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study of ce - intermetallics have drawn significant attention during past few decades due to the observation of many unusual properties such as valence fluctuations , kondo screening , heavy fermion superconductivity in these systems . such properties arise due to the proximity of ce 4@xmath3 level to the fermi level leading to strong hybridization between the ce 4@xmath3 states and the conduction electronic states @xcite . such hybridization often leads to a logarithmic enhancement of electrical resistivity at low temperatures in contrast to a decrease expected in a metal . this is known as kondo effect . in the case of strong kondo coupling , the antiparallel coupling of the ce moment with the conduction electrons forms a singlet ground state called kondo singlet that manifests as a sharp feature ( kondo resonance feature ) in the electronic structure in the vicinity of the fermi level , @xmath6 . a lot of success has been achieved to describe these systems within the anderson impurity models . here , the parameters defining the hybridization between 4@xmath3 states and valence electronic states are often estimated using band structure calculations based on local density approximations ( lda)@xcite . the scenario can be different if these materials contain transition metals : the @xmath2 electrons forming the conduction band are strongly correlated . the finite correlation strength among them suggests non - applicability of the band structure results within lda to derive the hybridization parameters and indicate the need to go beyond lda prescriptions . a kondo lattice compound , ce@xmath0cosi@xmath1 is a good candidate for this study . ce@xmath0cosi@xmath1 crystallizes in a alb@xmath0 derived hexagonal structure ( space group @xmath7 ) and is a mixed valent ( kondo lattice ) compound @xcite . the electrical transport measurements revealed temperature dependence@xcite typical of a mixed valent system @xcite . no signature of magnetic ordering was observed in the magnetic susceptibility measurements down to 0.5 k @xcite . interestingly , gradual substitution of rh at co sites leads to plethora of interesting features due to increasing dominance of indirect exchange interaction @xcite . for example , @xmath8 = 0.6 composition in ce@xmath0rh@xmath9co@xmath10si@xmath1 exhibits quantum critical behavior . intermediate compositions having higher rh concentration exhibit signature of spin density wave ( sdw ) state @xcite . clearly , the @xmath2 electronic states corresponding to the transition metals plays a key role in determining the electronic properties in this interesting class of compounds . it is thus , important to probe the role of electron correlation in the electronic structure of these compounds .    in this paper , we report our results of investigation of the electronic structure of ce@xmath0cosi@xmath1 using high resolution photoemission spectroscopy and _ ab initio _ band structure calculations . high energy resolution employed in our measurements enabled us to reveal kondo - resonance feature and the corresponding spin orbit satellite . the comparison of the experimental spectra and the calculated ones indicate that the correlation strength among co 3@xmath2 electrons is significant ( @xmath11 3 ev ) . the contribution of the co 3@xmath2 partial density of states ( pdos ) is small in the vicinity of the fermi level , where ce 4@xmath3 contributions are dominant . ce@xmath0cosi@xmath1 was prepared by melting together stoichiometric amounts of high purity ( @xmath12 99.9% ) ce , co and si in an arc furnace . the single phase was confirmed by the absence of impurity peaks in the @xmath8-ray diffraction pattern . the specimen was further characterized by scanning electron microscopic measurements and energy dispersive @xmath8-ray analysis @xcite . the photoemission measurements were performed using a gammadata scienta ses2002 analyzer and monochromatic laboratory photon sources . the energy resolutions were set to 0.4 ev , 5 mev and 5 mev at al @xmath13 ( 1486.6 ev ) , he ii@xmath14 ( 40.8 ev ) and he i@xmath14 ( 21.2 ev ) photon energies , respectively . the base pressure in the vacuum chamber was 3 @xmath15 10@xmath16 torr . the temperature variation down to 20 k was achieved by an open cycle he cryostat , lt-3 m from advanced research systems , usa . the sample surface was cleaned by _ in situ _ scraping using a diamond file and the surface cleanliness was ensured by the absence of o 1@xmath17 and c 1@xmath17 features in the @xmath8-ray photoelectron ( xp ) spectra and the absence of impurity features in the binding energy range of 5 - 6 ev in the ultraviolet photoelectron ( up ) spectra . the reproducibility of the spectra was confirmed after each trial of cleaning process . the electronic