Metallic carbon nanotube quantum dots under magnetic fields

Quantum dots made of individual metallic carbon nanotubes are theoretically studied under the influence of a magnetic field applied in the axial direction. After assessing the mechanical stability of the heterostructure by Monte Carlo simulations, the dependence of the electronic properties on the size of the nanotube quantum dot and applied magnetic field has been investigated within the Peierls approximation in a tight-binding model. The transport gaps induced by the magnetic field are found to be different from those of the perfect constituent tubes. Due to the presence of topological defects, some physical properties exhibit a lack of periodicity in the magnetic flux. The spin coupling to the magnetic field is also incorporated via a Zeeman term in the Hamiltonian; we have found huge differences between the up and down local densities of states which may be explored for future applications of carbon nanotube quantum dots as spintronic devices. Finally, the temperature dependence of the magnetic properties has also been addressed. We have found a diamagnetic response very similar to that of perfect tubes.

Figure 1

(Color online) Atomic configuration of the metallic quantum dot $(12,0)∕{(6,6)}_5∕(12,0)$ QD. Results are obtained from a numerical Monte Carlo simulation using the classic Tersoff potential.

Figure 2

LDOS as a function of the energy for (a) $N=2$ and (b) $N=6$ and magnetic flux varying from $\varphi ∕{\varphi }_o=0.0$ to 1.0.

Figure 3

(Color online) Conductance as a function of the Fermi energy for the $(12,0)∕{(6,6)}_{20}∕(12,0)$ QD and magnetic flux varying from $\varphi ∕{\varphi }_o=0.0$ to 1.0.

Figure 4

(a) LDOS as a function of the energy for a $(12,0)∕{(6,6)}_{20}∕(12,0)$ QD and null magnetic field (dashed line) and $\varphi ∕{\varphi }_o=0.3$ (bold curve). Spatial dependence of the LDOS for different energies $E=\left(\mathrm{b}\right)0.352\mathrm{eV}$, (c) $0.1\mathrm{eV}$, and (d) $0\mathrm{eV}$, and null magnetic field.

Figure 5

Spatial dependence of the LDOS for a $(12,0)∕{(6,6)}_{20}∕(12,0)$ QD, calculated at a peak energy $-1.42\mathrm{eV}$ and for a magnetic flux of $0.5{\varphi }_o$. Up- and down-spin components are shown with dotted and bold (blue online) curves, respectively.

Figure 6

(Color online) (a) Magnetization and (b) differential magnetization with respect to the magnetic flux versus magnetic flux for a $(12,0)∕{(6,6)}_6∕(12,0)$ QD at different temperatures.

Figure 7

(Color online) Magnetization dependence on the temperature for a $(12,0)∕{(6,6)}_6∕(12,0)$ QD.