Novel A-B type oscillations in a 2-D electron gas in inhomogenous magnetic fields

We present results from a quantum and semiclassical theoretical study of the $\rho_{xy}$ and $\rho_{xx}$ resistivities of a high mobility 2-D electron gas in the presence of a dilute random distribution of tubes with magnetic flux $\Phi$ and radius $R$, for arbitrary values of $k_f R$ and $F=e\Phi/h$. We report on novel Aharonov-Bohm type oscillations in $\rho_{xy}$ and $\rho_{xx}$, related to degenerate quantum flux tube resonances, that satisfy the selection rule ${(k_fR)}^2=4F(n+{1\over 2})$, with $n$ an integer. We discuss possible experimental conditions where these oscillations may be observed.Comment: 11 pages REVTE

Magnetoresistance of a 2-dimensional electron gas in a random magnetic field

We report magnetoresistance measurements on a two-dimensional electron gas (2DEG) made from a high mobility GaAs/AlGaAs heterostructure, where the externally applied magnetic field was expelled from regions of the semiconductor by means of superconducting lead grains randomly distributed on the surface of the sample. A theoretical explanation in excellent agreement with the experiment is given within the framework of the semiclassical Boltzmann equation.Comment: REVTEX 3.0, 11 pages, 3 Postscript figures appended. The manuscript can also be obtained from our World Wide Web server: http://roemer.fys.ku.dk/randmag.ht

Raman Fingerprint of Charged Impurities in Graphene

We report strong variations in the Raman spectra for different single-layer graphene samples obtained by micromechanical cleavage, which reveals the presence of excess charges, even in the absence of intentional doping. Doping concentrations up to ~10^13 cm-2 are estimated from the G peak shift and width, and the variation of both position and relative intensity of the second order 2D peak. Asymmetric G peaks indicate charge inhomogeneity on the scale of less than 1 micron.Comment: 3 pages, 5 figure

Universal Dynamic Conductivity and Quantized Visible Opacity of Suspended Graphene

We show that the optical transparency of suspended graphene is defined by the fine structure constant, alpha, the parameter that describes coupling between light and relativistic electrons and is traditionally associated with quantum electrodynamics rather than condensed matter physics. Despite being only one atom thick, graphene is found to absorb a significant (pi times alpha=2.3%) fraction of incident white light, which is a consequence of graphene's unique electronic structure. This value translates into universal dynamic conductivity G =e^2/4h_bar within a few percent accuracy

Stacking boundaries and transport in bilayer graphene

Pristine bilayer graphene behaves in some instances as an insulator with a transport gap of a few meV. This behaviour has been interpreted as the result of an intrinsic electronic instability induced by many-body correlations. Intriguingly, however, some samples of similar mobility exhibit good metallic properties, with a minimal conductivity of the order of $2e^2/h$. Here we propose an explanation for this dichotomy, which is unrelated to electron interactions and based instead on the reversible formation of boundaries between stacking domains (`solitons'). We argue, using a numerical analysis, that the hallmark features of the previously inferred many-body insulating state can be explained by scattering on boundaries between domains with different stacking order (AB and BA). We furthermore present experimental evidence, reinforcing our interpretation, of reversible switching between a metallic and an insulating regime in suspended bilayers when subjected to thermal cycling or high current annealing.Comment: 13 pages, 15 figures. Published version (Nano Letters

Lifting of the Landau level degeneracy in graphene devices in a tilted magnetic field

We report on transport and capacitance measurements of graphene devices in magnetic fields up to 30 T. In both techniques, we observe the full splitting of Landau levels and we employ tilted field experiments to address the origin of the observed broken symmetry states. In the lowest energy level, the spin degeneracy is removed at filling factors $\nu=\pm1$ and we observe an enhanced energy gap. In the higher levels, the valley degeneracy is removed at odd filling factors while spin polarized states are formed at even $\nu$. Although the observation of odd filling factors in the higher levels points towards the spontaneous origin of the splitting, we find that the main contribution to the gap at $\nu= -4,-8$, and $-12$ is due to the Zeeman energy.Comment: 5 pages, 4 figure

Quantum states in a magnetic anti-dot

We study a new system in which electrons in two dimensions are confined by a non homogeneous magnetic field. The system consists of a heterostructure with on top of it a superconducting disk. We show that in this system electrons can be confined into a dot region. This magnetic anti-dot has the interesting property that the filling of the dot is a discrete function of the magnetic field. The circulating electron current inside and outside the anti-dot can be in opposite direction for certain bound states. And those states exhibit a diamagnetic to paramagnetic transition with increasing magnetic field. The absorption spectrum consists of many peaks, some of which violate Kohn's theorem, and which is due to the coupling of the center of mass motion with the other degrees of freedom.Comment: 6 pages, 12 ps figure

Unimpeded permeation of water through helium-leak-tight graphene-based membranes

Permeation through nanometer pores is important in the design of materials for filtration and separation techniques and because of unusual fundamental behavior arising at the molecular scale. We found that submicron-thick membranes made from graphene oxide can be completely impermeable to liquids, vapors and gases, including helium, but allow unimpeded permeation of water (H2O permeates through the membranes at least 10^10 times faster than He). We attribute these seemingly incompatible observations to a low-friction flow of a monolayer of water through two dimensional capillaries formed by closely spaced graphene sheets. Diffusion of other molecules is blocked by reversible narrowing of the capillaries in low humidity and/or by their clogging with water