ELECTRONIC-STRUCTURE OF SUPERCONDUCTING

We have studied the actinide superconductor UBei3 with resonant photoemission using synchrotron radiation 30 «hv «120 eV. In agreement with the anomalously high y value of 1. 1 J/mole K2, we find a high density of 5 f states at the Fermi level EJ;. Uranium 6d electrons are not present at EJ; as can be derived from the zero intensity at EF in the off-resonance spectrum. The close resemblance of this off-resonance curve to a photoemission spectrum of pure Be suggests that the hybridization of U Sf electrons with Be s, p electrons is small. Indeed, the UBe&3 photoemission curves look like a superposition of U and Be curves. By using photoemission results from UO2 as a reference, we find that the valence-band emission contains about one Sf electron. The present results on UBe~3 are compared with those of CeCu2Si2, another member of the family of exotic superconductors.


I. INTRODUCTION
It was very recently discovered' that UBei3 becomes superconducting below 0.85 K, and below 1 K exhibits an anomalously high electronic specific-heat value of @=1. 1 J/mole K . Therewith, UBei3 is the first example of an actinide system containing 5 f electrons which falls into the class of so-called exotic superconductors.
Another example is the 4f system CeCu2Si2 (7 --I J/moleK'), ' in which superconductivity is claimed to be mediated by heavy fermions generated presumably by interactions between the localized 4f electron and the conduction electrons. 3 A clear understanding on a microscopic scale, however, is still lacking and the relevance of this understanding to the problem of "intermediate valence" in ceriumand uranium-based systems is evident. For CeT2Si2 (T=Cu, Ag, Au, Pd), it is the hybridization of Ce 4f electrons with T-derived d states which determines the ground-state properties (superconductivity for T=Cu, ordered magnetism for T=Ag, Au, Pd). This was concluded from resonant photoemission studies on these systems. 4 Here, we apply the same technique to UBe~3 using synchrotron radiation 30 eV (h v «120 eV from the National Synchrotron Light Source at the Brookhaven National Laboratory.

II. METHOD
Resonant photoemission provides a means to extract the n, I character our of the valence-band emission which usually consists of overlapping contributions, provided an n, (I -I) core level is available. ' Here, the electronic states are characterized by their main quantum number n and their orbital quantum number t. Experimentally, the resonance manifests itself as a minimum in the photoemissionintensity-versus-hv curve, which is followed by a maximum when hv is tuned through the core absorption edge. In case of an atomiclike uranium configuration, the minimum reflects Fano-type interference effects between direct valence-band photoemission Sd"5f'6d' 7s'+ht Sd"5f'6d'7s'+e~f,  1984 The American Physical Society U 5f and the 0 2p photoemission features are completely separated' and hence provide a way to determine quite accurately the 5 f to 2p photoemission cross-section ratio as well as the "pure" 5 f resonance behavior.

III. EXPERIMENTAL
The experiment was performed with the IBM twodimensional spectrometer now installed at the Brookhaven National Laboratory. The synchrotron radiation from the 750-MeV storage ring was monochromatized with a 3-m toroidal grating monochromator yielding a total energy resolution (electrons and photons) of 0.3 eV at he=40 eV and 0.75 eV at hv =100 eV. Single crystals of UBe&3 were fractured in a vacuum of 2X10 '0 Torr and then immediately transferred to the measuring position which held a vacuum better than I &&10 'o Torr. Angle-integrated (50 =86') energy-distribution curves (EDC's) were taken within two hours after cleavage.

