  The fractional quantum Hall effect (FQHE) is a canonical example of a
topological phase in a correlated 2D electron gas under strong magnetic field.
While electric currents propagate as chiral downstream edge modes, chargeless
upstream chiral neutral edge modes were recently observed only in
hole-conjugate states (states filling \nu, n+1/2<\nu<n+1, with n=0,1,2,...),
and in the even denominator state \nu=5/2. It is believed that spontaneous
'density reconstruction' near the edges of the 2D gas, leads to multiple
counter propagating edge channels, being separated from each other by
incompressible strips. Unavoidable disorder induces inter-channel tunneling;
accompanied by Coulomb interaction it renormalizes the multiple edge channels
to a downstream charge mode and upstream neutral edge mode(s), while
maintaining the parity requirements (e.g., the heat conductance along the edge)
dictated by the bulk. Here, we report of highly sensitive shot noise
measurements that revealed unexpected presence of neutral modes in a variety of
non-hole-conjugate fractional states. As already reported previously, we did
not observe neutral modes in any of the integer states. In addition to the
upstream neutral edge modes, we were surprised to find also neutral energy
modes that propagate through the incompressible bulk. While along the edge,
density reconstruction may account for the edge modes, we are not aware of a
model that can account for the bulk modes. The proliferation of neutral modes,
in every tested fractional state, changes drastically the accepted picture of
FQHE states: an insulating bulk and 1D chiral edge channels. The apparent
ubiquitous presence of these energy modes may account for decoherence of
fractional quasiparticles - preventing observation of coherent interference in
the fractional regime.
