Endre Tóvári

Snake trajectories in ultraclean graphene p–n junctions

Snake states are trajectories of charge carriers curving back and forth along an interface. There are two types of snake states, formed by either inverting the magnetic field direction or the charge carrier type at an interface. The former has been demonstrated in GaAs–AlGaAs heterostructures, whereas the latter has become conceivable only with the advance of ballistic graphene where a gap-less p–n interface governed by Klein tunnelling can be formed. Such snake states were hidden in previous experiments due to limited sample quality. Here we report on magneto-conductance oscillations due to snake states in a ballistic suspended graphene p–n junction, which occur already at a very small magnetic field of 20 mT. The visibility of 30% is enabled by Klein collimation. Our finding is firmly supported by quantum transport simulations. We demonstrate the high tunability of the device and operate it in different magnetic field regimes.

Fabrication of ballistic suspended graphene with local-gating

Abstract Herein we discuss the fabrication of ballistic suspended graphene nanostructures supplemented with local gating. Using in situ current annealing, we show that exceptional high mobilities can be obtained in these devices. A detailed description is given of the fabrication of bottom and different top-gate structures, which enable the realization of complex graphene structures. We have studied the basic building block, the p-n junction in detail, where a striking oscillating pattern was observed, which can be traced back to Fabry–Perot oscillations that are localized in the electronic cavities formed by the local gates. Finally we show some examples how the method can be extended to incorporate multi-terminal junctions or shaped graphene. The structures discussed here enable the access to electron-optics experiments in ballistic graphene.

Emergence of bound states in ballistic magnetotransport of graphene antidots

An experimental method for detection of bound states around an antidot formed by a hole in a graphene sheet is proposed via measuring the ballistic two-terminal conductance. In particular, we consider the effect of bound states formed by magnetic field on the two terminal conductance and show that one can observe Breit-Wigner like resonances in the conductance as a function of the Fermi level close to the energies of the bound states. In addition, we develop a new numeri- cal method utilizing a reduced computational effort compared to the existing numerical recursive Green’s function methods.

Gate-controlled conductance enhancement from quantum Hall channels along graphene p-n junctions

The conductance enhancement of QH states propagating far from disordered edges is directly observed. Separate biasing of channels, and gate-controlled transmission to contacts is demonstrated.

Coexistence of classical snake states and Aharonov-Bohm oscillations along graphene p − n junctions

Here we present measurements on p − n junctions in encapsulated graphene revealing several sets of magnetoconductance oscillations originating from quasiclassical snake states and edge state Aharonov-Bohm interferences. Even though some of these oscillations have already been observed in suspended and encapsulated devices including different geometries, their identification remained challenging as they were observed in separate measurements, and only a limited amount of data was available. Moreover, these effects have similar experimental signatures, therefore for their proper assignment their simultaneous observation and their detailed characterization is needed. The investigation of the charge carrier density, magnetic field, temperature, and bias dependence of the oscillations enabled us to properly identify their origin. Surprisingly we have found that snake states and Aharonov-Bohm interferences can coexist within a limited parameter range. We explain this using a unified picture of magneto-oscillations and confirm our findings using tight binding simulations. Since p − n junctions are the most important building blocks of graphene based electron-optical elements and edge state interferometers, our findings will be crucial for the design and understanding of future devices.

Characterization of SiO2/SiNx gate insulators for graphene based nanoelectromechanical systems

The structural and magnetotransport characterization of graphene nanodevices exfoliated onto Si/SiO2/SiNx heterostructures are presented. Improved visibility of the deposited flakes is achieved by optimal tuning of the dielectric film thicknesses. The conductance of single layer graphene Hall-bar nanostructures utilizing SiO2/SiNx gate dielectrics were characterized in the quantum Hall regime. Our results highlight that, while exhibiting better mechanical and chemical stability, the effect of non-stoichiometric SiNx on the charge carrier mobility of graphene is comparable to that of SiO2, demonstrating the merits of SiNx as an ideal material platform for graphene based nanoelectromechanical applications.

Exfoliation of single layer BiTeI flakes

Spin orbit interaction can be strongly boosted when a heavy element is embedded into an inversion asymmetric crystal field. A simple structure to realize this concept in a 2D crystal contains three atomic layers, a middle one built up from heavy elements generating strong atomic spin-orbit interaction and two neighboring atomic layers with different electron negativity. BiTeI is a promising candidate for such a 2D crystal, since it contains heavy Bi layer between Te and I layers. Recently the bulk form of BiTeI attracted considerable attention due to its giant Rashba interaction, however, 2D form of this crystal was not yet created. In this work we report the first exfoliation of single layer BiTeI using a recently developed exfoliation technique on stripped gold. Our combined scanning probe studies and first principles calculations show that SL BiTeI flakes with sizes of 100

Scalable Tight-Binding Model for Graphene

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Large scale nanopatterning of graphene

m were achieved which are stable at ambient conditions. The giant Rashba splitting and spin-momentum locking of this new member of 2D crystals open the way towards novel spintronic applications and synthetic topological heterostructures.