band structure of ce@xmath0cosi@xmath1 was calculated using _ state - of - the - art _ full potential linearized augmented plane wave ( flapw ) method using wien2k software@xcite within the local density approximations , lda . the convergence for different calculations were achieved considering 512 @xmath18 points within the first brillouin zone . the error bar for the energy convergence was set to @xmath19  0.2  mev per formula unit ( fu ) . in every case , the charge convergence was achieved to be less than 10@xmath20 electronic charge . the lattice constants used in these calculations are determined from the @xmath8-ray diffraction patterns considering alb@xmath0 derived hexagonal structure and are found to be @xmath21  =  8.104    and @xmath22 =  4.197  @xcite . the muffin - tin radii ( @xmath23 ) for ce , co and si were set to 2.5 a.u . , 2.28 a.u . and 2.02 a.u . , respectively . ( xps ) , and ( b ) he ii and he i excitation energies . the solid line in ( a ) represents the broadened he ii spectrum to take into account the energy resolution corresponding to xp spectrum . dashed line represent the calculated xp spectrum as described later in the text . ( c ) the ce 4@xmath3 spectral function obtained by subtracting he i spectrum from the he ii spectrum . ]    the valence band in ce@xmath0cosi@xmath1 consists of ce 5@xmath2 , ce 4@xmath3 , co 3@xmath2 and si 3@xmath5 electronic states . since the transition probability of the photoelectrons in the photo - excitation process strongly depends on the excitation energies , a comparison of the photoemission spectra collected at different excitation energies would help to identify experimentally the character of various features constituting the valence band . in fig . 1(a ) , we show the valence band spectra collected at 20 k using al k@xmath14 photon energy . the he ii@xmath14 and he i@xmath14 spectra are shown in fig . 1(b ) . in order to compare the spectral functions , it is important to subtract suitable background from the raw data as shown in fig . we have subtracted integral background from al k@xmath14 and he ii spectra . since the thermalized electrons lead to a large increase in background at low kinetic energies , the background function in the he i spectrum are often found to be better defined by a polynomial ( quadratic like ) as shown in fig . evidently , the background contributions in the energy range 0 - 3 ev binding energies is quite small in every case . moreover , different functional dependence of the background in the he i spectrum does not have significant influence on the conclusions as the calculated spectra are compared with the xp spectrum . excitation energies . the open circles represent raw data , lines represent the background function and the solid circles are the subtracted spectra . ]    in fig . 1(b ) , the intensity close to the fermi level , @xmath6 , exhibit significantly different behavior as a function of excitation energy . he ii spectrum exhibits large intensity and two distinct features c and k near @xmath6 , which are not visible in the other spectra . the high energy resolution employed in the he i and he ii measurements helped to resolve distinct signature of the features in the vicinity of @xmath6 . the photoemission cross section for ce 4@xmath3 states at he ii excitation energy is about 3 times larger than that at he i photon energy while it is almost the same for si 3@xmath5 states and double for co 3@xmath2 states @xcite . thus , the features c and k can be attributed to the photoemission signal primarily from the ce 4@xmath3 states . we have subtracted the he i spectrum from the he ii spectrum to delineate the ce 4@xmath3 contributions . the subtracted spectrum representing ce 4@xmath3 band is shown in fig . the distinct features , c and k corresponding to the spin orbit satellite of the abrikosov - suhl resonance ( asr ) and the main peak of asr , respectively , could clearly be identified @xcite . in addition to the resonance feature at @xmath6 , each spectrum exhibits two distinct features , a and b at about 2.3 ev and 1 ev binding energies , respectively . interestingly , the relative intensity of these features does not change with such a large change in photon energies . this is an important observation as the photoemission cross sections corresponding to ce 5@xmath2 , ce 4@xmath3 , co 3@xmath2 and si 3@xmath5 electronic states have significantly different excitation energy dependence @xcite . to verify the change in lineshape with better clarity , we have broadened the he ii spectrum by convoluting a gaussian of full width at half maximum ( fwhm ) = 0.4 ev to make the resolution broadening comparable to that of the xp spectrum . the broadened spectrum is shown by solid line superimposed over the xp spectrum in fig . the lineshapes of both the spectra are almost identical . in fig . 