Annika Kriisa

Magneto-transport characteristics of a 2D electron system driven to negative magneto-conductivity by microwave photoexcitation

Negative diagonal magneto-conductivity/resistivity is a spectacular- and thought provoking-property of driven, far-from-equilibrium, low dimensional electronic systems. The physical response of this exotic electronic state is not yet fully understood since it is rarely encountered in experiment. The microwave-radiation-induced zero-resistance state in the high mobility GaAs/AlGaAs 2D electron system is believed to be an example where negative magneto-conductivity/resistivity is responsible for the observed phenomena. Here, we examine the magneto-transport characteristics of this negative conductivity/resistivity state in the microwave photo-excited two-dimensional electron system (2DES) through a numerical solution of the associated boundary value problem. The results suggest, surprisingly, that a bare negative diagonal conductivity/resistivity state in the 2DES under photo-excitation should yield a positive diagonal resistance, with a concomitant sign reversal in the Hall voltage.

Electric fields enhance miscibility of polystyrene/poly(vinyl methyl ether) blends

How the presence of electric fields alters the miscibility of mixtures has been studied since the 1960s with conflicting reports on both the magnitude and direction of the shift in the phase separation temperature Ts. Theoretical understanding of the phenomenon has been hampered by the lack of experimental data with unambiguously large shifts in Ts outside of experimental error. Here, we address these concerns by presenting data showing that uniform electric fields strongly enhance the miscibility of polystyrene (PS)/poly(vinyl methyl ether) (PVME) blends. Reliable shifts in Ts of up to 13.5 ± 1.4 K were measured for electric fields strengths of E = 1.7 × 10(7) V/m in a 50/50 PS/PVME mixture. By using a sensitive fluorescence method to measure Ts, the PS/PVME blend can be quenched back into the one phase region of the phase diagram when the domains are still small allowing the blend to be remixed such that Ts can be measured repeatedly on the same sample. In this manner, highly reproducible Ts values at non-zero and zero electric field can be ascertained on the same sample. Our results agree with the vast majority of existing experimental data on various mixtures finding that electric fields enhance miscibility, but are opposite to the one previous study on PS/PVME blends by Reich and Gordon [J. Polym. Sci.: Polym. Phys. Ed. 17, 371 (1979)] reporting that electric fields induce phase separation, a study which has been considered anomalous in the field.

Hall Effects in Doubly Connected Specimens

We examine the impact on the Hall effect, when a hole is inserted inside the canonical Hall geometry, in 2-D electron gas specimens that exhibit an ultrahigh mobility (μ ~ 107 cm2/V·s) at liquid helium temperatures. We demonstrate that an “anti-Hall bar” configuration generates an ordinary Hall effect within the interior boundary and that the interior Hall voltage divided by the interior injected current equals the Hall resistance. Further, we show that it is possible to simultaneously realize two independent Hall effects in the high-quality specimen by injecting two currents into a single doubly connected device.

Topological Hall insulator

We examine physical transformations that preserve the well known properties of the Hall effect in the GaAs/AlGaAs 2D system and show that the Hall effect is immune to certain deformations such as the insertion of holes, inversion transformations, and superposition. From the results, we conclude that the ordinary Hall effect includes topological protection for the above mentioned operations.