Papers by Jeroen B. Oostinga
After the experimental discovery of graphene -a single atomic layer of graphite- a scientific rus... more After the experimental discovery of graphene -a single atomic layer of graphite- a scientific rush started to explore graphene’s electronic behaviour. Graphene is a fascinating two-dimensional electronic system, because its electrons behave as relativistic particles. Moreover, it is a promising material for future high-speed nano-electronic applications. In this thesis, several experiments are described to reveal graphene’s electronic transport properties. We have shown that we can control the bandstructure of bilayer and trilayer graphene. Simply by applying a perpendicular electric field in a graphene device, we could tune the bandgap in the bilayer and the bandoverlap in the trilayer. Furthermore, we have described transport measurements on graphene devices (length = 0.1-1 micrometer) showing that electronic transport in graphene is phase coherent at cryogenic temperatures (4 K or less). We have observed weak localization, bipolar supercurrents and the Aharonov-Bohm effect. We ha...
ABSTRACT The presence of strong disorder in graphene nanoribbons yields low-mobility diffusive tr... more ABSTRACT The presence of strong disorder in graphene nanoribbons yields low-mobility diffusive transport at high charge densities, whereas a transport gap occurs at low densities. Here, we investigate the longitudinal and transverse magnetoresistance of a narrow (60 nm) nanoribbon in a six-terminal Hall bar geometry. At B= 11 T, quantum Hall plateaux appear at $\sigma_{xy}=\pm2e^2/h$, $\pm6e^2/h$ and $\pm10e^2/h$, for which the Landau level spacing is larger than the Landau level broadening. Interestingly, the transport gap does not disappear in the quantum Hall regime, when the zero-energy Landau level is present at the charge neutrality point, implying that it cannot originate from a lateral confinement gap. At high charge densities, the longitudinal and Hall resistance exhibit reproducible fluctuations, which are most pronounced at the transition regions between Hall plateaux. Bias-dependent measurements strongly indicate that these fluctuations can be attributed to phase coherent scattering in the disordered ribbon.
Physical Review Letters, 2012
A strained and undoped HgTe layer is a three-dimensional topological insulator, in which electron... more A strained and undoped HgTe layer is a three-dimensional topological insulator, in which electronic transport occurs dominantly through its surface states. In this Letter, we present transport measurements on HgTe-based Josephson junctions with Nb as superconductor. Although the Nb-HgTe interfaces have a low transparency, we observe a strong zero-bias anomaly in the differential resistance measurements. This anomaly originates from proximity-induced superconductivity in the HgTe surface states. In the most transparent junction, we observe periodic oscillations of the differential resistance as function of an applied magnetic field, which correspond to a Fraunhofer-like pattern. This unambiguously shows that a precursor of the Josephson effect occurs in the topological surface states of HgTe. 73.25.+i, 72.80.Ey, 74.45.+c Topological insulators (TIs) are a recently discovered new class of materials [1]. In two dimensions (2-D), a TI is an insulating material with conducting helical states at its edges. The first experimental realization of a 2-D TI was a HgTe quantum well, in which the topological state of matter was unambiguously demonstrated by the observation of the quantum spin Hall effect in electronic transport experiments [2]. The first three-dimensional (3-D) TIs, Bi 1−x Sb x , Bi 2 Se 3 and Bi 2 Te 3 , were discovered soon after . 3-D TIs are bulk insulators with conducting states at their surfaces. Since these surface states are effectively described by Dirac Hamiltonians, the corresponding quasiparticles behave as chiral Dirac fermions, i.e. the spin and momentum of the electrons are locked [1].
Physical Review B, 2008
We investigate experimentally transport through ring-shaped devices etched in graphene and observ... more We investigate experimentally transport through ring-shaped devices etched in graphene and observe clear Aharonov-Bohm conductance oscillations. The temperature dependence of the oscillation amplitude indicates that below 1 K, the phase coherence length is comparable to or larger than the size of the ring. An increase in the amplitude is observed at high magnetic field, when the cyclotron diameter becomes comparable to the width of the arms of the ring. By measuring the dependence on gate voltage, we find that the Aharonov-Bohm effect vanishes at the charge neutrality point, and we observe an unexpected linear dependence of the oscillation amplitude on the ring conductance.