1(b ) , the he i and he ii spectra are superimposed over each other to investigate the change in lineshape when the energy resolution broadening is the same ( @xmath11  5  mev ) . the spectra in the binding energy range beyond 1 ev are found to be almost identical . all these observations are unusual . these observations can not be associated to the surface preparation via scraping for the following reasons . scraping helps to break the sample grains in an non - selective manner that helps to expose a clean surface for the measurements . in addition , scraping usually enhances the surface roughness . such a process will enhance the surface contribution in the spectra . in addition , the linewidth of the features may also get enhanced due to scraping . this later possibility can be ruled out as it is shown experimentally @xcite that cleaving and scraping have insignificant influence on the spectral lineshape at higher binding energies as the intrinsic linewidth of these features are already large due to various lifetime effects . as for the enhancement of surface contributions , it is already known that photoemission with he i and he ii photons have significant surface sensitivity ( probing depth is @xmath11 8 - 10  ) . thus , these spectra may contain large surface contributions due to scraping . however , the al k@xmath14 valence band spectra possess dominant bulk contributions ( probing depth @xmath11 25 ) . it is experimentally found that these spectra can often be considered as representative of the bulk electronic structure @xcite suggesting that the influence of scraping will have less influence in the al @xmath13 spectrum . thus , the observations of similar lineshape in the energy range of 1 to 3 ev binding energies in figs . 1(a ) and 1(b ) suggest that either ( i ) the surface sensitivity in the he i and he ii spectra are compensated by scraping or ( ii ) the surface and bulk electronic structures are quite similar and scraping has insignificant influence in the spectral lineshape . the former is unlikely as scraping enhances the surface contribution that will make the differences more prominent . thus , the reasonable conclusion will the later case . partial density of states ( pdos ) , ( b ) ce 4@xmath3 pdos , ( c ) co 3@xmath2 pdos and ( d ) si 3@xmath5 pdos . the thick solid line in ( b ) represents the ce 4@xmath3 pdos rescaled by 25 times to show the weak intensities at lower energies . ]    in order to verify the character of the features theoretically , we have calculated the electronic band structure using fplapw method . the calculated partial density of states ( pdos ) are shown in fig . the dominant contribution in this energy range arises from the ce 5@xmath2 , ce 4@xmath3 , co 3@xmath2 and si 3@xmath5 pdos as shown in fig . 3(a ) , 3(b ) , 3(c ) and 3(d ) respectively . all the other contributions are negligible in this energy range . evidently , ce 5@xmath2 contributions are small and almost equally distributed over the whole energy range shown . ce 4@xmath3 band is intense and narrow as expected . in order to provide clarity , we have rescaled the ce 4@xmath3 partial density of states ( pdos ) by 25 times and shown by thick solid line in fig . the 4@xmath3 pdos contribute essentially in the vicinity of the fermi level ( @xmath19 1 ev binding energy ) . the intensity of ce 4@xmath3 band is significantly weak at higher binding energies . this is consistent with the observation in fig . co 3@xmath2 states also have finite contributions in this energy range due to the hybridization between co 3@xmath2 and ce 4@xmath3 states . co 3@xmath2 and si 3@xmath5 electronic states are strongly hybridized and appear dominantly in the binding energy range larger than 0.5 ev . the bonding states contribute in the energy range higher than 2 ev binding energy , where the si 3@xmath5 pdos has large contributions . the antibonding features appear in the energy range 0.5 to 2 ev , where co 3@xmath2 contributions are dominant . this suggests that the feature b in fig . 1 has dominant co 3@xmath2 character and the intensities corresponding to si 3@xmath5 photoemission contribute to feature a. the dominance of co 3@xmath2 contributions in this whole energy range shown presumably leads to unchanged spectral lineshape with the change in photon energy as observed in fig 1(a ) and 1(b ) .    in order to compare the experimental spectrum with the calculated results , we have calculated the xp spectrum in the following way : the ce 5@xmath2 , ce 4@xmath3 , co 3@xmath2 and si 3@xmath5 pdos per formula unit were multiplied by the corresponding photoemission cross sections at al @xmath13 energy . the sum of all these contributions was convoluted by the fermi distribution function at 20 k and broadened by the lorentzian function to account for the photo - hole lifetime broadening . the resolution broadening is introduced via further broadening of the spectrum by a gaussian function of fwhm = 0.4 ev . the calculated spectrum is shown by dashed line in fig . 