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Pinning of charge density waves in irradiated TTF-TCNQ
~
Sol~d State Communmcat~ons, Voi.37, pp.405-408. Pergamon Press Ltd. ]98]. Printed ~n Great Britain.
PINNING OF CHARGE DENSITY UAVES IN IRRADIATED TTF-TCNQ S. BOUFFARD Centre
Sectlon d'Etude des Solmdes Irradl6s d'Etudes Nucl4amres de Fontenay-aux-Roses
(92260) - Frarce
R. CHIPAUX and D. ~EROME Laborato~re de Physique des Solmdes Unmvers~t4 Parms-Sud - 91405 Orsay - France K. BECHGAARD H.C. Oersted Instmtute - Unmvers~tetsparken DE-2100, Copenhagen, Denmark (Received
on Sept.
26, ]980, Revised version on Nov. Nov. ]9, ]980 by A. Bland~n)
5,
]3, ]980, Accepted
on
A smgnmf~eant fraction of the metallic conductmvmty of TTF-TCNQ ms due to fluctuatlng collect mve mode. Ue study the effets of mrradmatmon induced defects on the both components of the conductmv~ty : the fluctuatmng collectmve and the smngle-partmcle conductmvlt~es. We show that the blockm~@ effect of smngle-electrons ms a typmcal I-D effect and ms effectlve for concentratmons of about I~; and the p~nn~ng of charge densmty waves ~s a 3-D effect and occurs with concentratlons of an order of magnitude less.
The posslb111ty of electron transport in a collectlve way mn tetrathlafulvalene-tetracyanoqumnodlmethane (TTF-TCNQ) has strongly motlvated the development of the research effort. The exmstence of a slgnmfmcant collectlve contrlbutmon to the conductmon has been verlfmed through the possmbmllty of achmevmng commensurabmlmty under pressure [I, 2]. The phase dlagram of TTF-TCNQ obtalned from transport property measurements [3, 4] shows that above 15 kbar only one flrst order phase transltlon remains mn a 4 kbar range centered at 19 kbar. At 19 kbar, the transltlon temperature (Tc) reaches 74 K. The pesLkmng and I st order character of the transmtlon suggest a commensurabllmty between the charge densmty wave (C.D.W.) and the underlying lattlce due to a 2/3 charge transfer [3]. The longltudmnal conductmvmty versus pressure curves exhibit a large drop mn the metallic state (near the Pelerls transmtmon temperature) in the same pressure range [2 ]. Thms loss has been suggested as evmdence that a slgnmflcant fractlon of the metalllc conductlvmty mn uncommensurate TTF-TCNQ is due to a fluctuating C.D.W. collectmve transport mechanlsm. The commensurablllty (x3) provmdes a strong pmnnmng for the slmdlng C.D.W..The relatmve contrmbut~on of the smngle partzcle (o~ SP) and the cellectmve (o~ F) conductzvltles have been estlmated [2,3] from the behav~our of the transverse conduetmon ; in the uncommensurate state, these two oontrzbutmons are about s~m~lar at 150 K, but around 19 kbar, the slngle-part~cle conductmon dominates •n the total conductlvmty. Thins commensurab~imty phenomenon was also recently observed mn TSF-TCNQ near 7 kbar [5]. However, the enhancement of the maximum of
conductlvlty near 12 kbar is somewhat sample dependent [2]. One may argue that is a consequence of the fluctuatlon conductlvlty mechanlsm belng qulte sensmtlve to impurmtles or lattlee defects [6-10]. Thms effect was expected to become important at low temperature when the longltudlnal coherence l e ~ h t becomes comparable to the dlstance between impurltles. Untll now, thins C.D.W. plnnlng has been studled in the !norganlc low-dlmenslonal conductor NbSe 3 by conductlvlty frequency dependence and non-lmnear transport [11]. The most convenment way to mntroduce a controlled dlsorder mn organic compound is the irradmatlon, and more partmculary X-ray mrradlatmon. Although the nature of defects ms still unknown, the effects of irradlatlon induced defects are presently well understood [12-14]o We report an mnvestmgatlon of the plnnlng of the C .D.W. by X-ray Irradlatlon induced defects mn TTF-TCNQ. The room temperature irradmatmon was performed at Fontenay-aux-Roses by a eonventzonnal X-ray generator usm~@ a copper tube. The maxmmum dose (~ 2.105 kR~ntgens) corresponds to a 4 day irradmatlon. Fmgure I shows the dose dependence of the longltud~nal conductlvmty for two samples. The decrease of conductmvlty is approxlmatzvely 75 (Q.cm) -I ,but there is a small scatter in the results between dmfferent samples. The pressure experlments were carrled out mn the same manner as ~n reference 2. The pressure dependence of irradiated crystals at dlfferent temperatures is shown on flgure 2. The eonductmvmty of all the samples has been normalized to 400 (Q.cm) -I under a/nblent condition and before irradmatlon. The dotted llnes indlcate the conductmvlty of pure 405
406
PINNING OF CHARGE D E N S I T Y WAVES
0"/ (~.cm) -~
c
I
Vol. 37, No. 5
Tc (K) 70
4001 ..,,.
/ •
60 350 t
t*
I
ool
50 40.
250
I
0
10 ~ ~#(kRoentgen)
F~6ure 1 : L o n g i t u d i n a l conductmvlty as a f u n c t i o n of the m r r a d m a t m o n dose (two samples) at r o o m temperature. The u p p e r scale mnd~cates the calcul a t e d defect concentration.
O
~103(.Q cm) "1
3oL 0
110
p i (kbar)
rm6ure ~ : Phase d l a j r a m of i r r a d l a t e a 2TF-fCNO @ermved from the m a x i m u m of o(log p)/~(I/T). The results on pure sample are described by the dotted imae and by the stars (our r~sults). The olack dot shows the T c for a 8 HeV deuteron m r r a d l a t i o n at the same defect c o n c e n t r a t i o n
(ref. ~ ~).
0
1'0
'p (kbar)
Flgtare 2 : Pressure dependence of the lonsmtudznal conductlv~ty at constant temperature for mrradmated TTF-TCNQ. The various slgn c o r r e s p o n d to dmfferent samples. The stars mndicate our resulto on pure samples, and the dotted lines are the data for the "a" sample m~ ref. 2.
crystal at 10d K, 15u K an~ 29t K (samplp a in reference 2). All the sampleo we u s e d mn thls experiment come from the ~ame batch, and we can see that the behavior of the u n i r r a d l a t ~ d sampie (stars in f~gure 2 and 3) is ~u~te slm±lar to the best one i~ referenc~ [2]. The prlnclpal feature ~ m e r g m n s from t_~es~ data is the small decrease of th~ conductivity occurlng around 19 kbar by aoout 5C0 (Q.cm) -I i n s t e a d of 3200 (Q.cm) -I for the u n m r r a d m a t e d sample o b s e r v e d on tha ILO k c o n d u c t i v i t y versus pressure ourv~. At lov temperature, the irradiation effects ar~ more efficient in th~ uncom~ensurate stat~ (~ - 4560 (Q.cm) -I) t{~an In the commensurate one ~~ - 2000 ( Q . c m ) - ) . A satura-
tlon of the e o £ d u c t l v l t y ~an be oE,s~rve@ above a0 kbar mn all the temperature range. The phas~ transitions are brosCcned and snlfted to lower t e m p e r a t u r e s by m r r a d z a t m o n - m n d u c e d defects [I 5, 16],tflus the t r a n s i t i o n temperatures ar~ no longer w e l l defined. N e v e r t h e l e s s the phase diag r a m of irradiated ~amples deduced f r o m th~ ~la~ i m u m of o(log p)/e(I/~) sho,s at first, uelo~, 17 kbar a shmft of 8 K to,ards low temp~r~tur~ (~igure 3), and no peakmn~ ~ arounu 19 kbar. LJur results are coherent ~,zth the s~Iit m e a s u r e d at the same d~f~ct s o n c e n t r 4 t m o n ±o~ a 9 HeV d~ut ~ r o n i r r a d i a t i o n [I 5] (black ~Irclc in flturc
;). A~ ±al as t ~. d I<_t ~onc i_trat±on ¢ a l c u ± a tlol ms cons_rn~d c ~.av ~ u p i o ~ J that u ~ d c r ambi 11t ~onaltion_, t_~c i l u ~ u : t ± h 3 coil ~ t m v conductmvmiy is n ii mblc ~ c m i ~ r i tc th ±I~SO 'Jr maj b~ ~ d lmpl and ;, L 1 4 1 rrlod, 1 i t I , tly t , r o t o s ~ i for tL~ b o n d % ~ L ± v l t i ~ sol d i ~ old,~r d ,iuci~l-Oa~-,]irf~ i_ iofla] ~oL~tl.tor [~2]. ll!
