November 2008 EPL, 84 (2008) 37003 www.epljournal.org doi: 10.1209/0295-5075/84/37003 Observation of a kink in the dispersion of f-electrons T. Durakiewicz 1(a) , P. S. Riseborough 2 , C. G. Olson 3 , J. J. Joyce 1 , P. M. Oppeneer 4 , S. Elgazzar 4 , E. D. Bauer 1 , J. L. Sarrao 1 , E. Guziewicz 5 , D. P. Moore 1 , M. T. Butterfield 6 and K. S. Graham 1 1 Los Alamos National Laboratory - Los Alamos, NM 87545, USA 2 Department of Physics, Temple University - Philadelphia, PA 19122, USA 3 Ames Laboratory, Iowa State University - Ames, IA 50011, USA 4 Department of Physics and Materials Science, Uppsala University - Box 530, S-75121 Uppsala, Sweden, EU 5 Institute of Physics, Polish Academy of Sciences - 02-668 Warsaw, Poland, EU 6 Lawrence Livermore National Laboratory - Livermore, CA 94550, USA received 10 July 2008; accepted in final form 15 September 2008 published online 15 October 2008 PACS 79.60.-i – Photoemission and photoelectron spectra PACS 71.28.+d – Narrow-band systems; intermediate-valence solids PACS 74.25.Jb – Electronic structure Abstract – Strong interactions in correlated electron systems may result in the formation of heavy quasiparticles that exhibit kinks in their dispersion relation. Spectral weight is incoherently shifted awayfrom the Fermi energy, but Luttinger’s theorem requires the Fermi volume to remain constant. Our angle-resolved photoemission study of USb2 reveals a kink in a noncrossing 5f band, representing the first experimental observation of a kink structure in f -electron systems. The kink energy scale of 21 meV and the ultra-small peak width of 3 meV are observed. We propose the novel mechanism of renormalization of a point-like Fermi surface, and that Luttinger’s theorem remains applicable. Copyright c  EPLA, 2008 Introduction. – Since the remarkable discovery of heavy-fermion superconductivity in CeCu 2 Si 2 [1], the properties of f -electron materials were linked to the renormalization of electron effective masses due to strong electronic correlations. At low temperatures, a typical heavy-fermion system behaves as a Fermi liquid with heavy quasiparticles, while, at higher temperatures, it exhibits a local moment character. The cross-over between these two types of behavior is driven by the high entropy associated with the local moments that is removed at low temperatures when the electrons delocalize leading to the formation of the heavy Fermi liquid. Hence, the f -electrons exhibit a dual nature, and the physical properties of f -electron systems become sensitive to minute changes in parameters like temperature, pressure, magnetic field etc., exhibiting a plethora of exotic and often coexisting states. Details of the quasiparticle mass renormalization are central to understanding these properties. In the low-temperature electronic structure of strongly correlated d-electron systems, electron correlations may lead to band renormalization. Narrow, dispersive quasi- particle bands are formed in the vicinity of the Fermi (a) E-mail: tomasz@lanl.gov energy, exhibiting long lifetimes and increased effective masses. Numerous studies have been recently directed at a detailed spectral analysis of the quasiparticle disper- sion in strongly correlated systems, especially in the high-temperature superconductors. This effort revealed the existence of characteristic energy scales, or kinks, in the quasiparticle excitation spectrum. The observed kink energy scales range from 40 meV [2] in high-temperature superconductors up to 900 meV [3] in graphene. The nature of kink-like changes in the slope of band dispersion is being vigorously debated. Kinks are taken as evidence for the coupling of electrons to either phonons or spin fluctuations [4–8]. Multiple coexisting energy scales were reported for kink-like many-body interaction effects in cuprates [9]. Recently it was also proposed that within a coupling-free scenario [10], kinks emerge solely from elec- tron correlations. Spectroscopic evidence for the first two mechanisms is found in the d-electron high-temperature superconductors, but no kink structures have been observed in f -electron systems. Here we describe the first observation of a kink in the 5f quasiparticle excitation spectrum obtained by high-resolution angle-resolved photoelectron spectroscopy (ARPES), linked to the local spin density approximation (LSDA) band structure 37003-p1