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Submitted on 1 Jan 1971
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OPTICAL TRANSITIONS AND THE ENERGY LEVEL SCHEME OF THE EUROPIUM
CHALCOGENIDES
G. Busch, G. Güntherodt, P. Wachter
To cite this version:
G. Busch, G. Güntherodt, P. Wachter. OPTICAL TRANSITIONS AND THE ENERGY LEVEL SCHEME OF THE EUROPIUM CHALCOGENIDES. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-928-C1-929. �10.1051/jphyscol:19711331�. �jpa-00214365�
JOURNAL DE PHYSIQUE Colloque C 1, suppliment au no 2-3, Tome 32, Fivrier-Mars 1971, page C 1 - 928
OPTICAL TRANSITIONS AND THE ENERGY LEVEL SCHEME OF THE EUROPIUM CHALCOGENIDES
G. BUSCH, G. GUNTHERODT and P. WACHTER Laboratorium fur Festkorperphysik ETH, Ziirich, Switzerland
Rbsum6. - Les constantes optiques de monocristaux des chalcogknures d'europium ont kt6 dkterminks a la tempk- rature ambiante pour des energies de 1 k 6 eV. Pour l'interprktation des spectres d'absorption nous avons essay6 de correler les maxima du coefficient d'absorption avec des transitions entre bandes. Cette correlation est confirmke par une analyse des forces d'oscillateurs et des mesures d'absorption de couches minces ii basses tempkratures. Nous dauisons un schkma des niveaux d'energie qui permet une evaluation de la largeur des Ctats 5d et de leur skparation par le: champ cristallin.
Abstract. - The optical constants of Eu chalcogenide single crystals have been determined at room temperature for photon energies from 1 to 6 eV. To interpret the absorption spectra an attempt has been made to relate the maxima of the absorption coefficient to interband transitions. This assignment has been confirmed by analysis of the 'oscillator strengths and low temperature investigations on thin evaporated films. Thus we established a consistent energy level scheme which gives useful information about the width of the 5d-states and the crystal field splitting.
For optical investigations in the fundamental absorp- tion region of semiconducting single crystals, reflec- tance methods are more appropriate than transmission measurements. The optical constants of Eu chalco- genide single crystals have been computed from reflec- tivity data by means of Fresnel's equations. The measurements have been carried out at room tempera- ture for photon energies from 1 to 6 eV [I]. From the absorption index k of the four Eu chalcogenides we have calculated the absorption coefficient K = 4 n k / l , which is shown in figure 1. Within the wavelength
"I 2 3 4 5 eV 6
Photon Energy -
FIG. 1. - Absorption coefficient of the four Eu chalcogenides at room temperature.
range covered by the experiment two main absorption regions are observed, consisting of one single peak at low energies and a superposition of 3 to 6 peaks a t higher energies in going from EuO to EuTe (the peaks have been numbered e. g. for EuSe). The energy separation of these two absorbing regions is largest for EuO and decreases towards EuTe.
The transmission of thin evaporated films of the Eu chalcogenides (except for EuO) has also been measured for photon energies from 1 to 6 eV above and below the magnetic ordering temperature [2].
For T < T, and with unpolarized light the first absorption peak of e. g. EuS splits into a triplet,
while the fourth peak splits into a doublet. These splittings have been related t o spin-orbit and exchange interactions [ l ] and suggest transitions from the 4 f7-level of Eu2+ into the crystal field split 5dt2,- and 5de,-state, respectively.
As an interpretation of our absorption spectra an attempt has been made to establish an energy level scheme which was stimulated by the suggestion of Methfessel [3], the ionic crystal model of Wachter [4]
and Kasuya [5] and by the energy block diagram of Eastman et al. [6]. Generally maxima of the absorp- tion coefficient are found when the product of the joint density of states and the matrix element of the initial and final state is a maximum. Certainly for narrow bands the energy dependence of the matrix elements can be neglected compared with the one of the joint density of states. The same simplification has been assumed for broader bands, because the matrix element is a rather slowly varying function.
We attempt to relate the maxima of the absorption coefficient to interband transitions.
I t seems to be assured that the 4 f-levels in the' Eu chalcogenides are situated between the p-valence band of the anions and the 5d-conduction band of Eu2+
[3-71 and thus represent the highest occupied states.
