Magneto-oscillatory behavior of carbon nanotube bundles

ELSEVIER

Synthetic Metals 86 (1997) 2001-2002

Magneto-oscillatory Behavior of Carbon Nanotube Bundles M.Baxendale,V.Z.Mordkovich,

R.P.H.Chang* and SYoshimura

Yoshimura n-Electron Materials Project, ERATO, JRDC. clo Matsushita R.I.T.. Inc. Tama-ku, Kawasaki 214, Japan *Materials

Research Center, Northwestern

University,

Evanston, Illinois

60208, USA

Abstract Low-temperature magnetoresistance measurements on pristine bundles revealed non-oscillatory magnetoresistance behavior comprising positive and negative components. However, upon intercalation of K-atoms, random oscillations about a large that were also seen for some pristine bundles taken from the same source negative underlying trend were revealed -features material. When plotted as magneto-conductance, the amplitude of these oscillations is comparable to that of the ‘universal’ conductance fluctuations recently observed for a single multiwall nanotube [Langer, L. et al, Phys. Rev. Lett. 76 (1996) 4791. This amplitude is temperature-independent below Tc -3 K. An explanation of these observations is given within the framework of 3D weak localization theory. Keywords:

1.

Transport

measurements,

conductivity,

magnetotransport;

Introduction

Recently, magnetotransport measurements on single multiwall carbon nanotubes have been reported [l]. Nevertheless macroscpic bundles comprising ensembles of aligned closely-packed multiwall nanotubes remain an interesting system for study since they are much easier to handle and results should complement the mesoscopic measurements [2]. The recent demonstration of intercalation of guest atoms into such bundles also offers the possibility of modification of the electronic structure [3]. 2.

Experinleutal

The carbon nanotube bundles used in this study were produced by one of the authors by arc discharge technique in a He atmosphere [4]. The material consists of multiwall nanotubes close-packed into bundles with a mean outer diameter of several tens of nanometers. The bundles in turn form the bigger fiber-like structures of length l-3 mm and approximate diameter 0.1 mm to which electrical contacts were made. Measurements were performed with a standard dc two-contact technique with the magnetic field H applied perpendicular to the sample axis. Details of the K-intercalat -ion are given elsewhere [3]. Measurements were performed on the same bundle before and after intercalation. 3.

Results

and

and derivatives

components of MR at low temperature; (iii) T>70 K. MR is described by classical two-band model. K-intercalation produces dramatic changes in the above behavior. Fig.1 shows the temperature variation of G. Unlike the featureless characteristic of the pristine material, there are several noteworthy characteristics: (i) G=T0*12 for T>r, where r is the temperatureindependent elastic scattering time. Here p =0.24, suggesting that disorder-enhanced electron-electron scattering is present [G]. The abrupt incrcasc in G at T=lOO K occurs when rh,=r.

‘-: a

10-3 -

discussion

The magnetoresistance (MR) and zero-field properties of the pristine bundle are in good agreement to those reported by S.N. Song, et of for similar source material made by one of the authors [2]. These properties are reproducible and exhibited by the majority of samples. They can be summarized as: (i) semiconductor-like temperature variation of conductance G, often linear in 1nT for Tel0 K; (ii) negative and positive 0379-6779/97/%17.00 8 1997 Elsevier Science S.A AU rights nxerved SO379~779(%)04686-3

PII

Fullerenes

Fig. 1 Temperature variation carbon nanotube bundle.