Physical Review B, 2010
We investigate magneto-transport through graphene nano-ribbons as a function of gate and bias vol... more We investigate magneto-transport through graphene nano-ribbons as a function of gate and bias voltage, and temperature. We find that a magnetic field systematically leads to an increase of the conductance on a scale of a few tesla. This phenomenon is accompanied by a decrease in the energy scales associated to charging effects, and to hopping processes probed by temperature-dependent measurements. All the observations can be interpreted consistently in terms of strong-localization effects caused by the large disorder present, and exclude that the insulating state observed in nanoribbons can be explained solely in terms of a true gap between valence and conduction band. PACS numbers: 85.35.-p, 73.23.-b, 72.80.Vp, 73.20.Fz
Physical Review B, 2010
We have investigated shot noise and conduction of graphene field effect nanoribbon devices at low... more We have investigated shot noise and conduction of graphene field effect nanoribbon devices at low temperature. By analyzing the exponential I − V characteristics of our devices in the transport gap region, we found out that transport follows variable range hopping laws at intermediate bias voltages 1 < V bias < 12 mV. In parallel, we observe a strong shot noise suppression leading to very low Fano factors. The strong suppression of shot noise is consistent with inelastic hopping, in crossover from one-to two-dimensional regime, indicating that the localization length l loc < W in our nanoribbons.
Nature Materials, 2008
The potential of graphene-based materials consisting of one or a few layers of graphite for integ... more The potential of graphene-based materials consisting of one or a few layers of graphite for integrated electronics originates from the large room-temperature carrier mobility in these systems (~10 , 000 cm2/Vs). However, the realization of electronic devices such as fieldeffect transistors will require controlling and even switching off the electrical conductivity by means of gate electrodes, which is made difficult by the absence of a bandgap in the intrinsic material. Here, we demonstrate the controlled induction of an insulating statewith large suppression of the conductivity -in bilayer graphene, by using a doublegate device configuration that allows an electric field to be applied perpendicular to the plane. The dependence of the resistance on temperature and electric field, and the absence of any effect in a single-layer device, strongly suggest that the gate-induced insulating state originates from the recently predicted opening of a bandgap between valence and conduction bands.
Nature Communications, 2011
Three-dimensional topological insulators are characterized by the presence of a bandgap in their ... more Three-dimensional topological insulators are characterized by the presence of a bandgap in their bulk and gapless Dirac fermions at their surfaces. New physical phenomena originating from the presence of the Dirac fermions are predicted to occur, and to be experimentally accessible via transport measurements in suitably designed electronic devices. Here we study transport through superconducting junctions fabricated on thin Bi 2 Se 3 single crystals, equipped with a gate electrode. In the presence of perpendicular magnetic fi eld B, sweeping the gate voltage enables us to observe the fi lling of the Dirac fermion Landau levels, whose character evolves continuously from electron-to hole-like. When B = 0, a supercurrent appears, whose magnitude can be gate tuned, and is minimum at the charge neutrality point determined from the Landau level fi lling. Our results demonstrate how gated nano-electronic devices give control over normal and superconducting transport of Dirac fermions at an individual surface of a threedimensional topological insulators.
Nano Letters, 2010
We report an experimental study of 1/f noise in liquid-gated graphene transistors. We show that t... more We report an experimental study of 1/f noise in liquid-gated graphene transistors. We show that the gate dependence of the noise is well described by a charge-noise model, whereas Hooge's empirical relation fails to describe the data. At low carrier density, the noise can be attributed to fluctuating charges in close proximity to the graphene, while at high carrier density it is consistent with noise due to scattering in the channel. The charge noise power scales inversely with the device area, and bilayer devices exhibit lower noise than single-layer devices. In air, the observed noise is also consistent with the charge-noise model.
Bulletin of the American …, 2009
Graphene based materials are promising candidates for nano electronic applications. It is current... more Graphene based materials are promising candidates for nano electronic applications. It is currently unclear which layer thickness is better suited for a given application, as only the properties of monolayers and bilayers have been investigated systematically. For the ...
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Papers by Jeroen B. Oostinga