C60 composite doped by alkali metals low-field microwave absorption and SQUID study

PHYSICA® ELSEVIER

Physica C 264 (1996) 161-171

Multiphase superconductivity in OO-PPV/C60 composite doped by alkali metals Low-field microwave absorption and SQUID study Katsumi Yoshino a, *, Anvar A. Zakhidov a,1 Hirotake Kajii a, Hisashi Araki a Kazuya Tada a, Takanobu Noguchi b, Toshihiro Ohnishi b, Kyuya Yakushi c a Department of Electronic Engineering, Faculty of Engineering, Osaka University, Yamada-Oka, Suita, Osaka, 565 Japan b Tsukuba Research Laboratories, Sumitomo Chemical Co. Ltd., 6 Kitahara, Tsukuba, Ibaraki 300-32, Japan ¢ Institute for Molecular Science, Myodaiji, Okazaki, 444, Japan Received 26 February 1996

Abstract

Superconducting (SC) phases in poly(2,5-dioctyloxy-p-phenylene vinylene) (OO-PPV) composite with fullerene C6o are found upon alkali-metal A ( ffi K and Rb) vapor doping by sensitive low-field microwave absorption (LFMA) and proved by SQUID. LFMA of OO-PPV(C6o)yK x at small y < 0.005 shows a "normal out-of-phase" shape, and appears at Tc ffi 11 K. At larger C6o content 0.01 < y < 0.1, the LFMA revealed unusual multipeak spectra, with two components appearing at different Tc's. At T~l ffi 12-13 K for K dopant (and Tel ffi 21.5 K for Rb) the broad component appears with anomalous phase, which we assign to SC A3C6o grains weakly linked into Josephson media, containing ~r junctions. By SQUID measurements the paramagnetic Meissner effect has been observed, supporting the existence of ~ junctions. Below Tc2 = 6.5-8.5 K (for K) and T¢2 ffi 11 K (for Rb) the narrow component with a normal phase and hysteresis appeared in the LFMA spectra. The structure found by SQUID measurements at the same To2 supports this assignment to a second SC 2 phase. One possible origin of SC 2 is discussed in terms of nonstoichiometric AxC60 grains (with x :g 3), and C60 induced superconductivity, in which electrons of conducting polymer chains are actively involved in SC pairing via hybridization with C6o may be another origin for SC 2. Alternative explanations for multipeaked LFMA are also discussed.

I. Introduction

C60 is well known to exhibit superconductivity upon alkali [1,2] and alkali-earth [3] metal doping, which accelerated extensive studies of various dop-

* Corresponding author. Fax: +81 6 877 3542. IOn leave from Department of Thermophysics, Uzbekistan Academy of Sciences, Katartal 28, Chilanzar C, Tashkent, 700135, Republic Uzbekistan.

ing effects into fullerite. Solid C60 normally has been considered as a host for intercalation dopants. On the other hand conducting polymers (CP's) like polythiophene and polyphenylene vinylene have attracted attention from both fundamental and practical view points since they undergo an insulator-metal transition and exhibit many other interesting electronic and optical properties upon doping by acceptors (p-type) such as iodine, BF4- and C10~- and also donors (n-type) such as Na, K or T B A + [4]. Super-

0921-4534/96/$15.00 Copyright © 1996 Published by Elsevier Science B.V. All rights reserved PII S0921 - 4 5 3 4 ( 9 6 ) 0 0 2 3 I-6

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K. Yoshino et al./ Physica C 264 (1996) 161-171

conductivity was predicted in CP theoretically at moderate doping levels, x ~ 0.6 (just at the border of the insulator-metal transition) [5], but experimentally has never been observed. We proposed to use fullerene molecules C60 and C70 themselves as dopants to CP's [6-11] and found the quenching of photoluminescence [6,7], enhancement of photoconductivity [8,9] and some other photophysical phenomena, including persistent photoconductivity [10] and electroluminescence quenching [11] in such CP-C60 composites, due to photoinduced charge transfer (PCT). Ultrafast PCT has been found independently by complementary methods of photoinduced absorption [12] and light-induced ESR

