1. Activation energies and dissipation in biased quantum Hall bilayer systems at. B. Roostaei [1], H. A. Fertig [2,3], K. J. Mullen [1], S. Simon [4] [1] Department of Physics, University of Oklahoma, Norman, OK [2] Department of Physics, Indiana University, Bloomington, IN [3] Technion, Haifa,Israel [4] Lucent Tech., Murray Hill, NJ Supported by : NSF and the Center for Semiconductor Physics in Nanostructures (NSF-MRSEC) OSCER : OU Supercomputer Center. APS March Meeting 2007

2. Outline : Merons as Pseudospin excitations of double layer quantum Hall systems. Meron flavors. Experimental results of transport in separately contacted bilayers. Disorder and effect of merons on transport. Numerical HF results of activation energies of merons Model of meron-edge interaction ( ongoing work).

3. Anisotropic Heisenberg magnet in long wavelength approximation : Double layer electron gas in strong magnetic field : Pseudospin formalism : AlGaAsGaAs Energy Typical separation between electrons Two electron gases form a quantum coherent liquid when : Quantum Coherence

4. Pseudospin-z Pseudospin Excitations Uniform State Charged Excitations Topological Excitations : Bimerons : Meron-Meron Pairs They carry electric charge Their projection in the plane is a vortex-antivortex pair. 1  T

5. Meron Flavors Charge : vorticity : In real experiment disorder/Temperature likely to unbind merons. Each meron carries half electronic charge and an electric dipole moment. VorticityElectric Dipole moment Charge +1+1/2-e/2 +1/2+e/2 +1-1/2+e/2 -1/2-e/2 L U

6. Image from Senthil et al., Science 2004. Cartoon Picture of a Meron

7. Transport Experiments : Drag and Drive V V R. Wiersma, et. Al. PRL 93,266805(2004) Drag: symmetric in bias Drive: antisymmetric in bias Measured activation energies behave differently with respect to bias for drag and drive layer ! This may be due to dissipation caused by merons. Activation energies

8. Effect of Disorder Dopants form a smooth disorder potential inducing puddles of charge. This disorder excites meron-meron pairs and unbinds them in the system. Merons and antimerons can diffuse in the system independently. There is a barrier for merons in hopping over an incompressible region from one puddle to the other. + + + + + + H.A. Fertig,G. Murthy,PRL 95 (2005) + + + + - + - - Incompressible barrier

9. Checkerboard Bias Pseudospin-z Calculation of Activation barrier using meron lattice We can model the activation barrier using our lattice of merons in the Hartree-Fock approximation : We apply a checkerboard bias potential to the lattice, in-phase and out of phase. Analogy to spin in magnetic field : The activation barrier is : L. Brey, H.A. Fertig, R. Cote, and A.H. MacDonald, PRB 54, 16888 (1996)

10. The energy is linear in interlayer bias. Estimation of activation energies are higher than observation. HF Approximation over-estimates the exchange energy. The results are depends on the shape and width of the potential. Narrow potential W < (meron radius) W W ~ (meron radius)

11. Next question : Why the behavior of activation energy for drag and drive layer is so different ? Model of interaction of merons with the edge and its dynamics : R. Wiersma, et. Al. PRL 93,266805(2004) Counterflow current drives merons to the edges. Merons can dissipate edge current in drag and drive layer depending on their charge and electric dipole moment. VA A VA A -e/2 +e/2 -e/2 VorticityElectric Dipole moment Charge +1+1/2-e/2 +1/2+e/2 +1-1/2+e/2 -1/2-e/2 antisymmetric symmetric Ongoing work : V V

12. Summary Drag and drive layer resistivities are observed to be activated. The activation energies for drag and drive layer behave differently as overall bias changes. Activated behavior may be caused by mobile topological excitations of the bilayer system. These topological excitations have pseudospin nature( merons and antimerons). Behavior of activation energies can be understood by the fact that merons have electric dipole moment and charge. A model of interaction of the merons with edge electrons may explain the activated behavior consistently.