Therefore the first absorption peak at low energies can be attributed to a 4 f 7-4 f sdt,, transition. This has also been confirmed by analysis of the oscillator strengths [I]. We relate the second peak (with an esti- mated oscillator strength in the order of to a weakly forbidden 4 f-(6 s, 6 p) transition and the small fifth peak to a charge transfer transition from the p6-valence band of the anions into the (6 s, 6 p)- state. For these two transitions it is unknown up to now whether the maximum of the density of states of the final state has dominant 6 s- or 6p-character.
Finally we ascribe the third arid sixth peak to tran- sitions from the p6-valence band into the 5dt2,- and 5de,-state, respectively.
Hence we established an energy level scheme which is shown in figure 2, e. g. for EuS. The origin of the energy scale has been chosen at the middle of the 3 p6-valence band. The optical transitions are assumed as direct ones and have been indicated by arrows.
On the right hand side of the figure the density of
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711331
OPTICAL TRANSITIONS AND THE LJNEKGY LEVEL SCHEME OF THE I:UROPIUM CHALCOGENIDES C 1 - 929
FIG. 2. Encrgy level scheme of EuS at room temperature.
states is shown which has been assumed to be sym- metric to the maxima ; the form and width of the density of states is only schematic.
The consistency of the assigned transitions can be checked except for EuO. From the positions of the first to fourth peak determined experimentally it is possible to calculate the positions in energy of the fifth and sixth peak as sums and ditferences of the former ones. A comparison between calculated and experimentally observed positions shows good agreement to within about f 0.1 eV.
The proposed energy level schemes of the four Eu chalcogenides are plotted in figure 3. 'I'he origin of the energy scale has been chosen at the bottom of the p6-valence band of EuO. The width of the valence bands of the Eu chalcogenides was taken from photo- emission data of Eastman et al. [6] (except for EuTe where we assume the same width as for EuSe).
As a reference the 4 f7-level of the four compounds has heen placed at the same energy.
Thc absorption edge corresponds to the onset of the 4 f7-4 f6('FJ) 5 dt,, transition, where the final state is combined of one 5d-electron and six 4 f- electrons. Thc 'FJ-multiplet (J = 0, ..., 6) of the six 4 f-electrons with a total splitting of about 0.6 eV [8]
EuO €US EuSe E uTe
FIG. 3. - Energy level schemes of the fouriEu chalcogcnides at room temperature.
is superimposed o n the 5 d(t2,)-band as has also been assumed by 1)imrnock et al. [lo]. This is illustrated in our schemc by the ladder structure of the 5 d(ttg)- band. Hence, using also the energy of the absorption edge, we obtain the width of the 5d(t,,)-band amount- ing to 1.1 eV for EuO, 0.7 eV for EuS and EuSe arid 0.6 eV for EuTe. The 5 d-bands here turn out to be narrow as has also been concluded by Feinlcib et
al. [ 9 ] , otherwise the observed exchange splittings of
the 5 d-bands could not be resolved. The difference in energy between the 5dt2,-and the Sde,.level diminishes from EuO to EuTe in agreement with the decreasing crystal field splitting with increasing lattice constant. We obtain the following values of the crystal field splitting 10 Dq : 3.1 eV for EuO. 2.2 eV for EuS, 1.7 eV for EuSe and 1.5 eV for EuTe.
Another check of the consistency of the model is given by the computation of the intraionic energy difference between the 4f-level and the center of gravity of the crystal field split 5 d-levels (the center of gravity is obtained by dividing the energy diffe- rence 5de,-5dt2, by the ratio 3 : 2 ) . This difference amounts to (3.2 f 0.05) eV for all the four chalco- genides.
For a more detailed information we refcr to refe- rence [I].
Acknowledgement. - The authors are grateful to the << Schwcizerischer National fonds )) for financial support. They are obliged to Dr. 0 . Vogt for sup- plying the crystals and to Mr. J. Schoenes for preparing the thin films.
References
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J . Appl. Phys., 1970, 41, 1083.
(31 MI:.I.IIFESSEL (S.), 2. angew. Phys., 1965, 18, 414. [9] FEINLE~H (J.), SCOULER (W. J.), DIMMOCK (J. 0.).
[4] WA(.IITER (P.), IIabilitat~onsschrift ETH, Ziirich, HANUS (J.), REED (T. B.) and P I D G E ~ N (C. R.), Aug. 1969, to be published. Phys. Rev. Letters, 1969, 22, 1385.
[5] KASUYA (T.), J. Appl. Phys., 1970, 41, 1090. [lo] DIMMOCK (J. O.), HANUS (J.) and FEINI.EIR (J.), [6] EASTMAN (D. E.), HOLTZBERG (F.) and METHFESSEI. J . Appl. Phys., 1970, 41, 1088.
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