More Like this

1. Isoelectronic perturbations to f - d -electron hybridization and the enhancement of hidden order in URu ₂ Si ₂

   https://doi.org/10.1073/pnas.2026591118  

   Wolowiec, Christian T.; Kanchanavatee, Noravee; Huang, Kevin; Ran, Sheng; Breindel, Alexander J.; Pouse, Naveen; Sasmal, Kalyan; Baumbach, Ryan E.; Chappell, Greta; Riseborough, Peter S.; et al (May 2021, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Electrical resistivity measurements were performed on single crystals of URu 2– x Os x Si 2 up to x = 0.28 under hydrostatic pressure up to P = 2 GPa. As the Os concentration, x , is increased, 1) the lattice expands, creating an effective negative chemical pressure P ch ( x ); 2) the hidden-order (HO) phase is enhanced and the system is driven toward a large-moment antiferromagnetic (LMAFM) phase; and 3) less external pressure P c is required to induce the HO→LMAFM phase transition. We compare the behavior of the T ( x , P ) phase boundary reported here for the URu 2- x Os x Si 2 system with previous reports of enhanced HO in URu 2 Si 2 upon tuning with P or similarly in URu 2– x Fe x Si 2 upon tuning with positive P ch ( x ). It is noteworthy that pressure, Fe substitution, and Os substitution are the only known perturbations that enhance the HO phase and induce the first-order transition to the LMAFM phase in URu 2 Si 2 . We present a scenario in which the application of pressure or the isoelectronic substitution of Fe and Os ions for Ru results in an increase in the hybridization of the U-5 f -electron and transition metal d -electron states which leads to electronic instability in the paramagnetic phase and the concurrent formation of HO (and LMAFM) in URu 2 Si 2 . Calculations in the tight-binding approximation are included to determine the strength of hybridization between the U-5 f -electron states and the d -electron states of Ru and its isoelectronic Fe and Os substituents in URu 2 Si 2 . 

   more » « less
2. Global perspectives of the bulk electronic structure of URu ₂ Si ₂ from angle-resolved photoemission

   https://doi.org/10.1088/2516-1075/ac4315  

   Denlinger, J D; Kang, J-S; Dudy, L; Allen, J W; Kim, Kyoo; Shim, J-H; Haule, K; Sarrao, J L; Butch, N P; Maple, M B (January 2022, Electronic Structure)

   Abstract Previous high-resolution angle-resolved photoemission (ARPES) studies of URu 2 Si 2 have characterized the temperature-dependent behavior of narrow-band states close to the Fermi level ( E F ) at low photon energies near the zone center, with an emphasis on electronic reconstruction due to Brillouin zone folding. A substantial challenge to a proper description is that these states interact with other hole-band states that are generally absent from bulk-sensitive soft x-ray ARPES measurements. Here we provide a more global k -space context for the presence of such states and their relation to the bulk Fermi surface (FS) topology using synchrotron-based wide-angle and photon energy-dependent ARPES mapping of the electronic structure using photon energies intermediate between the low-energy regime and the high-energy soft x-ray regime. Small-spot spatial dependence, f -resonant photoemission, Si 2 p core-levels, x-ray polarization, surface-dosing modification, and theoretical surface slab calculations are employed to assist identification of bulk versus surface state character of the E F -crossing bands and their relation to specific U- or Si-terminations of the cleaved surface. The bulk FS topology is critically compared to density functional theory (DFT) and to dynamical mean field theory calculations. In addition to clarifying some aspects of the previously measured high symmetry Γ, Z and X points, incommensurate 0.6 a * nested Fermi-edge states located along Z – N – Z are found to be distinctly different from the DFT FS prediction. The temperature evolution of these states above T HO , combined with a more detailed theoretical investigation of this region, suggests a key role of the N -point in the hidden order transition. 

   more » « less
3. Orbital-selective Kondo lattice and enigmatic f electrons emerging from inside the antiferromagnetic phase of a heavy fermion

   https://doi.org/10.1126/sciadv.aaw9061  

   Giannakis, Ioannis; Leshen, Justin; Kavai, Mariam; Ran, Sheng; Kang, Chang-Jong; Saha, Shanta R.; Zhao, Y.; Xu, Z.; Lynn, J. W.; Miao, Lin; et al (October 2019, Science Advances)

