POSSIBILITY OF A ONE-DIMENSIONAL KONDO SYSTEM IN THE ALLOYS CuxNi1-x (PHTHALOCYANINATO)I

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POSSIBILITY OF A ONE-DIMENSIONAL KONDO

SYSTEM IN THE ALLOYS CuxNi1-x

(PHTHALOCYANINATO)I

G. Quirion, M. Poirier, K. Liou, M. Ogawa, B. Hoffman

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Supp16ment au no 12, Tome 49, decembre 1988

POSSIBILITY OF

ONE-DIMENSIONAL KONDO SYSTEM IN THE ALLOYS

CuxNil-x (PHTHALOCYANINAT0)I

G . Quirion (I), M. Poirier (I), K. K. Liou (2), M. Ogawa (2) and B. M. Hoffman (2) (I) Dipartement de Physique, Universite' de Sherbrooke, Sherbrooke, Qukbec J1K 2R1, Canada

(2) Chemistry Departement and Materials Research Center, Northwestern University, Evanston, Illinois 60201, U.S.A.

Abstract. - The family of the Cu,Nil-, (pc) I alloys are new quasi-one-dimensional conductors that incorporate in their structure a dense array of local moments ( c u + ~ , S = 1/2) interacting with free carriers. The microwave conductivity (17 GHz) of these compounds being metallic at room temperature suddenly drops and shows a large positive magnet* conductivity at low temperatures. The results on the alloys clearly indicate that the rapid decrease of the conductivity is closely related to the presence of the C U + ~ local moments. All these observations may possibly be consistent with a one-dimensional Kondo system.

We report some interesting electrical properties of new quasi-one-dimensional organic conductors. The Cu,Nil-, (pc) I alloys consist of organic phthalocyan- inato (pc) macrocycles with a metal ion located at its center. These planar molecules assemble to form linear conducting chains when partially oxidized by iodidne (IT) ; charge carriers are exclusively associated with the T-molecular orbitals of the pc-ring [I, 21. In these compounds, the ~ iions do not possess any local mo- + ~ ment and play no direct role on the electronic proper- ties. On the other hand C U + ~ (d9, S = 1/2) ions retain their local moment and form linear magnetic chains which are strongly coupled to the free carriers of the organic molecules [3]. Synthesis of alloy sam- ples (Cu,Nil-, (pc) I), where a fraction of the copper ions are replaced by nickel, gives thus the possibility to study the effects of the C U + ~ local moment concen- tration on the electrical properties. In this paper, we report and discuss the results obtained on three differ- ent alloys Cu,Nil-, (pc) I where x = 1, 0.65, 0.10 in the presence of an external magnetic field of 10 Teslas. The sample being extremely small (2 x 0.03 x

0.03) mm3 and fragile, we have used a standard microwave cavity perturbation technique [4] instead of the usual dc technique to get the conductivity (17 GHz) and the dielectric constant. This technique requires no contact on the crystal and thus reduces the mechanical stresses. The configuration of the experi- mental set-up [5] allowed measurements with the R F electrical field along the needle axis and the external magnetic field applied in the transverse direction.

In figure 1, we present the overall conductivity as a function of temperature on a log-log scale for con- venience. All the alloys present approximatively the same behavior. The conductivity is metallic at room temperature, reaches a maximum as the temperature is progressively decreased and rapidly drops upon further cooling. For the x = 1.0 and 0.65 compounds, we also observed a different temperature behavior below 10 K.

Fig. 1. - Microwave conductivity of the Cu,Nil-, (pc) I alloys: H = O ( m ) and 1 0 T (0).

As the C U + ~ concentration is reduced, we observe that a) the absolute conductivity increases, b) the con- ductivity maximum, as well as the low temperature regime, move at lower temperatures and c) the mag- nitude of the conductivity drop decreases. From these observations, it seems clear that the temperature be- havior a t low temperatures is closely related to the

C U + ~ concentration. The exact nature of the conduc- tivity decline is not fully understood, but it is not be- lieved to be associated to a metal-insulator or semicon- ductor transition since for Cu(pc)I the thermopower measurement [I] indicates a metallic behavior down to 10 K. Another interesting point is that the flattening of the conductivity at the lowest temperatures occurs approximatively at the same temperature where the EPR linewidth suddenly increases [3]; at these temper- atures the EPR signal is dominated by the C U + ~ local moments, an observation which reinforces the propo- sition that this regime is also related to the local mo- ments.

