Carbon nanotubes with ferromagnetic contacts – Niels Bohr Institute - University of Copenhagen

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Carbon nanotubes with ferromagnetic contacts

When electrodes are placed upon a carbon nanotube (CNT) the contact resistance between electrode and CNT will usually be high compared to the quantum resistance h/4e2, where h is Planck's constant and e is the charge of an electron. This high degree of isolation of the electrons on the CNT results in an electro-static energy of e2/C for removal or addition of just a single electron, where C is the capacitance of the CNT to its surroundings. When e2/C, often called the Coulomb blockade energy, becomes larger than the thermal energy, the CNT will behave as a quantum dot. If the contact resistance decrease towards h/4e2 higher order tunnelling effect though the CNT begins to play a role, and Kondo effect and level renor-malization can be observed.
In this presentation I will focus on CNT quantum dots in the Kondo regime (small contact resistances) with ferromagnetic nickel contacts. The Kondo effect is used as a probe to study the spin state of the CNT quantum dot. I have in my experiments observed that the ferromagnetic electrodes via the tunnel couplings to the CNT induce a sizeable magnetic field called the exchange field in the CNT. This field splits the normally degenerate spin states and leads to a splitting of the Kondo peak. By applying an ex-ternal field it is possible to compensate this splitting, thereby restoring the degenerate Kondo-peak, sig-nalling a vanishing effective magnetic field on the quantum dot. It is furthermore demonstrated in my experiments that it is possible to control and reverse this tunnelling induced exchange fields and thereby the splitting of the spin states, merely by tuning the gate-voltage. This demonstrates a very direct electri-cal control over the spin-state of a quantum dot which, in contrast to an applied external magnetic field, may allow for a rapid spin-reversal with a local gatevoltage.

PhD Defence by Jonas Rahlf Hauptmann