Low température magnetic susceptibility of quasi one-dimensional conductors

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Low température magnetic susceptibility of quasi one-dimensional conductors

M. Miljak, B. Korin, J.R. Cooper, K. Holczer, A. Jánossy

To cite this version:

M. Miljak, B. Korin, J.R. Cooper, K. Holczer, A. Jánossy. Low température magnetic sus- ceptibility of quasi one-dimensional conductors. Journal de Physique, 1980, 41 (7), pp.639-646.

�10.1051/jphys:01980004107063900�. �jpa-00209290�

Low température magnetic susceptibility ^of quasi one-dimensional conductors

M. Miljak, ^B. Korin, ^{J. R.} Cooper

Institute of Physics ^{of the} University, ^POB 304, Zagreb, Yugoslavia K. Holczer and A. Jánossy

Central Research Institute for Physics, Budapest, Hungary

(Reçu ^le 6 juin 1979, révisé le 3 décembre 1979, accepté ^le 27 février 1980)

Résumé.

Nous rendons compte ^{de mesures de la} susceptibilité magnétique (~s) ^entre 2,6 ^{et 300 K} pour dix conducteurs quasi unidimensionnels, ^{dont la} plupart ^{obéit à}

^{loi ~s}

^{AT-03B1, où}

^0,7, au-dessous de 20 K.

Les résultats pour TTF-TCNQ ^{irradié et autres évidences} indiquent que

comportement

produit pour des concentrations de spin ^de l’ordre de 1 % par molécule.

Ceci semble impliquer que dans ^ces matériaux, les interactions spin-spin ^sont soit d’une portée supérieure, soit que

ces

spins ^sont peu localisés.

Abstract.

We report magnetic susceptibility (~s) data from 2.6 to 300 K for ten quasi one-dimensional organic conductors, ^{most of which} obey the law ~s

AT-03B1 with

0.7 below about 20 K. Results for irradiated TTF-

TCNQ and other évidence indicate that such behaviour can occur for

small concentration of unpaired spins

at the level of 1 % per molecule.

This

to imply that in such materials there is either

long range interaction between localized spins, ^{or that the} spins themselves

only weakly ^localized.

Classification Physics ^Abstracts

75.20C - 75. 10J

Several _{years ago} Bulaevskii and co-workers [1, 2]

made a detailed study ^{of the} magnetic properties ^of quasi one-dimensional organic conductors based

on the tetracyanoquinodimethane ^molecule (TCNQ).

They discovered that at low temperatures ^the spin susceptibility (Xs) ^{did not} obey ^{a Curie law} typical

of non-interacting paramagnetic centres, but instead the law

was obeyed, ^{with a less than} unity. ^For complexes ^of quinolinium, acridinium and n-methyl phenazinium

with TCNQ, Qn(TCNQ)2, Ac(TCNQ)2 ^{and NMP-} TCNQ, ^{this law was} obeyed ^{over a wide} temperature

range (0.1 ^{to 10} K) with values of a equal ^to 0.73, 0.74 and 0.58 respectively.

The T-0152 law for the susceptibility and similar power laws for the specific ^{heat and} magnetization ^field (M-H) ^{curves at} low T were interpreted ^{in terms of the}

random one-dimensional antiferromagnetic ^Heisen- berg model described by the Hamiltonian

in which Jm ^{is the} antiferromagnetic exchange ^interac-

tion between nearest neighbour magnetic ^sites (m)

with spin Sm, ^{and H is the} applied field. For the mate- rials considered here g

^{2 and} Sm

i .

At that time the random variations in Jm ^were

attributed to some unspecified ^lattice irregularities.

It was suggested [2] that the interaction Jm ^between

some spins ^{could be} nearly ^{zero thus} breaking up the chains into weakly interacting sub-systems, ^{half of}

which would have an odd number of electrons, ^i.e.

spin 2 and that the weak interactions between these

spins would lead to a less than unity.

