Angle-resolved photoemission study of USb2 the 5f band struc(5)

Single crystal antiferromagnetic USb2 was studied at 15K by angle-resolved photoemission with an overall energy resolution of 24 meV. The measurements unambiguously show the dispersion of extremely narrow bands situated near the Fermi level. The peak at th

34eV and 60 eV is the combined result of 1) reduced momentum resolution, 2) larger lifetime broadening that appears at higher photon energy 3) reduced electron analyzer performance at greater magnification values (E k/E p). Previous USb2 data were taken with an energy resolution of 45 meV [14], so the apparent width of the photoemission structure at the Fermi edge was much greater, and dispersion relations and binding energies not as obvious. One of the significant improvements in this work is energy resolution near the value of the observed dispersion. The sharp near Fermi edge peak observed in the experiment is the sharpest photoemission feature found in uranium compounds up to date [15-20].

The nature of the sharp peak near Fermi level is shown in Fig.1. Several details are important; 1) The dotted lines are the Pt Fermi level and in all cases the peak in USb2 is below E F, 2) The peaks are very narrow but clearly show variation in binding energy as function of photon energy, 3) At the lowest energy (34 eV), the natural linewidth is conservatively estimated to be less than 10 meV when instrumental resolution is removed (and in reality may very possibly be much smaller than 10 meV). The dashed lines are the fitted data to the open circles after a Shirley background has been removed. The traditional Fermi liquid characteristics exhibit a E2 dependence of the linewidth. One may construct arguments for linewidths in excess of E2 arising from electron-electron, electron-phonon and electron-impurity interactions [21-22] and still maintain a Fermi liquid interpretation. However, the first peak linewidth being narrower than E2 would seem to place the interpretation of the electronic structure for USb2 outside the Fermi liquid regime.

3. Results and discussion

The USb2 photoemission spectra presented in Fig. 2, 3 and 4 were taken for lower photon energies (34 eV, 43 eV and 60 eV) where good angular resolution and small lifetime broadening allow observation of dispersion in the U5f bands. This is a suitable photon energy range to achieve an adequate compromise between the high U5f photoionization cross-section while still retaining substantial momentum resolution.

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