Prof. Efrat Shimshoni
Bar-Ilan University, Israel
Large magneto-conductance as a signature of magnetic phases in graphene

The emergence of magnetic ordering in two-dimensional electron systems in the quantum Hall regime is a remarkable consequence of interaction effects. Most prominently, these systems exhibit an intimate relation between spin and charge excitations enforced by a topological constraint. As a result,the nature of magnetic phases and transitions between them can be manifested in electric transport.

A particularly interesting realization of this phenomenon is observed in undopedgraphene,where owing to the presence of a valley isospin in addition to the real spin, a rich variety of magnetic phases can be formed. A transition between distinct phases may then be signified by large magneto-conductance. Indeed, recent experimental results indicate that graphene subject to a strong, tilted magnetic field exhibits an insulator-metal transition tunable by tilt-angle. This behavior is attributed to aquantum phase transition from a canted antiferromagnetic (CAF) to a ferromagnetic (FM) bulk quantum Hall state at filling factor zero.

Motivated by this observation, we develop a theoretical model accounting for the formation of conducting domain walls as a key to its understanding. We show the emergence of spin-valley edge textures in the two phases, and study the implied evolution in the nature of their collective excitations throughout the transition. In both phases, the low-energy edge excitations are found to be dominated by mutually coupled charged and neutral modes. In particular, we show that the CAF phase supports a gapped charge-carrying edge mode, constructed by the binding of a vortex (meron) in the bulk to a spin twist at the edge. The energy gap to excitation of this edge mode is therefore dictated by the bulk spin stiffness. At the transition to the FM state where the stiffnessvanishes, this charged edge mode becomes gapless. It isthus smoothly connected to the helical edge mode characteristic of the FM state, which exhibits almost perfect conduction. Our theory further suggests explanations to several experimental observations: most notably, the saturation of conductance in the FM phase below the ideal quantized conductance, and a doping-induced insulator-metal transition.


Prof. Efrat Shimshoni is a graduate of the Faculty of Physics at the Weizmann Institute of Science. After a postdoctoral research appointment as a Beckman fellow at the University of Illinois in Urbana-Champaign, she has been a faculty member in the University of Haifa during the years 1995-2008,including three years as a Head of Department. In 2008 she has joined the Department of Physics at Bar-Ilan University where she is presently leading a research group in Theoretical Condensed Matter physics. 

Prof. Shimshoni specialized on the theory of strongly correlated electronic systems, particularly in systems of reduced dimensionality and nanoscopic devices. Her research activities span a variety of topics including superconductivity, topological matter and quantum magnetism. In the recent years she has been primarily interested in magneto-transport phenomena in superconducting devices and spin-chain materials, and in quantum magnetic phases and phase transitions in graphene.

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