C8 - 1476 JOURNAL DE PHYSIQUE

Considering the assumption that these two temper- ature behaviors are correlated to the magnetic system, we have examined the effects of an external magnetic field on the dielectric properties (Fig. 1, H = 10 T). In the metallic regime magnetic effects are not ob- served. A positive magnetoconductivity, more pro- nounced with reduced concentration of C U + ~ , is how- ever observed when the conductivity decreases rapidly. For example the conductivity of Cuo.~oNio.~o(pc)I is increased by almost one order of magnitude at 4 K. The rapid drop of the conductivity may thus be quali- tatively associated to an independent Kondo impurity model [6]. As the temperature is decreased the mag- netic local moments become more efficient as scatter- ing centers for free carriers and a maximum is observed in the conductivity data. An external magnetic field should partially freeze the local moments and thus re- duce the scattering rate of the itinerant carriers: this results in a magnetoconductivity. Unfortu- nately, no exact model for the derivation of the con- ductivity may be found in the literature for such one- dimensional Kondo system. Furthermore no quanti- tative agreement is found when the conventional three dimensional Kondo model is used; no logarithmic tem- perature dependence is observed and the conductivity variation over three orders of magnitude is much larger that the value generally obtained in the three dimen- sional Kondo system (10 % ) [7]. These discrepancies could be attributed t o the low dimensionality of these compounds: scattering and localization effects are ex- pected to be more dramatic in one dimension.

In the iower temperature regime (flattening), the magnetoconductivity is rather negative and the mag- nitude is relatively important. For the moment, the exact nature of this regime of conductivity is uncer- tain since no other transport measurements (dc con- ductivity) are available at these temperatures. To un- derstand this conductivity regime one has also to con- sider the behavior of the dielectric constant. In fig- ure 2, we present the microwave dielectric constant as a function of temperature for the same compounds and for both magnetic field values. Since the conductivity is relatively high, we can only measure the dielectric constant at low temperatures. The dielectic constant is quite high and increases with temperature and mag- netic field; it is however a decreasing function of the C U + ~ concentration. These results are extremely sur- prising since they seem to imply an important coupling between the magnetic and the dielectric properties of the structure.

These one dimensional compounds seem t o be the origin of a lot of new phenomena and suprising results.

Fig. 2. - Dielectric constant of the Cu,,Nil-, (pc) I alloys:

H = 0 (.) and 10 T (a).

First the conductivity suddenly drops by a few orders of magnitude and shows a huge posntive magnetocon- ductivity. These results seem to be correlated to the presence of the C U + ~ local moments in the structure

and they qualitatively agree with a Kondo type scat- tering. The second interesting point is the presence of the low conductivity regime which: shows a negative magnetoconductivity and a dielectric constant highly affected by the magnetism. This regime of conduction might also imply a new type of mechanism between the free carriers and the C U + ~ local moments.

[I] Ogawa, M. Y., Martinsen, J., Pstlmer, S. M., Stan- ton, J. L., Tanaka, J., Greene, R. L., Hoffman, B. M., Ibers, J. A., J. Am. Chem. Soc. 109 (1987) 1115.

[2] Palmers, S. M., Ogawa, M. Y., Martinsen, J., Stanton, J. L.,

offm man,

Liq.

(1985) 427.

[3] Ogawa, M. Y., Hoffman, B. Ed., Lee, S., Yud- kowsky, M., Halperin, W. P., Phys. Rev. Lett.

57 (1986) 1117.

[4] Buravov L. I., Schegolev, I. F., Prib. Tekh. Eskp.

2 (1971) 171.

[5] Quirion, G., Poirier, M., Ogawa, M. Y., Hopff- man, B. M., Solid State Commrm. 4 (1987) 613. [6] Kondo, J., Solid State Phys. Eds F. Seitz,

D. Turnbull and H. Ehrenreich (Academic New York) 23 (1969) 183.

[7] Van dan Berg, G. J., Progress Low Temp. Phys.