Nils M. Freitag

Electrostatically Confined Monolayer Graphene Quantum Dots with Orbital and Valley Splittings

The electrostatic confinement of massless charge carriers is hampered by Klein tunneling. Circumventing this problem in graphene mainly relies on carving out nanostructures or applying electric displacement fields to open a band gap in bilayer graphene. So far, these approaches suffer from edge disorder or insufficiently controlled localization of electrons. Here we realize an alternative strategy in monolayer graphene, by combining a homogeneous magnetic field and electrostatic confinement. Using the tip of a scanning tunneling microscope, we induce a confining potential in the Landau gaps of bulk graphene without the need for physical edges. Gating the localized states toward the Fermi energy leads to regular charging sequences with more than 40 Coulomb peaks exhibiting typical addition energies of 7–20 meV. Orbital splittings of 4–10 meV and a valley splitting of about 3 meV for the first orbital state can be deduced. These experimental observations are quantitatively reproduced by tight binding calculations, which include the interactions of the graphene with the aligned hexagonal boron nitride substrate. The demonstrated confinement approach appears suitable to create quantum dots with well-defined wave function properties beyond the reach of traditional techniques.

Tuning the Pseudospin Polarization of Graphene by a Pseudomagnetic Field

One of the intriguing characteristics of honeycomb lattices is the appearance of a pseudomagnetic field as a result of mechanical deformation. In the case of graphene, the Landau quantization resulting from this pseudomagnetic field has been measured using scanning tunneling microscopy. Here we show that a signature of the pseudomagnetic field is a local sublattice symmetry breaking observable as a redistribution of the local density of states. This can be interpreted as a polarization of graphenes pseudospin due to a strain induced pseudomagnetic field, in analogy to the alignment of a real spin in a magnetic field. We reveal this sublattice symmetry breaking by tunably straining graphene using the tip of a scanning tunneling microscope. The tip locally lifts the graphene membrane from a SiO2 support, as visible by an increased slope of the I(z) curves. The amount of lifting is consistent with molecular dynamics calculations, which reveal a deformed graphene area under the tip in the shape of a Gaussian. The pseudomagnetic field induced by the deformation becomes visible as a sublattice symmetry breaking which scales with the lifting height of the strained deformation and therefore with the pseudomagnetic field strength. Its magnitude is quantitatively reproduced by analytic and tight-binding models, revealing fields of 1000 T. These results might be the starting point for an effective THz valley filter, as a basic element of valleytronics.

Large tunable valley splitting in edge-free graphene quantum dots on boron nitride

Coherent manipulation of the binary degrees of freedom is at the heart of modern quantum technologies. Graphene offers two binary degrees: the electron spin and the valley. Efficient spin control has been demonstrated in many solid-state systems, whereas exploitation of the valley has only recently been started, albeit without control at the single-electron level. Here, we show that van der Waals stacking of graphene onto hexagonal boron nitride offers a natural platform for valley control. We use a graphene quantum dot induced by the tip of a scanning tunnelling microscope and demonstrate valley splitting that is tunable from −5 to +10 meV (including valley inversion) by sub-10-nm displacements of the quantum dot position. This boosts the range of controlled valley splitting by about one order of magnitude. The tunable inversion of spin and valley states should enable coherent superposition of these degrees of freedom as a first step towards graphene-based qubits.The valley splitting in a stack of graphene and boron nitride can be controlled through a quantum dot induced by a scanning tunnelling microscope.

Graphene quantum dots: wave function mapping by scanning tunneling spectroscopy and transport spectroscopy of quantum dots prepared by local anodic oxidation

Graphene quantum dots are considered as promising alternatives to quantum dots in III-V semiconductors, e.g., for the use as spin qubits due to their consistency made of light atoms including spin-free nuclei which both imply relatively long spin decoherene times. However, this potential has not been realized in experiments so far, most likely, due to a missing control of the edge configurations of the quantum dots. Thus, a more fundamental investigation of Graphene quantum dots appears to be necessary including a full control of the wave function properties most favorably during transport spectroscopy measurements. Here, we review the recent success in mapping wave functions of graphene quantum dots supported by metals, in particular Ir(111), and show how the goal of probing such wave functions on insulating supports during transport spectroscopy might be achieved.

Impact of gender and genetics on emotion processing in Parkinson's disease - A multimodal study

Highlights • Understanding of the phenotypic heterogeneity of Parkinsons disease is needed.• Gender and genetics determine manifestation and progression of Parkinsons disease.• Altered emotion processing in Parkinsons disease is specific to male patients.• This is influenced by endocrinal and genetic factors in both genders.• This finding may impact the diagnosis and treatment of emerging clinical features.

Correction to Tuning the Pseudospin Polarization of Graphene by a Pseudomagnetic Field

T following NSF grant number should be added to the Acknowledgment: DMR-1508325 (D.F., D.Z., and N.S.). With this, the correct Acknowledgment section should read: We acknowledge discussions with M. I. Katsnelson, A. Bernevig, M. Kra ̈mer, W. Bernreuther, F. Libisch, C. Stampfer,and C. Wiebusch, assistance at the STM measurements and sample preparation by C. Pauly, C. Saunus, S. Hattendorf, V. Geringer. We acknowledge financial support by the Graphene Flagship (Contract No. NECT-ICT-604391) and the German Research Foundation via Li 1050/2-2 (A.G., P.N.I., M.P., M.L. and M.M.); DFG SPP 1459 and the A. v H. Foundation (M.S., S.V.K.); CNPq No.150222/2014-9 (D.F.); NSF No. DMR-1108285 (D.F., R.C-B., D.Z., and N.S.) and DMR-1508325 (D.F., D.Z., and N.S.); PRODEP (R.C.B). FNR Luxembourg INTER/ANR/13/20/NANOTMD (L.W).