1(a ) after normalizing by the total integrated area under the curve . the intensities near @xmath6 in the experimental spectrum appears to be captured reasonably well in the calculated spectra . in the higher binding energy region , the intensity around 1 ev is overestimated and that around 2.5 ev is underestimated ( see fig . since this energy range contains dominant contribution from the co 3@xmath2 states , it naturally indicates that co 3@xmath2 pdos region is not well described and correlations among co 3@xmath2 electrons may be important in determining the electronic structure in this energy range . in order to verify this , we have calculated the electronic density of states considering finite electron correlation , @xmath24 , among co 3@xmath2 electrons . the spectral functions corresponding to different @xmath24 values are calculated from the lda+@xmath4 results following the procedure described above .    -values . ( a ) the calculated spectral functions without ce 4@xmath3 contributions . ( b ) the calculated spectral functions contains ce 4@xmath3 contributions . clearly , the results in ( b ) provide better description than that in ( a ) revealing signature of ce 4@xmath3 contributions in the vicinity of the fermi level . ]    the lines in fig . 4 represent the spectral functions for different @xmath24 values which are superimposed on the experimental xp spectrum represented by open circles . in fig . 4(a ) we show the calculated spectra without ce 4@xmath3 contributions and the ones including ce 4@xmath3 contributions are shown in fig . it is evident from the figure that the signature of the feature around 2.3 ev ( feature a ) becomes more and more prominent with the increase in @xmath24 . consequently , the intensity of feature b reduces . thus , the feature a can be attributed to the photoemission signal from electron correlation induced co 3@xmath2 bands ( lower hubbard band ) in addition to the si 3@xmath5 contributions . it is to note here that although the feature a has comparable contributions from co 3@xmath2 and si 3@xmath5 states from band structure calculations , the photoemission intensity from the co 3@xmath2 states would be larger than that of si 3@xmath5 states at xps photon energies due to the matrix element effects . it is clear that the calculated spectral functions corresponding to @xmath24 @xmath11 3 ev is close to the experimental spectrum compared to all other cases . for higher values of @xmath24 , the correlation induced feature becomes stronger and appears at higher binding energies . the finding of such correlation among co 3@xmath2 electrons is not unrealistic as it is often found in the literature that co 3@xmath2 electrons are quite strongly correlated @xcite . it is to note here that the estimation of @xmath24 obtained within the lda+@xmath4 calculations in this study suggests signature of significant correlation among co 3@xmath2 electrons . an accurate estimate of @xmath24 requires further study using other methods e.g. lda+dmft . partial density of states for different electron correlation strength , @xmath25 among ce 4@xmath3 electrons . the results corresponding to the whole energy range is shown in ( a ) and that near fermi level is shown in ( b ) . ] the comparison of fig . 4(a ) and 4(b ) establishes that the intensities near @xmath6 essentially arise due to the photoemission from the occupied part of the ce 4@xmath3 band . the electron correlation among the co 3@xmath2 electrons has negligible influence on the spectral intensity at @xmath6 . consideration of ce 4@xmath3 bands in the spectral function calculation leads to a better description of the spectral intensities at the fermi level . electron correlation among ce 4@xmath3 electrons is also known to be strong . thus , we verify if such an effect leads to significant spectral intensity at higher binding energies . we show the ce 4@xmath3 pdos calculated for @xmath25 = 0 and 4 ev in fig . the unoccupied part of the spectral function exhibits large spectral redistribution . the occupied part exhibits a change in lineshape close to the fermi level as shown with better clarity in fig . evidently , the 4@xmath3 contributions appear essentially in the vicinity of the fermi level even if the 4@xmath3 correlations are considered . spectrum ( open circles ) exhibiting distinct signature of main and satellite peaks . the satellite intensity is shown by shaded region . solid line passing through the experimental data represents the fit comprising of a main peak ( dashed line ) and a satellite ( shaded peak ) . ] the signature of electron correlations can also be observed in the co 2@xmath5 core level spectra @xcite . in fig . 