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| URu2Si2 | |
|---|---|
| Name | URu2Si2 |
| Formula | URu2Si2 |
| Crystal system | Tetragonal |
| Space group | I4/mmm |
| Discovery | 1985 |
URu2Si2 is a heavy-fermion intermetallic compound containing uranium, ruthenium, and silicon, notable for hosting an enigmatic "hidden order" phase and unconventional superconductivity. First synthesized and characterized in the 1980s, URu2Si2 has become a focal point for condensed matter research involving correlated electrons, quantum criticality, and emergent phases. The compound is studied across a wide experimental and theoretical landscape including low-temperature transport, neutron scattering, angle-resolved photoemission, and first-principles modeling.
URu2Si2 crystallizes in the tetragonal ThCr2Si2-type structure, related to materials such as ThCr2Si2 structure, CeCu2Si2, BaFe2As2, SrRuO3, and YNi2B2C, and is isostructural with many intermetallics studied in the context of heavy-fermion physics and unconventional superconductivity. The lattice hosts uranium atoms on a body-centered tetragonal sublattice coordinated by ruthenium square nets and silicon layers, paralleling motifs found in CeRhIn5, PuCoGa5, LaFePO, Kondo lattice compounds, and High-temperature superconductors. Stoichiometry and sample quality affect properties; single crystals grown by the Czochralski method or flux techniques are compared to polycrystalline specimens in work by groups at institutions such as Los Alamos National Laboratory, Max Planck Institute for Chemical Physics of Solids, University of Cambridge, and Rice University.
URu2Si2 exhibits heavy-fermion behavior with large electronic specific heat coefficients reminiscent of CeAl3 and UPt3, electrical resistivity anomalies similar to those in Kondo insulators, and low-temperature superconductivity. The Sommerfeld coefficient gamma reaches several hundred mJ mol−1 K−2, comparable to YbRh2Si2 and CeCoIn5, and the system shows a Kondo lattice crossover and coherence onset analogous to phenomena in Anderson impurity model materials. Under applied pressure, magnetic field, or chemical substitution (e.g., Re, Rh, Fe doping) the phase diagram evolves, linking URu2Si2 to studies of quantum phase transitions, pressure-induced superconductivity, metamagnetism, and itinerant magnetism explored in compounds like MnSi and Sr3Ru2O7.
At T0 ≈ 17.5 K URu2Si2 undergoes a second-order transition into the so-called hidden order (HO) phase, a long-standing puzzle akin to order-parameter mysteries in pseudogap phase research and broken-symmetry problems studied in nematic order and charge-density wave systems. The HO phase produces large entropy release and reconstruction of the Fermi surface detected by quantum oscillation measurements paralleling work on de Haas–van Alphen effect in CeRu2Si2 and LaRhIn5. Competing proposals for the HO include multipolar order, density-wave formation, hastatic order, hybridization wave, and orbital currents, with theoretical input from groups associated with Princeton University, Institute for Advanced Study, University of Tokyo, Los Alamos National Laboratory, and ETH Zurich.
Below Tc ≈ 1.5 K URu2Si2 becomes superconducting, with unconventional pairing suspected by comparisons to UPt3, CeCoIn5, Sr2RuO4, and UBe13. The superconducting state coexists or competes with the HO phase and displays nodal gap signatures in heat capacity, thermal conductivity, and NMR Knight shift experiments similar to evidence gathered on d-wave superconductors and spin-triplet superconductors. Pressure and magnetic field studies performed at facilities including National High Magnetic Field Laboratory and European Synchrotron Radiation Facility map suppression of superconductivity and the emergence of antiferromagnetism, drawing parallels to the pressure phase diagrams of CeRhIn5 and UGe2.
Magnetism in URu2Si2 is subtle: small antiferromagnetic moments reported in early neutron diffraction work led to debates linking the HO to tiny-moment antiferromagnetism, a theme also seen in hidden-order problems of other correlated materials. Under pressure or chemical substitution URu2Si2 transitions to a large-moment antiferromagnetic phase, comparable to transitions studied in MnSi and CeIn3, and quantum critical points have been inferred by transport and thermodynamic scaling akin to analyses in YbRh2Si2 and CeCu6−xAux. High-field phases and metamagnetic transitions connect URu2Si2 to metamagnetism studies in Sr3Ru2O7 and CeRu2Si2.
The electronic structure of URu2Si2 features itinerant and localized 5f-electron character, placing it among f-electron systems like UBe13, UPd2Al3, PuCoGa5, and CeMIn5. Angle-resolved photoemission spectroscopy (ARPES), dynamical mean-field theory (DMFT), and density functional theory (DFT+U) studies map heavy bands, hybridization gaps, and Fermi surface pockets, connecting to methodologies applied to SmB6, FeSe, Actinide metals, and Topological Kondo insulators. Competing itinerant and localized scenarios motivate models invoking crystal-field splitting, spin–orbit coupling, multiplet physics, and hybridization with conduction electrons, drawing on theoretical frameworks from Anderson lattice model, Kondo insulator theory, and Hund's coupling studies.
Key experimental probes of URu2Si2 include neutron scattering, muon spin rotation (μSR), nuclear magnetic resonance (NMR), resonant x-ray scattering, ARPES, scanning tunneling microscopy (STM), de Haas–van Alphen and Shubnikov–de Haas oscillations, specific heat, thermal conductivity, and transport measurements. Landmark experiments were performed at facilities such as ISIS Neutron and Muon Source, Oak Ridge National Laboratory, Paul Scherrer Institute, Stanford Synchrotron Radiation Lightsource, National Institute of Standards and Technology, and Advanced Photon Source. Results from these techniques inform cross-disciplinary debates linking URu2Si2 to topics investigated by researchers at Columbia University, Massachusetts Institute of Technology, University of California, Berkeley, University of Michigan, and University of Tokyo.
Category:Actinide compounds