[13]. Recently we have initiated the doping of CP//C60 composites by alkali metals (A), like K and Rb aiming at the search for possible superconducting (SC) phases [14-18]. The ground-state charge transfer from A dopants should create a novel type of ~r electron system in which charged C6~0- anions should be coexisting (ideally) with the sea of mobile charges on CP chains and the associated rr electron network. Optical and conductive properties of such a system are very intriguing, while at low temperatures SC phases can be expected. We have found indeed that in the poly(3-alkyltiophene) (PAT 12) composite with C60 the SC phase appears at optimal K vapor doping [14,15] and from the LFMA behavior and features in SQUID the SC phase of PAT12(C60)yKx at large C60 content y > 0.025 was assigned to K 3 C 6 0 grains weakly coupled by Josephson junctions [15]. However, at small y < 0.005 the origin of the SC phase was not so clear due to a distinct LFMA behavior. It has been suggested that in such composites a novel two-component C60 induced SC phase may exist [15,16], in which Cooper pairs may reside not only on C60 but also on CP chains due to strong ~ - r r overlapping hybridization, which has in fact been proved by previous PCT studies [6-13]. C60 may play the role of negative U centers, and thus induce the SC pairing in the CP host network, provided the latter is in a conductive state and resonating with electronic levels of C60 [15,18], in the spirit of

two-component SC models developed recently for HTSC ceramics [19-21]. Our studies of conductivity and ESR of the PAT12(C60)yK ~ composite [14,17] have shown that the PAT12 matrix is actually lightly doped (semiconductive) or even in undoped (insulating) states, since due to the strong electron affinity C60 clusters behave as "electronic sponges", absorbing all K dopants from the CP host and withdrawing electrons from K to their LUMO levels. We suggested to use the CP matrix which is more n-type dopable (i.e. it has a better ability to withdraw electrons than PAT12) as a host for C60 composite, with the aim to achieve effective A doping of both CP chains and C60, thus creating favorable conditions for C60 induced SC phases [16,17,19]. Poly(2,5-dioctyl-oxy-p-phenylene vinylene) (OOPPV) is known to possess a larger electron affinity than PATI2 [7,9,22], and our previous studies of the OO-PPV(C60)y composite, such as stronger photoluminescence quenching [7] and a much more pronounced effect of photoconductivity enhancement [9], have demonstrated a much larger PCT compared to (the other CP)/C60 composites. In the present paper we study such an OOPPV(C60)y composite upon n-type doping, namely by K and Rb emphasizing the search for the novel SC phases using L F M A / E S R combined spectroscopy [23,24] and SQUID magnetometry. It is found that at low y = 0.005 the LFMA spectra is of normal type, similar to the case of PAT12 matrices, but already at the moderate value y = 0.01 the LFMA spectra becomes of multicomponent type, showing an anomalous phase (AP) broad wing below T~l, and revealing a central narrow (CN) peak below a certain temperature T * < Tel. This behavior is interpreted as due to the existence of two spatially separated SC phases with Tel and Tc2 = T*, the origins of which are discussed below.

2. Experimental Conducting polymer OO-PPV, prepared and purified by the method already reported [9,22], is soluble

Fig. 1. DerivativeLFMA( d P / d H ) spectra of SC compositeOO-PPV(C6o)yKx at different T below Tc for (a) small y = 0.005 and (b) moderate y = 0.01. The shapes of direct absorption P(H) for those spectra are shown in the upper part for (a) and in (c) for (b).

K. Yoshino et al./ Physica C 264 (1996) 161-171

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K. Yoshino et a l . / Physica C 264 (1996) 161-171

in conventional solvents such as toluene and chloroform. C60 prepared by an arc discharge utilizing graphite as an electrode and washed with toluene, and provided by Science Laboratory Co. Ltd., was used in this experiment. Both fullerene and conducting polymer of appropriate molar ratio were dissolved in toluene, and thin films were prepared by casting the solution on PTFE (teflon) seats. The free-standing films of OO-PPV(C60)y with various C60 contents ( y -- 0.005, 0.01, 0.05, 0.1 molar ratio) of the average weight of 15-20 mg were placed into 5 mm diameter ESR quarts tubes, on the other end of which metallic K or Rb was inserted. The tubes thus prepared were then heated in a double furnace, which enables us to control the temperatures of both the A ( = K, Rb) and OO-PPV(C60)y ends of the tube independently. The derivative MA measurements were carried out using a Varian E-112 and a Bruker ESR-300 spectrometer operating at 9.1-9.6 GHz with a field-modulation frequency of 100 kHz. DPPH was used as a marker of the g factor and a reference of the ESR intensity. An Oxford Instruments ESR900 cryostat allowed one to vary the temperature from 3 K to 300 K with +_0.2 K precision. Additionally equipped external DC Helmholz coils allowed H field scanning to be made through zero from - 5 0 to +50 G (see Refs. [14-18,24] on details of the LFMA method).

