We have measured the dynamic alignment properties of single-walled carbon nanotube (SWNT) suspensions in pulsed high magnetic fields through linear dichroism spectroscopy. Millisecond-duration pulsed high magnetic fields up to 56 T as well as microse
cond-duration pulsed ultrahigh magnetic fields up to 166 T were used. Due to their anisotropic magnetic properties, SWNTs align in an applied magnetic field, and because of their anisotropic optical properties, aligned SWNTs show linear dichroism. The characteristics of their overall alignment depend on several factors, including the viscosity and temperature of the suspending solvent, the degree of anisotropy of nanotube magnetic susceptibilities, the nanotube length distribution, the degree of nanotube bundling, and the strength and duration of the applied magnetic field. In order to explain our data, we have developed a theoretical model based on the Smoluchowski equation for rigid rods that accurately reproduces the salient features of the experimental data.
We have investigated excitons in highly-aligned single-walled carbon nanotubes (SWCNTs) through optical spectroscopy at low temperature (1.5 K) and high magnetic fields ($textbf{textit{B}}$) up to 55 T. SWCNT/polyacrylic acid films were stretched, gi
ving SWCNTs that are highly aligned along the direction of stretch ($hat{n}$). Utilizing two well-defined measurement geometries, $hat{n}paralleltextbf{textit{B}}$ and $hat{n}perptextbf{textit{B}}$, we provide unambiguous evidence that the photoluminescence energy and intensity are only sensitive to the $textbf{textit{B}}$-component parallel to the tube axis. A theoretical model of one-dimensional magneto-excitons, based on exchange-split `bright and `dark exciton bands with Aharonov-Bohm-phase-dependent energies, masses, and oscillator strengths, successfully reproduces our observations and allows determination of the splitting between the two bands as $sim4.8$ meV for (6,5) SWCNTs.
