Sample-Specific and Ensemble-Averaged Magnetoconductance of Individual Single-Wall Carbon Nanotubes

We discuss magnetotransport measurements on individual single-wall carbon nanotubes (SWNTs) with low contact resistance, performed as a function of temperature and gate voltage. We find that the application of a magnetic field perpendicular to the tube axis results in a large magnetoconductance of the order of $e^2/h$ at low temperature. We demonstrate that this magnetoconductance consists of a sample-specific and of an ensemble-averaged contribution, both of which decrease with increasing temperature. The observed behavior resembles very closely the behavior of more conventional multichannel mesoscopic wires, exhibiting universal conductance fluctuations and weak localization. A theoretical analysis of our experiments will enable us to reach a deeper understanding of phase-coherent one-dimensional electronic motion in SWNTs.

Figure 1

Gate voltage dependence of the conductance of an individual SWNT (shown in the inset) measured at different temperatures [1.6 (bottom curve), 4.2, 20, 45, and 90 K (top curve)]. Note the aperiodic conductance fluctuations whose amplitude decreases with increasing temperature.

Figure 2

(a) Individual magnetoconductance traces measured at 1.6 K, for different values of the gate voltage ($A$, $V_{\mathrm{G}}=-16.8\mathrm{V}$; $B$, $V_{\mathrm{G}}=-15\mathrm{V}$; $C$, $V_{\mathrm{G}}=-12\mathrm{V}$, $D$, $V_{\mathrm{G}}=-6.8\mathrm{V}$), showing large aperiodic fluctuations, whose precise shape strongly depends on $V_{\mathrm{G}}$. Note that both a maximum and a minimum is observed around $B=0$. (b) Magnetoconductance traces measured at $T=45\mathrm{K}$, for the same values of $V_{\mathrm{G}}$: at higher temperature, the magnetoconductance has always a minimum around $B=0$.

Figure 3

Magnetoconductance curves measured at $V_{\mathrm{G}}=-17.7\mathrm{V}$ for different temperatures, showing that the magnitude of the aperiodic conductance fluctuations decreases with increasing temperature.

Figure 4

(a) Ensemble-averaged magnetoconductance curves measured at different temperatures. In all cases the average is taken over 34 magnetoconductance traces measured at different values of $V_{\mathrm{G}}$. It is apparent that a positive magnetoconductance remains after averaging. (b) amplitude of the aperiodic conductance fluctuations and of the weak-localization signal as a function of temperature. The lines are a guide to the eye.