LFS), in fullerides has proved to be a unique method for the search and study of novel SC phases [1418,23-28]. This method has the highest sensitivity as compared to the resistivity measurements or even SQUID magnetometry of the SC state, and can detect negligibly small quantities of SC phases [23,24]. LFMA is known to have numerous advantages: (1) It can detect submicrogram quantities of SC phase in SC and non-SC matrices, quickly determine T~, and distinguish multiple T~ phase [23,30] contactlessly and in a high vacuum, which is important for air-sensitive fullerides [25-28]. So we have found recently by LFMA the new SC in a ternary compound of LiENa(Ny)C60 [25]. (2) It may check the evolution of SC fractions upon various treatments, like annealing [26] or alloying [24b),271. (3) It can provide information about the quality of the SC state (granular nature, glassy behavior, etc. [23]), which is particularly important in our case of a composite structure [14-18]. However, only the fact of LFMA appearing does not necessarily mean the SC phase occurs, if the correct hysteretic behavior is not proved. So LFMA without hysteresis may appear in CP [29], due to the negative magnetoresistance connected with spin selective polaron-bipolaron transformations, but this can be ruled out easily due to the distinct temperature dependence.

3. Results and discussion

The following strategy [24] for the SC search is used: the K doping has been performed step by step at doping temperatures Ta = 130/120°C, rather low compared to the typical TO for vapor A ( = K, Rb) doping of fullerides [24] (to avoid melting of the OO-PPVmatrix), and each step has been monitored by ESR study to analyze the appearance and evolution of the spins (and hence charge carriers) both on OO-PPV and C6o components of the composite, while at each step in the same ESR spectrometer the LFMA has been checked for SC phase existence. 3.1. Low-field microwave absorption (LFMA)

The measurement of nonresonant microwave absorption in low magnetic field (so called LFMA or

3.1.1. Low y The characteristic SC LFMA appeared upon optimal doping in dilute C60 composites with y = 0.005 (i.e. 0.5 mol%). Typical LFMA spectra at various temperatures below T~ are shown in Fig. 1 for OO-PPV(C60)yKx at optimal doping time (td). Note that at small y the hysteresis is negligible, probably due to the small SC volume fraction. If one cannot observe hysteresis of LFMA, there is always uncertainty whether this is really SC LFMA or not, since a rather similar type absorption, also without any hysteresis and with a different temperature dependence, may appear in CP's upon doping due to a distinct non-SC origin [29]. To be sure that LFMA is not misinterpreted, one

K. Yoshino et al. / Physica C 264 (1996) 161-171

should check the temperature dependence and additionally prove the SC phase existence by some independent method, like SQUID magnetometry. In our case LFMA abruptly appears at To, which is typical to LFMA of SC origin, contrary to the smooth exponential decrease characteristic for magnetoresistance type LFMA, found in poly(p-phenylene) [29]. The Tc found as LFMA onset changes only slightly upon further increase of td. Tc strongly decreased compared to Tc = 19 K in bulk K 3 C 6 0 and T~ slightly changes with increase of to for each y composition. The intensity of LFMA in composites is higher than the intensity of LFMA in bulk K 3 C 6 0 of the same mass of C60, which indicates that this intensive LFMA does not come from the bulk, but from the

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intergrain weak links which are known to contribute largely to the LFMA intensity [30-32] through the very low viscosity of fluxons in Josephson junctions. The inset of Fig. 1 shows that direct MA P(H) has the minimum at H = 0, increasing with [ H I, due to the larger density of vortices at higher H ( > Hc~ ), which corresponds to the normal "out-of-phase" d P / d H (derivative LFMA). 3.1.2. Medium and large y (0.01 < y < 0.1)

With a small increase of C6o content to y = 0.01, the shape of LFMA changes qualitatively as shown in Fig. 1(b): the LFMA spectra shows two coexisting components: a central peak (CN) which is narrow and has a normal "out-of-phase" shape of d P / d H