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Vol
37, No. 5
PINNING OF CHARGE D E N S I T Y WAVES
@/~ 140
I
t
C
5~
407
(9" x103(~ cm) "1
2 0
2
4
kRoentg
0 0
,
,
,p
1'0
(kbar)
~10s
F16ure 4 : Dose dependence of the anlsotropy (o~ /ox). The llne shows the calculated variation from equation I.
Figure > : Pressure dependence of the total longltudlnal eonductlvmty of irradlated TTF-TCNQ (contlnuous imne) and of slngle-par~icle conduotlvlty of pure samples, as derlved from reference [3].
fit, and we flnd for the parameters
measured the pressure dependence of the transverse conductlvlty of irradiated samples, but we may compare the longltudlnal eonductlvlty of damaged crystals to the slr4~le-partzcle one of unlrradlated TTF-TCNQ. In flgure 5 we have dlsplayed the pressure dependence of the slngleparticle conductlvlty (derlvated from the behavlour of the transverse c~nductlvlty [3] and normalized to 400 (~.cm)- under amblent condltlons) together wmth the total eonductmvlty of irradlated samples. The above comparlson of the measured and calculated curves shows that below 17 kbar, the single-particle conduetmvlty P-dependence agrees well wlth the irradlated eonductlvzty results (at 100 K) :
:
~ = lo -6 ( ~ ) kRontgen and ~ - 2.104 K ; thus the maxlmum dose corresponds to 2.16 -3 molecular defects. Along a chaln, one over 500 molecules only ms affectCd by a defect , but in three dlmenslons the dmstance bet ~een damage molecules ms about 4.5 parameters. A defect or an Impurity in a quasi-one-dimensional conductor mnduces a spacmal oscmllatmon of the charge density around mt (namely, Frmedel oscillation [17]). The ,~avelen~ht of the oscillation ms ~/k F. Due to Coulomblc lnterchamn coupiing a long-ranged distortion is also induced in nemghbourlnj stacks [10]o In the strong pmnnlr~ case, the C.D.U. phase ms pmlmed to each of tL~se Frm~del oa~mllatmons, and ~-han the interd~fects dmztance and the C.D.W. correlation lenght become comparable, ~ve may expect a substantmal reluctmon of the conductivity related to t~,~ i~ictmon lees slldmng of fluctuating C.D.J.'s. I~ our experiment, ~e are very close to thm~ condition . th~ distance between damaged molecules me about 50 A and at 100, }[, the correistmon itm3ht is o0 ~ [18]. Since the smnglepartmcult conductivity ms only affected by the h z m n ~ m~i~tatmon [15] we may expect that the le~reas~ of th~ fluctuatmng collective conductlvmtg m~ lar.~r than the dabrease oi the singlelartmol~ ccnductmvmty for the present expermment~l ~ituatmon. Th~ couplmn ~ b£t.ecn irradmatmon lnltc6d d f~cts and C.D.L.'s ~hzch ms taken as a thr,~ -dlmenbmonal effect mn the present ~ ork, mu~t not b confused ~ t h the 3-D Coulomb couplln{" bet ~ en 7.D. 's of ne~gbbourlng chains h±eh occurs only in the very vicinity of the p~as~ transition m n a pure TTF-TCHQ crystal° It ha~ been proposed [19] that mn a 4uasmone-dlmen~monal conductor, the diffusive transvers~
o[£n o~.
