Tuesday 23.02.2010, 13:00-14:00 SE2.

Electronic structure and hidden-order transition in URu₂Si₂

Andrés F. Santander-Syro

CSNSM – Université Paris Sud

and

LPEM-ESPCI (Paris)

The heavy-fermion semi-metal URu₂Si₂ presents an enigmatic second-order phase transition at T_o = 17.5 K to a `hidden order' state whose order parameter remains unknown after 25 years of intense research. Various transport and thermo-transport experiments point to the reconstruction and partial gapping of the Fermi surface when the hidden-order establishes. However, up to now, the question of how this transition affects the electronic structure at the Fermi surface has not been directly addressed by a spectroscopic probe. We used angle-resolved photoemission spectroscopy to study the hidden-order transition in URu₂Si₂. We discovered that a band of heavy quasi-particles shifts from E>E_F at T>T_o to E<E_F in the ordered state. Our data provide thus the first direct evidence of a large reorganization of the electronic structure across the Fermi surface of URu₂Si₂ occurring during the hidden-order transition. The data further suggest that the observed heavy-electron band results from the hybridization of a light hole-like conduction band with a band of localized states, pointing to the importance of the “localized-itinerant” interplay of electrons in this material. We will discuss these observations, their relation to other experiments, and their implications in the understanding of the hidden-order transition.