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contrary to the phase of conventional ESR (similar to that in Fig. 1) and a broad component which has an unusual "in-phase" shape, called here the anomalous phase (AP) peak. The CN component disappears at a certain T* = 6.9 K and at higher T only AP remains; this disappears at Tel -- 11.5 K. The appearance of such AP type signals in SC LFMA is rather unusual, since this behavior of dP/dH (which is positive for H < 0, and negative for H > 0) corresponds to a maximum in P(H) at H = 0 . The evolution of P(H) with T is schematically sketched in Fig. l(b). We already observed such an unusual LFMA in PATI2(C60)yKx composites [15-18] and explained the anomalous phase in terms of so called Josephson -rr junctions (see Refs. [36] and [37] and the discussion below). But what is really new in OO-PPV case, is the appearance of a narrow CN peak below T*, which is slightly different from T¢ of the low-y case of Fig. 1. Since there is still no hysteresis, one may hesitate on the SC origin of CN component LFMA. However, at larger y the similar two-component behavior is found (as shown in Fig. 2), which now shows a correct SC type "clockwise" hysteresis. This hysteresis is observed mainly in the CN part at y = 0.05, of Fig. 2(a), but at larger y = 0.1 hysteresis becomes very broad and shows up in the full spectra. Note that T*, at which CN appears, slightly changes with y, while T¢~ is nearly the same: Tel = 12-12.5 K. The temperature dependences of the two peaks of LFMA are plotted separately in Fig. 3. It can be seen that the AP component has a tendency to decrease at lower T for any y, and in general the relative intensity of AP increases with y. AP has a maximum at a certain H = H ; P, and this maximum shifts towards higher H with decrease of T (this behavior has been found also in PAT12 matrix case as well [15]), while CN always shows a narrow peak at the same HpCN= 0 . 8 - 1 G. The hysteresis of LFMA is well known to be dependent on the modulation amplitude (Hmod), it becomes smaller at large Hmod, and at small Hmo~, the hysteresis usually changes sign. The same behavior is found for y = 0.1 (shown in Fig. 4), proving again the clear SC origin of LFMA. We also checked the behavior of the LFMA intensity with Hmoo, and found it to be consistent with a typical SC behavior, as shown in the inset of Fig. 4. The critical magnetic

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field Hc~ of the Josephson media can be found from the plot of Fig. 4 and it is estimated as Hc] = 0.3-0.4 G.

3.2. Superconducting hysteresis of LFMA In the y = 0.1 composite doped by K, LFMA shows the typical normal-phase CN component at small H ( = 0) at low T and what is most important, it has a correct hysteresis which can be readily observed at low Hmod. Due to the larger SC volume fraction at larger y, the hysteresis can clearly be found and studied. Fig. 4 shows the hysteretic behavior of the normal-phase LFMA, which changes in a

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field (see Fig. 4). The amplitude of the hysteresis loop decreases with increase of Hint d. Josephson junctions allow the flux to enter at a very low effective Hc~ [23]. The change of the LFMA sign at the reverse sweep is related to the change of the critical state at the surface [23], i.e. to the pinning and depinning of fluxons during each modulation cycle. Since Hmw and Hintd are always applied to the sample in a cavity, the critical current Jcs already flows at the surface of the grains at these low fields and contributes to the MA. The surface critical current reverses with reversing the field sweep only after a certain field interval equal to twice the Josephson critical field [31] of 2nc] = ( 4 1 r / c ) A, Jcs,

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characteristic way with decreasing Hint ~, typically to that of granular superconductors with a contribution from the Josephson vortices. From this hysteresis dependence on Hint a, one can obtain the value of the critical field of the Josephson junction H ~ . At small Hmod < Hc] LFMA changes sign whenever there is a change in direction of the scanning DC magnetic

167

(1)

where Aj is the Josephson penetration depth. The same change of the critical current occurs over a modulation field cycle. Usually the dependence of LFMA intensity on Hmod exhibits two linear parts. These two regions involve two different contributions: the signal induced by the surface critical current for Hint a < Hc], and the signal caused by changing the fluxon density for nmod > 2 n c ] , where MA is proportional to the concentration of fluxons [35]. The minimum point between these two regions corresponds to Hint a = 2H¢]. On increasing the temperature, the minimum point shifts to lower field. In our case the two linear regions are not so clearly resolved, as seen in Fig. 4 which probably reflects the more complicated geometry of our granular SC phase. Although the minimum does not appear clearly, it still is possible to estimate the value of Hc] = 0.2-0.4 G, which is smaller than the similar quantity in HTSC. Since the flux is known to penetrate into the Josephson junction at the first critical field, derived as [32] He] = (~)0//(4TrALAj),

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the critical current Jc, can be estimated from Eqs. (3) and (4). Using Hc] = 0.3 G found experimentally and taking the representative AL = 200 nm, we can get Aj = 20 ixm, and Jc~ = 102 A/cm2. 3.3. SQUID magnetometry

