ELECTRON-PHONON INTERACTION IN SmS

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ELECTRON-PHONON INTERACTION IN SmS

G. Güntherodt

To cite this version:

G. Güntherodt. ELECTRON-PHONON INTERACTION IN SmS. Journal de Physique Colloques,

1980, 41 (C5), pp.C5-65-C5-66. �10.1051/jphyscol:1980512�. �jpa-00219947�

JOURNAL DE PHYSIQUE Colloque CS, supplkment au n ^O 6 , Tome 41, juin 1980, page C5-65

ELECTRON-PHCNON INTERACTION IN SmS

G. Gintherodt

Max-Planck-Institut fiir Festkdrperforsclxng, 7000 Stuttgart 80, R.F.A.

Extended abstract.- Strong electron-phonon coupling is generally expected for rare earth (RE) compounds exhibiting valence fluctuations. Fluctuations in the 4f occu- pation number due to degenerate 4f and n

4fn-l5d electron con£ 1 igurations imply drastic changes (>lo%) in the ionic radius of the RE ions. Consequently, there results a strong coupling to the lattice which ob- viously is evidenced for instance in Sml-x YxS by a) the "soft" bulk modulus B for x 0.15 /I/, b) the anomalous mean square displacements of the S ions at low tempera- ture for x = 0.3 /2/, and c) the anomalies in the phonon dispersion curves for x = 0.25 /3/. However, most remarkably, a 15% smaller B in integral-valence, semiconducting SmS as compared to EuS /4/ is accompanied by a

17% "softening" of LO (L) phonons /5-7/, de- spite the same lattice constants. This ano- malous behavior has called for a more gene- ral concept of electron-phonon interaction in semiconducting as well as intermediate- valence SmS or Sml-xYxS, not readily asso- ciated with valence fluctutations only.

Raman scattering in EuS /8/ and SnS /6,7/ has provided clear evidence for a locally full symmetric ( rl) 4f-electron-

+

n-1 1

phonon coupling upon 4fn + 4f 5d optical excitation. This coupling involves LO

phonons, dominantly from the L point of the Brillouin zone, where S ion planes are vibrating with respect to Eu or Sm ion planes at rest. The "softening" of LO (L) ph~nons in semiconducting SmS as conpared to EuS has been attributed to a stronger 4f-5d hybrkdization, giving rise to an enhanced isotropic compressibility K = 1/B of the Sm ion. The f-d pronotion energy of 0.1 eV in SmS is much smaller than that of 1.65 eV in EuS. Besides the nearest neigh- bor Sm(4f)

-

Sm(5d) hybridization in the

b-aC1 lattice with inversion symmetry, we have emphasized the importance of on-site f-d hybridization due to electron-phonon interaction / 7 / . This could be confirmed by a model calculation, showing that the deformation-potential type electron-phonon interaction of

r l +

symmetry can account within a factor of three for the LO(L) phonon frequency renormalization in semi- conducting SmS

.

A more general approach to electron- phonon interaction in EuS and Sm 1-x x _{Y S,} for 0 ( x 2 1, has been taken recently in a

lattice dynamical model in terms of specific short-range electron-lattice interactions

("deformabilities") / 9 / . In this model the response of the electronic charge of the RE ion and the S iossis described by three local deformabilities of dipolar-, "brea- thingn- and quadrupolar-type symmetry. High symmetry points in the Brillouin zone allow for an unambiguous identification of the contributions of specific deformabilities to the phonon self-energy in the correspon- ding phonon branches.These three deforma- bilities, together with two nearest neigh- bor force constants give a clear descriptia of the electron-lattice interactions and the phonan anomalies in the above compounds.

In particular, in semiconducting SmS a dipolar and "breathing" deformability are responsible for the reduced L O ~ O splitting at

r

and the "softening" of LO phonons at L, respectively. These deforma- bilities are related to virtual f-d excita- tions.

While the above model accounts quite well for the phonon anomalies in Sm

0 -75' 0.25S, a question still remains concerning the difference in B and LO(L) "softening"

between Smo .75Y0 25S and metallic SmS

.

^{T he}

bulk modulus of SmS of 520 kbar at a hydro-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980512

JOURNAL DE PHYSIQUE

static pressure of 6.5 kbar /lo/ is much larger than B = 70 kbar of Smo .75Y0. 25S /1/

and even larger than B = 420 kbar of semi- conducting SmS /lo/. In addition, the LO (L)

"softening" is 2% in metallic SmS, but 15%

in S% .75Y0. 25S as compared to a stable- valence, metallic RE3+s / 7 / . The importance of Y in Sml-xYxS in breaking the inversion symmetry and enhancing the on-site f-d hybridization has been pointed out /7/. The role of the Y(d) states in the f-d hybridi- zation is still subject to further investi- qations /9,11/.

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