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Title: From hidden order to antiferromagnetism: Electronic structure changes in Fe-doped URu 2 Si 2
In matter, any spontaneous symmetry breaking induces a phase transition characterized by an order parameter, such as the magnetization vector in ferromagnets, or a macroscopic many-electron wave function in superconductors. Phase transitions with unknown order parameter are rare but extremely appealing, as they may lead to novel physics. An emblematic and still unsolved example is the transition of the heavy fermion compound U R u 2 S i 2 (URS) into the so-called hidden-order (HO) phase when the temperature drops below T 0 = 17.5 K. Here, we show that the interaction between the heavy fermion and the conduction band states near the Fermi level has a key role in the emergence of the HO phase. Using angle-resolved photoemission spectroscopy, we find that while the Fermi surfaces of the HO and of a neighboring antiferromagnetic (AFM) phase of well-defined order parameter have the same topography, they differ in the size of some, but not all, of their electron pockets. Such a nonrigid change of the electronic structure indicates that a change in the interaction strength between states near the Fermi level is a crucial ingredient for the HO to AFM phase transition.
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Proceedings of the National Academy of Sciences
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National Science Foundation
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  1. Abstract

    Previous high-resolution angle-resolved photoemission (ARPES) studies of URu2Si2have characterized the temperature-dependent behavior of narrow-band states close to the Fermi level (EF) 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 globalk-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 2pcore-levels, x-ray polarization, surface-dosing modification, and theoretical surface slab calculations are employed to assist identification of bulk versus surface state character of theEF-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,ZandXpoints, incommensurate 0.6a* nested Fermi-edge states located alongZNZare found to be distinctlymore »different from the DFT FS prediction. The temperature evolution of these states aboveTHO, combined with a more detailed theoretical investigation of this region, suggests a key role of theN-point in the hidden order transition.

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  2. Electrical resistivity measurements were performed on single crystals of URu2–xOsxSi2up tox= 0.28 under hydrostatic pressure up toP= 2 GPa. As the Os concentration,x, is increased, 1) the lattice expands, creating an effective negative chemical pressurePch(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 pressurePcis required to induce the HO→LMAFM phase transition. We compare the behavior of theT(x,P) phase boundary reported here for the URu2-xOsxSi2system with previous reports of enhanced HO in URu2Si2upon tuning withPor similarly in URu2–xFexSi2upon tuning with positivePch(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 URu2Si2. 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-5f-electron and transition metald-electron states which leads to electronic instability in the paramagnetic phase and the concurrent formation of HO (and LMAFM) in URu2Si2. Calculations in the tight-binding approximation are included to determine the strength of hybridization between the U-5f-electron states and thed-electron states ofmore »Ru and its isoelectronic Fe and Os substituents in URu2Si2.

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  3. Nontrivial topology in condensed-matter systems enriches quantum states of matter to go beyond either the classification into metals and insulators in terms of conventional band theory or that of symmetry-broken phases by Landau’s order parameter framework. So far, focus has been on weakly interacting systems, and little is known about the limit of strong electron correlations. Heavy fermion systems are a highly versatile platform to explore this regime. Here we report the discovery of a giant spontaneous Hall effect in the Kondo semimetal C e 3 B i 4 P d 3 that is noncentrosymmetric but preserves time-reversal symmetry. We attribute this finding to Weyl nodes—singularities of the Berry curvature—that emerge in the immediate vicinity of the Fermi level due to the Kondo interaction. We stress that this phenomenon is distinct from the previously detected anomalous Hall effect in materials with broken time-reversal symmetry; instead, it manifests an extreme topological response that requires a beyond-perturbation-theory description of the previously proposed nonlinear Hall effect. The large magnitude of the effect in even tiny electric and zero magnetic fields as well as its robust bulk nature may aid the exploitation in topological quantum devices.
  4. 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 .
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