Fig. 5 shows the typical DC magnetic susceptibility ( X ) of the optimally K doped y = 0.05 compos-

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K. Yoshino et a l . / Physica C 264 (1996) 161-171

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ite. The sample was cooled to 5 K in zero magnetic field and the magnetization was measured up to 20 K in an external field of H = 20 G (Fig. 5(a)) and H = 10 G (Fig. 5(b)). Measurements were repeated in a field-cooled (FC) run, and the hysteresis is clearly seen between ZFC and FC which is typical for type-II superconductors due to flux trapping in FC. Moreover the FC run clearly shows the paramagnetic Meissner effect (PME), i.e. paramagnetism occurs, which disappears at T~, instead of a diamagnetic response. It should be noted that T~ found at low H = 10 G is 12 K, which is the same as T~ determined by LFMA onset for AP LFMA in Fig. 3. From Fig. 5(a) one can already see that at low T the paramagnetic tail appears in the x(T) curve and that the SC transition is rather broad, typical for the multiphase case. Moreover, x(T) shows a type of a

cusp (both in ZFC and FC ran) around T* = 7 - 8 K at which the CN appeared in the LFMA spectra, and also reveals some systematic fluctuations around T = 16-17 K (i.e. close to Tc of bulk K 3 C 6 0 ) . The SC shielding fraction estimated by comparison with the ideal diamagnetic X0 = 1/4~r, corresponds to 0.008% of the whole sample. Taking into account that only C60 can give a SC phase we arrive at the value of 0.14% SC fraction relatively to C60 only, which is much lower than we observed in PAT12(C60)yK x composites. For y = 0.01, and 0.05 composites which showed clear SC phases with Tc = 12-13 K by LFMA method, we were not able to detect SC phase by SQUID magnetization, proving once again that LFMA is a much more sensitive method, since the fraction of SC phase is very low. Also it has been observed that the SQUID response is large if measured quickly after the doping procedure, and it disappears upon aging, indicating that slow diffusion processes of A atoms (at a moderate Td) result in a nonequilibrium distribution of A dopants just after doping.

4. D i s c u s s i o n

The above results on multipeaked LFMA and SQUID magnetization with structures can be understood as follows. The most reasonable explanation for the two LFMA peaks with different phases appearing at two different T's, is the existence of two spatially separated SC phases with different Tel and Tc2. Normally when a new peak appears in LFMA spectra at T different from Tc, it has been interpreted as a new SC phase [39,40], and this helped to find coexisting phases in several HTSC ceramics [39]. However, it is more difficult to assign each of those SC phases. It is clear that the SC phase at Tel = 11-12 K is connected with SC K3C60 grains. Imperfections disorder and defects within each grain decrease Tc, as was observed in granular thin films of A3C60 and in LB films of K3C60 [28], in which Tc is to a larger extent suppressed (T~ = 8.5 K). At small y grains are well separated and mainly intragrain damped motion of vortices (Josephson vortices in defects like twin

K. Yoshino et al./ Physica C 264 (1996) 161-171

boundaries within each single grain) is responsible for the microscopic mechanism of MA in a H field. At larger y some grains form intergrain junctions and are weakly coupled into loops, so that quantized penetration of the H field into weakly linked loops gives an additional contribution to LFMA, normally at higher H fields [34,35]. In this regime the phase of LFMA can be reversed [41] if loops contain so called w junctions, i.e. local spins in the Josephson tunneling barriers [36,37]. We earlier suggested that polarons in CP chains may play the role of ~r junctions [15]. A larger y naturally results in a larger AP intensity (as observed) due to the more extended network of weak-linked loops. Note that spin-carrying C6~0- radicals at the interfaces of grains or at twin boundaries within grains also can act as -tr junctions. We do not have any other reasonable explanation for AP of LFMA which survives to the lowest temperatures below T~. It has been discussed that phase reversal close to T~ can be understood as a result of redistribution of different microwave losses in the percolative media [42]. (Close to T~, grains are in the critical state, leading to a larger contribution of normal-state losses.) But in our case AP exists far below T~, making ~r junctions the only possible reason for an opposite phase as discussed in detail in our Refs. [15] and [16]. Also PME observed by SQUID strongly supports this picture. Now as to CN below T~2 = 7 - 8 K, from its normal phase and very small width one can see that this phase does not show a granular behavior, otherwise weak linked loops would have a too large radius, r = [t~0//('ffHCN)]I/2 ~ 10 -4 cm, at the observed H CN = 0.5-1 G. The large hysteresis found namely within CN suggests that this phase absorbs microwaves due to damped vortices, and hysteresis appears due to fluxon pinning-depinning processes. So it might be a SC phase of small largely defected single grains, with associated temperature close to T~2. One needs more studies to clarify the origin of the second SC 2 phase, with low Tc. We cannot exclude at the moment that this phase is due to C60 induced SC pairing of electrons in CP chains, as we earlier proposed, due to a strong pairing interactions known for C6o anions. Another possibility is that small KxC60 grains, with x distinct from 3, may give a lower Tc, since in small grains, the stoichiometric K3C60 phase need not necessarily be the most stable

