Spectroscopy of the Peierls transition order parameter in the quasi-one dimensional organic conductor (FA)2PF6

Solid State Communications, Vol. 97, No. 10, pp. 863-867, 1996 Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/96 $12.00 + .SMJ

0038-1098(95)00727-X

SPECTROSCOPY

OF THE PEIERLS TRANSITION ORDER PARAMETER IN THE QUASI-ONE DIMENSIONAL ORGANIC CONDUCTOR (FA)2PF6

D. Berner. V.M. Burlakov,’ G. Scheiber, K. Widder, H.P. Geserich, J. Gmeiner2 and M. Schwoerer2 Institut fur Angewandte Physik, Universitat Karlsruhe, Kaiserstr. 12, 76128 Karlsruhe, Germany ‘Institute of Spectroscopy Russian Academy of Sciences, 142092 Troitsk, Moscow Region, Russia 2Physikalisches Institut and BIMF, Universitat Bayreuth, 95440 Bayreuth, Germany (Received 19 September 1995; accepted 26 October 1995 by P. Wachter)

The broad band spectrum of the optical conductivity of the quasi-one dimensional organic conductor (FA)2PF6 (FA = fluoranthene) has been investigated in a wide temperature range including the Peierls transition temperature Tp = 180 K. A semiconducting gap has been observed both below and above Tp varying between 150meV at 10K and 80meV at 300 K. The ( Tp - T)1/4 temperature dependence for the order parameter has been deduced from an analysis of the integrated band intensity of phase phonons thus giving evidence that the Peierls transition is a 2nd order phase transition near the tricritical point. Keywords: A. organic crystals, A. metals, D. phase transitions, E. light absorption and reflection.

1. INTRODUCTION THE RADICAL cation salt (FA)2PF6 (FA = fluoranthene) is one of the most anisotropic quasione-dimensional organic conductors, which show charge-density-wave (CDW) ground state properties [l-3]. The CDW becomes static below the Peierls transition temperature Tp 2: 180K due to 3-D ordering of the CDWs in adjacent chains [4]. The anisotropy of the dc. conductivity of the fluoaranthene salt at room temperature is about 104. Furthermore, a strong anisotropy of the dynamical conductivity has also been well established in [5] for the energy range between 0.1 and 5 eV where it has been found that for the electric field E parallel to the direction of the high dc. conductivity, i.e. the direction of the molecular stacks (a-direction), the reflectance spectrum at temperatures both below and above Tp shows sharp plasma edge near 1 eV. In the polarization perpendicular to the a-direction the spectrum is typical for inorganic semiconductor. The temperature dependence of the d.c. conductivity and of the spin susceptibility of an 1-D conductor can be described in terms of an effective energy gap A@(T) at temperatures above the Peierls

transition temperature [6, 71. Since the temperature range of interest is usually below the mean-field transition temperature TCMF, the effective gap is thought to be related to fluctuation-perturbed meanfield gap AMF( T) and therefore it is exactly equal to the mean-field value at T = 0. Below Tp the d.c. conductivity behavior can be described using just the mean-field gap behavior scaled from T,“” to the transition temperature Tp as has been demonstrated for Ko,,Mo03 [6]. The use of the mean field-like approach below Tp seems quite reasonable since 3-D ordering should suppresses the 1-D fluctuations. Thus, the d.c. conductivity requires two different regimes for its temperature behavior description with the crossover point at Tp. The d.c. conductivity temperature dependence in (FA)2PF6 has been described above Tp in terms of pseudogap and below Tp by the mean-field approach [3]. Thus, the appropriate order parameter (OP) is supposed to have a BCS-like temperature dependence, i.e. proportional to dv in close vicinity of Tp. Detailed investigation of the IR reflectance spectra of (FA)2PF6 performed in the present study allowed us to suggest the ( Tp - T)‘j4 temperature dependence for the Peierls transition OP in fair agreement with

863

864

PEIERLS TRANSITION

ORDER PARAMETER

X-ray data [S]. Thus, the Peierls transition under investigation appears to be a second order phase transition near the tricritical point. The Peierls gap value has been estimated to be about 80 meV at room *_-_-___~____ _-J I*zn__-_%T ^. ,fi\TI Lelrlperawrt: a11u JU IIK v a1 I u h. 3 -.