   Novel electronic phenomena frequently form in heavy-fermions because of the mutual localized and itinerant nature of f -electrons. On the magnetically ordered side of the heavy-fermion phase diagram, f -moments are expected to be localized and decoupled from the Fermi surface. It remains ambiguous whether Kondo lattice can develop inside the magnetically ordered phase. Using spectroscopic imaging with scanning tunneling microscope, complemented by neutron scattering, x-ray absorption spectroscopy, and dynamical mean field theory, we probe the electronic states in antiferromagnetic USb 2 . We visualize a large gap in the antiferromagnetic phase within which Kondo hybridization develops below ~80 K. Our calculations indicate the antiferromagnetism and Kondo lattice to reside predominantly on different f -orbitals, promoting orbital selectivity as a new conception into how these phenomena coexist in heavy-fermions. Finally, at 45 K, we find a novel first order–like transition through abrupt emergence of nontrivial 5 f -electronic states that may resemble the “hidden-order” phase of URu 2 Si 2 . 

   more » « less
4. On structure and phase transformation of uranium doped La ₂ Hf ₂ O ₇ nanoparticles as an efficient nuclear waste host

   https://doi.org/10.1039/c8qm00266e  

   Abdou, Maya; Gupta, Santosh K.; Zuniga, Jose P.; Mao, Yuanbing (November 2018, Materials Chemistry Frontiers)

   The design and development of efficient and stable nuclear waste hosts has drawn intensive interest for long-lived lanthanides and actinides. A detailed investigation of their structure and potential structural evolution are crucial. In this study, we have synthesized lanthanum hafnate La 2 Hf 2 O 7 nanoparticles (NPs) doped with uranium at different concentrations (0–10%) and investigated their structural transition. We have discovered that in our La 2 Hf 2 O 7 :U NPs, the uranium dopants are stabilized at both U 4+ and U 6+ oxidation states in which the U 6+ oxidation state exists in octahedral uranate UO 6 6− form. We also confirmed that the U 4+ ions substituted the Hf 4+ ions with a lifetime of ∼1.0 μs and the UO 6 6− ions resided at the La 3+ sites with a lifetime of ∼9.0 μs. More interestingly, the proportion of the U 4+ ions in the La 2 Hf 2 O 7 :U NPs was higher than that of the UO 6 6− ions at low doping level, but at the doping level higher than 2.5%, the fraction of the UO 6 6− ions was greater than that of the U 4+ ions. Furthermore, we studied the structural phase transformation from order pyrochlore to cotunnite of these La 2 Hf 2 O 7 :U NPs with increasing uranium doping level, and found that ordered pyrochlore phase favors the U 4+ ions whereas disordered cotunnite phase favors the UO 6 6− ions. We further used in situ Raman spectroscopy to confirm the reversible cotunnite to pyrochlore phase transformation of the La 2 Hf 2 O 7 :10%U NPs at 900 °C. Therefore, this work demonstrated the successful development of uranium doped La 2 Hf 2 O 7 NPs and thorough characterization of the fundamental spectra of uranium ions, doping induced phase transformation, and structure–optical property correlation. 

   more » « less
5. Interplay between light and heavy electron bands in magic-angle twisted bilayer graphene

   https://doi.org/10.1038/s41567-025-02912-x  

   Merino, Rafael Luque; Călugăru, Dumitru; Hu, Haoyu; Díez-Mérida, Jaime; Díez-Carlón, Andrés; Taniguchi, Takashi; Watanabe, Kenji; Seifert, Paul; Bernevig, B Andrei; Efetov, Dmitri K (July 2025, Nature Physics)

   Abstract Recent studies have suggested that the strongly correlated flat bands of magic-angle twisted bilayer graphene may host coexisting light and heavy carriers. Although transport and spectroscopic measurements have hinted at this behaviour, distinct signatures of incoherent heavy carriers have not been reported. Here we provide evidence of this by performing thermoelectric transport measurements of magic-angle twisted bilayer graphene using the photo-thermoelectric effect in gate-defined p–n junctions. At low temperatures, we observe sign-preserving, filling-dependent oscillations of the Seebeck coefficient at non-zero integer fillings of the moiré superlattice. This suggests the preponderance of one carrier type even when the Fermi level is tuned through the charge neutrality point of the correlated states. At higher temperatures, the thermoelectric response provides evidence of strong electron correlations in the unordered, normal state. Our observations are explained by the interplay between light, long-lived electron states and heavy, short-lived hole excitations near the Fermi level of the symmetry-broken ground states. These findings are in qualitative agreement with the topological heavy fermion model. 

   more » « less