More recently ^the validity ^of equation (1) ^{has been}

demonstrated experimentally by Clark and col-

leagues [3, 4] ^for Qn(TCNQ)2 and other materials to very low temperatures. ^On the theoretical side Theodo-

rou and Cohen [5-8] have shown how the random

Heisenberg hamiltonian (eq. (2)) ^{and the} probability

distribution P(J.) ^can be derived microscopically using ^{the more} fundamental disordered Hubbard model and a cluster approximation.

Following ^{several other} groups [9, 10] they ^consi-

dered that in materials such as Qn(TCNQ)2 ^{and NMP-} TCNQ, the disorder was intrinsic and arose because the asymmetric donor molecules have two possible

orientations. This disorder was thus responsible ^for (a)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01980004107063900

640

1D localization of the electron states and (b) ^random

variations in site _energy, leading ^{to a random} spin

distribution along ^{the chains, since} only ^{sites in a}

certain energy range would be singly occupied. ^The

fits to experimental ^{data were made} using the disorder- ed Hubbard model in thè limits t U W or t W U where in the usual notation, t ^{is the elec-}

tron tight-binding ^transfer integral along the chain : U is the effective Coulomb repulsion ^{between two}

electrons on the same site and W, the standard devia- tion in the site _energy, is a measure of the disorder

potential. ^{These limits seem to} correspond ^to strongly

localized electronic states and rather a high ^concentra-

tion of spins.

The latter authors also state [5] ^that within the

Heisenberg ^{model the} general conditions for a T-’

law are : (a) ^a random distribution of spins along

a line, ^and (b), ^an exponential decrease of the (anti- ferromagnetic) exchange interaction J with distance between spins, ^{i.e. :}

where n is the number of lattice spacings (d) ^between spins. Then statistical arguments ^{lead to a} probability

distribution P(J)

A’ J-" which in turn leads to

equation (1). ^The exponent ^a depends ^on spin ^concen-

tration (c) [4, 5, 8]. ^{It will} only ^be substantially ^less

than unity (e.g. -0.7) ^{when the mean distance}

between spins (11 = d/c) ^is comparable ^with 12

^the

range of J.

Clark et al. [4] ^derived P(J) ^{in a similar} way ^to Theodorou and Cohen but derived the T-" law using

an interacting pair ^model. They (arbitrarily) ^set

altemate values of J equal ^{to zero thus} breaking up ^a

given ^{chain of} spins ^into interacting pairs. ^{Then the}

total spin susceptibility and other thermodynamic properties ^{can be} easily ^{obtained as} integrals ^over P(J) ^{of the} well known singlet-triplet ^formulae [2]

for isolated pairs. ^This procedure gives good agree- ment with experimental thermodynamic properties

of Qn(TCNQ)2 ^{but it is} thought ^{to be} inadequate ^for dynamic properties ^{such as} relaxation rates [4].

In the present paper ^we report experimental ^results

for the magnetic susceptibility ^{of ten linear} organic

conductors from 2.6 to 300 K. Although they ^have quite ^different conducting properties ^{most of them} obey a ^T-a law below about 20 K. Comparison ^{with a} TTF-TCNQ sample containing radiation induced defects, ^{and other} evidence, ^{leads to the conclusion}

that in all these materials the concentration of weakly interacting spins responsible for the low temperature susceptibility ^{is not more than a few} %.

The magnitude ^{of the T-a term} is discussed in terms of the interacting pair ^model [4] ^{and a} closely ^related

isolated pair model. In order to account for the observed values of a ( £r 0.7) ^at these concentrations the exponentially decaying exchange interaction J must be long range, and two possible mechanisms for this are mentioned.

1. Experimental details and results.

The mole- cular structures of the various materials measured are

recalled in figure 1, they ^were prepared and charac- terized in several different laboratories indicated in table I.

