SUPERCONDUCTING PROPERTIES OF BROMINATED (SN)x

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SUPERCONDUCTING PROPERTIES OF BROMINATED (SN)x

R. Greene, J. Kwak, W. Fuller

To cite this version:

R. Greene, J. Kwak, W. Fuller. SUPERCONDUCTING PROPERTIES OF BROMINATED (SN)x.

Journal de Physique Colloques, 1978, 39 (C6), pp.C6-1401-C6-1403. �10.1051/jphyscol:19786579�.

�jpa-00218070�

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JOURNAI. DE PHYSIQUE _ColloqueC6, supp1c;ment uu no 8, Tome 39, rrolit 1978, prqe C6- 140 1

SUPERCONDUCTI NG PROPERTIES OF BROMI NATED (SN) ,

R.L. Greene, J.F. Kwak, and W.W. Fuller

IBM Research Laboratory, Sun Jose, CaZifornia 95193, U. S. A.

RBsum6.- Les mesures de T

,

AT et des champs critiques parallble et perpendiculaire de (SNBr ) en

0.4 x fonction de la tempdraturg song prdsentges. De la comparaison des rdsultats avec ceux obtenus pour la pristine (SN)x il ressort que T est augmentde de 10 % environ, AT est rdduite et 116volution du champ critique perpendiculaire avel la tempdrature est fortement modiFi6e. Un couplage plus fort en- tre les fibres dans le compos6 bromd permet dVinterpr6ter ces r6sultats.

Abstract.- We report measurements of T

,

AT

,

the pressure dependence of T

,

and the parallel and perpendicular critical fields versus t$mper$ture for (SNBr )

.

As compares to pristine (SN)

,

T is

0 4 X

enhanced about 10 % and has the opposite sign of pressure dehendence, ATc is reduced, and thextem6e- rature dependence of the perpendicular critical field is dramatically altered. These results are ex- plained by stronger coupling between fibers in the brominated material.

The properties of the novel superconducting

1

.o.

^R/RN--- polymer (SN) have been actively studied during the

X

past three years

111.

Originally this compound was thought to be a quasi-one-dimensional metal, like

KCP or TTF-TCNQ, but later work showed that (SN)x (SNBr0.4)x

has sufficient coupling between chains to be consi- ^T, ⁼^0.316K AT, = 11.5 rnK dered a three-dimensional (although highly anisotro-

pic) semi-metal. Because of the great interest in the properties of ID metals and the hope of finding organic or new polymeric superconductors, conside-

Fig. 1 : The four-probe resistance of a typicalcrys- rable chemical effort has been expended in trying to tal of (SN) before and after bromination. The re- produce analogues of, or intercalation compounds of sistance isXnormalized to the value at 1 K.

(SN)x 121. For example, one might naively expect

that most molecules intercalated between the (SN) Cp(300)lp(4.2)] of 20. The (SNBr0.4)x samples had

X

chains would cause weaker interchain coupling and room temperatureconductivities 5 to 10 times larger than (SN) while the defect limited conductivity at hence more quasi ID properties (perhaps even a x

Peierls transition instead of superconductivity). 4.2 K was essentially unchanged. The important chan- For the most part the chemical effort has been un- ges illustrated in figure 1 are the s 10 % increase successful, however, recently it was discovered that in T and the dramatic decrease in width of the the properties of (SN)-- could be significantly modi- transition (AT ) . Similar changes were found in all

*

fied by chemical interaction with halogens 131. The measured.

In figure 2 we show the critical field of material that has been investigated in most detail -

to date is (SNBro.4)x. Here we report measurements (SN) perpendicular to the fiber axis,

Hcl. both before and after bromination. A significant change on the superconducting properties of this new com-

pound and compare our results to those previously in the shape of the

Hcl

vs. curve and in the ex- found for pristine (SN)x. trapolated T = 0 value of H

I

is observed, even

c-

though Tc increased only 'L 5 % in this sample. In Figure 1 shows our result for the variation

ia the superconductivity for a representative crys- figure we show the change in the parallel criti- tal of (SN) before and after bromination. The data cal Hc//y after bromination' In 'Ontrast

-

^'X

were obtained by a standard 4-probe resistivity rnea-

Hc.

there is Only a 'light change in the and surement along the b, or fiber, axis of the crystal. magnitude of

Hell

VS. T even though T increases by The samples were of the usual fibrous mar- 30 % in the sample used for this measurement. In phology

i l l

and typically had a resistivity ratio figure 4 we show the pressure dependence of Tc for

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786579

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C6- 1402 JOURNAL DE PHYSIQUE two crystals of (SNBr ) and compare it with pres-

0.4 X

sure data obtained elsewhere on pristine (SN) /4,5/.

X

The experimental pressure techniques were the same as employed previously by our group at IBM /4/. As can be seen from figure 4, there is a striking change in the pressure dependence of T upon bromination.

