying the bulk 4f-electron properties of Ce-based compounds.

Despite extensive research on the heavy fermion superconductor URu2Si2 in the past three decades, the nature of the hidden order (HO) phase transition occurring at 17.5 K remains ambiguous. Here we report a comparative scanning tunneling microscopy/spectroscopy (STM/STS) study on different terminations of the parent URu2Si2 and Fe-doped samples. A small gap, which was ascribed to the HO parameter by previous STM/STS studies, emerges in both the HO and large moment antiferromagnetic phases on the U terminations, indicating it is not the unique hallmark of the HO parameter. Moreover, a peak-gap-peak structure is observed on the Si terminations. Variations of the two spectral features with Fe concentration and temperature show that they stem from the alteration of f-c hybridization. The higher vanishing temperatures and larger sizes of the gap in the Fe-substituted samples indicate stronger f-c hybridization strength compared to URu2Si2. Our studies demonstrate hybridization is not the driving force for the HO phase transition.

Heavy fermion systems can exhibit abundant attractive quantum ground states by tuning external parameters such as dimension. High-quality USb2 thin films were prepared on graphene/6H-SiC(0001) surface by molecule beam epitaxy. Combining the reflection high energy electron diffraction, X-ray diffraction, electric transport and X-ray photoelectron spectroscopy measurements, it is demonstrated that the grown USb2 films are high-quality single crystals. Furthermore, the surface topography, atomic structure and band structures of USb2 films were characterized by scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy (ARPES). Results show that the surface atomic structure, electric transport property and band structure of the grown USb2 films are similar to those of bulk USb2 single crystals. The preparation and characterization of high-quality USb2 films provide precious experimental experiences for exploring fantastic properties of low-dimensional uranium-based heavy fermion systems by growing ultrathin films with desirable thickness in the future.

The mysterious properties of individual U atoms on transition metal surfaces play indispensable parts in supplementing our understanding of uranium-transition metal systems, which are important subjects for both nuclear energy applications and fundamental scientific studies. By using scanning tunneling microscopy and density functional theory calculations, the adsorptions, reactions and electronic properties of individual U atoms on Cu(111), Ag(111), Au(111) and Ru(0001) surfaces were comparatively studied for the first time in this work. Upon the deposition of a small amount of U onto Cu(111) or Ag(111) at 8 K, individual U atoms show relatively high activity and can either be adsorbed on intact substrate surfaces or induce various surface vacancies surrounded by clusters of substrate atoms. By contrast, the majority of U atoms tend to dispersedly adsorb on intact surfaces of Au(111) and Ru(0001) rather than producing surface vacancies at the same temperature. In all cases, Kondo resonance manifested as asymmetric dip feature around Fermi energy is only observed in the differential tunneling conductance spectra of single U adatoms on Ag(111).

As a novel quantum state in condensed matter physics, Majorana zero mode has become a popular research topic at present because of its potential value in topological quantum computing. Theory predicts that Majorana zero mode appears in the vortex core of the topological superconductor as a unique bound state. However, due to various factors such as the existence of conventional low energy bound states or impurity states, it is difficult to identify the Majorana zero mode and to put it into the specific applications. Nowadays, it is still urgent to find a suitable topological superconducting system and identify the clean Majorana zero mode in experiment. In this paper, we study the vortex states of electron-doped iron-selenium-based superconductors (Li, Fe)OHFeSe and single-layer FeSe/SrTiO3 with extremely high energy resolution STM. There exists a robust and clean Majorana zero mode in the free vortex core of (Li, Fe)OHFeSe, which has the quantized conductance. As for single-layer FeSe/SrTiO3 film, it has only conventional Caroli-de Gennes-Matricon (CdGM) bound states without zero energy mode. These experimental results provide a suitable platform for further studying the physical properties of Majorana zero mode, and also shed light on the source of topological superconductivity in iron-based superconductors.

The physical mechanism driving the γ – α phase transition of face-centre-cubic (fcc) cerium (Ce) remains controversial until now. In this work, high-quality single crystalline fcc–Ce thin films were grown on Graphene/6 H -SiC(0001) substrate, and explored by XRD and ARPES measurement. XRD spectra showed a clear γ – α phase transition at T γ − α ≈ 50 K, which is retarded by strain effect from substrate comparing with T γ − α (about 140 K) of the bulk Ce metal. However, APRES spectra did not show any signature of α -phase emerging in the surface-layer from 300 to 17 K, which implied that α -phase might form at the bulk-layer of our Ce thin films. Besides, an evident Kondo dip near Fermi energy was observed in the APRES spectrum at 80 K, indicting the formation of Kondo singlet states in γ –Ce. Furthermore, the DFT + DMFT calculations were performed to simulate the electronic structures and the theoretical spectral functions agreed well with the experimental ARPES spectra. In γ –Ce, the behavior of the self-energy’s imaginary part at low frequency not only confirmed that the Kondo singlet states emerged at T K S ≥ 80 K, but also implied that they became coherent states at a lower characteristic temperature ( T coh ~40 K) due to the indirect RKKY interaction among f – f electrons. Besides, T coh from the theoretical simulation was close to T γ−α from the XRD spectra. These issues suggested that the Kondo scenario might play an important role in the γ– α phase transition of cerium thin films.