Superconductivity in Thin Films of Boron-Doped Carbon Nanotubes

Superconductivity in carbon nanotubes (CNTs) is attracting considerable attention. However, its correlation with carrier doping has not been reported. We report on the Meissner effect found in thin films consisting of assembled boron (B)-doped single-walled CNTs (B-SWNTs). We find that only B-SWNT films consisting of low boron concentration leads to evident Meissner effect with $T_c=12\mathrm{K}$ and also that a highly homogeneous ensemble of the B-SWNTs is crucial. The first-principles electronic-structure study of the B-SWNTs strongly supports these results.

Figure 1

(a),(b) Room temperature Raman spectra of semiconducting $\mathrm{B}$-SWNT bundles synthesized from targets containing $N_{\mathrm{B}}$ of $\sim$ (i) 0, (ii) 1.5, (iii) 2.0, (iv) 3.0, and (v) 4.5 at. %. ${\lambda }_{\mathrm{ext}}=514.5\mathrm{nm}$ is the excitation wavelength. The $\mathrm{B}$-induced changes in the $\mathrm{RBM}$ (a) and the D ($\sim 1350{\mathrm{cm}}^{-1}$) and G bands ($\sim 1600{\mathrm{cm}}^{-1}$) are shown in (b). (c) Three peaks labeled as $a$, $b$, and $c$ are evident in the NMR spectrum of $\mathrm{B}$-SWNTs synthesized with the $N_{\mathrm{B}}\sim 1.5\mathrm{at}.\%$ target.

Figure 2

Scanning electron microscope (SEM) images of thin films consisting of assembled $\mathrm{B}$-SWNTs prepared on Si substrates (a) without spin coating and (b) by spin coating (500 rpm). Insets: Cross-sectional SEM images of individual samples observed at a 30° tilt.

Figure 3

Normalized magnetization as a function of temperature at magnetic fields ($H$) of 100 Oe in FC and ZFC regimes in thin films of assembled $\mathrm{B}$-SWNTs, which are synthesized from a target with $N_{\mathrm{B}}=\sim 1.5\mathrm{at}.\%$ and prepared following the method used in Fig. 2b.

Figure 4

Normalized magnetization vs magnetic field for various temperatures in the sample shown in Fig. 3. Inset: Upper critical field ($H_{\mathrm{c}2}$) vs the temperature relationship estimated from the main panel. We determine $H_{\mathrm{c}2}$ at the $H$ value for which $M_N=0$ at each temperature by extrapolating the linear slope of the $M_N–H$ relationship for $H>\sim 1500\mathrm{Oe}$ in the main panel.

Figure 5

Correlation of normalized magnetization drops with $N_{\mathrm{B}}$ in targets. Contribution of graphite structure on magnetization is not subtracted [7b]. Inset: $H_{c2}$ vs temperature relationships estimated from $M_N–H$ relationship in the thin films consisting of high $N_{\mathrm{B}}$-value SWNTs (2 and 3 at. %).

Figure 6

Electronic density of states of individual semiconducting $\mathrm{B}$-SWNT, ${\mathrm{BC}}_{159}$ obtained by using the local-density approximation in the framework of the density-functional theory [18]. Plane-wave basis with the cutoff energy of 50 Ry is used. The host CNT is (10, 0) SWNT (i.e., zigzag nanotube) with the $N_{\mathrm{B}}$ value as low as 0.625 at. %. Energy is measured from the $E_F$. Inset: Fermi-level density of states as a function of $N_{\mathrm{B}}$.