Effect of an impurity in a quantum resonator.

We present a numerical study of the influence of disorder on quantum-mechanical transmission by considering the presence of an impurity in a T-shaped quantum-wire junction. The device is modeled as an electron waveguide of finite cross section. Transmission and reflection probabilities are computed by use of a waveguide-matching technique. The calculations show that the transmission of the two-dimensional electron gas through the narrow-wide-narrow geometry can be both enhanced and suppressed by the presence of a repulsive impurity potential. Although perfect transparency is not generally achieved with the presence of a single impurity, we find a 100% modulation of conductance as one device parameter is changed, which is robust against the scattering. If the impurity is located in the narrow-wire region, a periodic oscillation appears in transmission due to multiple reflections between the impurity and the stub region. We also examine an attractive scatterer.