Coulomb Drag (etc.)

Mindy Kellogg

Coulomb Drag is a measurement configuration designed to directly probe interactions between the electrons in two closely spaced 2D electron gases (2DEGs). High quality double quantum wells of GaAs/AlGaAs provide two parallel 2DEGS separated by just 100 Angstroms and yet with extremely little tunneling between them. Selective depletion technique allows us to drive current through just one of the layers while measuring the voltage that builds up in the other layer via the interlayer Coulomb interactions.

My research focuses on the special case in which the total filling factor of the two layers is unity. When the layers are close enough together, the system is believed to enter a ferromagnet state in which the pseudospins (the eigenstates of the layer index) are aligned in the XY plane. Applying the 2D XY model to our system leads us to anticipate a superfluid mode in the form of a perfectly dissipationless antisymmetric current. Our 2002 drag measurements (shown below - Figure 1) give indirect evidence for this superfluid mode, manifesting most dramatically as the observation of "Quantized Hall Drag" (curve B). In our latest experiments we have measured resistances while in a counterflowing, antisymmetric current configuration and found that the dissipation does go to zero (as the temperature goes to zero) while the conductivity goes to infinity. This is the most direct evidence yet of superfluidity in this system (see Figure 2 below and our PRL 93, 036801).

FIGURE 1: Conventional and Coulomb drag resistances of a low density double layer 2D electron system. Trace A: Conventional longitudinal resistance R_xx measured with current in both layers. Trace B: Hall drag resistance R_xy,D. Trace C: Longitudinal drag resistance R_xx,D. Trace D: Hall resistance R_xy* of single current-carrying layer (displaced vertically by 5 kΩ for clarity). Trace B reveals the quantization of Hall drag in the ν =1 excitonic QHE. Insets schematically illustrate the measurement configurations: Current is injected and withdrawn at the open dots; voltage differences between the solid dots are recorded. Traces A,B, and D obtained at T=20 mK; trace C at 50mK. Layer densities: N₁=N₂=2.6 x 10¹⁰ cm^-2, giving d/l=1.6 at ν =1.

FIGURE 2: Superfluidity as evidenced by vanishing Hall and longitudinal resistances when currents in the layers go in opposing directions. Main figure shows the Hall resistance R_xy (measured in just one layer) versus magnetic field in the parallel (dotted line) and counterflow (solid line) configuration for n=2.46 x 10¹⁰ cm^-2 at T=30mK. At total filling factor one, the Hall resistance becomes quantized in the parallel configuration, but vanishes in the counterflow configuration. The inset shows the longitudinal resistances, both of which vanish at total filling factor one.

Papers

Onset of Interlayer Phase Coherence in a Bilayer Two-Dimensional Electron System: Effect of Layer Density Imbalance
I.B. Spielman, M. Kellogg, J.P. Eisenstein, L.N. Pfeiffer, and K.W. West, Phys. Rev. B 70, 081303 (2004).
Vanishing Hall Resistance at High Magnetic Field in a Double Layer Two-Dimensional Electron System
M. Kellogg, J.P. Eisenstein, L.N. Pfeiffer, and K.W. West, Phys. Rev. Lett. 93, 036801 (2004).
Bilayer Quantum Hall Systems at ν_T = 1: Coulomb Drag and the Transition from Weak to Strong Interlayer Coupling
M. Kellogg, J.P. Eisenstein, L.N. Pfeiffer, and K.W. West, Phys. Rev. Lett. 90, 246801 (2003).
Evidence for 2k_F Electron-Electron Scattering Processes in Coulomb Drag
M. Kellogg, J.P. Eisenstein, L.N. Pfeiffer, and K.W. West, Solid State Commun. 123, 515 (2002).
Observation of Quantized Hall Drag in a Strongly Correlated Bilayer Electron System
M. Kellogg, I.B. Spielman, J.P. Eisenstein, L.N. Pfeiffer, and K.W. West, Phys. Rev. Lett. 88, 126804 (2002).
Watch my Everhart Lecture ("Stalking the Exciton Condensate" - 3/11/04) or just look at my slides .
Look at my thesis. Here are the Fortran programs from Appendix K of my thesis.

Last modified: May 12, 2005