Oscillatory Hall effect in high-mobility two-dimensional electron gases

Abstract

We report an unexpected anomaly in the zero-field Hall coefficient of two-dimensional electron systems in high-mobility GaAs/AlGaAs heterostructures. Our device layout allows the investigation of mesoscopic systems with variable Fermi energy as well as with tunable Hall probes. At very low temperature, both positive and negative deviations from the noninteracting Hall coefficient ␥ H 0 are observed, which can be twice as large as ␥ H 0 itself. A distinct regularity in the deviations and their temperature dependence are interpreted as the spontaneous formation of localized spins and their indirect exchange interaction.

Highly Enhanced Thermopower in Two-Dimensional Electron Systems at Millikelvin Temperatures

“…The latter is plausible at G ≪ e 2 /h, when the 2DES becomes inhomogeneous deep into the band tail, and transport is dominated by classical percolation [28,29,30]. In our case however, G ≫ e 2 /h, and direct Hall measurements also indicate the charge distribution to be uniform [23]. Moreover, the TP never reverses its sign, ruling out sequential or co-tunneling effects in unintentional quantum dots within mesoscopic region [30,31].…”

“…Silicon delta-doped heterostructures with thick (≈ 80 nm) spacer layer of undoped AlGaAs (to minimize remote impurity scattering) were employed in our experiments. Similar devices were earlier used for nonequilib- rium transport and Hall measurements [16,17,23]. The mobility of the electrons in these wafers were found to be in the range of 1 − 3 × 10 6 cm 2 /V-s at the as-grown electron sheet density, n s ≃ 1 × 10 11 cm 2 resulting in a long elastic mean free path ( 10µm).…”

“…We complement thermal measurements with conventional nonequilibrium transport, and show that these results point strongly towards the existence of localized spins in 2D mesoscopic systems. Our findings may have a direct impact on the understanding of many experimentally reported, but not fully understood, phenomena in low-dimensional quantum systems, such as the '0.7-state' [14] and breakdown of Wiedemann-Franz law in quantum point contacts (QPC) [22], the zero-bias anomaly [16,17,18], or the anomalous Hall effect in 2DESs at ultra-low temperatures [23].…”

Anomalous transport in mesoscopic inhomogeneous two-dimensional electron systems at low temperature

“…This has recently been observed. 27 With dc bias, if transport is indeed through a network of barriers and metallic domains, one might expect to observe features also seen in quantum dots and quantum point contacts, namely, a zero-bias anomaly. 28 This has also been seen in experiments by Ghosh et al 29 Finally, if transport is through metallic domains with discrete states, then one might expect to see an oscillatory thermopower as a function of the gate bias.…”

Field-tunable magnetic phases in a semiconductor-based two-dimensional Kondo lattice

“…The lowest field ranges show strongest fluctuations, and increasing field range decreases the intensity of the fluctuations. Since the interspin interaction itself couples to very small transverse fields [22], the fluctuations are only meaningful when the slope is computed over smallest field ranges, i.e., leading to the zero field Hall coefficient. Since very low field ranges do also increase the error in the data, we use 2-5 mT range for all γ H data.…”