CAMP1 FS _QJn L_ __.__ _-L-

E?(PEp_I_&!Er\TTb_L

Vol. 97, No. 10

IN (FA)2PFs u (lo3 cm-l) 1

100 . 1 . . ., I,

,

10 . . . . ...(

t

SETUP

The (FA)2PF6 crystals were grown electro11..iis ^_ UCSc;II”CU .._^__11_1CIDeWIIC1C _,“___.L___ mm _I_- Ul _CIl__ LllC sI‘c LllC chemically ty,. vl__ crystals was about 1.5 x 2 x 1 mm3 and the surfaces were shiny and black in color. For optical m%s_?rements as grown crystals with well reflecting surfaces have been selected. Reflectance measurements were carried out using a single-beam prism spectrometer for room tempera_ -_ ture measurements in the energy range from 0.i to 6 ev and a Fourier transform spectrometer BRUKER ,,,+;“,.n.,” an.., rr..r\o+n+ TFC 111.7 ,“..;s-...m4 ..,;el. 11 u-1 ,_I* t,qu’~pAl vv,cu a ~“I,cIIIu”uJ-II”w wy”“caL for low-temperature measurements range between 0.02 and 2 eV.

,lOK

lldJLl__

in the energy

r

. ..

I.

50

3. RESULTS AND DISCUSSION

100

500

fiwime'di

The reflectance spectra of (FA)2PF6 for polarization both parallel and perpendicular to the a-direction are shown in Fig. 1 for 300K and 10K. The general shape of the room temperature spectra is very close to that obtained in [5]. A strong difference between the spectra in two polarizations due to the strong anisotropy

Of the COIiipOiid

is ObSHWd.

a-_ ?__I_ Tnc rllgn

energy part of the RI, spectrum (E 11a polarization) is dominated by a steep reg_ectivity edge be!ow 1 PV, due to a strong oscillator related to one-particle excitations across the Peierls gap (we will refer to it as to pseudogap, but it should not necessarily coincide with the value of effective energy gap used for d.c. conductivity). ‘The smooth increase in Kil at i0w energy we attribute to thermally excited free carriers Ax.n+;.r:+., l-ho ..,h:,.h .%..*..,,...n...Gl.lp cn.. tl.A ,. v*lli~U CUL ‘GD~““J’“‘L I”1 LIIb u.Ls. CGfiUU~CI*ILy. 1 IIG

. ..I

1000 .

Fig. 1. Polarized reflectance spectra of (FA)*PF6 at 300K and at 10K. determined by excitations across the energy gap. Only the low energy tail is enhanced by optical absorption due to thermally excited free carriers. The gap energy can be roughly estimated at room temperature as the rr(tw) peak position. This kind of estimation faiis in the case of the iOK spectrum

intermediate energy range between 50 and 250meV shows a slight modulation of the reflectivity related to intramolecular vibrations of the fluoranthene molecules. The reflectivity in E _La polarization R, is roughly an order of magnitude lower than RI\. No pronounced electronic features are observed in the _- __I.._..__ sptxmum as tiie iiiosi Sirikiiig fkaiiire iipon coding a sharp modulation of RI, is developing in the intram.olecular vibrations region rapidly increasing with decreasing temperature (Fig. 1 lower panel and Fig. 3). The optical conductivity spectra gll(hw) calculated from the corresponding RI, spectra via standard Kramers-Kronig transformation are shown in Fig. 2. The room temperature spectrum is mainly

10

50 100 'nwimev;

500

1000

Fig. 2. Optical conductivity spectra for E ((a polarization caicuiated from the corresponding refiectance spectra.

