Ballistic electron focusing by elliptic reflecting barriers

We investigate reflection of ballistic electrons off an elliptic barrier in a high quality AlGaAs/GaAs heterostructure. Electrons injected at one foca l point of an ellipse are collected at the second focal point, or at the mi dpoint. Application of a magnetic field perpendicular to the plane of the e llipse modifies the focusing effects by distorting the electrons' paths, re sulting in a novel four-terminal negative magnetoresistance phenomenon. Sev eral secondary features in the magnetoresistance are observed and are inter preted in the light of numerical path simulations. (C) 1999 American Instit ute of Physics. [S0003-6951(99)03809-7].