SP)/dP =
O(In @/ irr)/dp =
.LI kbar -I .~5 kbar -I
These results show that the conductlvmty of fluctuatlng collective ormgln ms much more sensitmve to defects than the conductmon of classical origin. The blocking effect of single ~lectrons ms a typmcal I-D effect and becomes effectmve for concentratlons of I% [12]. On the contrary the pm~mmng of C.D.W. by charged impurities ms a 3-D effect and occurs with concentrations of an order of magnitude less. At temperature belo~ the phase transition the C.D.W. are also pinned on the lattlce and ordered mn three dlmensmons. The radiation induced defects disturb the coherence needed for the achlevement of the phase transltlon, consequently the phase transmtmons are smeared and shifted to lower temperatures. The analysms of the shape of tie damage curves mn the insulating state shots that the defect ms surrounded by a transformed volume of about 120 molecules [16]. Thms number whmch charactermbes the pmnnmng effmcmency of the defects zs mn agreement with the result of our experiment. ,~e have demonstrated that for ~.I0 -3 molecular defect, practically the whole coll~ctmve contrmbutmon ms removed from the total con~uctmvmty, fhe three-dzmensmonnal effect of irra-
PINNING OF CHARGE DENSITY WAVES
408
diatlon-mnduced d~fects provides a strong plllnlng of the sliding C.D.W.'s, whmle t le blngleparticle conductlvmty which is only ~noitive to the chain breaking due to on-chain defects remains hardly effeoted.
Vol. 37, No. 5
Ackno~,legments . ~e '~i~h to t~ank L. ~UPPIR(LI a~d H.J. SCHULZ for useful dmocu~olono and A. ~IDRImUX and C. os EDEK for tc~±~nical assistance.
REFEP~;I~,3ES
I
A. A~DRIEUX, H.J. SCIIULZ, D. JEROME and K. B E C H G A A R D Phys. Rev. Lett. 43, z27 (1979).
II
P.M. CHAIKI]], ~.J. FULLER, G. GRU]]ERo.A.P.S. 25, 3 (1980) March Meetlng in [e~J-kork.
2
A. ANDRIEUX, H.J. SCHULZ, D. JEROME and K. B E C H C A I R D J. Physique Lett. 40, L 585 (1979).
12
L. ZDPPIROLI, S. rUFFARD, K. LmCH]AARD, ~. filLTI and C.W. ~4AYER Phys. kev. (1989) in press.
3
D. J E R 0 ~ in The Physics s~'~d Chemlstry of Low Dimensional Solids - L. Alcacer (ed.), D. Reldel Publishing Comp. (1980) and D. O~tRO~S al~d H.J. SCHULL, Proceedings of Fribourg Conference (1980).
13
C. MII{ALY, S. 0UFEARD, L. ZUPPIROLI K. L F C H C A A R D J. Physique (198C) in preso.
14
C. F~IHALY, L. Z U P P I R j L I To be published.
15
C.K. CHIAI]G, M.J. CCKEt, P.R. NE~>£A]I and H.J. H E E C E R Phys. R~v. B I~ , 51t3 [1977).
It
L. LUPPIROLI and S. BOU~'FARDJ. Physl~ue 41, 291 (1980).
17
J. F R I E D E L Phil. Mag. 43, 153 (195~).
18
S.K. KI{A~]I~A, J.P. POUGET, R. C O ~ S , GARIT0 and A.J. HE~]GER Phys. Rev. ~ It, 14o8 (1977).
