Transport and spin-pseudospin domain walls in integer quantum Hall systems

Quantum Hall systems are characterised by a 2D electron gas (2DEG) which is incompressible in the bulk, i.e., there is an energy gap to creating low energy excitations. However gapless excitations can be created at the edges of the sample and are theoretically described by the chiral Luttinger liquid. The currents in the sample flow via these chiral edge modes and it has been an interesting issue to study electron transport when counter propagating edge modes are brought close together in space.; In the first part of this thesis we discuss transport in the presence of such a narrow and uniform barrier in the 2DEG. The chiral counter propagating modes on either side of the barrier, for an infinitely high barrier, may be modeled as a Luttinger liquid. The gapless mode of the Luttinger liquid is a signature of a broken symmetry state associated with an uncertainty in which edge the electron resides. The idea here is similar to the interlayer phase coherent state formed in double layer quantum Hall systems for narrow layer separations. Just as in the double layer case, we find it convenient to use a pseudospin language to describe the physics in the vicinity of the barrier. Pseudospin up (down) corresponds to the electron being on the left (right) side of the barrier and the broken symmetry state is characterised by a finite pseudospin domain wall width even in the absence of interedge tunneling. A finite barrier however gives rise to a finite gap in the excitation spectrum and this is described by the sine-Gordon model. We find that electron-electron interactions play an important role in tunneling and enhance the tunnel gap by a factor of 2 over the size expected for non-interacting electrons.; The second part of this thesis discusses the possibility of creating electron spin domain walls in a nu = 1 quantum Hall ferromagnet by using optical NMR techniques to create a spatially varying nuclear spin profile. As a result of the hyperfine coupling between the electron and nuclear spins, a spatially varying nuclear field would appear as a spatially varying effective Zeeman field for the electrons. (Abstract shortened by UMI.)...