POLYACETYLENE : LA CHASSE AUX SOLITONS

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POLYACETYLENE : LA CHASSE AUX SOLITONS

S. Roth, K. Ehinger, K. Menke, M. Peo, R. Schweizer

To cite this version:

S. Roth, K. Ehinger, K. Menke, M. Peo, R. Schweizer. POLYACETYLENE : LA CHASSE AUX SOLI- TONS. Journal de Physique Colloques, 1983, 44 (C3), pp.C3-69-C3-76. �10.1051/jphyscol:1983311�.

�jpa-00222619�

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Colloque C3, supplement au n06, Tome 44, juin 1983 _pageC3-69

POLYACETYLENE : LA CHASSE AUX SOLITONS

S. Roth, K . E h i n g e r , K. Menke, M. Peo and R.J. Schweizer

Max-Planck-Institut fiir Festkbrperforschung, HeisenbergstraBe 1, 0-7000 Stuttgart 80, F.R.G.

g s m 6

-

Des experiences Blectriques, magnetiques et optiques dans le polyacBtylSne sont recensees et des r6sultats nouveaux sont present&

concernant lqisom6risation cis-trans par dopage AsF (etude R m a n ) ;la coexistence des spins localls6s et itingrants (exp6r?ence RPE) mSme dans des 6chantillons dopBs doucement ; et la variation en temps (Bchelle des nanosecondes) de l'effet photoelectrique. Des arguments experimentaux pro- soliton sont critiqu6s et l'avis est exprim6 qu'il manque des Bvidences directes de l'existence des solitons mobiles dans le polyac6tylSne.

Abstract - Electric, magnetic and optical experiments on polyacetylene are reviewed and new h t a are presented concerning Raman studies of the AsF5- induced isomerisation of cis samples, ESR results implying the coexistence of localized and itinerant spins even in slowly doped films at all doping levels, and time-resolved photoresponse of trans-polyacetylene in the nsec regime. It is argued that at present there is still very little direct experimental evidence for soliton motion in polyacetylene.

Introduction:

Since several years it has been noticed that a perfect polyene chain is a tempting model to illustrate some field-theoretical phenomena and to carry out gedankenex- perimente of cosmological importance / I / . Schrieffer, Su, and Heeger /2/ and Rice /3/ have independently and nearly simultaneously shown that excitations in such a chain would have soliton-like properties. In this context a "soliton" is a bond- alternation defect as shown in Fig. 1: a domain-wall separating -regions from regions. Other authors might prefer to call this soliton "free radical", "dangling bond" or "Pople-Walmsley defect" / 4 / .

(neutral) soliton in trans-(CHIx

As a nonbonding electron this defect will carry a magnetic moment and give rise to an ESR signal. If the polyene chain is doped (oxidized) this dangling electron is likely to be the first one to be removed. Then there still remains a domain-wall, but now it carries a charge and no spin. The separation of electric and magnetic properties is one of the fascinating features of solitons in a polyene chain /5/.

Other idealized solid state systems have been proposed, where solitons are even more spectacular and carry fractional charges /6/.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983311

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C3-70 JOURNAL DE PHYSIQUE

During the l a s t ten years about two and a half thousand papers have appeared on s o l i t o n s , f i v e hundred on polyacetylene and a hundred on s o l i t o n s i n polyacetylene

(Fig. 2). There is l i t t l e s u r p r i s e about the t h e o r e t i t i a n s ' and experimentalists' hope t h a t s o l i t o n s would be not only a beautiful mental construct but a l s o e n t i t i e s somehow r e l a t e d t o physical observation and thus r e s i s t a n t t o Ockham's razor /7/.

I t i s the tendency of t h i s report t o point out t h a t up t o now s o l i t o n s i n polyacetylene have p r e t t y well managed t o escape d i r e c t detection and t h a t most evi- dences of t h e i r existence could a l s o be interpreted i n a non-solitonic way. This s t a t e of a f f a i r s w i l l be found t o be reminiscent of the well-kown French folk song

"La chasse a m papillons" where what is chased turns out t o be something other than b u t t e r f l i e s /8/. Of course, t h i s shortcoming has t o be blamed t o the demiurge, who has created the material world a s a but imperfect copy of the pure ideas, a s de- monstrated by previous authors / 9 / . The relevant aspects of the s t a t e d imperfection a r e the f i n i t e length of r e a l polyene chains and the existence of various defects, which a c t a s s o l i t o n traps.

