Janner, A.-M.

Jonkman, H.T.

Koopmans, H.

Sawatzky, G.A.

English

We studied the dynamics of the electric dipole (ED) forbidden 1T1g excitonic state of C60 at ħω=1.8 eV using temperature dependent Second-Harmonic Generation (SHG). We found a strong temperature dependence of the SH intensity and a splitting of the resonance structure below the first order rotational ordening phase transition at 260 K. We interpret this splitting as Factor Group or Davydov splitting. The magnitude of the splitting (40 meV) and the large exciton band width (100 meV) will be explained with a charge transfer mediated propagation mechanism

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   University of GroningenExciton Propagation Studied by Temperature Dependent Optical Second-HarmonicGeneration from C60 FilmsJanner, A.-M.; Jonkman, H.T.; Sawatzky, G.A.; Koopmans, H.Published in:Physical Review BIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:1995Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Janner, A-M., Jonkman, H. T., Sawatzky, G. A., & Koopmans, H. (1995). Exciton Propagation Studied byTemperature Dependent Optical Second-Harmonic Generation from C60 Films. Physical Review B, 52(24).CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.234EXCITON PROPAGATION STUDIED BYTEMPERATURE DEPENDENTOPTICAL SECOND-HARMONIC GENERATION FROM C60 FILMSA.-M. JANNER, B. KOOPMANS, H. T. JONKMAN and G. A. SAWATZKYLaboratory of Solid State Physics, Materials Science Centre, University ofGroningen,Nijenborg 4, 9747 AG Groningen, The NetherlandsAbstractWe studied the dynamics of the electric dipole (ED) forbidden 1T1g excitonicstate of C60 at •ω=1.8 eV using temperature dependent Second-Harmonic Gen-eration (SHG). We found a strong temperature dependence of the SH intensityand a splitting of the resonance structure below the first order rotational orden-ing phase transition at 260 K. We interpret this splitting as Factor Group orDavydov splitting. The magnitude of the splitting (40 meV) and the large ex-citon band width (100 meV) will be explained with a charge transfer mediatedpropagation mechanism.1. IntroductionIn recent years considerable interest has been generated in the study of the physicalproperties of C60 solids. These materials seem to exhibit a dualistic behaviour, on theone hand they behave like weakly van der Waals bound molecular solids, while on theother hand they behave like semiconductors with a moderate (2.3 eV [1]) band gap.The properties of the fullerene compounds show some resemblance to the chargetransfer type of molecular solids like the much studied TCNQ salts. In this paper wepresent evidence that the low lying excitonic states of solid C60 exhibit such dualisticbehaviour. There is experimental evidence that the energies of the excitonic states arevery close to the corresponding excited states in the gas phase, emphasing thepreservation of the molecular character in the solid. But the propagation of the ex-citons results in abnormally large excitonic bandwidths for ED forbidden transitions inmolecular solids.Koopmans et al. [2,3] has shown that the ED forbidden 1T1g excitonic state at1.81 eV is readily accessible by magnetic dipole selection rules and can be probed in adoubly resonant SHG experiment. With this technique we probe the dynamicalproperties of the 1T1g excitonic state in solid C60 by studying the temperature de-pendence of its dispersional width and the observed splitting of the excitonic bandbelow the rotational ordening phase transition [7].2. ExperimentalC60 with a purity better than 99.99% was evaporated from a Knudsen cell onto asubstrate  (fused quartz or MgO)  at  UHV pressures  below 4·109 mbar.  For  the  SHG235experiments a Nd:YAG laser was used to pump a dye laser, producing 7 ns pulseswith an energy of approximately 6 mJ/pulse and a repetition rate of 10 Hz. We usedfrequencies in the range •ω= 1.7...2.0 eV. SH intensities were calibrated using the SHsignal of a reference Quartz crystal. All SHG experiments were performed using afixed angle of incidence (45° to the surface normal) and a mixed (i.e. s + p) inputpolarization. The p-polarized component of the specular reflected SH was detected.The temperature dependent SHG experiments were performed by using a He-flowcryostat (4 - 500 K), More experimental details can be found in [12].Fig.1. The temperature dependent SHG of C60 thinfilm using m-p polarization combination. ∆ = 430 K," = 265 K, ∇= 203 K, ☼ = 120 K, • = 27 K.Fig.2.  The temperature dependenceof the total SH intensity, which isstrongly enhanced just below thephase transition at Tc = 260 K3. ResultsThe SH intensity measured as a function of the fundamental frequency (•ω) is shownin Fig.1 for different temperatures. The resonance (at about 1.8 eV) clearly shows ablue shift at low temperatures. Below the rotational phase transition temperature theresonance structure splits up into two peaks. At low temperatures this splittingamounts to 40 meV. The total dispersional bandwidth of about 100 meV (at the base)is large for a molecular crystal where an ED forbidden transition is involved.The temperature dependence of the total SH intensity is shown in Fig.2. Notice thestrong SH intensity enhancement just below the phase transition temperature.2364. DiscussionNeutron diffraction experiments have shown that going through the