PHYSICAL REVIEW B VOLUME 26, NUMBER 1 1 JULY 1982

Cekuz and CeCo2. Superconductors with 4f electrons

J. W. AllenXerox Palo Alto Research Centers, Palo Alto, California 94304

S.-J. Oh' and I. LindauStanford Synchrotron Radiation Laboratory, Stanford, California 94305

M. B. Maple and J. F. SuassunaInstr'tute for Pure and Applied Physical Sciences, University of California, San Diego, La Jolla, California 92093

S. B. HagstromXerox Palo Alto Research Centers, Palo Alto, California 94304

(Received 26 April 1982)

CeRu2 and CeCo2 have been considered to have tetravalent Ce because of their superconduc-

tivity, collapsed volume, and lack of magnetic moments. Resonant photoemission studies show

these compounds to be mixed valent, suggesting the need to reexamine the mechanism of their

superconductivity.

CeRu2 and CeCo2 are superconductors with transi-tion temperatures T, of 6 and 1 K, respectively. ' Ithas generally been assumed that Ce in these materi-als is tetravalent, i.e., 4f, because studies in whichCe is introduced into various superconducting ma-trices have shown that a localized 4f electron, eithermagnetic or nonmagnetic, drives the host T, to zerofor only a few atomic percent Ce.' CeRu2 and CeCo2also display collapsed volume and a loss of magneticmoments, properties which are common to a groupof Ce intermetallic compounds and which are oftentaken to signal tetravalent Ce for the group. 4 Alter-natively, mixed valence has been proposed for vari-ous of these compounds, ' and, based on a reexam-ination of the cerium atomic volume and on their 2pcore-level absorption spectra, there is a recent con-tention that no intermeta11ic tetravalent cerium com-pounds exist. This severely challenges the tradi-tional understanding of superconductivity in CeRu2and CeCo2, especially since superconductivity hasbeen reported for only one other material knownwith certainty to be mixed valent, CeCu2Si2, forwhich the T, of —0.5 K is less than a tenth that ofCeRu2. '

This Communication describes the results ofresonant photoemission measurements on CeRu2 andCeCo2, made to search for 4f emission. It is con-venient to compare with a material known to have 4felectrons, and therefore data are also presented forCeA13, which is interesting in its own right because itdisplays 4f electron demagnetization below —5 K.

The photoemission experiments reported here wereperformed at the Stanford Synchrotron Radiation La-boratory using equipment, operating conditions, andexperimental procedures exactly as described previ-

ously. " The samples were polycrystalline ingots,fracturedin situ in a vacuum of 1&10 ' Torr, andsuperconductivity with the correct T, was verified forthe samples of CeRu2 and CeCo2. Figures 1, 2, and3 show the valence-band photoemission spectra ofCeAl3, CeRu2, and CeCo2, respectively, for severalphoton energies h v between 80 and 130 eV.

The interpretation of resonant photoemission datafor any cerium material, and especially one believedto have no 4f electrons, must take account of the

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26 O1982 The American Physical Society

RAPID COMMUNICATIONS 26

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FIG. 3. CeCo~ valence-band photoemission spectra forseveral photon energies. The resolution is 0.83 eV for the115—120-eV spectra, and is 0.41, 0.45, and 0.46 eV for the80-, 112-, and 121-eV spectra, respectively.

fact that the Ce Sd emission resonates with multiplicat«

tive gain as large as 0.5 that of the 4f electrons. Thatthis is so is known experimentally from studies of Gdand Tm, where the 4f and Sd emission do not over-lap. ""It is therefore necessary to have a spectro-scopic criterion to identify 4f emission. In CeA13, as

in cerium itself and many other compounds, the mostprominent feature in the on-resonant spectrum (pho-ton energy h v =121—122 eV) occurs at —2-eV bind-ing energy and all workers who have made resonantphotoemission studies of cerium compounds agreethat this peak has 4f origin. "'4 " An identifyingfeature of its resonance behavior is that this peak be-comes predominant only for br & 118 eV, althoughthe entire valence band resonates for h v & —112 eV,the Fano minimum.

