Imaging Localized States in Graphene Nanostructures

Probing techniques with spatial resolution have the potential to lead to a better understanding of the microscopic physical processes and to novel routes for manipulating nanostructures. We present scanning-gate images of a graphene quantum dot which is coupled to source and drain via two constrictions. We image and locate conductance resonances of the quantum dot in the Coulomb-blockade regime as well as resonances of localized states in the constrictions in real space.Comment: 18 pages, 7 figure

Interplay between nanometer-scale strain variations and externally applied strain in graphene

We present a molecular modeling study analyzing nanometer-scale strain variations in graphene as a function of externally applied tensile strain. We consider two different mechanisms that could underlie nanometer-scale strain variations: static perturbations from lattice imperfections of an underlying substrate and thermal fluctuations. For both cases we observe a decrease in the out-of-plane atomic displacements with increasing strain, which is accompanied by an increase in the in-plane displacements. Reflecting the non-linear elastic properties of graphene, both trends together yield a non-monotonic variation of the total displacements with increasing tensile strain. This variation allows to test the role of nanometer-scale strain variations in limiting the carrier mobility of high-quality graphene samples

Cable Extraction of Harvester-Felled Thinnings: An Austrian Case Study

A time study of the cable extraction of thinnings in short corridors was carried out in the Neuberg an der Mürz forest area, Austria. Both the yarder and the choker-setter(s) were studied. Six options were compared. For the "standard" option the timber was felled, cut to length, and pre-bunched by the harvester on a 20-meter-wide corridor, and was yarded downhill. Two choker-setters were employed. The five variations included: (1) "larger" bundles, (2) in-creased lateral hauling distance, (3) one choker-setter, (4) the harvester cutting-to-stem length and the timber yarded uphill with only one choker setter, and (5) trees in a 30-meter-wide corridor felled and bucked by motor-manual methods. The harvester used was a Skogsjan 687 XL with a 601 head; the medium-sized yarder was a Syncrofalke with a Sherpa U3 carriage. The time study results showed that the corridors felled and cut to length by the harvester, in comparison to the motor-manually cut corridor, provided a significant improvement in the cable extraction cycle times: 3.7 min compared to 4.6 min. Additionally, an average turn volume increase of 26% was achieved by the improved presentation of the timber. A 20-meter lateral-hauling distance increased the cycle time by only 7%. The use of one choker-setter increased the delay-free cycle time by just 10%, however it significantly decreased the work-related waiting time for the choker-setter to just 5%. Uphill stem extraction using one choker-setter had the same cycle time as the downhill cut-to-length extraction using two choker-setters, although a 5% greater average turn volume was recorded

Impact of Many-Body Effects on Landau Levels in Graphene

We present magneto-Raman spectroscopy measurements on suspended graphene to investigate the charge carrier density-dependent electron-electron interaction in the presence of Landau levels. Utilizing gate-tunable magneto-phonon resonances, we extract the charge carrier density dependence of the Landau level transition energies and the associated effective Fermi velocity $v_\mathrm{F}$. In contrast to the logarithmic divergence of $v_\mathrm{F}$ at zero magnetic field, we find a piecewise linear scaling of $v_\mathrm{F}$ as a function of charge carrier density, due to a magnetic field-induced suppression of the long-range Coulomb interaction. We quantitatively confirm our experimental findings by performing tight-binding calculations on the level of the Hartree-Fock approximation, which also allow us to estimate an excitonic binding energy of $\approx$ 6 meV contained in the experimentally extracted Landau level transitions energies.Comment: 10 pages, 6 figure

Fabrication of comb-drive actuators for straining nanostructured suspended graphene

We report on the fabrication and characterization of an optimized comb-drive actuator design for strain-dependent transport measurements on suspended graphene. We fabricate devices from highly p-doped silicon using deep reactive ion etching with a chromium mask. Crucially, we implement a gold layer to reduce the device resistance from $\approx51.6$ k$\mathrm{\Omega}$ to $\approx236$ $\mathrm{\Omega}$ at room temperature in order to allow for strain-dependent transport measurements. The graphene is integrated by mechanically transferring it directly onto the actuator using a polymethylmethacrylate membrane. Importantly, the integrated graphene can be nanostructured afterwards to optimize device functionality. The minimum feature size of the structured suspended graphene is 30 nm, which allows for interesting device concepts such as mechanically-tunable nanoconstrictions. Finally, we characterize the fabricated devices by measuring the Raman spectrum as well as the a mechanical resonance frequency of an integrated graphene sheet for different strain values.Comment: 10 pages, 9 figure

Coulomb oscillations in three-layer graphene nanostructures

We present transport measurements on a tunable three-layer graphene single electron transistor (SET). The device consists of an etched three-layer graphene flake with two narrow constrictions separating the island from source and drain contacts. Three lateral graphene gates are used to electrostatically tune the device. An individual three-layer graphene constriction has been investigated separately showing a transport gap near the charge neutrality point. The graphene tunneling barriers show a strongly nonmonotonic coupling as function of gate voltage indicating the presence of localized states in the constrictions. We show Coulomb oscillations and Coulomb diamond measurements proving the functionality of the graphene SET. A charging energy of $\approx 0.6$ meV is extracted.Comment: 10 pages, 6 figure

Local gating of a graphene Hall bar by graphene side gates

We have investigated the magnetotransport properties of a single-layer graphene Hall bar with additional graphene side gates. The side gating in the absence of a magnetic field can be modeled by considering two parallel conducting channels within the Hall bar. This results in an average penetration depth of the side gate created field of approx. 90 nm. The side gates are also effective in the quantum Hall regime, and allow to modify the longitudinal and Hall resistances

Tunable mechanical coupling between driven microelectromechanical resonators

We present a microelectromechanical system, in which a silicon beam is attached to a comb-drive actuator, that is used to tune the tension in the silicon beam, and thus its resonance frequency. By measuring the resonance frequencies of the system, we show that the comb-drive actuator and the silicon beam behave as two strongly coupled resonators. Interestingly, the effective coupling rate (~ 1.5 MHz) is tunable with the comb-drive actuator (+10%) as well as with a side-gate (-10%) placed close to the silicon beam. In contrast, the effective spring constant of the system is insensitive to either of them and changes only by $\pm$ 0.5%. Finally, we show that the comb-drive actuator can be used to switch between different coupling rates with a frequency of at least 10 kHz.Comment: 5 pages, 4 figures, 1 tabl

Gate-defined graphene double quantum dot and excited state spectroscopy

A double quantum dot is formed in a graphene nanoribbon device using three top gates. These gates independently change the number of electrons on each dot and tune the inter-dot coupling. Transport through excited states is observed in the weakly coupled double dot regime. We extract from the measurements all relevant capacitances of the double dot system, as well as the quantized level spacing