大型放射光施設 SPring-8

Speaker: Dr. Bareille Cédric

Language: English

Affiliation: 東京大学物性研究所

Title: Combined laser, VUV and soft-X-ray photoemission investigation of the conduction band of the heavy fermion β-YbAlB4

Abstract:
　Heavy fermion compounds are very strongly correlated electronic systems, such that their ground states have pretty small energy scale, i.e. of the order of 10-3 to 10-4 electron-Volt. Therefore, investigation of the relevant interactions by spectroscopy requires ultra-high energy resolution, and thus low photon energies. However, from bulk sensitivity to resonant photoemission, higher photon energies has proven to be necessary to probe details inaccessible otherwise [1,2]. In this seminar, we will see how photoemission spectroscopy using each of laser, VUV and soft-X-ray sources allow the access of different and complementary information over the conduction band of β-YbAlB4.
　This heavy fermion system harbors an unconventional quantum criticality under ambient conditions below about 4 K, characterized by an unexpected T/B-scaling [3] and unusual critical exponents [4]. Intriguingly, it occurs away from any magnetic order [5], definitely discarding the usual picture of magnetic instability. Several theoretical proposals have been advanced, from the radical breakdown of the fermionic quasiparticles, to the valence criticality, to a fermionic condensate. The variety of exotic tentative models shows just how much this quantum criticality challenges theoretical descriptions.
　Thanks to the ultra-high energy resolution of the laser-ARPES, we resolved a quasiparticle peak bounded 4 meV below the Fermi level, with an onset of coherence at about 40 K. Comparison with previous Hall effect measurements [6] identifies the corresponding carriers as responsible of the non-Fermi liquid behavior. Photon energy dependence in the VUV range allows to locate the laser measurements around the Z point.
　Further VUV-ARPES mapping over the whole Brillouin zone highlights a limited agreement with DFT calculation. This is in contrast with other Yb-compounds, as YbRh2Si2 where despite several differences a general agreement between ARPES and DFT is found [2]. In order to elucidate this point, we investigate the Yb 5d density of state in the conduction band thanks to resonant photoemission in the soft-X-ray range. It reveals a large gap in the Yb 5d density of state, absent from the DFT calculation, and therefore suggests the existence of some unexpected correlation.
Finally, all our results are discussed with existent theoretical models.

[1] I. Kawasaki et al. Band structure and Fermi surface of URu2Si2 studied by soft x-ray angle-resolved photoemission spectroscopy. Phys. Rev. B 83, 235121 (2011).
[2] A. Yasui et al. Observation of bulk band dispersions of YbRh2Si2 using soft x-ray angle-resolved photoemission spectroscopy. Phys. Rev. B 87, 075131 (2013)
[3] Y. Matsumoto et al. Field Evolution of Quantum Critical and Heavy Fermi-Liquid Components in the Magnetization of the Mixed Valence Compound β-YbAlB4. Journal of the Physical Society of Japan 84, 024710 (2015)
[4] S. Nakatsuji et al. Superconductivity and quantum criticality in the heavy-fermion system β-YbAlB4. Nature Physics 4, 603-607 (2008)
[5] T. Tomita et al. Strange metal without magnetic criticality. Science 349, 506-509 (2015).
[6] E. C. T. O'Farrell et al. Evolution of c-f Hybridization and Two-Component Hall Effect in β-YbAlB4. Phys. Rev. Lett. 109, 176405 (2012)