Fig. 2 : The temperature dependence of the perpendicular critical magnetic field of (SN) before and after bromination X

Fig. 3 : The temperature dependence of the parallel critical field of (SN)x before and after bromination

How can we explain these changes in superconducting properties ? At this stage we can only propose qua- litative answers since it is not yet known exactly where the bromine resides in the (SN) lattice. It appears likely that some bromine is surrounding the fibers /3/ and some is intercalated within the fibers, among the (SN)x chains / 6 / . It is known that the superconducting properties of pristine (SN)

X

are strongly influenced by the fibrous nature of the material. The width of the transition (ATc) and the behavior of H_I (T) and H//(T) both have been

Fig. 4 : The pressure dependence of T for two samples of (SNBro.4)x Golid line] compargd with the data of references /4/ and /5/ on pristine (SN)x

[dashed line].

interpreted by a model of weakly coupled supercon-

0

ducting fibers (of diameter d%100 A) /7,8/. Figure 1 shows that the transition in (SNBr ) is much

0.4 X

sharper than in (SN)

.

this despite the fact that

X '

the brominated samples are known to be significantly more disordered, as is seen from both X-ray and electron diffraction measurements / 3 / . In fact, it has not been possible to determine the crystal structure of (SNBr ) due to the extensive disor-

0.4 x

der, whereas the structure of (SN) is known. From

X

this one deduces that the transition width in (SN)

X

is due not to disorder or inhomogeneity but to the fluctuations expected in a quasi-one-dimensional superconductor and that the reduction of ATc in (SNBro.b)xmust be due to the suppression of these fluctuations. That is, the brominated material is a more 3D-like superconductor because of increased coupling between fibers.

The critical field behavior of (SNBrQ.,) is consistent with this interpreation since H

cl

.s: T has the usual shape. A more detailed analysis sug- gests that H is paramagnetic limited in both

c //

(SN)x and (SNBr,.4)xat temperatures below Tc/2.

From the slope of H (T) near T we find 11

cl(0) % 100 A for the perpendicular coherencelength.

This implies good coupling over many fibers of es- timated 131 diameter d%30 in (SNBro.4)x.Thepres- sure dependence of the T of (SNBro.4)xis typical of that found in three-dimensional s-p superconductors, i.e., it decreases with pressure. The unusual behavior of pristine (SN) is probably caused by

X

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the weak coupling between the superconducting fibers /I/. The origin of the sharp drop in T aroung 8kbar

References

is unknown. If it is caused byaphase transition /I/ For a review see Greene, R.L., and Street, G.B., then it is clear that the bromine inhibits this in Chemistry and Physics of One-Dimensional Me-

tals, edited by H.J. Keller (Plenum Press) 1977;

transition (at least up to 12 kbar). Geserich, H.P.,and Pintschovius, L., in

The physical origin of this increased cou- Festk;jrperprobleme, Vol. 16, edited by J. Treusch (Vieweg) 1976

pling is not understood, partly because of the un-

/ 2 / Street, G.B. and Greene, R.L., IBM J. of Res.

certainty about where the bromine resides in the and Dev.

2

(1977) 99 (SN) lattice. Independent of this uncertainty it

X ^{/ 3 /} Gill, W.G. et al., Phys. Rev. Lett.

2

(1977)

seems likely that the bromine affects the intrinsic 1305 ; Chiang, C.K. et al., Solid State Commun.

23 (1977) 607 electronic band structure of (SN) as well as the

x /4/ Bickford, L.R., Greene, R.L. and Street, G.B., interfiber coupling. Raman scattering /6,9/ and in- Phys. Rev.

=

(1978) 3525

frared absorption experiments /lo/ suggest that most /5/ Miiller, W.H.G. et al., Solid State Comun.

2

of the bromine is in the form Brr. If all the bromi- (1978) 119

ne is in the lattice as ~ r ; then 0.12 electrons per /6/ Igbal, Z., Baughman, R., Kleppinger, J. and Macdiarmid, A.G., Solid State Commun (In press) SN unit must have been removed from the (SNIx chains.

/7/ Civiak, R.L. et al., Phys. Rev. (1976) 5413 From a rigid band viewpoint this implies a decrease

/8/ Azevedo, L. J. et al., Solid State Commun.

19

of almost 1 eV in the Fermi energy /11/. This shift (1976) 197

completely removes the electron pocket at Z in the /9/ Temkin, H., Fitchen, D.B. and Street, G.B., to Brillouin zone and leads to a slight increase in be published

the Fermi level density of states. We believe this /10/.Macklin, J.J., private communication large perturbation of the Fermi surface is the cause /I1/ private

dHc 11

of the large increase in room temperature conducti- 1121 For example

[ /%I

^{~ 6 0 .}

vity upon bromination, i.e., the electron-hole scat- Tc tering mechanism has been eliminated. The slight in-

crease in N(E ) may explain the observed increase in F

Tc, although other explanations are possible. For example, increased interaction between weakly coupled fibers could also lead to an increase in T

.

In summary we find that the changes in superconductivity caused by bromination of (SN) are ex- plained primarily by increased coupling between fibers. The (SNBr ) is still a highly anisotropic

0.4 x

material / 1 2 / ; however, this may reflect the intrinsic anisotropy of a crystal not dominated by interfiber coupling.

ACKNOWLEDGEMENTS.- We thank the Office of Naval Research for their partial support of this work and Dr. G.B. Street for providing the (SN) crystals.

We are also grateful for useful discussion with Dr. P.M. Grant and Dr. W.D. Gill and the technical assistance of J. Vasquez and C. Pike.