  We report the observation of an unpredicted behavior of interfering 2D
electrons in the integer quantum Hall effect (IQHE) regime via a utilization of
an electronic analog of the well-known Mach-Zehnder interferometer (MZI). The
beauty of this experiment lies in the simplicity of two path interference.
Electrons that travel the two paths via edge channels, feel only the edge
potential and the strong magnetic field; both typical in the IQHE regime. Yet,
the interference of these electrons via the Aharonov-Bohm (AB) effect, behaves
surprisingly in a most uncommon way.
  We found, at filling factors 1 and 2, high visibility interference
oscillations, which were strongly modulated by a lobe-type structure as we
increased the electron injection voltage. The visibility went through a few
maxima and zeros in between, with the phase of the AB oscillations staying
constant throughout each lobe and slipping abruptly by at each zero. The lobe
pattern and the 'stick-slip' behavior of the phase were insensitive to details
of the interferometer structure; but highly sensitive to magnetic field. The
observed periodicity defines a 'new energy scale' with an unclear origin. The
phase rigidity, on the other hand, is surprising since Onsager relations are
not relevant here.