IV. RESULTS AND DISCUSSION
In Fig. 1(a), we present an EDC of UBet3 at hv =40 eV, a photon energy at which the p, d, and f photoemission cross sections are roughly equal, ' and hence a picture of the total density of states (DOS) is obtained. We note two spectral features in Fig. 1(a), one sharp peak right at EF and a broader one centered at about -7-eV initial energy. From the cross-section dependence at low excitation energies hv (60 eV, as well as the resonance behavior (to be discussed below), we can immediately assign U 5 f, 6d character to the Fermi peak, and Be s, p character to the second feature at -7 eV. So far, no theoretical DOS calculation has been performed. Therefore, in Fig. 1 Be. ' Inoue and Yamashita'4 have performed bandstructure calculations for Be as a function of the lattice constant. They find essentially no change of the DOS when varying the lattice constant from 0.8 to 1.2 times its nominal value. In addition, hypothetically fcc Be has a DOS almost equal to hcp Be. ' Hence, to a first approximation, the sand p-like electrons of Be will have the same wave-function overlap and form bands at approximately the same energy positions in UBe~3 as they were determined for Be [cf. Fig.   1(b)]. Noteworthy is the low DOS of Be at EF, exactly where we recognize the U Sf contribution in Fig. 1(a). In addition, the 2s-like maximum of states in Be corresponds to the -7-eV feature in UBe~3 to which we had already previously assigned s,p character (see above).
To derive the mere 5 f contribution, in Fig. 2 we use the EDC's of UBe~3 at he=98 eV (on resonance) and hv =92 eV (off resonance). These photon energies were determined from the maximum and minimum, respectively, in the 5 f intensity-versus-hv curve (constant initial-state spectrum) obtained from EDC's measured in steps of 2 eV through the 5d absorption edge and then normalized to the photon flux. ' The EDC's in Fig. 2(a) have been normalized to equal Be 2s intensity at -7 eV. The Be 2s emission does not change at the Sd Sf Pano resonance (because of different atoms, different main quantum numbers, and the AI-selection rule). Most striking is the zero intensity at EF in the off-resonance spectrum of Fig. 2(a). Such a zero intensity at her=92 eV has so far only been observed in UO2, ' which is known to be purely f-like with no other overlapping states. Usually, at least some U 61contribution accounts for some spectral intensity (see, e.g. , UTe in Ref. 5 or o,-uranium in Ref. 15). Therefore, we conclude that no U 6d states are present at EF. In going away from the Fermi level, increasing spectral intensity can be seen in the off-resonance EDC of Fig. 2(a). Although, in principle, some U 6d's could account for this emission, comparison of this El3C at by=92 eV with the pure Be spectrum of Fig.   1(b) reveals a lot of resemblance. As pointed out above, Be 2s or 2p states are not affected by the 5d Sf resonance.
Therefore it is tempting to attribute 5 f character to the entire difference curve of the onand off-resonance curves, which is shown in Fig. 2 To obtain a measure for the number of 5f electrons per formula unit n5f(UBCI3) in UBe13, we use as a reference the photoemission spectrum of UQ2 at hp=40 eV, 5 ]6 obtained under the same conditions as the EDC in Fig. 1(a).
For the intensity ratio, we find I5f/I2~=0. 5 in UO2. Thts ratio can also be expressed in terms of the photoemission cross sections o f and 0 p aIld thc 11UIllbcl of clcctrons pcI' formula unit nsf(UO2) and n2p(UO»: Separating the UBe13 EDC of Fig. 1(a)  The ratio of the 2s to 2p photoemission cross sections has been calculated' for carbon at hI =40.8 CV: o. , /a.~=0.788.
The total nun1ber of' s and p electrons in UBe I3 is 2 x 13 = 26. From the x-ray photoemission spectroscopy results on Be, it was deduced" that 40'k of the total number of electrons is contained in the s-like em~ss~on feature yield- From one localized f electron, a free-ion moment of 2.54@, s is derived in Russel-Saunders coupling.
Finally, the f number obtained at hI =40 CV [cf. Fig. 1(a)] is consistent with the pure f contribution derived from the resonance curves (see above).
One 5f electron in UBe13 points towards a very close resemblance to CCCu2Si2 (and Ce systems, in general) which presumably has one 4f electron. " Although the width of the 5 f emission in the 40-eV EDC [ Fig. 1(a)l is rather broad ( -2eV) as compared with the expectedly narrow f band of millivolt width in the ground state, it is known that the 4f photoemission in Ce systems is very broad (2 -3) CV and exhibits the anomalous two-peak structure (cf. , e.g. , Refs. 4 and 8) which is currently a matter of great interest. '7 So, in principle, effects similar to those proposed ' '8 for photoemission from Ce systems may account for the broad width of the 5f emission in UBe».
The high-f DOS is compatible with the anomalously high y value, while some localized character manifests itself in the effective moment of 3.1p.~per U ion. Such localization effects and the degree of correlation can be studied by high-energy photoemission on the 4f core levels in 5 f systems. ' It is the possible existence of a satellite and its intensity relative to the 4f main line which gives a measure for the localization of the 5f's. In the valence-band region, " no such satellites are discernible (cf. Figs. 1 and 2), therefore future x-ray photoemission experiments on the core levels are welcomed. Complementary bremsstrahlung isochromat spectroscopy will allow determination of the effective Coulomb correlation energy. As a final remark, it is worthwhile recalling that the band-structure calculation for Be (Refs. 11 and 14), as well as thc pho«emission DOS [cf. Fig. 1