Magnetotransport measurements of charge carriers at the interface of a LaAlO3/SrTiO3 heterostructure with 26 unit cells of LaAlO3 show Hall resistance and magnetoresistance which at low and high temperatures is described by a single channel of electr
on-like charge carriers. At intermediate temperatures, we observe non-linear Hall resistance and positive magnetoresistance, establishing the presence of at least two electron-like channels with significantly different mobilities and carrier concentrations. These channels are separated by 6 meV in energy and their temperature dependent occupation and mobilities are responsible for the observed transport properties of the interface. We observe that one of the channels has a mobility that decreases with decreasing temperature, consistent with magnetic scattering in this channel.
Magneto-transport experiments on ABC-stacked suspended trilayer graphene reveal a complete splitting of the twelve-fold degenerated lowest Landau level, and, in particular, the opening of an exchange-driven gap at the charge neutrality point. A quant
itative analysis of distinctness of the quantum Hall plateaus as a function of field yields a hierarchy of the filling factors: u=6, 4, and 0 are the most pronounced, followed by u=3, and finally u=1, 2 and 5. Apart from the appearance of a u=4 state, which is probably caused by a layer asymmetry, this sequence is in agreement with Hunds rules for ABC-stacked trilayer graphene.
We present temperature-dependent magneto-transport experiments around the charge neutrality point in graphene and determine the amplitude of potential fluctuations $s$ responsible for the formation of electron-hole puddles. The experimental value $s
approx 20$ meV is considerably larger than in conventional semiconductors which implies a strong localization of charge carriers observable up to room temperature. Surprisingly, in the quantized regime, the Hall resistivity overshoots the highest plateau values at high temperatures. We demonstrate by model calculations that such a peculiar behavior is expected in any system with coexisting electrons and holes when the energy spectrum is quantized and the carriers are partially localized.
The low-temperature resistance of a conducting LaAlO3/SrTiO3 interface with a 10 nm thick LaAlO3 film decreases by more than 50% after illumination with light of energy higher than the SrTiO3 band-gap. We explain our observations by optical excitatio
n of an additional high mobility electron channel, which is spatially separated from the photo-excited holes. After illumination, we measure a strongly non-linear Hall resistance which is governed by the concentration and mobility of the photo-excited carriers. This can be explained within a two-carrier model where illumination creates a high-mobility electron channel in addition to a low-mobility electron channel which exists before illumination.
We have measured the quantum Hall activation gaps in bilayer graphene at filling factors $ u=pm4$ and $ u=pm8$ in high magnetic fields up to 30 T. We find that energy levels can be described by a 4-band relativistic hyperbolic dispersion. The Landau
level width is found to contain a field independent background due to an intrinsic level broadening and a component which increases linearly with magnetic field.
The temperature dependence of the magneto-conductivity in graphene shows that the widths of the longitudinal conductivity peaks, for the N=1 Landau level of electrons and holes, display a power-law behavior following $Delta u propto T^{kappa}$ with
a scaling exponent $kappa = 0.37pm0.05$. Similarly the maximum derivative of the quantum Hall plateau transitions $(dsigma_{xy}/d u)^{max}$ scales as $T^{-kappa}$ with a scaling exponent $kappa = 0.41pm0.04$ for both the first and second electron and hole Landau level. These results confirm the universality of a critical scaling exponent. In the zeroth Landau level, however, the width and derivative are essentially temperature independent, which we explain by a temperature independent intrinsic length that obscures the expected universal scaling behavior of the zeroth Landau level.
We have measured a strong increase of the low-temperature resistivity $rho_{xx}$ and a zero-value plateau in the Hall conductivity $sigma_{xy}$ at the charge neutrality point in graphene subjected to high magnetic fields up to 30 T. We explain our re
sults by a simple model involving a field dependent splitting of the lowest Landau level of the order of a few Kelvin, as extracted from activated transport measurements. The model reproduces both the increase in $rho_{xx}$ and the anomalous $ u=0$ plateau in $sigma_{xy}$ in terms of coexisting electrons and holes in the same spin-split zero-energy Landau level.
We have performed a metrological characterization of the quantum Hall resistance in a 1 $mu$m wide graphene Hall-bar. The longitudinal resistivity in the center of the $ u=pm 2$ quantum Hall plateaus vanishes within the measurement noise of 20 m$Omeg
a$ upto 2 $mu$A. Our results show that the quantization of these plateaus is within the experimental uncertainty (15 ppm for 1.5$ mu$A current) equal to that in conventional semiconductors. The principal limitation of the present experiments are the relatively high contact resistances in the quantum Hall regime, leading to a significantly increased noise across the voltage contacts and a heating of the sample when a high current is applied.
We use an atomic force microscope (AFM) to manipulate graphene films on a nanoscopic length scale. By means of local anodic oxidation with an AFM we are able to structure isolating trenches into single-layer and few-layer graphene flakes, opening the
possibility of tabletop graphene based device fabrication. Trench sizes of less than 30 nm in width are attainable with this technique. Besides oxidation we also show the influence of mechanical peeling and scratching with an AFM of few layer graphene sheets placed on different substrates.
