Qiu-Yun Chen

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 ${\mathrm{UFeGa}}_{5}$, a layered uranium intermetallic compound, using high-resolution angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations. Three-dimensional electronic structure of ${\mathrm{UFeGa}}_{5}$ was systematically studied and fine structures of the $\mathrm{U}5f$ bands near the Fermi energy were clearly resolved. Our results reveal that ${\mathrm{UFeGa}}_{5}$ exhibits a predominantly itinerant nature of $5f$ electrons, with strong band dispersion and significant contributions to the Fermi surface. Theoretical calculations assuming itinerant $5f$ 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).

Utilizing ultrafast optical pump-probe spectroscopy, we present a comprehensive investigation on the photoexcited quasiparticle dynamics in single-crystal ${\mathrm{CeAgSb}}_2$. We elucidate the emergence of collective $c\text{−}f$ hybridization between conduction electrons and localized $f$ moments below a critical temperature $T^{*}$ ($\approx 65$ K), as indicated by the onset of an indirect gap of $\sim 9$ meV via the fluence-dependent quasiparticle relaxation and phonon renormalization processes. Our experimental data also suggest that the $c\text{−}f$ hybridization fluctuation should appear at a much higher-temperature value, i.e., $T^{†}$ ($\approx 180$ K). Below $\sim 50$ K, we further detect an additional quasiparticle relaxation channel with sub-ps timescale, which can be attributed to the crystalline electric field (CEF) excitation. These observations provide important insights into the Kondo lattice coherence and CEF effect in ${\mathrm{CeAgSb}}_2$, shedding light on its unique properties and behaviors.

Uranium-based compounds display rich and exotic physical properties, but remain less studied compared with $4f\text{−}\mathrm{electron}$ 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 $\mathrm{UC}{\mathrm{u}}_5$ (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 moiré patterns are observed in the $\mathrm{UC}{\mathrm{u}}_5$ (111) films, which gradually become weaker with the increase of film thickness of $\mathrm{UC}{\mathrm{u}}_5$. $dI/dV$ spectra of $\mathrm{UC}{\mathrm{u}}_5$ 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 $5f$ electrons and surrounding conduction electrons. Different terminated surfaces can be obtained by sputtering $\mathrm{UC}{\mathrm{u}}_5$ thick films with $\mathrm{A}{\mathrm{r}}^{+}$ 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.

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 $\mathit{Ln}\mathrm{SbTe}$ ($\mathit{Ln}$ denotes lanthanide) compounds exhibit intriguing characteristics due to the presence of $\mathit{Ln} 4f$ 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 $4f$ 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 $\gamma =2.6231\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$. Our results provide more detailed information about the $\mathit{Ln}\mathrm{SbTe}$ family, which gives a deeper understanding of the interaction between $\mathit{Ln} 4f$ electrons and the topological states.

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.

In actinide-based materials, the multiplicity of $5f$ orbitals provides a rich setting to elucidate the large and outstanding issues of the competition and interplay among different channels of interactions. Among them, the dual character of $5f$ electrons has been studied in some $5f$ electron Kondo lattice compounds. Here, by using scanning tunneling microscopy/spectroscopy (STS), we present the indication of Kondo effect driven by $5f$ electrons and subatomic features of single U atoms on graphene/6H-SiC(0001), ${\mathrm{Cu}}_2\mathrm{N}/\mathrm{Cu}\left(100\right)$, and Bi(110). For all three systems, an asymmetric-dip spectral feature can be observed around the Fermi energy, which is an indication of Kondo resonance in STS. The size of the “screening cloud” for single U atoms on Bi(110) is much larger than that observed on graphene/6H-SiC(0001) and ${\mathrm{Cu}}_2\mathrm{N}/\mathrm{Cu}\left(100\right)$, indicating the $5f$ electrons of single U atoms on Bi(110) are more itinerant than that on the other two substrates. In addition to the asymmetric spectrum feature around the Fermi energy, various subatomic spectrum features are also observed from the $dI/dV$ maps of U atoms on different substrates. Further density-functional theory calculations show that these distinct subatomic features arise from the orbital selective hybridizations between U atoms and the substrates. Our results present the distinct observation of the orbital selective $5f$ electron character, indication of Kondo effect, and together with the subatomic features of individual $5f$ impurity magnetic atoms adsorbed on nonmagnetic surfaces.

Despite extensive research on the heavy fermion superconductor $\mathrm{U}{\mathrm{Ru}}_2{\mathrm{Si}}_2$ 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 $\mathrm{U}{\mathrm{Ru}}_2{\mathrm{Si}}_2$ 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 $f\text{−}c$ hybridization. The higher vanishing temperatures and larger sizes of the gap in the Fe-substituted samples indicate stronger $f\text{−}c$ hybridization strength compared to $\mathrm{U}{\mathrm{Ru}}_2{\mathrm{Si}}_2$. Our studies demonstrate hybridization is not the driving force for the HO phase transition.

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 ${\mathrm{CeSb}}_2$ has been studied by high resolution angle-resolved photoemission spectroscopy (ARPES) and dynamical mean-field 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 low-temperature itinerant electrons in ${\mathrm{CeSb}}_2$ 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.