Magnetic susceptibility measurements were made from 2.6 to 300 K on 5-20 _mg of powder ^{or small} randomly ^oriented single crystals using ^the Faraday

method in fields _up to 9.4 kG [16]. Magnetization-

Fig. ^1.

^Chemical structures and abbreviations for the eight donor molecules and two acceptors forming ^the compounds ^measured

in this work.

Table 1.

Summary of transport ^and magnetic ^data for the materials studied here. Most of ^the symbols ^are

standard. ⁶ values refer ^{to the} highly conducting direction and 4E is the activation energy of ^the conductivity

below TMAX- C4.2 ^{is the} spin concentration at 4.2 K calculated assuming free spins. ^{As discussed in} the text, for

a

0.7 the concentration c from equation (4) ^{is 3.4 to 4.2 times} larger if Jo ^{is 100 to 200 K.} AXRT ^{is the correction}

made for ferromagnetic impurities.

The diamagnetic value used for ^the TCNQ ^{molecule was} that calculated using ^{Pascal’s constants} namely

The value measured for ^solid TCNQ ^{at room} temperature ^{was - 0.99 x} 10-4 emu/mole.

Sample preparation :

(a), (6) ^K. Ritvay-Emandity ^et al., Budapest.

0, (e) ^{L. Giral et} al., Montpellier.

(d), (f ) ^K. Bechgaard, Copenhagen.

(*) ¹⁰ % larger than Pascal value.

References : Transport ^data. (°) ^Refs. [10] ^and [11], (b) ^Ref. [12], (J ^K. Holczer et al., unpublished ^{and Ref.} [20], (d) ^Ref. [13], (e) ^Ref.

[14], (f) C. Jacobsen (private

1978).

Magnetic ^data (a), 0 ^and (d), ^this work, (b) ^Ref. [15], (e) this work and Ref. [14].

field (M-H) plots ^{were made at selected} temperatures in order to correct for ferromagnetic contamination.

The magnitude of these corrections at 295 K are

listed in table I. They generally ^varied by up ^to 5 %

with temperature, leading ^to corresponding ^uncer- tainty ⁱⁿ xs in between the selected temperatures (4.2, 77 and 300 K).

The spin susceptibility xs ^was obtained from the measured values in the usual way by subtracting ^the

calculated core diamagnetism XDIA [17], whose values

are also listed in table I.

As shown in the log xs log ^T plots ⁱⁿ figure 2, many of the materials obey equation (1) ^{with a} significantly

less than unity ( - 0.7). Only ^{for two cases} (Fig. 3),

pure TTF-TCNQ ^-and pure HMTTF-TCNQ, ^{is a} equal ^to unity ^within experimental ^error. The results for HMTTF-TCNQ agree with those of reference [14].

Because these materials have only ^{a small} upturn ⁱⁿ susceptibility ^{at low T, an error} of only ^{± 10- 5} emu/

mole _{in XDIA} leads to an uncertainty ^{of about ± 0.1}

in a. In fact our results for HMTTF-TCNQ ^{do not}

fit a power law very well and if anything ^{a is} greater than 1 with _xDIA taken as - 2.73 x 10-’ emu/mole (the ^Pascal value). ^In figure ^{3 we have used an} upper limit _XDIA

2.93 ^x 10-4 for this material.

One batch of TTF-TCNQ crystals ^{was irradiated}

at the Budapest ^{reactor with a dose of 1 x 1012 neu-} trons/cm’/s ^{for neutrons above 1 MeV.} According ^to

the scale established in several different _ways by ^the Budapest group [11, 18] ^the exposure time of 3 h.

used corresponds ^{to a} defect concentration of 0.6 mole %. ^{The nature} of the radiation induced defects is presently ^{not known} exactly. They ^are probably strong perturbations associated with mole- cular decomposition [19].

