Carbon nanotubes (CNTs) have been extensively studied due to their
excellent properties such as ballistic transport. Electrically induced charge carriers in
CNTs and the relation between the spin states and the ballistic transport, however, have
not yet been microscopically investigated owing to experimental difficulties. Here we
review electron spin resonance (ESR) spectroscopy of semiconductor single-walled
CNT (SW-CNT) thin films and their transistors. We have investigated the spin states
and the electrically induced charge carriers in the SW-CNTs by utilizing a transistor
structure under device operation. The electrically induced ESR method is useful for the
microscopic investigation into CNTs because it is capable of directly observing the
spins in CNTs. We have observed a clear reverse correlation between the ESR intensity
and the transistor current under high charge-density conditions. This result directly
demonstrates electrically induced ambipolar spin vanishment in CNTs, providing a first
clear evidence of antiparallel spin fillings of the electrically induced charges’ spins and
the vacancies’ spins in CNTs. The ambipolar spin vanishment is considered to improve
the transport properties of CNTs because it seems to greatly reduce carrier scatterings.
Similar spin vanishment has been observed in single-layer graphene transistors. Thus,
this result suggests that the electrically induced ambipolar spin vanishment is a
universal phenomenon for carbon materials.
Keywords: Ambipolar spin vanishment, Anisotropy, Antiparallel spin fillings,
Carbon nanotubes (CNTs), Charge carriers, Electron spin resonance (ESR), Gate
voltage, Number of spins, Single-walled CNTs (SW-CNTs), SW-CNT thin films,
SW-CNT transistor, Spin-orbital interaction, Spin states, Spin susceptibility,
Temperature dependence, Transport properties, Thin films, Tomonaga-Luttinge-
-liquid (TLL) states.
