Observation of 5f electrons in the itinerant limit: Three-dimensional electronic structure of UB 2 Takuo Ohkochi, 1 Shin-ichi Fujimori, 1 Hiroshi Yamagami, 1,2 Tetsuo Okane, 1 Yuji Saitoh, 1 Atsushi Fujimori, 3 Yoshinori Haga, 4 Etsuji Yamamoto, 4 and Yoshichika Ōnuki 5 1 Synchrotron Radiation Research Unit, Japan Atomic Energy Agency, Sayo, Hyogo 679-5148, Japan 2 Department of Physics, Faculty of Science, Kyoto Sangyo University, Kyoto 603-8555, Japan 3 Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 4 Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan 5 Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Received 12 May 2008; published 13 October 2008 We have derived the three-dimensional band structure and Fermi surfaces of itinerant uranium compound UB 2 by soft x-ray angle-resolved photoelectron spectroscopy. We have observed clear energy dispersions and Fermi surfaces with large contribution from the U 5 f states. The obtained results have been compared with the result of band-structure calculation within the local-density-functional approximation as well as the results of de Haas–van Alphen dHvA study. The qualitative agreement has been obtained in the size and topology of the Fermi surfaces with the band-structure calculation and the dHvA measurement. Meanwhile, their band struc- ture near the Fermi level is slightly different from the calculation. This might be due to a dynamic renormal- ization beyond the local-density approximation band calculations even in the very itinerant 5 f compound. DOI: 10.1103/PhysRevB.78.165110 PACS numbers: 71.18.+y, 71.20.-b, 71.27.+a, 79.60.-i I. INTRODUCTION In recent years, a different class of unconventional super- conductors has been discovered in transuranium compounds and attracted much attention. 1 These superconductors share a unique position due to their relatively high superconducting transition temperatures T c 18 K for PuCoGa 5 , 2 8.7 K for PuRhGa 5 , 3 and 5 K for NpPd 5 Al 2 Ref. 4 compared with uranium heavy-fermion superconductors. These remarkable discoveries recall the long-standing question of f -electron physics: How are actinide 5 f electrons involved in their band structures and Fermi surfaces FSs, and how can they be described theoretically? To address this question, it is essen- tial to clarify, first of all, the validity of the theoretical de- scription of the 5 f electronic states within a local-density approximation LDA when the 5 f electrons are in the itin- erant limit. Angle-resolved photoemission spectroscopy ARPES has been applied for various actinide compounds since the detection of the 5 f band dispersion is the most direct test of various theoretical models for the 5 f states. 5–7 Nevertheless, the detection of energy dispersion in actinide 5 f states has not been successful due to the drawbacks in the conventional ARPES experiments such as the high surface sensitivity or the low photoionization cross section of the actinide 5 f states in comparison with ligand s, p, and d states. In this study, we have performed h-dependent ARPES in the soft x-ray region SX-ARPES on a very itinerant 5 f compound UB 2 . We have observed unambiguous 5 f -selective band dispersions and found that they form FSs of this compound. It is demonstrated that they are basically described by a band-structure calculation based on LDA with renormalization due to a static electron correlation. SX- ARPES is a recently developed technique which can be uti- lized to observe bulk electronic structures of materials. 8,9 This is a particularly powerful experimental technique for uranium compounds due to its high sensitivity to the 5 f states. 10,11 While we have measured the SX-ARPES spectra of the itinerant 5 f compound UFeGa 5 in previous work 10 and found that the gross features of the band structure and FSs can be well described by the band-structure calculation, only a limited part of the Brillouin zone was explored, and a de- tailed comparison between the experiment and the band- structure calculation could not be made. UB 2 is a paramagnetic compound with the hexagonal AlB 2 -type structure in which a two-dimensional uranium- and boron-atoms network are stacked alternately along the 0001 direction. The interatomic U-U spacing in the two- dimensional plane is 3.123 Å, which makes a large direct f - f overlap. Its electronic specific-heat coefficient  e is as low as 10.3 mJ / K 2 mol which is comparable to the value obtained by the LDA band-structure calculation 7.29 mJ / K 2 mol. 12 The de Haas–van Alphen dHvA measurement was performed for this compound, and the ob- served branches were well explained also by the LDA band- structure calculation. 13 Therefore, UB 2 is considered to have itinerant 5 f electronic states, and it gives a unique opportu- nity to observe directly the itinerant 5 f electronic states. II. EXPERIMENTAL The ARPES experiments were carried out at the SPring-8 BL23SU. The energy resolution was 120–130 meV in the ARPES experiments with h =450–500 eV. The angular resolution was 0.15°, and the corresponding momentum resolution for the k  direction was about 0.06 Å -1 . Since the size of the Brillouin zone of UB 2 for the -K direction is 1.34 Å -1 , about 20 individual k points along this direction can be resolved in the present experiments. The momentum broadening for the k  direction due to the finite escape depth of photoelectrons 10–15 Å is estimated to be 0.06–0.1 Å -1 . This is much smaller than the size of the Brillouin zone for the c axis 1.58 Å -1 , but substantial con- tributions from surrounding band structures are not negli- gible when the bands have strong dispersions along the k  direction. 15 The position of the ARPES scan in the Brillouin zone was estimated by assuming a free-electron final-state PHYSICAL REVIEW B 78, 165110 2008 1098-0121/2008/7816/1651105 ©2008 The American Physical Society 165110-1