19
G. SODA, D. JEROI4E, £. ,fEgER, J.ALIZON, J. GALLICE, H. R0~ERT, J.M. FAzPS and L. G I R A L J. Physi,~ue 38, 931 [1977).
4
C. WEYL, P.M. CHAIKIII and D. JEROME To be published.
5
J.F. THOMAS, D. JEROME To be puolished in Solld State Cor~m.(1980).
6
P.A. LEE, T.M. RICE and P.W. AI'!DERS0I!Solld State Comm. 14, 705 (1974).
7
J.B. S O K O L O F F Solld State Comm.
16, 375 (1975).
M. FUKUYAMA
8
and P.A. LEE Phys. Rev. B 17, 535 (1978).
9
T. TSUZLrKI and K. SASAKI Solid State Coma~. 33, 1063 (1980).
10
T. TSUZUU~I and K. SASAKI Solld State Comm. 34, 219 (1980).
and
A.F.
Sol~d State Communmcat~ons, Voi.37, pp.405-408. Pergamon Press Ltd. ]98]. Printed ~n Great Britain.
PINNING OF CHARGE DENSITY UAVES IN IRRADIATED TTF-TCNQ S. BOUFFARD Centre
Sectlon d'Etude des Solmdes Irradl6s d'Etudes Nucl4amres de Fontenay-aux-Roses
(92260) - Frarce
R. CHIPAUX and D. ~EROME Laborato~re de Physique des Solmdes Unmvers~t4 Parms-Sud - 91405 Orsay - France K. BECHGAARD H.C. Oersted Instmtute - Unmvers~tetsparken DE-2100, Copenhagen, Denmark (Received
on Sept.
26, ]980, Revised version on Nov. Nov. ]9, ]980 by A. Bland~n)
5,
]3, ]980, Accepted
on
A smgnmf~eant fraction of the metallic conductmvmty of TTF-TCNQ ms due to fluctuatlng collect mve mode. Ue study the effets of mrradmatmon induced defects on the both components of the conductmv~ty : the fluctuatmng collectmve and the smngle-partmcle conductmvlt~es. We show that the blockm~@ effect of smngle-electrons ms a typmcal I-D effect and ms effectlve for concentratmons of about I~; and the p~nn~ng of charge densmty waves ~s a 3-D effect and occurs with concentratlons of an order of magnitude less.
The posslb111ty of electron transport in a collectlve way mn tetrathlafulvalene-tetracyanoqumnodlmethane (TTF-TCNQ) has strongly motlvated the development of the research effort. The exmstence of a slgnmfmcant collectlve contrlbutmon to the conductmon has been verlfmed through the possmbmllty of achmevmng commensurabmlmty under pressure [I, 2]. The phase dlagram of TTF-TCNQ obtalned from transport property measurements [3, 4] shows that above 15 kbar only one flrst order phase transltlon remains mn a 4 kbar range centered at 19 kbar. At 19 kbar, the transltlon temperature (Tc) reaches 74 K. The pesLkmng and I st order character of the transmtlon suggest a commensurabllmty between the charge densmty wave (C.D.W.) and the underlying lattlce due to a 2/3 charge transfer [3]. The longltudmnal conductmvmty versus pressure curves exhibit a large drop mn the metallic state (near the Pelerls transmtmon temperature) in the same pressure range [2 ]. Thms loss has been suggested as evmdence that a slgnmflcant fractlon of the metalllc conductlvmty mn uncommensurate TTF-TCNQ is due to a fluctuating C.D.W. collectmve transport mechanlsm. The commensurablllty (x3) provmdes a strong pmnnmng for the slmdlng C.D.W..The relatmve contrmbut~on of the smngle partzcle (o~ SP) and the cellectmve (o~ F) conductzvltles have been estlmated [2,3] from the behav~our of the transverse conduetmon ; in the uncommensurate state, these two oontrzbutmons are about s~m~lar at 150 K, but around 19 kbar, the slngle-part~cle conductmon dominates •n the total conductlvmty. Thins commensurab~imty phenomenon was also recently observed mn TSF-TCNQ near 7 kbar [5]. However, the enhancement of the maximum of
