Spin-Sensitive Photoluminescence Microscopy of Quantum Hall Liquids
November 29th, 2017 NICHOLAS MOORE Tohoku University

Quantum Hall (QH) liquids form when electrons are confined to two dimensions and placed in a strong perpendicular magnetic field. They are among the topological states of matter which are insulating in their bulk due to a gapped density of states, and conducting at the boundary due to gapless edge states. Transport measurements of QH liquids have informed decades of research, but the insulating nature of the bulk has been a challenge to transport experiments seeking to learn what physics occurs away from the edge. We use a novel and highly sensitive form of photoluminescence microscopy to spatially image the polarization of electron and nuclear spins and study spin phenomena occurring in bulk QH liquids at millikelvin temperatures in GaAs. The small electron g-factor in this material creates competition between Zeeman and Coulomb energy which leads to a variety of spin phases and to mechanisms for dynamic nuclear polarization.

Here I discuss a transition between QH ferromagnetic and nonmagnetic phases that occurs in the state of Landau level filling factor equal to 2/3. We have observed these two phases separate into distinct domains at the critical point of the transition, and we discovered that applying sufficient current can excite these domains to form as a stripe pattern perpendicular to the current. We then developed a powerful technique to image the nuclear polarization that is generated by electrons flipping their spins as they traverse the boundaries of the striped domains. Further, we studied a regime in which the domains are in motion, and showed that the mechanism of their motion is also mediated by hyperfine interaction with the nuclear spin lattice.

Last, I will discuss a transition between QH ferromagnetic and skyrmion phases near to filling factor 1. QH skyrmions are the result of competition between Zeeman and exchange energy, and are topological spin quasiparticle excitations of the QH ferromagnet. We have unprecedentedly observed in real space the QH ferromagnet discontinuously transition into a skyrmion-like phase.

Seminar, November 29, 2017, 15:00. ICFO’s Seminar Room

Hosted by Prof. Dmitri Efetov