Electron focusing by a magnetic fie

Electron focusing by a magnetic field may also play a role in geometries other than the double-point contact geometry of Fig. 52. One example is mentioned in the context of junction scattering in a cross geometry in Section III.E. Another example is the experiment by Nakamura et al.108 on the magnetoresistance of equally spaced narrow channels in parallel (see Fig. 59). Resistance peaks occur in this experiment when electrons that are transmitted through one of the channels are focused back through another channel. The resistance peaks ocur at B = (n/m)Bfocus, where Bfocus is given by Eq. (3.25) with L the spacing of adjacent channels. The identification of the various peaks in Fig. 59 is given in the inset. Nakamura et al.108 conclude from the rapidly diminishing height of consecutive focusing peaks (which require an increasing number of specular reflections) that there is a large probability of diffuse boundary scattering. The reason for the difference with the experiments discussed previously is that the boundary in the experiment of Fig. 59 is defined by focused ion beam lithography, rather than electrostatically by means of a gate. As discussed in Section II.A, the former technique may introduce a considerable boundary roughness.