Some unknown ferromagnetic contamination was

inadvertently introduced during ^the irradiation _pro- cedures. A second set of susceptibility of measurements

was made after cleaning ^the irradiated TTF-TCNQ

sample by immersion in acetonitrile for 15 min. at

642

Fig. ^2.

^{Plots of} spin susceptibility X. (

xMEAS - XDIA) for different compounds

log-log scale. The solid lines show fits to

T -a law. The dashed lines show fits to

Curie law plus

arbitrary constant, X.

A + B/T ^For Qn(TCNQ)2 it is known [1, 4] that the latter formula is inadequate ^{at low T. It is not} quite ^such

good fit for the other materials either, although measurements to lower temperatures would provide

good check. Rather large values of A

required : 0.725, 0.4, 0.8 and 3.0

10-4 emu/mole ^for HMTSF-TNAP, ^TTF-

TCNQ, TIT-I1.s ^and Qn(TCNQ)2 respectively.

Fig. ^3.

Log xg log ^T plots ^for pure materials (TTF-TCNQ ^and HMTTF-TCNQ) ^and

alloy, showing ^that

^{1 in the} pure limit.

60 °C, which resulted in half of the sample being

dissolved. The ferromagnetic contamination was

thereby ^reduced by ^{a factor of 3 to the value} given ⁱⁿ

table I. In both cases the value of a ^was the same

within the experimental ^{error but the} coefficient A was

30 % smaller in the second measurement. We suppose that there could be a lower defect concentration towards the centre of the samples or, perhaps, ^some

of the radiation damage ^{was annealed out} by heating

to 60 °C.

In figure ^{2 we also show} by ^{dashed lines the extent}

to which _xs can be fitted to a Curie law plus ^{an arbi-}

trary ^{constant. Such a} behaviour could arise from free spins plus ^a Pauli-like contribution associated with, ^band tailing ^for example. The dashed lines do not

provide ^{such a} good ^{fit as the T-a law below 5 K}

and the following discussion is based entirely ^{on the}

latter law. However _X., of irradiated TTF-TCNQ ^or

HMTSF-TNAP should be measured down to lower temperatures ^{as was done for} Qn(TCNQ)2 [1, 4] ⁱⁿ

order to verify ^this point.

An independent guide ^to the defect concentration for the irradiated TTF-TCNQ samples ^{can be obtained}

from figure ⁴ where the derivative dxs jdT ^is compared

with that obtained in the same way ^{on a} pure sample.

The double peaked ^structure associated with the phase

transitions at 53 and 38 K is still just visible for the irradiated material. This is consistent with conduc-

tivity ^results of the Penn _group at similar defect levels [20] namely ^{for their} highest dose, ^{5 x} 1014 deu- terons (of energy 8 MeV) per cm2, leading ^{to an esti-}

mated defect concentration of 0.3 %.

Fig. ^4.

Derivatives of the measured susceptibility (without ^sub-

traction of the low T uptum)

temperature for nonnal and irradiated TTF-TCNQ. Tlie experimental points

^smoothed

over an

interval of ± 4 K in order to reduce the scatter in the derivatives. This procedure

reduction of 30 % ^{in the} height

of the lower peak for the pure sample and its width (FWHM) ^is

increased by ^{about 3 K} while the upper peak ^is hardly affected.

2. Discussion.

The well defined T -a law

(ce = ^0.74 ± 0.03) ^{for the} irradiated TTF-TCNQ

is evidence that it can be associated with defect levels

of 1 % ^{or so. This statement} is also consistent with the studies of the low temperature dielectric constant (e)

made recently by ^the Budapest group [10, 11]. They

have concluded that in materials such ^as Qn(TCNQ)2

and others listed in table I, ^the large, temperature

dependent ^{values of e} (measured ^{at 9} GHz) ^{are asso-}

ciated with a coherence length of the order of 100 lat- tice spacings along ^the chains. The nature of this electronic coherence length ^{is at} present ^uncertain

but it is clearly ^reduced by - ¹ % of radiation induced defects. The results for unannealed TTT2-I3 [12] ^and

HMTSF-TNAP [13], ^both of which remain good

conductors (u L-- 100 and 200-2000 (Q.cm)-l ^res- pectively) ^{at low T,} provide ^a further indication that the T-a law can occur in weakly disordered systems.

