SPring-8, the large synchrotron radiation facility

Speaker : Dr. Andrés F. Santander-Syro

Language : ENGLISH

Affiliation : CSNSM, Université Paris-Sud and CNRS/IN2P3

Title : Momentum-resolved 'hidden-order' gap structure, symmetries, and entropy loss in URu₂Si₂

Abstract:
　Due to their exceptionally strong correlations, ƒ-electron systems present a wide realm of original phase transitions and often poorly understood states of matter. One of the most intriguing is the so-called hidden-order (HO) state forming below T_HO =17.5 K in URu₂Si₂. Extensive macroscopic characterizations gathered during the last 30 years show a reduction of almost 60% in the electronic specific heat across the transition, and suggest that a gap of about 10 meV opens over more than 50% of its Fermi surface. However, the identification of the associated broken symmetry and gap structure remain longstanding riddles [1-3].
　In this talk, I will present our state-of-the-art ARPES measurements imaging the reconstruction of the electronic structure of URu₂Si₂ across the hidden-order transition [4-6]. We observe a change of the electronic structure symmetry from body-centred tetragonal in the paramagnetic state to simple-tetragonal in the ordered state, described by the ordering vector Q₀ = (1, 0, 0) (reciprocal-lattice units). This is accompanied by the opening of a gap of 7 meV over 70% of a large heavy-fermion Fermi surface, and the formation of four small “Fermi petals” at the incommensurate wave-vector Q₁ = (0.6, 0, 0) showing another gap of 5 meV with respect to the states that defined the heavy-fermion Fermi surface above T_HO. Furthermore, the Fermi sheets measured in the HO state are in quantitative agreement with those determined by Shubnikov–de Haas experiments. Thus, our results provide a unified microscopic picture of the large entropy loss in the HO state, of the emergence of sharp inelastic peaks in the magnetic excitation spectrum at Q₀ and the gap of magnetic excitations at Q₁ observed by inelastic neutron scattering, and of the similarity found by magneto-transport measurements between the HO phase and the high-pressure antiferromagnetic phase, which would then be a consequence of both phases having the same simple-tetragonal electronic symmetry described by the ordering vector Q₀.

[1] T. M. Palstra et al. Superconducting and magnetic transitions in the heavy-fermion system URu₂Si₂. Phys Rev Lett. 55, 2727-2730 (1985).
[2] M. B. Maple et al. Partially gapped Fermi surface in the heavy-electron superconductor URu₂Si₂. Phys. Rev. Lett. 56, 185-188 (1986).
[3] J. A. Mydosh and P. M. Oppeneer. Colloquium: Hidden order, superconductivity, and magnetism: The unsolved case of URu₂Si₂. Rev. Mod. Phys. 83, 1301 (2011).
[4] A. F. Santander-Syro et al. Fermi-surface instability at the ’hidden-order’ transition of URu₂Si₂. Nature Phys. 5, 637-641 (2009).
[5] F. L. Boariu et al. Momentum-resolved evolution of the Kondo lattice into “hidden-order” in URu₂Si₂. Phys. Rev. Lett. 110, 156404 (2013).
[6] C. Bareille et al. Momentum-resolved “hidden-order” gap structure, symmetries, and entropy loss in URu₂Si₂. Nature Communications (Accepted, 2014).

Organizer : Shin-ichi Fujimori
Mail : fujimori@spring8.or.jp
PHS : 3914