HIGH-ENERGY SPECTROSCOPIC STUDY OF THE ELECTRONIC-STRUCTURE OF UBE13

PHYSICAL REVIEW B VOLUME 29, NUMBER 9 High-energy 1 MAY 1984 spectroscopic study of the electronic structure of UBe$3 E. Wuilloud and Y. Baer Universite de Neuchatel, Rue A. -L. Breguet 1, CH-2000 Neuchatel, Switzerland Institut de Physique, H. R. Ott Laboratorium fur FestkOrperphysik, EidgenDssische Technische Hochschule-Honggerberg, CH-8093 Zurich, Switzerland Z. Fisk and J. L. Smith Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 7 February 1984) X-ray photoemission and bremsstrahlung isochromat spectroscopies have been used to probe the occu- pied and unoccupied states of UBe». Between two and three electrons are found to populate the tail of a 5 eV) of extended states. With our resolution ( & 0. 5 eV) it is not possible surprisingly broad 5f band ( to observe directly at the Fermi energy any peculiarity of the density of states explaining the extraordinary properties of this compound. However, the drastic differences between the core-level spectra of U and Be indicate that the Sf states remain essentially confined around the U atoms and are only weakly hybridized with the sp-band states originating from the Be atoms. The recent discovery of bulk superconductivity in UBe» 1 K (Ref. 1) has given clear evidence for the ex- istence of strongly interacting electrons in some metallic materials, giving rise to anomalous properties at low tem- Other examples for this type of materials are peratures. CeA13 (Ref. 2) and CeCu2Si2 (Ref. 3), again two com- electrons, obviously a prerequisite for pounds containing the observation of the behavior to be discussed here. All these compounds show an anomalously large specific heat In CeA13, c~ ( T) decreases c~ ( T) at low temperatures. linearly with decreasing temperature below 1 K but, in com- parison with normal metals, with a very large coefficient y of the order of 1. 5 J/mole K, indicating a considerable re- normalization of the electronic subsystem. In UBet3 (Ref. 1) and CeCu2Si2, this linear decrease with a slope of about 1 J/mole K is intercepted by a discontinuity Ac due to a su- In both cases, the magnitude of perconducting transition. Ac is compatible with the large values of c~/T just above the transitions and the experimental verification that the entro- and the normal py difference between the superconducting state is zero below T, demonstrates that it is indeed the strongly interacting electrons that are involved in the super- conducting state. These high y values are necessarily also based on large densities of electronic states at the Fermi energy E+, imply- ing very narrow features in the energy dependence of the electronic structure at E+. The possible occurrence of such features is well known from theoretical work concerning the electronic structure of simple metals containing transition- metal impurities in the dilute limit. Recent work consider- claims that similar narrow reso- ing concentrated systems nances at E~ also appear in this case. Information on the energy dependence of the electronic structure of a metal is provided by using photoemission techniques. A first at- tempt concerning UBe» involved resonant photoemission, favoring the emission of electrons with symmetry, by scanning the energy range below EF to about 12-eV binding Since also core-electron spectra and particularly the energy. below f f energy distribution of empty electron states provide valuable information, we chose to map parts of the electronic spec- trum of UBet3 by using x-ray photoemission (XPS) and bremsstrahlung-isochromat spectroscopy (BIS). The sample investigated in the present study was a platelet cut from the same polycrystalline batch of material that was used for previous specific-heat measurements. The XPS and BIS spectra were obtained in a combined in- strument described elsewhere. The contamination was re- moved from the sample surface by scraping it in situ with an 1s and C 1s XPS signals could no A1203 file until the longer be detected in a 5-min scan. The base pressure of 1x10 Torr in the instrument allowed us to accumulate the different spectra during periods of many hours without any sizable degradation of the surface cleanliness. The occupied valence-band states of UBe» have recently been studied by resonant photoemission. The spectra have been interpreted as revealing a UBe» density of states (DOS) looking rather like a superposition of the DOS of the two pure constituents U and Be. A calibration procedure of the intensity of the different signals in the spectra has yield- ed a Sf population of roughly one electron. Our XPS valence band spectrum of UBe» is shown on the left side of Fig. 1. In the energy range from the Fermi energy EF to — 4 eV it is in good agreement with the previously pub- lished spectrum excited at 40 eV. The sp states of Be forming a broad band' have a very weak cross section at the Al Eo. photon energy. From atomic cross-section calcu- lations they are estimated to represent less than 10'/0 of the total intensity of the XPS spectrum and for this reason they do not emerge from the background of inelastically scattered electrons extending at higher binding energies. In U, the atomic cross section for the 5 states is substantially larger than the one for the 6d states [a (Sf')itr(6d') =6] (Ref. 11) and the XPS peak must be mainly attributed to the tail of the Sf band cut by EF. This spectrum of UBet3 shows a striking similarity with the spectrum of n-U metal' despite the different environment of the U atoms and their f S228 O1984 The American Physical Society