The main characteristics of good qubits are long coherence times in combination with fast operating times. It is well known that carbon-based materials could increase the coherence times of spin qubits, which are among the most developed solid-state qubits. Here, we propose how to form spin qubits in graphene quantum dots. A crucial requirement to achieve this goal is to find quantum-dot states where the usual valley degeneracy in bulk graphene is lifted. We show that this problem can be avoided in quantum dots based on ribbons of graphene with armchair boundaries. The most remarkable new feature of the proposed spin qubits is that, in an array of many qubits, it is possible to couple any two of them via Heisenberg exchange with the others being decoupled by detuning. This unique feature is a direct consequence of the quasi-relativistic spectrum of graphene.

Spin qubits in graphene quantum dots

Even though graphene is a low-energy system consisting of a two-dimensional honeycomb lattice of carbon atoms, its quasiparticle excitations are fully described by the (2+1)-dimensional relativistic Dirac equation. In this paper we show that, while the spin-orbit interaction in graphene is of the order of 4 meV, it opens up a gap of the order of 10(-3) meV at the Dirac points. We present a first-principles calculation of the spin-orbit gap, and explain the behavior in terms of a simple tight-binding model. Our result also shows that the recently predicted quantum spin Hall effect in graphene can occur only at unrealistically low temperature.

Spin-orbit gap of graphene: First-principles calculationsYugui Yao

Hysteretic switching in the magnetoresistance of short-channel, ferromagnetically contacted individual single wall carbon nanotubes is observed, providing strong evidence for nanotube spin transport. By varying the voltage on a capacitively coupled gate, the magnetoresistance can be reproducibly modified between +10% and -15%. The results are explained in terms of wave vector matching of the spin polarized electron states at the ferromagnetic / nanotube interfaces.

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-002635.pdf

Gated Spin Transport through an Individual Single Wall Carbon Nanotube

We investigate the interplay between the edge and bulk states, induced by the Rashba spin-orbit coupling, in a zigzag silicene nanoribbon in the presence of an external electric field. The interplay can be divided into two kinds, one is the interplay between the edge and bulk states with opposite velocities, and the other is that with the same velocity direction. The former can open small direct spin-dependent subgaps. A spin-polarized current can be generated in the nanoribbon as the Fermi energy is in the subgaps. While the later can give rise to the spin precession in the nanoribbon. Therefore, the zigzag silicene nanoribbon can be used as an efficient spin filter and spin modulation device.

Interplay between edge and bulk states in silicene nanoribbon

Spin dependent transport through triangular graphene quantum dot in the presence of Rashba type spin–orbit coupling

We have calculated spin-dependent conductance in zigzag graphene nanoribbons, attached to two leads, in the presence of Rashba spin−orbit interaction, based on the green function method. A tight binding model including Rashba spin-orbit interaction is used. It is shown that elements of conductance matrix crucially depend on the width and the length of the graphene nanoribbon for strong Rashba spin−orbit interaction.

Spin-polarized transport in zigzag graphene nanoribbons with Rashba spin–orbit interaction

Effect of single magnetic atom on spin-polarized transport of armchair graphene nanoribbons

By using a simple method, the effects of a substitutional magnetic impurity on the conductance of metallic single-wall carbon nanotubes, lying between two spin-polarized electron reservoirs, are studied. It is demonstrated how the differential conductance depends sensitively on the radius and length of nanotube as well as the position of impurity. It is shown that magnetic impurity produces more effect for the spin-polarized current between the antiparallel reservoirs than the parallel ones.

M. Kariminezhad

Papers

Spin-polarized transport in zigzag graphene nanoribbons with Rashba spin–orbit interaction

Effect of single magnetic atom on spin-polarized transport of armchair graphene nanoribbons

Effect of magnetic impurity on spin-polarized transport in armchair single-wall carbon nanotubes