Quantum ballistic transport in nanowire junctions

Abstract

The quantum ballistic transport of electrons through nanowire junctions formed by putting one nanowire (with square, rectangular, and circular cross sections) on top of another wire (the two wires are not lying in the same plane) is studied using tight-binding models and the Green's function approach. Different tight-binding models are considered to find the optimum number of tight-binding sites in the transverse plan for approximating the continuum model in a finite range of energy and also to find the minimal site number for qualitative description of the transport characteristics. Resonant dips and peaks found in the interwire and intrawire conductances can be explained by the formation of bound and quasibound states at the cross junction. How the conduction channels of the wires are coupled together in the formation of the bound and quasibound states are analyzed using the projected Green's functions. Quasibound states unbound in both wires give rise to resonant peaks in the interwire conductance as well as resonant dips in the intrawire conductance. Quasibound states unbound in only one wire give rise to only resonant dips in the intrawire conductance without corresponding resonant peaks in the interwire conductance. Some exception in the latter case is discussed. Reduction of interwire coupling strength is shown to suppress the conductance at energy far from the subband edges. In the weak interwire coupling regime, larger conductances are found at energies close to the subband edges. A comparison of the square and rectangular wire junctions studies is made. Increase in the dimension perpendicular to a square wire junction reduces the interwire conductance and enhances the intrawire conductance. On the other hand, degeneracy of subbands due to the higher symmetry of a square cross section enhances the interwire conductance. Comparison of circular wire junctions with square wire junctions show that some conductance features found in square wire junctions disappear in circular wire junctions owing to the weaker interwire coupling in circular wire junctions, which is the result of a smaller contact area between wires. The energies and probability densities of some bound and quasibound states are also determined and analyzed in the present study.