Alexander Epping

Dielectric screening of the Kohn anomaly of graphene on hexagonal boron nitride

Kohn anomalies in three-dimensional metallic crystals are dips in the phonon dispersion that are caused by abrupt changes in the screening of the ion cores by the surrounding electron gas. These anomalies are also present at the high-symmetry pointsand K in the phonon dispersion of two-dimensional graphene, where the phonon wave vector connects two points on the Fermi surface. The linear slope around the kinks in the highest optical branch is proportional to the electron-phonon coupling. Here, we present a combined theoretical and experimental study of the influence of the dielectric substrate on the vibrational properties of graphene. We show that screening by the dielectric substrate reduces the electron-phonon coupling at the high-symmetry point K and leads to an upshift of the Raman 2D line. This results in the observation of a Kohn anomaly that can be tuned by screening. The exact position of the 2D line can thus be taken also as a signature for changes in the (electron-phonon limited) conductivity of graphene.

Limitations to Carrier Mobility and Phase-Coherent Transport in Bilayer Graphene

We present transport measurements on high-mobility bilayer graphene fully encapsulated in hexagonal boron nitride. We show two terminal quantum Hall effect measurements which exhibit full symmetry broken Landau levels at low magnetic fields. From weak localization measurements, we extract gate-tunable phase-coherence times τϕ as well as the inter- and intravalley scattering times τi and τ*, respectively. While τϕ is in qualitative agreement with an electron-electron interaction-mediated dephasing mechanism, electron spin-flip scattering processes are limiting τϕ at low temperatures. The analysis of τi and τ* points to local strain fluctuation as the most probable mechanism for limiting the mobility in high-quality bilayer graphene.

Etched graphene quantum dots on hexagonal boron nitride

We report on the fabrication and characterization of etched graphene quantum dots (QDs) on hexagonal boron nitride (hBN) and SiO2 with different island diameters. We perform a statistical analysis of Coulomb peak spacings over a wide energy range. For graphene QDs on hBN, the standard deviation of the normalized peak spacing distribution decreases with increasing QD diameter, whereas for QDs on SiO2 no diameter dependency is observed. In addition, QDs on hBN are more stable under the influence of perpendicular magnetic fields up to 9 T. Both results indicate a substantially reduced substrate induced disorder potential in graphene QDs on hBN.

Identifying suitable substrates for high-quality graphene-based heterostructures

We report on a scanning confocal Raman spectroscopy study investigating the strain-uniformity and the overall strain and doping of high-quality chemical vapour deposited (CVD) graphene-based heterostuctures on a large number of different substrate materials, including hexagonal boron nitride (hBN), transition metal dichalcogenides, silicon, different oxides and nitrides, as well as polymers. By applying a hBN-assisted, contamination free, dry transfer process for CVD graphene, high-quality heterostructures with low doping densities and low strain variations are assembled. The Raman spectra of these pristine heterostructures are sensitive to substrate-induced doping and strain variations and are thus used to probe the suitability of the substrate material for potential high-quality graphene devices. We find that the flatness of the substrate material is a key figure for gaining, or preserving high-quality graphene.

Etched graphene single electron transistors on hexagonal boron nitride in high magnetic fields

We report on the fabrication and electrical characterization of etched graphene single electron transistors (SETs) of various sizes on hexagonal boron nitride (hBN) in high magnetic fields. The electronic transport measurements show a slight improvement compared to graphene SETs on SiO2. In particular, SETs on hBN are more stable under the influence of perpendicular magnetic fields up to 9 T in contrast to measurements reported on SETs on SiO2. This result indicates a reduced surface disorder potential in SETs on hBN, which might be an important step toward clean and more controllable graphene quantum dots (QDs).

Quantum transport through MoS2 constrictions defined by photodoping

We present a device scheme to explore mesoscopic transport through molybdenum disulfide (MoS2) constrictions using photodoping. The devices are based on van-der-Waals heterostructures where few-layer MoS2 flakes are partially encapsulated by hexagonal boron nitride (hBN) and covered by a few-layer graphene flake to fabricate electrical contacts. Since the as-fabricated devices are insulating at low temperatures, we use photo-induced remote doping in the hBN substrate to create free charge carriers in the MoS2 layer. On top of the device, we place additional metal structures, which define the shape of the constriction and act as shadow masks during photodoping of the underlying MoS2/hBN heterostructure. Low temperature two- and four-terminal transport measurements show evidence of quantum confinement effects.

Aharonov-Bohm oscillations and magnetic focusing in ballistic graphene rings

We present low-temperature magnetotransport measurements on graphene rings encapsulated in hexagonal boron nitride. We investigate phase-coherent transport and show Aharonov-Bohm (AB) oscillations in quasi-ballistic graphene rings with hard confinement. In particular, we report on the observation of

Gate-Defined Electron–Hole Double Dots in Bilayer Graphene

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Graphene Quantum Dots

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