Vol. 97, No. 10

PEIERLS TRANSITION

ORDER PARAMETER

865

IN (FA)2PF6

v (cm‘i)

300K

20 L

2 6000 > .s 4000

60 F

180K

40 180K

20 100

-jj6000 .-

80

54000

60 40 20 0

100

200 hw (meV)

Fig. 3. Reflectance spectra in E (( a polarization in the vibrations of energy range of intramolecular (F&PFe. because of strong and sharp peaks related to intramolecular vibrations coupled to electronic continuum. These vibrations are shown in a more expanded scale in Fig. 3 in the reflectivity spectrum and in Fig. 4 in the +,J) spectrum. The latter obviously indicates the fact that these vibrations are really coupled to electronic excitations: all the peaks have well pronounced asymmetry in the 10K spectrum and looking like deeps, or Fano resonances, in the 180 K spectrum. Taking into account the pronounced interaction of the intramolecular vibrations with electronic excitations and obvious increase of their IR intensity upon cooling we assign the peaks to the so-called phasephonons. Phase phonons are those which mainly involve totally symmetric intramolecular vibrations perpendicular to the a axis coupled to the charge density wave. They are suggested to be involved into stabilization of the CDW [l 1, 121. The IR activity of the phase phonons in the polarization along the chains results from small-amplitude phase vibrations of partial charge densities with respect to the total CDW [13]. The observed temperature behavior of the phase phonons is determined by the temperature dependence of the electronic part of o(hw) via the temperature dependence of the Peierls gap. One can roughly

hw CmeV)

Fig. 4. Conductivity spectra for E /( a polarization in the energy range of intramolecular vibrations of (FQPF,. estimate the gap value at 10K using the fact that at energies below 150meV the peaks in the 10 K spectrum are relatively narrow while at energies above this value they are much broader and have a different shape. According to [14] this behavior results from an overdamping of phase phonons with energies above the gap energy due to a decay into one-particle excitations. Using this gap value of 150meV at 10K one can estimate the mean-field critical temperature T,“” from the expression 2AMF(0) = 3.52kBTcMF

(1)

to be about 490K. The integrated intensity of the phase phonons depends on the CDW amplitude, and can be used as an indicator of the Peierls transition [12,15,16]. In the case of the fluoranthene salt we regard the intensity of the phase phonons to be proportional to the square of the OP, i.e. to (A(T))2. The value of (A(T)) derived from the IR spectra however, does not necessarily correspond to A@(T) used in [3] to describe the conductivity above TP since the averaging somehow takes into account the fact that this value relates to rather high frequency conductivity. While below TP where the fluctuations are suppressed the integrated intensity of the phase phonon is a measure of the square of real long-range order parameter AMF( T).

866

PEIERLS TRANSITION

ORDER PARAMETER

The number of phase phonons in the spectrum shown in Fig. 4 is rather large (about 30) and some of them are covered by the broad-band electronic continuum to which they are coupled. Therefore, it is very unreliable to measure the integrated intensity of a particular phase phonon, because the correct determination of the phase phonon parameters requires precise knowledge of the shape of the electronic continuum and all constants of interaction between the continuum and the intramolecular vibrations. Since the phase phonon structure creates a high frequency modulation of the conductivity spectrum we performed a complex Fourier transformation of the @w) spectrum Z+(w)] = a(fl)e’@)

(2)

and calculated an integrated intensity Z, of the high frequency part of the spectrum a(R). Obviously IX is proportional to the integrated intensity of relatively sharp features in the a(hw) spectrum and if the integration limits are chosen in a proper way to avoid the contributions from both smooth continuum and high frequency noise it gives the measure of the integrated intensity of the phase phonons. An advantage of this method is that the spectrum of the rather smooth electronic continuum on the one hand and that of the sharp phase phonon structure on the other are well separated in the R space and both peaks, i.e. normal phase phonons, and deeps related to Fano-type resonances can be measured simultaneously. The temperature dependence of IX is shown in Fig. 5. It is clear from Fig. 5 that around T = 180 K (this point corresponds to a maximum of d2Zc( T)/ dT*) some change in temperature dependence of IX takes place. We interpret it as a crossover from the 120

\. 8

100

?

1.

\ ‘1

80

‘\ A‘\.

‘1

La

“t \. l

w H

20 a

~.,I.*..I...,‘....‘*,,.l.,,.‘,.,,’...*~

j-A.

30

60

90

.