HMTSF-TNAP is the only pure ^two chain charge

transfer salt in which a T-« law has been observed.

As we have only ^{measured one} sample, experimental problems ^{cannot be} entirely ^{ruled out.} On the other hand the TNAP molecule has a lower symmetry ^than TCNQ and this could be a source of weak disorder.

For the alloy system

oc

0.8 although ^{there is} considerable uncertainty arising ^{from the} uncertainty ⁱⁿ xDIA mentioned earlier.

However it is noteworthy that in this 11 % alloy ^the magnitude ^{of the T - a term} (i.e. A ) ^is only 1 /2 ^{of that}

in the irradiated TTF-TCNQ containing ^0.6 % ^defects.

Thus the weaker potentials associated with Se-S substitution in alloys ^are much less effective in pro-

ducing unpaired spins ^than radiation induced defects.

The changes ⁱⁿ conductivity ^on alloying ^{are also} correspondingly ^smaller (Table I). ^{For the 11} % HMTTF-TCNQ alloy QRT is reduced by ^{a factor of}

1.3 whereas in the irradiated TTF-TCNQ sample ^{it is}

reduced by ^{a factor of} approximately ^2.2 [20]. ^Both

of these results emphasize that the defects produced by alloying and irradiation are qualitatively ^different.

In order to discuss these results quantitatively

we will consider a slightly ^different pair ^{model to that}

of reference [4]. Namely ^{for a} random distribution

(of concentration c) of well localized spins along ^a

line the probability ^{that a} given spin ^{is one of an}

isolated pair separated by ^{n lattice} distances, ^{with no} other spins ^{within n + 1 distances an} either side is

simply ² c(l - c)3n-l (Fig. 5). ^Thus the concentration of these pairs (dropping ^{a factor two to} avoid double

counting ^is c2(1 - c)3n-1. ^{As c --+ 0}

Fig. ^5.

^Isolated pair ^of spins ^{with n}

^{3. The} probability ^that

a

given spin ^{A is} part ^{of such}

pair ^is (1 - c)" (1 - c)"-1 c(1 - c)"

multiplied by ^{2 since} spin B

^{be to the left}

right.

644

so 2/3 ^{of all} spins ^{occur in such} pairs. ^{It is} physically plausible ^{that these} pairs ^will give ^a singlet-triplet ^like

contribution to xs with the value of J determined by ^the pair spacing ^{nd as in refs.} [4-6]. ^This approach ^is complementary ^to that of reference [4] ^since although

it avoids the assumption that alternate values of J

are zero, it neglects 1/3 ^{of the} spins ^{which are not in}

isolated pairs. Thus it should give ^a lower limit to the

thermodynamic properties ^{such as} xs(T). ^Similar pair approximations ^{have been} quite successful in 3D dilute magnetic alloys [21]

^{in 1D} they ^should

hold _up to higher concentrations since there are only

two nearest neighbours ^{for each atom.}

Using the standard formula

and following ^the steps described in reference [4]

we obtain

The approximation ! ln (1 - c) I

^{c holds} up ^to quite high ^{values of c} (c 5 0.3). ^Thus for c 0.3 ^and kT « Jo ^{e -1/12 the} magnetic susceptibility ^is given by :

where

has the values 0.26, 0.27 and 0.28 for a

0.9, 0.8,

0.7 respectively. NAv ^is Avogadro’s number. These results are very similar to those of reference [4]. ^In

our equations (3) ^and (4) ^{there are extra} factors of 3 and 2/3 respectively.

