Quantum Transport in Graphene Quantum Dots

Abstract

Graphene exhibits unique electrical properties and offers substantial potential as building blocks of nanodevices owing to its unique two-dimensional structure (Geim et al., 2007; Geim et al., 2009; Ihn et al., 2010). Besides being a promising candidate for high performance electronic devices, graphene may also be used in the field of quantum computation, which involves explo‐ ration of the extra degrees of freedom provided by electron spin, in addition to those due to elec‐ tron charge. During the past few years, significant progress has been achieved in implementation of electron spin qubits in semiconductor quantum dots (Hanson et al., 2007; Hanson et al., 2008). To realize quantum computation, the effects of interactions between qubits and their environment must be minimized (Fischer et al., 2009). Because of the weak spin-orbit coupling and largely eliminated hyperfine interaction in graphene, it is highly desirable to co‐ herently control the spin degree of freedom in graphene nanostructures for quantum computa‐ tion (Trauzettel et al., 2007; Guo et al., 2009). However, the low energy quasiparticles in single layer grapheme behave as massless Dirac fermions (Geim et al., 2007; Geim et al., 2009), and the relativistic Klein tunneling effect leads to the fact that it is hard to confine electrons within a small region to form quantum dot in graphene using traditional electrostatical gates (Ihn et al., 2010; Trauzettel et al., 2007). It is now possible to etch a grapheme flake into nano-constrictions in size, which can obtain electron bound states and thus act as quantum dots. As a result, usually a dia‐ mond-like characteristic of suppressed conductance consisting of a number of sub-diamonds is clearly seen (Stampfer et al., 2009; Gallagher et al., 2010), indicating that charge transport in the single graphene quantum dot device may be described by the model of multiple graphene quan‐ tum dots in series along the nanoribbon. The formation of multiple quantum dot structures in the nanoribbons may be attributed to edge roughness or local potential. The rough edges also lift the valley degeneracy, which could suppress the exchange coupling between spins in the gra‐ pheme quantum dots (Trauzettel et al., 2007; Ponomarenko et al., 2008). Recently, there was a