conductlvlty near 12 kbar is somewhat sample dependent [2]. One may argue that is a consequence of the fluctuatlon conductlvlty mechanlsm belng qulte sensmtlve to impurmtles or lattlee defects [6-10]. Thms effect was expected to become important at low temperature when the longltudlnal coherence l e ~ h t becomes comparable to the dlstance between impurltles. Untll now, thins C.D.W. plnnlng has been studled in the !norganlc low-dlmenslonal conductor NbSe 3 by conductlvlty frequency dependence and non-lmnear transport [11]. The most convenment way to mntroduce a controlled dlsorder mn organic compound is the irradmatlon, and more partmculary X-ray mrradlatmon. Although the nature of defects ms still unknown, the effects of irradlatlon induced defects are presently well understood [12-14]o We report an mnvestmgatlon of the plnnlng of the C .D.W. by X-ray Irradlatlon induced defects mn TTF-TCNQ. The room temperature irradmatmon was performed at Fontenay-aux-Roses by a eonventzonnal X-ray generator usm~@ a copper tube. The maxmmum dose (~ 2.105 kR~ntgens) corresponds to a 4 day irradmatlon. Fmgure I shows the dose dependence of the longltud~nal conductlvmty for two samples. The decrease of conductmvlty is approxlmatzvely 75 (Q.cm) -I ,but there is a small scatter in the results between dmfferent samples. The pressure experlments were carrled out mn the same manner as ~n reference 2. The pressure dependence of irradiated crystals at dlfferent temperatures is shown on flgure 2. The eonductmvmty of all the samples has been normalized to 400 (Q.cm) -I under a/nblent condition and before irradmatlon. The dotted llnes indlcate the conductmvlty of pure 405
406
PINNING OF CHARGE D E N S I T Y WAVES
0"/ (~.cm) -~
c
I
Vol. 37, No. 5
Tc (K) 70
4001 ..,,.
/ •
60 350 t
t*
I
ool
50 40.
250
I
0
10 ~ ~#(kRoentgen)
F~6ure 1 : L o n g i t u d i n a l conductmvlty as a f u n c t i o n of the m r r a d m a t m o n dose (two samples) at r o o m temperature. The u p p e r scale mnd~cates the calcul a t e d defect concentration.
O
~103(.Q cm) "1
3oL 0
110
p i (kbar)
rm6ure ~ : Phase d l a j r a m of i r r a d l a t e a 2TF-fCNO @ermved from the m a x i m u m of o(log p)/~(I/T). The results on pure sample are described by the dotted imae and by the stars (our r~sults). The olack dot shows the T c for a 8 HeV deuteron m r r a d l a t i o n at the same defect c o n c e n t r a t i o n
(ref. ~ ~).
0
1'0
'p (kbar)
Flgtare 2 : Pressure dependence of the lonsmtudznal conductlv~ty at constant temperature for mrradmated TTF-TCNQ. The various slgn c o r r e s p o n d to dmfferent samples. The stars mndicate our resulto on pure samples, and the dotted lines are the data for the "a" sample m~ ref. 2.
