Theory for electron hopping through nanometer-scale contacts: From tunneling regime to ballistic regime

Using the recursion-transfer-matrix (RTM) method combined with nonequilibrium Green's function (NEGF) method, we study the electronic states and current-voltage (I-V) characteristics of junction systems with atomic-scale nanocontacts as a function of the distance between electrodes. We observe a strong nonlinear behavior in the I-V characteristics and correspondingly a gap structure appears in conductance. We find that such a nonlinear behavior emerges when the transport properties change from tunneling to ballistic regimes.