PHYSICS ABSTRACTS 1.12.1982 I

n 727374E%?7X379%I81 Q YEAR

1

-

2 Intra

-

^Chain

2

-

³ ^Inter

-

^Chain

3

-

⁴ ^Inter

-

^Fibre

-

1

-

⁴ Superposition of above

Fig. 2: Last decade's topic-relevant Fig. 3: A ~ t i s t ' s view of a magnified s c i e n t i f i c output p i c t u r e of polyacetylene

E l e c t r i c Conductivity:

-

Fig, 3 shows an a r t i s t ' s view of a magnified picture of "real" polyacetylene. Chains of f i n i t e lengths a r e somehow bundeled t o form f i b r e s . Hypothetical subminiature robes a r e inserted t o measure the e l e c t r i c resistance: 1 and 2 along a chain seg- ment, 2 and 3 between chains and 3 and 4 between f i b r e s . I t i s reaso&ble t o expect a t l e a s t three d i f f e r e n t transport mechanisms: intra-chain, inter-chain, and i n t e r - f i b r e . Ifeasurements - even a t high frequencies - can only be carried out between prcbes 1 and 4, so t h a t there w i l l be a superposition of a l l mechanisms. I f the theory f o r each mechanism has e.g. two f r e e parameters, any s e t of experimental data can be f i t t e d .

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nism is dominant. In undoped trans polyacetylene the resistance between 2 and 3 might be the most important. For this case Kivelson has developed the theory of

intersoliton hopping /lo/: there are charged and neutral solitons, both free to move along their chain segments, but they cannot leave these segments. They can, however, exchange their charge: the charge hops from a charged soliton to a neutral one. Epstein has carried out measurements of the temperature and frequency dependence of the conductivity of undoped trans polyacetylene and finds remarkably good agreement with Kivelsons's theory /11/.

Very recently, Summerfield /12/ has worked out an alternative theory: general hopping between random sites (their energy density being open to a fit). This theory does not make specific use of solitons, the sites between which to hop could be just any impurity. Summerfield's theory, see and behold, fits Epsteins's data not worse than Kivelson's theory (Fig. 4)! Moreover, Smerfield's theory also fits the imaginary part of the conductivity, which has become available since Chroboczek's measurement, some weeks ago /13/, and which is less consistent with intersoliton hopping.

The conductivity of doped samples can be explained by several non-solitonic mechanisms, amongst others fluctuation-induced tunneling between metallic particles /14/.

Apparently the claim of having observed solitons cannot be based on conductivity measurements.

FREQUENCY (Hz)

Fig. 4 : Summerfield's general -- hopping theory fitted

to Epstein conductivity data

Pauli

-

^Suscept.

doped into cis

'[ZZ

¹ ^cis~trans

\

0 -

0 5 lo 15

DOPING CONC. (at %I

Fig. 5: Correlation of steps when doped into cis polyacetylene

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Magnetic Properties:

I n undoped trans polyacetylene an ESR signal is seen, corresponding t o several hundred oum of f r e e electron spins. The l i n e appears t o be motionally narrowed and t h e G A i s an Overhauser e f f k c t /lS/. Are these spins s o l i t o n s ? Why not ? But the j u s t i f i c a t i o n of c a l l i n g them so l i e s e n t i r e l y i n our knowledge of the dy- namics of a polyene chain: mobility along the chain seems t o imply solitonhood.

If our phantasy were wide enough t o imagine other spin c a r r i e r s we would be l e s s sure!

Doping reduces the number of f r e e spins i n polyacetylene /16/, consistent with the above mentioned concept of converting neutral s o l i t o n s i n t o charged but spinless ones. Admittedly t h i s i s an evidence f o r s o l i t o n s , but a very i n d i r e c t one: it is always i n s a t i s f a c t o r y t o deduce the existence of something from the non-observation of something e l s e . What, i f doping somehow were t o produce such a crazy band struc- t u r e t h a t most electrons went i n t o band s t a t e s or doubly occupied localized s t a t e s below the Fenni l e v e l ? Even without the help of s o l i t o n s most of the paramagnetism would vanish !

I n 1980 Peo e t a l . have reported a t various spring meetings /17/ t h a t i n polypara- phenylene and a l s o i n polyacetylene no Pauli s u s c e p t i b i l i t y could be observed up t o a c e r t a i n doping l e v e l . These r e s u l t s were reproduced by several groups /18/, and contradicted by others /19/. In some cases a sudden r i s e of the Pauli s u s c e p t i b i l i - t y was found a t a c r i t i c a l doping concentration. This s t e p could be correlated with a similar behaviour i n the NMR l i n e s h i f t and l i n e width /20/ and was interpreted a s insulator-to-metal t r a n s i t i o n due t o the overlap of s o l i t o n s i n trans polyacetylene.

