Mollie Schwartz

Infrared Spectroscopy of Landau Levels of Graphene

We report infrared studies of the Landau level (LL) transitions in single layer graphene. Our specimens are density tunable and show in situ half-integer quantum Hall plateaus. Infrared transmission is measured in magnetic fields up to B=18 T at selected LL fillings. Resonances between hole LLs and electron LLs, as well as resonances between hole and electron LLs, are resolved. Their transition energies are proportional to sqrt[B], and the deduced band velocity is (-)c approximately equal to 1.1 x 10(6) m/s. The lack of precise scaling between different LL transitions indicates considerable contributions of many-particle effects to the infrared transition energies.

Interaction-Induced Shift of the Cyclotron Resonance of Graphene Using Infrared Spectroscopy

We report a study of the cyclotron resonance (CR) transitions to and from the unusual n=0 Landau level (LL) in monolayer graphene. Unexpectedly, we find the CR transition energy exhibits large (up to 10%) and nonmonotonic shifts as a function of the LL filling factor, with the energy being largest at half filling of the n=0 level. The magnitude of these shifts, and their magnetic field dependence, suggests that an interaction-enhanced energy gap opens in the n=0 level at high magnetic fields. Such interaction effects normally have a limited impact on the CR due to Kohns theorem [W. Kohn, Phys. Rev. 123, 1242 (1961)], which does not apply in graphene as a consequence of the underlying linear band structure.

EXPERIMENTAL STUDIES OF ADVECTION-REACTION-DIFFUSION SYSTEMS

We present the results of experiments on the effects of chaotic fluid mixing on the dynamics of reacting systems. The flow studied is a chain of alternating vortices in an annular geometry with drifting and/or oscillatory time-dependence. The dynamical system is the oscillatory or excitable state of the well-known Belousov-Zhabotinsky chemical reaction. Results from two sets of experiments are as follows: (1) Fronts propagating in the oscillating vortex chain are found to mode-lock onto the frequency of the external oscillations. It is also found that the presence of a significant “wind” (drift of the vortices in the lab frame) causes fronts propagating against the wind to freeze. (2) Synchronization of oscillating reactions in an extended flow (vortex chain with large number of vortices) is found to be enhanced significantly by the presence of superdiffusive transport characterized by Lévy flights that connect different parts of the flow.