T(K) of conductance

for K-intercalated

2002

A4. Baxendak

et al. /Synthetic

Metals 86 (1997) 2001-2002

Magnetotransport measurements revealed lowtemperature random conductance fluctuations about an underlying positive trend, Fig.2. The amplitude of the random fluctuations is comparable to that recently observed for a single multiwall carbon nanotube [l]. We find that the nonoscillatory component of magneto-conductance is described by 3D WL theory [71;

absolute value of conductivity can be calculated. Thus we calculate r=10-13s, Zin=3X10-13T-o*24S and D=6x10s4 m2s-l Since the above analysis allows calculation of the underlying magneto-conductance, the root mean square amplitude, rms[SG]. of the random fluctuations can be

Aa(H,T)=-Aa,(H)+Aaw,(H,T)

determined with accuracy. Theory predicts that rms[GG] is temperature independent below a certain temperature Tc when the canier coherence length, lo. is greater that the confining

(1)

where Au, =~u(qr)~, oo=ne21tn* and q=eHlm* (other symbols have their usual meanings), is the ‘normal’ component. The weak localization term is given by

dimension L. At T=T,-, lq=L and for TBT,. nns[SG] is

(2) where x = 3ctlJ eH4DG

and D is the diffusion coefficient.

When xc< 1, AC,,,,_(H)= 0.918dH (Q-lcrn-l)

with H in kGe

single nanotube (Tc=0.3 K) due to the enhancement of qn described above. It should be noted that random conductance fluctuations were also seen for some pristine samples taken from the same source material.

(3)

Note that Eq.(3) is independent of the system parameters. When x>> 1, A%,_ (H,T)=o,(zi,

controlled by the shorter of f+ and the thermal diffusion length. Fig.3 clearly shows these two temperature regions and Tc =3.1 K. Note that T, is higher than that observed in a

/r)3R(cycr)2/12’/3

0.1 -

(4)

The features of both the pristine and K-intercalated material can be explained within the framework of the theory. K-intercalation introduces change to the WL term through the quantity x which is a function of rin ,r and H. The dominant effect of K-intercalation is to enhance the inelastic scattering time as described.

z ?p . . G Tc=3.1K

.

??

0.

f

0.01, T(K)

10 --

Fig.3 Tcmpenture variation of the rms amplitude of the random conductance fluctuations. -

1OK

Dau

7K 4K 2K 1.6K

Hcrc 1&(3Drm) themfore we calculate L-20 nm. This value is much less than the sample and individual nanotube dimensions therefore the sample is best modeled as a series and parallel combination of units of dimension L. The same value of L was found for a single nanotube [l] and is also the average diameter of the nanobeads observed after intercalation of K guest atoms. We speculate the defects responsible for confining the nanobeads along the length of the nanotube are also those that confine the charge carriers. References

o.oI 0

’

2

’ 4

’ 6

’ 8

’ 10

’ 12

’ ’ ’ ’ 14 16 18 20

B(T) Fig.2 Temperature variation of magneto-conductance for Kintercalated nanotube bundle showing random fluctuations. Fig.2 shows the smooth curves generated by a leastsquare fit of the model described in Eq. (l-4). Such a fitting procedure is appropriate in this case since the integrated area between the random fluctuations and the smooth curves should be zero for the range of field used. Since Eq.(3) is independent of the system parameters, the appropriate factor relating to the sample dimensions that allows determination of the

[l] L. Langer. V. Bayot. E. Grivei. J-P Issi, J.P. Heremans, C.H. Olk, L. Stockman, C. Van Haesendonk and Y. Bruynseraede. Phys. Rev. Len. 76 (1996) 479. T.W. Ebbesen, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi and T. Thio, Nufure 382 (1996) 54. [21 S.N. Song, X.K. Wang, R.P.H. Chang and J.B. Ketterson, Phy.r. Rev. Len. 72 (1994) 697. [3] V.Z. Mordkovich, M. Baxendale, R.P.H. Chang and S. Yoshimura. Synrh. Mer. (1997) to be. published. [4] X.K. Wang, X.W. Lin, V.P. David, J.B. Ketterson and R.P.H. Chang. Appf. Phys. Left. 62 (1993) 1881. [5] P.A. Lee and T.V. Ramakrishnan, Rev. Mod. Phys. 57 (1985) 287. [6] B.L. Al’tshuler, A.G. Aronov and P.A. Lee, Phys. Rev. Lert. 44 (1980) 1288. [7] A. Kawabata, Sol. St. Commun. 34 (1980) 431.