Interlayer tunneling in counterflow experiments on the excitonic condensate in quantum Hall bilayers

The effect of tunneling on the transport properties of} quantum Hall double layers in the regime of the excitonic condensate at total filling factor one is studied in counterflow experiments. If the tunnel current $I$ is smaller than a critical $I_C$, tunneling is large and is effectively shorting the two layers. For $I > I_C$ tunneling becomes negligible. Surprisingly, the transition between the two tunneling regimes has only a minor impact on the features of the filling-factor one state as observed in magneto-transport, but at currents exceeding $I_C$ the resistance along the layers increases rapidly

Carbon-doped high mobility two-dimensional hole gases on (110) faced GaAs

Carbon-doped high mobility two-dimensional hole gases grown on (110) oriented GaAs substrates have been grown with hole mobilities exceeding 10^6 cm^2/Vs in single heterojunction GaAs/AlGaAs structures. At these high mobilities, a pronounced mobility anisotropy has been observed. Rashba induced spin-splitting in these asymmetric structures has been found to be independent on the transport direction

Carbon doped symmetric GaAs/AlGaAs quantum wells with hole mobilities beyond 10^6 cm^2/Vs

Utilizing a novel carbon doping source, we prepared two-dimensional hole gases in a symmetric quantum well structure in the GaAs/AlGaAs heterosystem. Low temperature hole mobilities up to 1.2 x 10^6 cm^2/Vs at a density of 2.3 x 10^11 cm^-2 were achieved on GaAs (001) substrates. In contrast to electron systems, the hole mobility sensitively depends on variations of the quantum well width and the spacer thickness. In particular an increase of the quantum well width from an optimal value of 15 nm to 18 nm is accompanied by a 35 % reduction of the hole mobility. The quality of ultrahigh-mobility electron systems is not affected by the employed carbon doping source

Nonlinear growth with the microwave intensity in radiation-induced magnetoresistance oscillations

We report the observation of inverse-magnetic-field-periodic, radiation-induced magnetoresistance oscillations in GaAs/AlGaAs heterostructures prepared in W. Wegscheider's group, compare their characteristics with similar oscillations in V. Umansky's material, and describe the lineshape variation vs. the radiation power, $P$, in the two systems. We find that the radiation-induced oscillatory $\Delta R_{xx}$, in both materials, can be described by $\Delta R_{xx} = -A exp(-\lambda/B)sin(2 \pi F/B)$, where $A$ is the amplitude, $\lambda$ is the damping parameter, and $F$ is the oscillation frequency. Both $\lambda$ and $F$ turn out to be insensitive to $P$. On the other hand, $A$ grows nonlinearly with $P$.Comment: 6 pages, 6 color figure

Enhanced spin-orbit scattering length in narrow Al_xGa_{1-x}N/GaN wires

The magnetotransport in a set of identical parallel AlGaN/GaN quantum wire structures was investigated. The width of the wires was ranging between 1110 nm and 340 nm. For all sets of wires clear Shubnikov--de Haas oscillations are observed. We find that the electron concentration and mobility is approximately the same for all wires, confirming that the electron gas in the AlGaN/GaN heterostructure is not deteriorated by the fabrication procedure of the wire structures. For the wider quantum wires the weak antilocalization effect is clearly observed, indicating the presence of spin-orbit coupling. For narrow quantum wires with an effective electrical width below 250 nm the weak antilocalization effect is suppressed. By comparing the experimental data to a theoretical model for quasi one-dimensional structures we come to the conclusion that the spin-orbit scattering length is enhanced in narrow wires.Comment: 6 pages, 5 figure

Extra Spin-Wave mode in Quantum Hall systems. Beyond the Skyrmion Limit

We report on the observation of a new spin mode in a quantum Hall system in the vicinity of odd electron filling factors under experimental conditions excluding the possibility of Skyrmion excitations. The new mode having presumably zero energy at odd filling factors emerges at small deviations from odd filling factors and couples to the spin-exciton. The existence of an extra spin mode assumes a nontrivial magnetic order at partial fillings of Landau levels surrounding quantum Hall ferromagnets other then the Skyrmion crystal.Comment: 9 pages, 4 figure

Large Bychkov-Rashba spin-orbit coupling in high-mobility GaN/AlGaN heterostructures

We present low temperature magnetoconductivity measurements of a density-tunable and high mobility two-dimensional electron gas confined in the wide bandgap GaN/AlGaN system. We observed pronounced anti-localization minima in the low-field conductivity, indicating the presence of strong spin-orbit coupling. Density dependent measurements of magnetoconductivity indicate that the coupling is mainly due to the Bychkov-Rashba mechanism. In addition, we have derived a closed-form expression for the magnetoconductivity, allowing us to extract reliable transport parameters for our devices. The Rashba spin-orbit coupling constant is $\alpha_{so}$ $\sim$ 6$\times$ 10$^{-13}$eVm, while the conduction band spin-orbit splitting energy amounts to $\Delta_{so}$ $\sim$ 0.3meV at n$_e$=1$\times10^{16}$m$^{-2}$.Comment: Accepted for publication in PR