Page 126 - Carbon Nanotubes

116 X. K. WANC et al.
the center of a quartz tube (holder). No susceptibil- the MR is negative at low fields followed by an upturn
ity difference between the c60 sealed in vacuum and at another characteristic field that depends on temper-
unsealed samples was observed. Cm is a large band- ature. Our low-temperature MR data has two striking
gap (1.4 eV) semiconductor[29,37], and the paramag- features. First, at low temperatures and low fields
netic upturn at very low temperature is probably due Ap/po depends logarithmically on temperature and
to a very small concentration of foreign paramagnetic Bright’s model predicts a VTdependence at low fields.
impurities. Clearly, the data cannot be described by 1D WL
We conclude this section stating that buckytubes in theory. Second, from Fig. 6 (b), we see that the char-
a bundle have a large diamagnetic susceptibility for H acteristic magnetic field at which the MR exhibits an
both parallel to and perpendicular to the buckybun- upturn is smaller for lower temperatures. On the con-
dle axis. We attribute the large susceptibility of the trary, the transverse MR at different temperatures for
buckytubes to delocalized electrons in the graphite the pyrocarbon exhibits quite a different behavior[40]:
sheet[38]. The increase in the diamagnetism at low the upturn field decreases with temperature. With in-
temperature is attributed to an increasing mean free creasing temperature, the Ap/po vs B curves shift up-
path. C70, which is formed by 12 pentagons and 25 ward regularly and there is no crossover between the
hexagons, exhibits a larger diamagnetic susceptibility curves measured at different temperatures. All these
than that of c60, which consists of 12 pentagons and facts indicate that the buckytubes show 2D WL behav-
20 hexagons. This suggests that the diamagnetic sus- ior at low temperatures.
ceptibility of fullerenes may increase with an increas- It is also seen that above 60 K the MR is positive;
ing fraction of hexagons. The susceptibility of the it increases with temperature and tends to saturate at
buckytubes is likely the largest in this family. a characteristic magnetic field that is smaller at lower
temperatures. Based on a simple two-band model, this
3.3 Transport properties means that unequal numbers of electrons and holes are
Theory predicts that buckytubes can either be met- present and that the difference in electron and hole
als, semimetals, or semiconductors, depending on di- concentrations decreases with increasing tempera-
ameter and degree of helicacy. The purpose of our ture[41]. The temperature dependence of the conduc-
study is to give a preliminary answer to this question. tance (shown by the right scale in Fig. 7) cannot be
A detailed analysis has been published elsewheret391. described by thermal excitation (over an energy gap)
In this section, we present mainly experimental results. or variable range hopping. Instead, above 60 K con-
The transverse magnetoresistance data, p/po (Ap = ductance, u (T), increases approximately linearly with
- lic and that the hopping between the tubes within the
p (B) - po), measured at different temperatures, are temperature. The absence of an exponential or a vari-
shown in Fig. 6. It is seen that, at low temperatures, able range-hopping-type temperature dependence in
the conductivity indicates that the system is semimetal-

bundle is not the dominant transport mechanism.
40 From our transport measurements, we can con-
(a)
35 - clude that at low temperatures, the conductivity of the
bundle of buckytubes shows two-dimensional weak lo-
calization behavior and the MR is negative; above
3 25 60 K the MR is positive and increases approximately
Y
a- 20
-...
8 15
10 1.20 ,
5 I I I 1 I i 0.14
0
E 1.00 0.12 -i
h
0 E c
2 0.80
-1 J 0.10 Y
- -2 5 0.60 8
e C
. m 0.08 5
v
rd
-3
&s
8 -4 0.40 3
v 0
-5 0.06
E= 0.20
-6 0
-7 I I ? I 1 0.04
0 50 100 150 200 250 300
T (K)
Fig. 6. The magnetic field dependence of the high- and low-
temperature MR, respectively; the solid lines are calculated. Fig. 7. The Hall coefficient (left scale) and conductance
The inset shows a schematic of the contact configuration for (right scale) vs temperature. R, was determined using the
the transport measurements. measured sample dimensions without any correction.