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Séminaire MCBT

Mardi 11 septembre à 11h00,
Salle Louis Weil, E424

Orateur : Dr. Jonathan Buhot, High Field Magnet Laboratory (HFML-EMFL)
"Exploring hidden orders by Raman spectroscopy under extreme conditions"


In condensed matter physics, new electronic phases, so-called “exotic” or “hidden” phases where the order parameter is not identified, occasionally appears at low temperature and/or under high pressure. In this talk, I will present two systems which exhibit such exotic orders : the heavy fermion URu2Si2 and the skutterudite compound PrRu4P12.
For three decades, the identity of the ordered phase, the so-called “hidden order” (HO), found in URu2Si2 at temperature below T0 = 17.5 K has eluded researchers despite intense experimental and theoretical investigations [1, 2]. Using polarised Raman spectroscopy, we have unveiled novel excitations emerging from the HO, a sharp collective mode at 1.7 meV and a gap in the electronic continuum below 6.8 meV, both in the A2g symmetry [3]. More recently, we have also addressed the question of the symmetry of the Kondo pseudo-gap that is typically observed in the Eg symmetry below ̴ 100 K, suggesting a strong momentum anisotropy [4]. In light of these results, I will discuss the possible symmetries of the f electron ground state and excited states within the HO state.

PrRu4P12 undergoes a metal-insulator transition (MIT) at a temperature TMI = 63 K [5, 6], that has been associated with some form of hidden electronic order, akin to that realised in URu2Si2. While a hexadecapolar scenario have been proposed [7], no consensus has yet been reached on this problem by the scientific community. Thanks to the development of two unique Raman scattering experiments, capable of probing low energy excitations (0.6 meV) in extreme conditions of low temperatures (4.2 K), high pressures (17 GPa) (in the laboratory Materials and Quantum Phenomena, Paris) and high magnetic fields (30-37 T) (in the High Field Magnet Laboratory, Nijmegen), we have followed the evolution of both the phonon modes and the crystal electric field excitations across the entire complex phase diagram of PrRu4P12, giving us crucial new information about the crystalline structure and the electronic f states of Pr3+ ions.

[1] Mydosh and Oppeneer, Reviews of Modern Physics 83, 1301 (2011) [2] Mydosh, Philosophical Magazine 94, 3640 (2014) [3] Buhot et al., PRL 113 266405 (2014) [4] Buhot et al., arXiv:1805.11307 (2018) [5] Sekine et al., PRL 79, 3218 (1997) [6] Miyake et al., Physica B 403, 1298 (2008) [7] Takimoto et al., J. Phys. Soc. Jpn. 75, 34714 (2006)

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