A t t h a t time, a t l e a s t a l l the r e s u l t s of our group were obtained on samples o r i g i - n a l l y i n the c i s modification. I t was believed t h a t even moderate doping completely converted c i s i n t o trans. The P&R investigation, however, has shown t h a t - up t o the c r i t i c a l concentration - a l a r g e f r a c t i o n of the samples remained i n the c i s configuration. Subsequent work of Clarke and Scott /21/ has confirmed t h i s view even i f there remains some questions on the amount of the cis-to-trans conversions. Re- cent Raman measurements /22/ of various concentrations of AsF5 doped i n t o c i s polyacetylene show a pronounced s t e p a t the same c r i t i c a l concentration, when the r a t i o of c i s and t r a n s Raman l i n e s i s plotted. Fig. 5 shows the c o r r e l a t i o n of these s t e p s i n s u s c e p t i b i l i t y , N M R l i n e s h i f t , and Raman i n t e n s i t y . Therefore it i s rea- sonable t o assume s t r u c t u r a l changes a t Peo's Step rather than s o l i t o n overlap.

This summer, together with D. Davidov, we have been involved i n new ESR studies /23/.

Absolute s u s c e p t i b i l i t y measurements a r e very d i f f i c u l t by the ESR technique because of spindepth problems. So the existence of the above s t e p could not y e t be checked by t h i s method (Fig. 6 ) . In a l l samples, however, both i n c i s and i n trans (thermal- l y converted before doping), two ESR l i n e s a r e seen: a broad one with l i t t l e motio- nal narrowing, Curie-law temperature dependence and long relaxation time; the other narrow, f a s t relaxing and with temperature-independent i n t e n s i t y (Pauli suscepti- b i l i t y ) . So we conclude t h a t i n doped polyacetylene there is "always" coexistence of i t i n e r a n t electrons i n extended band s t a t e s and of localized spins. This does not necessarily imply the formation of metallic islands i n an otherwise undoped matrix

(Tomkiewicz's "Thousand Islands" hypothesis /19/), but it reveals inhomogeneities a t l e a s t on a microscopic l e v e l , e.g. due t o chain length d i s t r i b u t i o n and random c l u s t e r s of the dopant.

I f , i n s p i t e of the above c r i t i c i s m of i n d i r e c t proofs of existence, magnetic measurements could s a t i s f a c t o r i l y corroborate the s o l i t o n concept, a l l evidence would very l i k e l y be masked by these inhomogeneities

-

a t l e a s t up t o the present day t h i s seems t o have widely occured.

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(CH.yAsF5), XPQUli

"

_e^cis_trans

t

DOP. CONC. 1 y 1

Fig. 6: Coexistence of localized and i t i n e r a n t spins i n polyacetylene

Fig. 7: Nanosecond photoresponse of undoped trans polyacetylene

(room temperature)

Optical Behaviour:

Optical investigations a r e expected t o probe very local properties and so they a r e - ~ e r h a ~ s

-

most h e l ~ f u l i n the search f o r solitons in. alas. imperfect polyacety- lgne. usually the mihgap absorption /24/ and soliton-iAduced'infkared vi6rationai modes predicted by Mele and Rice /25/ a r e considered a s signature of solitons. Both features have been observed t o occur upon doping /25, 26/. Because it was suspected, however, t h a t immobile chemical defects might lead to similar optical properties as solitons would, attempts have been made t o look f o r optical e f f e c t s a f t e r a presumed photogeneration of solitons. Photogeneration of solitons in polyacetylene has been proposed by Su and Schrieffer /27/ and used by Etemad t o explain h i s finding of photoconductivity i n trans but not i n c i s samples /28/. Kiess e t a l . /29/ have t r i e d t o see the Mele-Rice l i n e during photoconductivity. They failed, however, t o obser- ve it, although t h e i r l i g h t intensity was high enough t o produce more photoconduc- t i v i t y than the doping-induced conductivitiy i n a p a r a l l e l experiment, where the l i n e had been seen. We have repeated Kiess' experiment with a pulsed laser using even three orders of magnitude more intensity and s t i l l did not see any Mele-Rice l i n e !