For the spectra of CeCo~ and CeRuq the broademission at —2.6-eV binding energy behaves in ex-actly the same fashion and is therefore assigned as 4femission. Although in these cases the 4f state ismore difficult to pick out because it is broad and lo-cated on a large background of Co or Ru d states, thesimilarity to the behavior in CeA13 is unmistakable.The finding of 4f emission in CeRur and CeCor, andits greater width than in CeA13, is qualitatively con-sistent" with all reported core-level photoemissionand photoabsorption spectra, which show both 4f0and 4f ' structure for CeCoq (Refs. 8 and 18) andCeRuq, '9 but only 4f' structure for CeA13.'0

There is not complete agreement about the assign-ment of the resonating emission nearer to the Fermilevel in Ce valence-band photoemission. Some work-ers" assign it as Sd emission, and this assignment issupported by the fact that strongly resonating 5demission is expected in the spectrum, ' ' and by thebehavior pointed out above, that its resonance beginsand peaks at slightly lower photon energies than forthe 2-eV feature. Such a difference for n =4 and 5shell emission was predicted theoretically ' and ob-served experimentally' for the 4d, Ss, Sp, and Sd lan-thanum emission of La86. Other workers ' "haveasserted that, in some cerium materials, the emissionnear the Fermi level is also of 4f origin. Based onprevious work, then, the sharp feature near E+ in theCeRur spectrum could be assigned either as 4f or Sd.We prefer the Sd assignment for the reasons givenabove, but at present this is an unresolved issue.

Since the resonance technique has been used hereto identify 4f emission well away from the Fermi lev-el, it should be mentioned that Baer and his colla-borators' have given a number of arguments that inseveral cerium materials, including y-Ce and CeA13,the 4f emission lies very close to the Fermi level,with the remainder of the spectrum being of Sd ori-gin. Of necessity these arguments are all indirect,having been undertaken to overcome the fact thatthe small spectral weight and lack of spectral struc-ture of the 4f' 4f0 transition have made it impossi-ble to identify by direct spectroscopic means in con-ventional x-ray photoemission (XPS). Other XPSworkers have also confronted this problem andhave found cause to place the 4f emission at 2 eV iny-Ce, in agreement with the conclusions fromresonant photoemission.

26 RAPID COMMUNICATIONS

Considering now these materials' superconductivi-

ty, at present we can only point out certain relevantaspects of the problem. Translational symmetry isthe most obvious new ingredient capable of superced-ing the impurity behavior described in the openingparagraph, because it implies coherent 4f states with

vanishing quasiparticle scattering as T 0. The de-tails of the 4f coherence have yet to be elucidated.Although a simple 4f band model could account forthe rather temperature-independent magnetic suscep-tibilities of CeCo2 and CeRu2, such a model is inap-propriate if the 4f Coulomb energy U is —5—6 eV,as claimed from Bremsstrahlung isochrornat spectros-copy on severa1 cerium materials. Presumably somesort of Fermi-liquid picture applies at low tempera-ture and T, is then material dependent, beinggoverned by the quasiparticles' residual interactions,about which little is known, and by their coupling tophonons, possibly strong in a mixed valent system.As has been emphasized for CeCu2Si2 (Ref. 9) aFermi-liquid enhancement of y may be important;the values of y for CeRu2 and the isostructural 4focompound LaRu2 are, respectively, 23.3 + 0.61 and

13.6+ 0.13 m J/mole K, in roughly the same ratio asthe values of T„which are 5.92 and 3.2 K, respec-tively. The Debye temperatures of CeRu2 and LaRu2are nearly equal, implying similar phonon spectra,although the possibility is not ruled out that a partic-ular feature of the phonon spectrum may account forthe higher T, of CeRu2. Finally, the fact that rare-earth substitutions in CeRu2 initially raise T„evenwhen the rare-earth ions carry localized moments, ' ismotivation to speculate that the response of the ceri-um lattice Fermi liquid to magnetic impurities is dif-ferent than that of other Fermi liquids.

It is a pleasure to thank R. D. Parks for the CeA13sample and R. M. Martin for helpful discussions.The Stanford Synchrotron Radiation Laboratory issupported by the NSF through the Division of Ma-terials Research. The work is supported at Stanfordby the NSF under Contract No. DMR 79-13102 andat the University of California, San Diego, by the U.S. Department of Energy under Contract No. DE-AT03-76ER70227. J. F. S. is also supported byFAPESP (Brazil).

'Present address: Xerox Palo Alto Research Centers, PaloAlto, Calif. 94304.

~On leave from Institute de Fisica "Gleb Wataghin, "Universidade Estadual de Campinas, 13100 Campinas,Sao Paulo, Brazil.

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RAPID COMMUNICATIONS 26

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