No abstract

CVD graphene growth typically uses commercially available cold-rolled copper foils, which includes a rich topography with scratches, dents, pits, and peaks. The graphene grown on this topography, even after annealing the foil, tends to include and reflect these topographic features. Further, the transfer of such CVD graphene to a flat substrate using a polymer transfer method also introduces wrinkles. Here, we examine an electropolishing technique for reducing native foil defects, characterize the resulting foil surface, grow single-crystal graphene on the polished foil, and examine the quality of the graphene for such defects.

We examined the influence of microwave radiation on both the amplitude of Shubnikov-de Haas (SdH) oscillations and the null field longitudinal magnetoresistance at liquid helium temperatures, in GaAs/AlGaAs Hall bar devices. Microwave radiation over the frequency range 25 ≤ f ≤ 50 GHz with source power 0 ≤ P ≤ 4 mW served to photo-excite the high mobility (10 7 cm 2 /V s) 2D electron system (2DES) as magnetoresistance traces were obtained as a function of the microwave power P and temperature T. Lineshape study of SdH oscillations has been carried out over the span 2.3 < ωc/ω ≤ 5.2, where ωc = eB/m * , ω = 2πf , B is the magnetic field, m * is the effective mass and f is the microwave frequency. Here, fits of the SdH lineshape served to determine the electron temperature (Te) as a function of P and T. Theory has proposed that, in the ωc/ω ≥ 1 regime, both the electron temperature and radiation energy absorption rate (Sp) exhibit relatively small response, while in the ωc/ω ≤ 1 regime, both Te and Sp are enhanced and exhibit oscillatory behavior. We compare the experimental results with these theoretical predictions, and comment upon relative role of electron heating in the microwave photo-excited high mobility 2DES.

A metallic state with a vanishing activation gap, at a filling factor $$\nu = 8/5$$ ν = 8 / 5 in the untilted specimen with $$n= 2 \times 10^{11} cm^{-2}$$ n = 2 × 10 11 c m - 2 , and at $$\nu = 4/3$$ ν = 4 / 3 at $$n=1.2 \times 10^{11} cm^{-2}$$ n = 1.2 × 10 11 c m - 2 under a $$\theta = 66^{0}$$ θ = 66 0 tilted magnetic field, is examined through a microwave photo-excited transport study of the GaAs/AlGaAs 2 dimensional electron system (2DES). The results presented here suggest, remarkably, that at the possible degeneracy point of states with different spin polarization, where the 8/5 or 4/3 FQHE vanish, there occurs a peculiar marginal metallic state that differs qualitatively from a quantum Hall insulating state and the usual quantum Hall metallic state. Such a marginal metallic state occurs most prominently at $$\nu =8/5$$ ν = 8 / 5 , and at $$\nu =4/3$$ ν = 4 / 3 under tilt as mentioned above, over the interval $$1 \le \nu \le 2$$ 1 ≤ ν ≤ 2 , that also includes the $$\nu = 3/2$$ ν = 3 / 2 state, which appears perceptibly gapped in the first instance.

In addition to the photo-excited zero-resistance states and radiation-induced magnetoresistance oscillations, which can be observed in the high-quality GaAs/AlGaAs two-dimensional electron system (2DES), magnetotransport studies of this 2DES also exhibit interesting dark magnetoresistance effects. Here, a narrow negative magnetoresistance (MR) effect that appears around zero field, and spans over about À0.02 T B 0.02 T is examined. This experimental work aims to study the influence of microwave (MW) photoexcitation on this narrow negative-MR effect in high-mobility GaAs/AlGaAs 2DES. Experimental data exhibit that the observed negative magnetoresistance effect disappears with increasing MW power. For example, the change in magnetoresistance (ΔR xx ) due to the narrow negative-MR effect drops by %50% upon increasing the source power up to about 8 mW. Further analysis shows that the zero-field resistance monotonically increases with increasing the power, suggesting that electron heating due to the energy absorbed from the radiation field accounts for the observed quenching of the narrow negative-MR effect.