Quantum Hall Mach-Zehnder interferometer at fractional filling factors

We use a Mach-Zehnder quantum Hall interferometer of a novel design to investigate the interference effects at fractional filling factors. Our device brings together the advantages of usual Mach-Zehnder and Fabry-Perot quantum Hall interferometers. It realizes the simplest-for-analysis Mach-Zehnder interference scheme, free from Coulomb blockade effects. By contrast to the standard Mach-Zehnder realization, our device does not contain an etched region inside the interference loop. For the first time for Mach-Zehnder interference scheme, the device demonstrates interference oscillations with $\Phi^*=e/e^*\Phi_0=\Phi_0/\nu$ periodicity at fractional filling factor 1/3. This result indicates that we observe clear evidence for fractionally charged quasiparticles from simple Aharonov-Bohm interference.