6 , we show the co 2@xmath5 spectrum collected at 20 k using al @xmath13 radiation . the spectrum consists of two spin orbit split features co 2@xmath26 and co 2@xmath27 at 778 ev and 792.9 ev binding energies , respectively ( energy separation of about 14.9 ev ) . these binding energies are identical to those found in elemental co metals @xcite . this indicates that the valence state of co in this material is very similar to that of the elemental co metals . in addition , every spin orbit split feature exhibits a weak but distinct shoulder at higher binding energies . this has been shown more clearly by rescaling and shifting this energy region in the same figure . although the satellite intensity looks small , it appears to be quite intense compared to the ones observed in earlier studies @xcite . the energy separation between the main peak and the satellite is about 4 ev . it is well known that the core level spectra reveal multiple features representing differently screened final states . the intense feature at lower binding energy is usually called the well screened feature and the ones at higher binding energies are called poorly screened features / satellites . such satellites appear due to the finite electron correlations . the finite intensity of the satellite feature in the present case , independently establishes the presence of electronic correlations among co 3@xmath2 electrons as concluded from valence band spectra @xcite . thus , our results establish that the electron correlation strength among co 3@xmath2 electrons are significant and needs consideration to derive the electronic structure of these systems . the mixed valency and/or kondo effect in this class of compounds depends on the kondo coupling strength , @xmath28 , where @xmath29 is the hybridization strength between conduction and the 4@xmath3 electronic states , @xmath30 is the binding energy of the 4@xmath3 electrons . in the present case , co 3@xmath2 electrons contribute significantly in the formation of the conduction band . electron correlation introduces local character among 3@xmath2 electronic states ; the band dispersion will be weaker than the case without electron correlation and hence the hybridization strength will be weaker . in addition , the formation of the lower hubbard band at higher binding energies relative to the uncorrelated band due to the electron correlation will influence the effective energy separation between the ce 4@xmath3 band and the conduction electrons . thus , @xmath31 and various hopping parameters involving co 3@xmath2 electronic states are expected to reduce leading to an effective reduction in the degree of mixed valency of ce . a quantitative estimation of such effects requires detailed modeling of this case presumably within the periodic anderson model . we believe that these results will help to initiate such research by the theoreticians . in summary , we have studied the electronic structure of ce@xmath0cosi@xmath1 using high resolution photoemission spectroscopy and _ ab initio _ band structure calculations . the experimental results indicate the dominance of co 3@xmath2 contributions in the valence band . si 3@xmath5 states appear at higher binding energies ( 2 - 3 ev ) . the ce 4@xmath3 contributions appear 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kondo systems are usually described by the interaction of strong correlation induced local moment with the highly itinerant conduction electrons . here , we study the role of electron correlations among conduction electrons in the electronic structure of a kondo lattice compound , ce@xmath0cosi@xmath1 , using high resolution photoemission spectroscopy and _ ab initio _ band structure calculations , where co 3@xmath2 electrons contribute in the conduction band . 
 high energy resolution employed in the measurements helped to reveal signature of ce 4@xmath3 states derived kondo resonance feature at the fermi level and dominance of co 3@xmath2 contributions at higher binding energies in the conduction band . 
 the line shape of the experimental co 3@xmath2 band is found to be significantly different from that obtained from the band structure calculations within the local density approximations , lda . 
 consideration of electron - electron coulomb repulsion , @xmath4 among co 3@xmath2 electrons within the lda+@xmath4 method leads to a better representation of experimental results . 
 signature of electron correlation induced satellite feature is also observed in the co 2@xmath5 core level spectrum . 
 these results clearly demonstrate the importance of the electron correlation among conduction electrons in deriving the microscopic description of such kondo systems .