rotational orden-ing phase transition the number of buckyballs per primitive unit cell changes fromone (in a FCC lattice) to four (in a SC lattice) [7]. In the isolated C60 molecule themolecular wave functions form the basis of the irreducible representations of themolecular point group (Ih). In the solid in a simple cubic lattice we can constructcrystal wavefunctions, which are linear combinations of the molecular wavefunctionsin a unit cell in such a way that they form the basis for the irreducible representationsof the Factor Group (Th). In the interpretation of recent two photon experimentsMuccine et al. have discussed the splitting of the three fold degenerate 1T1g excitonband in Th [8]. They show that the 1T1g exciton band correlated with 12 bands of 1Ag,1Eg and 3 1Tg symmetry. Only the 3 1Tg bands will be accessible by magnetic dipoleselection rules. In consequence we interpret the splitting at low temperature as acrystal field induced Factor Group or Davydov splitting between two of the 1T1gexcitonic states. The measured dispersional bandwidth of 100 meV and the largeDavydov splitting seems to be remarkable for an optical transition with an almost zeroED transition oscillator strength. With the usual for molecular crystals used excitonpropagation mechanism (a transition dipole - transition dipole or multipolemechanism) one would expect an only very small dispersional width of the excitonicstates and a Davydov splitting of a few meV. Lof et al. [1] suggested for this case acharge transfer mediated propagation mechanism, which also was used for the lowestsinglet state of Naphthalene [4], and the charge transfer TCNQ salts [5]. The neteffective exciton hopping integral is given byZtethTexciton = ——— (1)U - Vwhere te(h) are the single electron (hole) nearest-neighbour hopping integrals, Z is ageometrical constant and the energy U - V is the difference between the onsiteCoulomb interaction and the nearest-neighbour Coulomb repulsion [6].The strong temperature dependent SH intensity can be explained in terms of dy-namic rotational disorder. The rotational disorder above the phase transition will leadto a depression of the SH response by dephasing processes (T2) within the timescaledetermined by the excitation transition matrix elements. Photoluminescency exper-iments show also a strong temperature dependence of the intensity. These changesoccur at some lower temperature compared with the phase transition and the inter-pretation is still controversial [9,10,11].5. ConclusionsWe have studied the dynamics of the by magnetic dipole selection rules accessible 1T1g237excitonic state at •ω = 1.81 eV by doubly resonant temperature dependent SHG ex-periments. We found a very strong temperature dependence of this resonance. ItsSH intensity increases dramatically with decreasing temperature. We explained thisby a rotational disorder induced T2 dephasing mechanism. Below the rotational or-dering phase transition the SH resonance structure splits up into two bands. Weattribute this splitting to the Factor Group or Davydov splitting which finds its originin the presence of four molecules per unit cell in the low temperature phase. Theobserved unexpected large Davydov splitting of 40 meV, combined with the unusualdispersional bandwidth of 100 meV, are explained with a charge transfer mediatedmechanism for the propagation of the exciton.6. AcknowledgementsWe thank the two Erasmus students Roberta Guardini and Christoph Strecha fortheir contribution to the measurements during their stay in our laboratory. This in-vestigation was supported by the Netherlands Foundation for Fundamental Researchon Matter (FOM) with financial support from the Netherlands Organization for theAdvancement of Pure Research (NWO).7. References1. R.W. Lof, M.A. van Veenendaal, B. Koopmans, H.T. Jonkman and G.A. Sawatzky,Phys. Rev. Lett. 68, (1992) 3924.2. B. Koopmans, A. Anema, H.T. Jonkman, G.A. Sawatzky and F. van der Woude,Phys. Rev. B 48, (1993) 2759.3. B. Koopmans, A.-M. Janner, H.T. Jonkman, G.A. Sawatzky and F. van der Woude,Phys. Rev. Lett. 71, (1993) 3569.4. R. Silbey, J. Jortran, M.T. Valaad, S.A. Rice, J. Chem. Phys. 42, (1965) 2948.5. T. Hibma, G.A. Sawatzky and J. Kommandeur, Chem. Phys. Lett. 23, (1973) 21.6. P.A. Brühwiler, A.J. Maxwell, P. Rudolf, C.D. Gutleben, B. Wästberg and N.Mårtensson, Phys. Rev. Lett. 71, (1993) 3721.7. W.I.F. David, R.M. Ibberson, T.J.S. Dennis, J.P.Hare and K. Prassides, Europhys.Lett. 18, (1992) 219.8. M. Muccini, R. Danielli, R. Zamboni, C. Taliani, H. Mohn, W. Müller and H.U.ter Meer, submitted to Chem. Phys. Lett. (1995).9. K. Pichler, S. Graham, O.M. Gelsen, R.H. Friend, W.J. Romanow, J.P. McCauleyjr.,  N.  Coustel,  J.E.  Fisher  and  A.B.  Smith,  J.  Phys.:  Condens.  Matter 3, (1991)9259.10. J.L. Sauvajol, Z. Hricha, N. Coustel, A. Zahab and R. Arnar, J. Phys.: Condens.Matter 5, (1993) 2045.11. M. Matus, H. Kuzmany and E. Sohmen, Phys. Rev. Lett. 68, (1992) 2822.12. A.-M. Janner, B. Koopmans, H.T. Jonkman and G.A. Sawatzky, to be published.

Exciton Propagation Studied by Temperature Dependent Optical Second-Harmonic Generation from C60 Films

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Exciton Propagation Studied by Temperature Dependent Optical Second-Harmonic Generation from C60 Films

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