Applying equation (4) ^to the results for Qn(TCNQ)2

at T 20 K, ^leads to c

6.8 % ^{mole-’ or} 3.4 %

of the TCNQ ^molecules (1) ^{for a} reasonable value of Jo (100 K). This value of c is in reasonably good agree- ment with the inverse characteristic length ^{of 60 lat-}

tice spacings deduced from dielectric constant measu- rements [11].

An interesting point ^about equation (4) is that the

magnetic interactions have little effect on the magni- tude of xs in the helium temperature range. For

(1) Actually ^{recent work}

irradiated n-propyl Qn(TCNQ)2

shows

change

^from

^{1 to}

^{0.7 for}

sample,

^the temperature is lowered [25]. ^{For the} region ^where

¹

estimate

directly and find that

1.4 % per mole corresponds ^to

A

26

10-4 emu/mole/deg.

So from this

conclude that in Qn(TCNQ)2 the concentration of

weakly interacting spins ^{is about} 3 %

^{factor of two less that} given ^above.

example ^with Jo

^{100 K and a}

0.7, Xs/Xfree _spin

from equation (4) ^{is 0.38 at 10 K and 0.19 at 1 K.}

Thus the correct order of magnitude ^{for c can be}

obtained simply ^from /freespin? ^{and the} spin ^concen-

trations in irradiated materiafs, ^estimated assuming

free spins [20], ^{are the correct order of} magnitude

even though ^{at T-a law} may provide ^a better fit to the data in some cases.

Within the pair ^{model the} physical ^{reason for this}

is that _many pairs ^of spins ^{have J} JO. By integrat- ing ^the distribution function P(J)

A’ J-« it can

be seen that (for ^a

0.7) ⁵⁰ % ^{have J} Jo/10, ²⁵ %

have J Jo/100, ^{etc. Since} spins ^with J ^{kT still}

give substantial contributions to xs ^{it is} necessary ^to go down to temperatures, ^{kT -} 10-3 Jo in order for X, ^to be a factor of ten less than x free spin.

At first sight (2) ^{we would} expect ^{J to} arise from the usual virtual mixing process invoked in the singlet- triplet ^model whereby :

The overlap integral t ^falls rapidly ^as the lattice _spac-

ing ^increases (for ^the TCNQ ^{chains in} TTF-TCNQ d In tl/ In d = - 6 [22] ^{and so} according ^{to for-}

mula (5) the range of J is _very short, /2

d /12 leading

to a

0.99 for c

0.04 in equation (3)).

Thus there are two serious difficulties in applying

the above model to our results :

a) Understanding how values of a

0.7 can arise at such low defect concentrations, ^{i.e. how} l2, ^the range of J, ^{can be of the same order as} 11 ^{the mean dis-}

tance between spins, ^at low concentrations.

b) Understanding why all the materials in figure ²

have ^a quite ^{close to 0.7 while A} (i.e. c) ^varies by ^over

a factor of ten. This seems to imply that l2 ^and 11

decrease proportionally ^{as c increases.}

If the systems studied here remained metallic at low T then localized spins could interact via the long range RKKY interaction. In 1D systems ^{the RKKY} spin polarization ^{about a} magnetic impurity ^{shows the} peculiar feature that it tends to a constant value at

large ^distances [23]. ^{In real} systems ^defect scattering

or localization would reduce this constant value to an

exponential decay, exp - R/L, ^{where L is} inversely proportional ^{to the} defect concentration. Hence in

principle, ^RKKY interactions in a 1D metallic system could lead to an antiferromagnetic exchange ^interac-

tion which decays exponentially ^{with a} range inversely proportional ^{to the} impurity concentration. In prac- (2) ^A proper ^treatment [5-8] using the Hubbard model leads to

(neglecting the disorder parameter) which is valid in the limit t « U. So again 12 ^is generally ^short (/2

dl2 ^In U/t). ^{The fit} actually ^used [6] ^for NMP-TCNQ corresponded ^to /2

^{d. Even in}

this

we still have

0.94 for c

0.02 in equation (3).

tice of _course, the systems ^{studied are} mostly ^insulat- ing ^{at low} T, ^so the above mechanism seems to be ruled out, although ^{it is not} entirely clear that a small gap ^at the Fermi level would completely ^suppress

RKKY oscillations. This point ^seems worthy ^{of a}

more detailed theoretical study.

