Fig. 1. Ce 4f spectra of CeRu 2 Si 2 and CeRu 2 obtained from Ce 3d - 4f resonance photoemission with an energy resolution of 200 meV. The Ce 4f contributions are e s t i m a t e d b y s u b t r a c t i n g t h e o f f - resonance spectra taken at h ν = 875 eV from the on-resonance spectra at h ν = 882.6 eV. The baseline photoemission level is shifted for clarity. 57 BULK 4 f STATES OF CORRELATED Ce SYSTEMS REVEALED FOR THE FIRST TIME BY UNPRECEDENTED HIGH-RESOLUTION RESONANCE PHOTOEMISSION SPECTROSCOPY Photoemission spectroscopy (PES) is a direct means to probe the electronic structures of new functional materials which exhibit high-transition- temperature superconductivity, heavy fermion behavior, metal-to-insulator transitions and other unique properties. High-resolution PES is required to investigate their detailed electronic states near the Fermi level ( E F ). However, conventional high- resolution PES using low-energy excitation is limited by its surface-sensitivity due to a short photoelectron mean free path. For example, previously reported 4 f spectra (h ν≤ 120 eV, with surface contribution of ~55%) for correlated Ce compounds have similar shapes, when they should be quite different due to effects of hybridization with other valence bands. This problem is solved by the bulk-sensitive high-resolution 3 d -4 f resonance photoemission (surface contribution of ~15 %), which has became available recently at beamline BL25SU [1]. Here we present unprecedented high- resolution Ce 3 d -4 f resonance photoemission spectra of moderately and strongly hybridized Ce compounds (CeRu 2 Si 2 and CeRu 2 ), in which the genuine bulk Ce 4 f states are revealed. CeRu 2 Si 2 is a typical heavy fermion system with a Kondo temperature ( T K ) of ∼ 20 K. This material is thought to reside at a boundary between the localized and itinerant Ce 4 f states. CeRu 2 is a typical material of strongly valence-fluctuating 4 f systems, for which T K is of the order of 1000 K. High-resolution Ce 3 d -4 f resonance PES was performed on single crystal samples with an overall energy resolution of about 100 meV at h ν∼ 880 eV. The samples were cooled down to 20 K and cleaned in situ by scraping or cleavage. Surface cleanliness was confirmed before and after the measurements. The Ce 4 f spectra of CeRu 2 Si 2 and CeRu 2 in a wide energy region for the Ce 3 d -4 f resonances are shown in Fig. 1. In the CeRu 2 Si 2 spectrum, there is a sharp peak near E F and a broad tail ranging from − 1 to − 5 eV. According to the single impurity Anderson model (SIAM), the former corresponds to the contribution of both the tail of the Kondo peak ( f 5/2 1 ) and its spin-orbit partner ( f 7/2 1 ). The latter is ascribed to the f 0 final state. Such a f 0 broad tail is greatly suppressed in the spectrum of CeRu 2 compared with that of CeRu 2 Si 2 , indicating that the bulk Ce 4 f states are considerably hybridized in CeRu 2 . Energy Relative to E F ( eV) Photoemission Intensity (arb. units) CeRu 2 -8 -6 -4 -2 0 CeRu 2 Si 2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 Energy Relative to E F ( eV) Photoemission Intensity (arb. units) (T K ∼ 1000 K) CeRu 2 (T K ∼ 20 K) CeRu 2 Si 2 Fig. 2. High-resolution Ce 3d - 4f on resonance spectra with an resolution of 100 meV of CeRu 2 Si 2 and CeRu 2 . 58 Akira Sekiyama a , Shigemasa Suga a and Yuji Saitoh b (a) Osaka University (b) SPring-8 / JAERI E-mail: sekiyama@mp.es.osaka-u.ac.jp References [1] Y. Saitoh et al. , J. Synchrotron Rad. 5 (1998) 542. [2] A. Sekiyama, T. Iwasaki, K. Matsuda, Y. Saitoh, Y. Onuki and S. Suga, Nature 403 (2000) 396. The detailed high-resolution spectra near E F are s h o w n i n F i g . 2 . T h e s p e c t r a o f t h e t w o compounds are drastically different from one another. For CeRu 2 Si 2 , there are a prominent peak in the vicinity of E F and a weak shoulder near − 0.3 eV. These prominent and weak structures originate from the bulk f 5 /2 1 and f 7/2 1 final states; that is, from the tail of the Kondo peak and its spin- orbit partner as predicted from SIAM. Such a strong tail of the bulk Kondo peak has clearly been observed for the first time owing to the bulk- s e n s i t i v e 3 d - 4 f r e s o n a n c e P E S w i t h a n unprecedented high-resolution of 100 meV. The spectral shape near E F of CeRu 2 is, however, rather surprising since no structure is seen except for a broad feature centered at − 0.5 eV. One would have expected that the tail of the Kondo peak should be much stronger in the bulk-sensitive 3 d -4 f resonance PES spectrum for such a strongly hybridized system like CeRu 2 , as predicted from SIAM. The fact that the spectrum demonstrates a rather conventional Fermi cut-off together with the broad peak at − 0.5 eV cannot be explained by SIAM. Thus, the bulk 4 f spectrum of CeRu 2 is interpreted to represent the itinerant 4 f -band due to the very strong hybridization effect, which is beyond the scope of SIAM. This behavior, which has not been revealed by any previous 4 d -4 f resonance photoemission studies, reflects the real T K and bulk properties of these materials. Further detailed studies, including a comparison between these 3 d -4 f spectra and surface-sensitive 4 d -4 f resonance PES spectra of the same compounds are described elsewhere [2]. In summary, a new photoemission technique using high-resolution, high-h ν excitation has revealed the bulk electronic states of high- and low- T K Ce compounds, which were inaccessible by conventional high-resolution PES. SIAM is applicable to the low- T K system, but not to high- T K system, which demonstrates significant itinerant 4 f character. This technique can be applied as well to systems such as strongly correlated transition metal compounds.
home
- JP
- EN