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Accueil > Groupes de recherche > Physique des solides > Strongly correlated systems and new electronic states of matter > Exotic electronic states and phase transitions in heavy-fermion materials

Exotic electronic states and phase transitions in heavy-fermion materials

Heavy-fermion systems are materials in which the electron interactions are so strong that their effective mass becomes hundreds or thousands of times the free-electron mass. Due to such strong interactions, these materials display numerous exotic classical and quantum phase transitions. The resulting broken symmetries and electronic collective phenomena are, often, not well understood.

In our team, we employ high-resolution low-temperature angle-resolved photoemission spectroscopy (ARPES) to study the microscopic origin of those phase transitions. This technique allows imaging directly the band-structure and electron interactions in a solid. For instance, using ARPES, we could recently determine that a novel kind of Fermi-surface instability takes place across the enigmatic “hidden-order” transition of URu2Si2 [1]. This ordered state of electrons arises below 17.5K. However, the identification of the associated broken symmetry and gap structure remain longstanding riddles, owing it the nickname of “the Higgs boson of condensed-matter physics”. We are now studying in detail the hidden broken symmetries and gap [2]. We are also extending our studies to quantum-phase transitions in other heavy-fermion systems, for which an experimental description from an electronic-structure viewpoint is still missing.

For our research in heavy-fermion systems, we hold longstanding fruitful collaborations with the high-resolution ARPES group of Prof. Dr. Friedrich Reinert at Würzburg University, and the crystal growth group of Dr Pascal Lejay at Institut Néel (Grenoble).

[1] A. F. Santander-Syro, M. Klein, F. Boariu et al. Fermi surface instability at the ‘hidden-order’ transition of URu2Si2”. Nat. Phys. 5, 637 (2009).

[2] F. L. Boariu, C. Bareille, H. Schwab et al. Momentum-resolved evolution of the Kondo lattice into “hidden-order” in URu2Si2. Phys. Rev. Lett. 110, 156404 (2013).

PNG - 128.3 ko

Fermi-surface instability at the «hidden-order» transition in URu2Si2

In URu2Si2 a heavy-electron band forms at high temperatures, possibly from the hybridization, via the formation of a Kondo-lattice, of localized f-electron states with a conduction band. Upon entering the hidden-order state, below 17K, the heavy electron band shifts from the unoccupied to the occupied states, causing an instability in the zero-energy states that are responsible for the macroscopic behavior of the material, and forming the M-shaped band seen in the color plot. From Ref. [1].