crystal at 10d K, 15u K an~ 29t K (samplp a in reference 2). All the sampleo we u s e d mn thls experiment come from the ~ame batch, and we can see that the behavior of the u n i r r a d l a t ~ d sampie (stars in f~gure 2 and 3) is ~u~te slm±lar to the best one i~ referenc~ [2]. The prlnclpal feature ~ m e r g m n s from t_~es~ data is the small decrease of th~ conductivity occurlng around 19 kbar by aoout 5C0 (Q.cm) -I i n s t e a d of 3200 (Q.cm) -I for the u n m r r a d m a t e d sample o b s e r v e d on tha ILO k c o n d u c t i v i t y versus pressure ourv~. At lov temperature, the irradiation effects ar~ more efficient in th~ uncom~ensurate stat~ (~ - 4560 (Q.cm) -I) t{~an In the commensurate one ~~ - 2000 ( Q . c m ) - ) . A satura-
tlon of the e o £ d u c t l v l t y ~an be oE,s~rve@ above a0 kbar mn all the temperature range. The phas~ transitions are brosCcned and snlfted to lower t e m p e r a t u r e s by m r r a d z a t m o n - m n d u c e d defects [I 5, 16],tflus the t r a n s i t i o n temperatures ar~ no longer w e l l defined. N e v e r t h e l e s s the phase diag r a m of irradiated ~amples deduced f r o m th~ ~la~ i m u m of o(log p)/e(I/~) sho,s at first, uelo~, 17 kbar a shmft of 8 K to,ards low temp~r~tur~ (~igure 3), and no peakmn~ ~ arounu 19 kbar. LJur results are coherent ~,zth the s~Iit m e a s u r e d at the same d~f~ct s o n c e n t r 4 t m o n ±o~ a 9 HeV d~ut ~ r o n i r r a d i a t i o n [I 5] (black ~Irclc in flturc
;). A~ ±al as t ~. d I<_t ~onc i_trat±on ¢ a l c u ± a tlol ms cons_rn~d c ~.av ~ u p i o ~ J that u ~ d c r ambi 11t ~onaltion_, t_~c i l u ~ u : t ± h 3 coil ~ t m v conductmvmiy is n ii mblc ~ c m i ~ r i tc th ±I~SO 'Jr maj b~ ~ d lmpl and ;, L 1 4 1 rrlod, 1 i t I , tly t , r o t o s ~ i for tL~ b o n d % ~ L ± v l t i ~ sol d i ~ old,~r d ,iuci~l-Oa~-,]irf~ i_ iofla] ~oL~tl.tor [~2]. ll!
% I~l~
irradla~lOh
k
'~'¢N(
r%
LOIS"
b
r
£[<('~)
fo=lo
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in
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, (t.[1)
-- } [ [ 0 , ~ ) i ' 4,i ~ ~J-Z± ~ o t l k ) l' o t)l r[lr '~lj~ b ~ l - {~ zh~ ~oi~ _fiat lelc~ t ~ )no _,tratlon r l ~ t d to tm irraJlnt±o~ ~ os,~ [_) bj
- ~
in iI]ur
i~ th, tlan ~ I , hOpl411
6, th
oil< lln
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_it~ th~~ b,~t
Vol
37, No. 5
PINNING OF CHARGE D E N S I T Y WAVES
@/~ 140
I
t
C
5~
407
(9" x103(~ cm) "1
2 0
2
4
kRoentg
0 0
,
,
,p
1'0
(kbar)
~10s
F16ure 4 : Dose dependence of the anlsotropy (o~ /ox). The llne shows the calculated variation from equation I.
Figure > : Pressure dependence of the total longltudlnal eonductlvmty of irradlated TTF-TCNQ (contlnuous imne) and of slngle-par~icle conduotlvlty of pure samples, as derlved from reference [3].