Alternatively ^an approach ^{based on the weak 1D}

localization of all the electron states, including ^those

with unpaired spins, ^{could be} appropriate. ^{The wave-}

functions of localized states decay ^as exp - X/L along the chains [24] and for weak scatterers at least [24], ^the localization length ^{L is of the same order}

as the mean free path calculated from scattering theory. ^In general ^{a localized state} of energy EF ^would

be singly ^or doubly occupied ^{at T}

⁰ depending ^on

which alternative best satisfied charge neutrality ^over

a length = ^L (e.g. ^{in the} interrupted strand model

according ^{to whether there were an even or odd num-}

ber of electrons _per strand). ^{Thus in a random} system,

spin ^{values of 0} or 2 1 are equally likely ^{and the} spin

concentration c - d/L. ^The exchange interaction J between two spins separated by ^a distance R > ^L will decrease as exp - R/L.

Thus the two main conditions for aT-x law referred to in the introduction would be satisfied, ^{and further-}

more the _range of J(12) ^{and the} average spin separation (11) would both be N L independent of concentration.

In the case of strong potentials associated with radiation induced defects it seems that, experimentally,

the number of spins is roughly equal ^{to the number}

of defects [11]. On the other hand for weaker poten- tials, associated with _say, substitution of Se for S in HMTTF-TCNQ, experimentally, ^{the defect con-}

centration seems to be larger ^{than the} spin ^concentra-

tion. This latter point ^can be understood in the above

picture ^{because L will be} larger ^{than the mean defect} separation ^{for weak} potentials.

The latter picture ^{is not} very different from the

original physical discussion of Schegolev [2]. ^{It is}

similar to that of Theodorou and Cohen [5, 8] also,

the main difference being ^{that the} experimental

evidence presented here shows that we are dealing

with larger localization lengths ^{L - 100} d. However

we should point ^out that it may ^not be possible ^to

account for the dielectric constant measurements within such a simple model. Also ^we emphasize ^that

the pair approximation leading ^to equation (4) ^{is less} appropriate ^for weakly ^localized spins.

Throughout this discussion we have only ^considered

on chain effects. It is possible that defects on neigh- bouring chains increase the disorder on a given ^chain.

Also at sufficiently ^low temperatures interchain spin

interactions could be important.

In conclusion we have presented ^{evidence to show}

that the low temperature susceptibility ^{of some 1D}

systems changes ^{from T -1 to T-a.} (with ^{a =} 0.7) dependence ^even when there is only ^{about one} per cent of unpaired spins. These interactions occur either via a long range interaction between localized spins

or because the spins themselves are only weakly

localized.

Acknowledgments.

^{We are} grateful ^{to G. Grüner}

for initiating this work and for _many discussions and to K. Bechgaard, Copenhagen, ^L. Giral, Montpellier

and K. Ritvay Emandity, Budapest, ^for supplying

the samples. ^{We thank W. G. Clark for comments on}

the manuscript.

Note added in proof.

Since this _paper was sub- mitted work performed ^on n-propyl Qn(TCNQ)2 ^has

also demonstrated the changeover ^{from a}

^{1 to}

a

0.7 and has shown the spin concentration to be

-

1 % ^{in a} direct, ^model independent, way [25].

Also detailed calculations for a one dimensional

spin glass, ^{that is a} system of localized spins interacting

via the conduction electrons, ^have recently ^been published by ^Abrikosov [26].

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