169

one and it may be expected that an other x (:~ 3) can result in the SC state. An alternative explanation for multipeak LFMA is possible in terms of only one SC phase, due to K3C60 grains, but with two different microscopic mechanisms of microwave losses. In such a case at small H the CN may arise due to damped fluxon motion of the vortices, penetrating into an intergrain Josephson junction at small Hc~ (which means small critical tunneling currents), which corresponds to a small effective T~2, of weakly coupled three-dimensional array of grains, different from the larger bulk T~l of each grain. Then above Tc2 this weak Josephson junction is decoupled and this contribution to LFMA disappears. In other word SC becomes zerodimensional. But still the contribution from damped bulk fluxons, a n d / o r quantized flux penetration into Josephson junction loops in the bulk, giving a phase-slippage contribution survives till T~j and shows up as an AP component at larger H. However, what is bad with this scenario is the reversed phase of higher-T~t LFMA, which requires the existence of ~r junctions inside each grain. Those rr junctions can be C~o or C~o spins at the twin boundaries in grains (instead of the suggested spin polarons in CP chains), but why do they appear only at large y? One has to postulate that only large-size SC grains may have intragrain ~ junctions, and in general have stronger intragrain Josephson junctions. Then one arrives also at the conclusion that at small y, smaller single clusters have no 7r junctions but only weak intragrain normal junctions. It does not seem natural that with increase of y those intragrain junctions become stronger and ~ junctions appear. By transmission electron microscopy (TEM) we have observed intergrain loops at large y, and so the intergrain origin of the -rr junctions, and T~2 originating from a separate SC phase, seems more realistic, but of course the evidence is not conclusive yet. More study is needed to clarify the origin of two different Tc'S in our system.

5. Conclusions (1) At small C60 content y < 0.005 only one SC phase is found by LFMA with Tc = 11.5 K. LFMA has a normal "out-of-phase" shape and can be

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K. Yoshino et aL / Physica C 264 (1996) 161-171

assigned either to very small largely disordered KxCr0, with nonstoichiometric x < 3, or to hypothetic C60 induced SC phase. (2) At large y > 0.05, the multiphase LFMA with AP and CN components is found. The AP has a higher Tcl= 12-14 K, while the CN has a lower Tc2 = 6 - 8 K. (3) The AP is assigned to SC K3C60 granules weakly coupled to each other either through Josephson links or through a proximity effect across the CP fibrils. The AP is due to the existence of the orbital SC currents in the ground state, which is typical for granular SC Josephson arrays with rr junctions. (4) The CN is supposed to be spatially separated from the AP, and might originate either from very small disordered or even noncrystalline K xC60 clusters with nonstoichiometric x < 3, or from exotic C60 induced SC phases. (5) PME is found in the y = 0.1 sample in an FC run in a small external field H < 35 G, and apparently originates from -rr junctions, observed as an AP component of LFMA. (6) The SC volume fraction and Tc are in general much lower compared to those in a SC composite with PAT12 matrix [14-16], which is probably due to the distinct morphology of A x C 6 0 granules in OO-PPV host matrix, which we indeed observed by TEM.

Acknowledgements Part of this study was supported by JSPS Program for Supporting University-Industry Cooperative Research project and by a Grant-in-Aid for Scientific Research on New program from the Ministry of Education, Science and Culture of Japan. The authors also thank Kansai Electric Power Co. Ltd. for making the facilities available for ESR and LFMA measurements in this study by Bruker ESP 300. The valuable assistance of K. Niihara and T. Sekino of Osaka Univ. with TEM is highly appreciated. Thanks are due to Low Temperature Center of the Institute for Molecular Science, Okazaki, where the SQUID measurements have been performed. AAZ is particularly thankful for the hospitality of Osaka University.

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