fit, and we flnd for the parameters
measured the pressure dependence of the transverse conductlvlty of irradiated samples, but we may compare the longltudlnal eonductlvlty of damaged crystals to the slr4~le-partzcle one of unlrradlated TTF-TCNQ. In flgure 5 we have dlsplayed the pressure dependence of the slngleparticle conductlvlty (derlvated from the behavlour of the transverse c~nductlvlty [3] and normalized to 400 (~.cm)- under amblent condltlons) together wmth the total eonductmvlty of irradlated samples. The above comparlson of the measured and calculated curves shows that below 17 kbar, the single-particle conduetmvlty P-dependence agrees well wlth the irradlated eonductlvzty results (at 100 K) :
:
~ = lo -6 ( ~ ) kRontgen and ~ - 2.104 K ; thus the maxlmum dose corresponds to 2.16 -3 molecular defects. Along a chaln, one over 500 molecules only ms affectCd by a defect , but in three dlmenslons the dmstance bet ~een damage molecules ms about 4.5 parameters. A defect or an Impurity in a quasi-one-dimensional conductor mnduces a spacmal oscmllatmon of the charge density around mt (namely, Frmedel oscillation [17]). The ,~avelen~ht of the oscillation ms ~/k F. Due to Coulomblc lnterchamn coupiing a long-ranged distortion is also induced in nemghbourlnj stacks [10]o In the strong pmnnlr~ case, the C.D.U. phase ms pmlmed to each of tL~se Frm~del oa~mllatmons, and ~-han the interd~fects dmztance and the C.D.W. correlation lenght become comparable, ~ve may expect a substantmal reluctmon of the conductivity related to t~,~ i~ictmon lees slldmng of fluctuating C.D.J.'s. I~ our experiment, ~e are very close to thm~ condition . th~ distance between damaged molecules me about 50 A and at 100, }[, the correistmon itm3ht is o0 ~ [18]. Since the smnglepartmcult conductivity ms only affected by the h z m n ~ m~i~tatmon [15] we may expect that the le~reas~ of th~ fluctuatmng collective conductlvmtg m~ lar.~r than the dabrease oi the singlelartmol~ ccnductmvmty for the present expermment~l ~ituatmon. Th~ couplmn ~ b£t.ecn irradmatmon lnltc6d d f~cts and C.D.L.'s ~hzch ms taken as a thr,~ -dlmenbmonal effect mn the present ~ ork, mu~t not b confused ~ t h the 3-D Coulomb couplln{" bet ~ en 7.D. 's of ne~gbbourlng chains h±eh occurs only in the very vicinity of the p~as~ transition m n a pure TTF-TCHQ crystal° It ha~ been proposed [19] that mn a 4uasmone-dlmen~monal conductor, the diffusive transvers~
o[£n o~.
SP)/dP =
O(In @/ irr)/dp =
.LI kbar -I .~5 kbar -I
These results show that the conductlvmty of fluctuatlng collective ormgln ms much more sensitmve to defects than the conductmon of classical origin. The blocking effect of single ~lectrons ms a typmcal I-D effect and becomes effectmve for concentratlons of I% [12]. On the contrary the pm~mmng of C.D.W. by charged impurities ms a 3-D effect and occurs with concentrations of an order of magnitude less. At temperature belo~ the phase transition the C.D.W. are also pinned on the lattlce and ordered mn three dlmensmons. The radiation induced defects disturb the coherence needed for the achlevement of the phase transltlon, consequently the phase transmtmons are smeared and shifted to lower temperatures. The analysms of the shape of tie damage curves mn the insulating state shots that the defect ms surrounded by a transformed volume of about 120 molecules [16]. Thms number whmch charactermbes the pmnnmng effmcmency of the defects zs mn agreement with the result of our experiment. ,~e have demonstrated that for ~.I0 -3 molecular defect, practically the whole coll~ctmve contrmbutmon ms removed from the total con~uctmvmty, fhe three-dzmensmonnal effect of irra-
PINNING OF CHARGE DENSITY WAVES
408
diatlon-mnduced d~fects provides a strong plllnlng of the sliding C.D.W.'s, whmle t le blngleparticle conductlvmty which is only ~noitive to the chain breaking due to on-chain defects remains hardly effeoted.
Vol. 37, No. 5
Ackno~,legments . ~e '~i~h to t~ank L. ~UPPIR(LI a~d H.J. SCHULZ for useful dmocu~olono and A. ~IDRImUX and C. os EDEK for tc~±~nical assistance.
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