120 150 180 210 240 270

Temperature

Vol. 97, No. 10

fluctuational regime above this temperature to the mean-field-like behavior at low temperatures due to 3-D ordering of CDWs in adjacent chains representing the Peierls transition [4]. The theoretical meanfield regime is shown by a dashed line in Fig. 5 for the order parameter A(T) - (Tp - T)‘14. The value of the critical index p = 0.25 is characteristic for a second order phase transition near the tricritical point [17]. The experimental data shown in Fig. 5 are very close to the ( Tp - T)“*(Zc N (A(T))*) curve at temperatures below 150 K therefore we consider the temperature range below T = 150 K as being well described within the mean-field approach where the critical index of the order parameter has a value of p = 0.25. Remarkable deviations of experimental data from the curve above Tp = 180 K may be due to some OP fluctuations or, in other words, to the short-range order parameter. The latter is also responsible for the residual intensity of the phase phonons and to the presence of the effective energy gap well above Tp. 4. CONCLUSIONS The optical conductivity of the fluoranthene cation salt (FA)*PF6 has been investigated in a wide temperature range including the Peierls transition temperature. A strong feature related to one-particle excitations across the semiconducting energy gap has been observed both below and well above the Peierls transition temperature Tp = 180 K. Values of the optical energy gap were estimated to be about 80 meV and 150 meV at room temperature and at 10K respectively. The phase phonon integrated intensity shows clear mean-field behavior of the long-range order parameter below 180 K with a critical index p = 0.25. Why this value of p differs so much from that obtained from the fit of d.c. conductivity [3] is still an open question. Thus, for the proper understanding of the CDW properties in fluoranthene further investigations by various experimental techniques are encouraged. - This work was supported by the Commission of the European Communities under Contract No. CIl-0526-M (CD) and by the Deutsche Forschungsgemeinschaft. One of the authors (V.M.B.) acknowledges financial support and hospitality of the Karlsruhe University for his recent visit in the Institut fur Angewandte Physik. Acknowledgements

A \. . \

'O

IN (FA)2PF6

(K)

Fig. 5. Temperature dependence of the integrated intensity of the phase phonons. Triangles are experimental results, the dashed line is the theoretical curve - (T, - T)“*.

REFERENCES 1.

U. Kiibler, J. Gmeiner & E. Dormann, .Z.Mugn. 189 (1987).

mugn. Mat. 69,

Vol. 97, No. 10 2. 3. 4.

5. 6. 7. 8. 9. 10.

PEIERLS TRANSITION

ORDER PARAMETER

W. Riess, W. Schmid, J. Gmeiner & M. Schwoerer, Synth. Met. 41-43, 2261 (1991). W. Bruetting, W. Riess & M. Schwoerer, Ann. Physik 1, 409 (1992). V. Ilakovac, S. Ravy, J.P. Pouget, W. Riess, W. Bruetting & M. Schwoerer, J. Physique IV, Collogue C2, supplement au Journal de Physique I, 3, 137 (1993). Th. Schimmel, B. Koch, H.P. Geserich & M. Schwoerer, Synth. Metals 33, 311 (1989). D.C. Johnston, Phys. Rev. Lett. 52,2049 (1984). D.C. Johnston, J.P. Stokes & R.A. Klemm, J. Magn. Magn. Mat. 54-57, 1317 (1986). S. Ravy, Private communication. T.C. Chiang, A.H. Reddoch & D.F. Williams, J. Chem. Phys. 54,205l (1971). P.A. Lee, T.M. Rice & P.W. Anderson, Phys. Rev. Lett. 31, 462 (1973).

11. 12. 13. 14. 15. 16. 17.

IN (FA)*PF6

867

M.J. Rice, Phys. Rev. Lett. 37, 36 (1976). M.J. Rice, L. Pietronero & P. Bruesch, Solid State Commun. 21, 757 (1977). M.J. Rice, in Proc. Int. Conf on Quasi OneDimensional Conductors, Dubrovnik 1978. Springer Verlag, Berlin (1979). B. Horovitz, H. Gutfreund & M. Weger. Phys. Rev. B17, 2796 (1978). R. Bozio, C. Pecile & P. Tosi, J. Physique IV, Collogue C3, supplement au No. 6 44, C3-1453 (1983). W.A. Challener & P.L. Richards, Solid State Commun. 52, 117 (1984). A.D. Bruce & R.A. Cowley, Structural Phase Transitions, Taylor and Francis Ltd., London (1981).