Therefore we decided t o do a time-resolved photoconductivity measurement t o get an estimate of the lifetime of photogenerated solitons. Fig. 7 shows the response of undoped trans polyacetylene t o l a s e r pulses of about 2 nsec (nitrogen pumped dye l a s e r , hv " 2 eV)/30/. As i s clearly seen, there i s no photoresponse longer than the 2 nsec l a s e r pulse (except f o r a heating t a i l a t high i n t e n s i t i e s ) . From the pulse height and the electrode separation the migration distance of the photogenerated c a r r i e r s (Schubweg] 4can be obtained. Assuming unity (! ) quantum e flclency we

5 . -

thus obtain 117

-

⁸^x10 cm/V. If the mobility 1, i s guessed t o be 1 /Vsec (from conductivity and doping concentration /31/), we would get ^T

-

8 x 10- sec !

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This value f o r T i s extremely short, but it i s not a t a l l unreasonable. The l a s e r pulse w i l l preliminarily excite electrons and holes. These should relax-jgto s o l i - tons and a n t i s o l i t o n s within the time of a l a t t i c e vibration, i . e . 10 sec.

Most of the electrons and holes could recombine immediately, before they have a chance t o get separated by the f i e l d and before they r e l a x i n t o solitons. A few, however, do escape ^{( T}i s an average value), they w i l l migrate t o chain ends o r other traps, where they c e r t a i n l y w i l l a r r i v e within a nanosecond (before the l a s e r pulse is over). P a r t of t h i s migration the charge c a r r i e r s might carry out i n the form of s o l i t o n s , but it w i l l be very d i f f i c u l t t o "prove" t h i s . From the time on when the charge c a r r i e r s s i t on chain ends there is no more need t o c a l l them s o l i t o n s : they a r e j u s t trapped charges. They w i l l g e t slowly released from t h e i r traps and i n constant trap-and-release cycles give r i s e t o long-time photoconduc- t i v i t y a s measured by Etemad e t a l . /32/. The gap s t a t e i s c e r t a i n l y not s e n s i t i v e t o whether charge c a r r i e r s migrate as s o l i t o n s o r whether they s i t a s electrons on chain ends. Therefore the electron-energy-loss experiment of Salaneck /33/ and the absorption measurements of Orenstein and Baker /34/ do not r e a l l y reveal the existence of s o l i t o n s even though the c a r r i e r s have been photogenerated. The bleach- ing experiment of Vardeny e t a l . / 3 5 / does not indicate s o l i t o n motion i n the sense used here e i t h e r : it j u s t t e l l s t h a t dipoles diffuse. Therefore it is s a f e t o s t a t e t h a t the optical evidence f o r solitons i n polyacetylene is not y e t over- helming.

Discussion:

During the discussion a t the end of the o r a l presentation of t h i s r e p o r t A. Heeger drew the a t t e n t i o n of the audience t o a poster of Vardeny e t a l . / 3 6 / , where the Mele-Rice l i n e is found i n a photoconductivity experiment (the e f f e c t s a t u r a t e s e a s i l y so t h a t the intense l i g h t i n Zurich and especially i n S t u t t g a r t did not f a c i l i t a t e observation). When I was willing t o admit t h i s important experimental r e s u l t as strong evidence f o r the existence of s o l i t o n s , B. Horovitz pointed out, t h a t the Mele-Rice l i n e i s not s p e c i f i c f o r s o l i t o n s : any charge on the chain w i l l induce the l i n e , including charges trapped on chain ends.

Conclusion:

Incorporating a l l the discussions i n Les Arcs i n which I have been involved, men- tioning Popper's /37/ caveat, t h a t a theory can never be proved but only disproved, and leaving open the possible usefulness of finite-polyene soliton-algorithms i n future bio-computers /38/, I s t i l l can terminate t h i s report with the statement t h a t there seems t o be a profou~ld difference between b u t t e r f l i e s and s o l i t o n s : both a r e very l i k e l y t o e x i s t (tertium comparationis), but b u t t e r f l i e s do not

only e x i s t , they a l s o have been observed i n a r a t h e r d i r e c t way (differenta specifica)

.

Acknowledgement:

We have greatly p r o f i t e d from cooperation with our summer v i s i t o r s D. Davidov, Y. Yacoby, and S. S m e r f i e l d , with J. Hocker and with the Vienna group of K.

Seeger. We want t o thank D. Baeriswyl, H. Bilz, K. Dransfeld, P. Fulde, M. Nehring and I-:. Six1 f o r many stimulating discussions and M. Schmid and Andrea-Maus Stark f o r t h e i r valuable help i n the laboratory. The support of t h i s work by the Stiftung Volkswagenwerk und by the Bundesministerium f u r Forschung und Technologie i s great- f u l l y acknowledged.

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,

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