Session A11: Graphene: Transport in Devices and Structures

Transport in Graphene: Ballistic, Anomalous, or Diffusive?

The discovery of graphene, consisting of a layer of carbon atoms arranged in a honeycomb lattice, represents a conceptually new class of materials that are only one atom layer thick. Recent experiments achieved extremely high mobilities in suspended graphene leading to ballistic transport. In this talk, we study implications of diffusive and ballistic transport in graphene devices and ask what could be possible signatures of ``anomalous'' (or superdiffusive) transport. Experimental setups to differentiate between these stochastic processes are discussed.   

Transport in high-mobility mesoscale graphitic devices

Recent advances in graphene fabrication have allowed for high-mobility structures to be created. We report on the fabrication and measurement of mesoscale devices of graphene on boron-nitride. We present the details by which graphene is transferred on to boron-nitride substrates, where we observed enhanced mobility over similar devices fabricated on silicon dioxide substrates. Transport measurements at low temperature are described with focus given to reconfigurable, mesoscale devices in graphene.   

Gate defined quantum confinement in suspended bilayer graphene

Devices that confine electrons in graphene have sparked substantial interest due to applications ranging from spin-based quantum computation to valley filters. However, the absence of an intrinsic bandgap in graphene has limited such devices to on-chip nanopatterned structures to date. Here we present high quality quantum dots in suspended bilayer graphene with tunnel barriers defined by external electric fields that break layer inversion symmetry, thereby eliminating both edge and substrate disorder. We demonstrate clean electron confinement in two regimes: at zero magnetic field B using the single particle energy gap induced by a perpendicular electric field and at B$>$0 using the quantum Hall ferromagnet v=0 gap. Our devices exhibit clean quantum transport behavior at magnetic fields ranging from zero to seven Tesla, including a highly resistive v=0 quantum Hall state and over forty consecutive Coulomb blockade oscillations with symmetric source-drain coupling. The data indicate that the dots are defined by local top gating and are not disorder formed. Geometric control over oscillation periodicity is confirmed by electrostatic simulations based on lithographic gate geometry.   

Quantum Transport in Double-gated Trilayer Graphene \textit{pnp} Junctions

Using trilayer graphene \textit{pnp} junctions with suspended top gates, we perform transport measurements. At a magnetic field B=0, by an applied perpendicular electric field, the conductance is increased that it is suggestive of a semi-metallic band overlap. At B=8T we observe quantum Hall conductance with fractional values, which can be explained equilibration of edge state between differentially-doped regions, and the presence of an insulating state at filling factor $\nu $=0.   

Superconducting proximity effect in graphene: Injecting Cooper pairs in quantum Hall edge states

A superconductor-graphene(SG) hybrid system, such as an SGS junction or an SG interface, provides an ideal platform to investigate the relativistic nature of Dirac fermions combined with superconductivity. Instead of the retro-reflection of carriers in an ordinary superconductor-normal metal interface, an SG interface is theoretically predicted to show the specular reflection of quasiparticle carriers. We show that a supercurrent flows through a SGS junction with Nb electrodes even through a very long graphene distance of 1.2$\mu $m, more than 3 times the length previously reported. This supercurrent disappears in the vicinity of the Dirac point, indicating a strong sensitivity of the transmission of Andreev pairs to the formation of charge puddles with size greater than the superconducting coherence length. We also present data on similar size graphene samples with superconducting electrodes with a high critical field (more than 7Tesla) for which the properties of the normal state are dominated by quantum Hall physics. Whereas the behavior of the supercurrent is similar to the Nb/Graphene/Nb system in zero field, new features are observed in the high field quantum Hall regime.   

Gate-Tunable Superconducting-Insulating Transition in Tin-Decorated Graphene

We report the measurement of electrostatically tuned superconducting-insulating transition in macroscopic, CVD-Grown samples of graphene which decorated with tin nanoparticles. The self assembled network of Tin islands generates superconducting correlations locally in the Graphene by means of proximity effect. Correlations eventually leads to percolation of a supercurrent This system exhibits features related to granular superconductivity, a giant magnetoresistance peak, as well as an intermediate metallic behavior. We emphasize outstanding dynamics of the transition, which exhibit a change in resistance of more than 7 orders of magnitude within 40V of gate voltage, thus realizing a real electrostatically driven superconducting-insulating transition. The intense positive magnetoresistance observed for fields below the critical field of Tin nanoparticles is a signature of the localization of Cooper pairs. This hybrid superconductor provides a model system to better understand the physics of inhomogeneous superconductivity, as crossing the transition by adjusting the carrier density is conceptually simpler than using a magnetic field. It also allows to cross the transition continuously and under constant disorder.   

Transport through Andreev Bound States in a Graphene-base Quantum Dot

We perform tunneling spectroscopy on a graphene-quantum dot (QD)-superconductor junction, a system in which sharp, gate-tunable Andreev bound states (ABS) in the spectra have been observed [1]. Here we extend previous results, particularly regarding the origins of the QD. In particular, we discuss how a discontinuous layer of AlO$_{x}$ between the superconductor and the graphene plays a role in the formation of the QD. We also discuss additional spectroscopic features that may be due to multiple QDs and energy levels. Finally, we show that a robust superconducting tunneling junction can be created in a lead-graphene structure, without the explicit deposition of a tunneling barrier. \\[4pt] [1] Dirks, T., Nature Physics 7, 386--390 (2011)   

Direct Measurement of the Fermi Energy in Graphene Using a Double Layer Structure

The Fermi energy is a fundamental property of an electron system. Here we introduce a direct measurement technique of the relative Fermi energy as a function of carrier density, using transport measurement in a double layer structure where one of the layers is graphene. The principle of this method is that the Fermi energy in the target material is equal to the applied inter-layer bias required to bring the graphene layer to charge neutrality point. Using a double layer graphene structure, we illustrate the technique by measuring the Fermi energy in one of the graphene layers. By mapping the top graphene layer zero density line as a function of bottom and inter-layer bias, we measure the Fermi energy as a function of carrier density at zero and in high magnetic fields. We extract the Fermi velocity, Landau level spacing and Landau level broadening.   

Magnetic field dependence of the carrier's effective mass and the g-factor in graphene

It has been established that the intrinsic Zeeman energy is one half of the cyclotron energy for ``\emph{bare}'' electrons in graphene. Consequently, there could be Landau-level mixing between the energy bands. We investigate how the band mixing is affected by the Coulomb interaction. Pairing of the electrons and holes in the presence of a circularly polarized light is also considered for several filling factors. We calculate the quasiparticle effective mass and effective $g$-factor for dressed electrons and holes in monolayer graphene. As an intermediate step of these calculations, we obtain the dielectric function for the case of electron dressed states and investigate how the magetoplasmons modes are affected by the electron-photon interaction.   

Magnetic-field tunable electron-phonon coupling in graphite

Electron-phonon coupling (EPC) plays a pivotal role in condensed matter physics, governing intriguing phenomena such as superconductivity, ballistic transport, and excited-state dynamics. In graphitic systems, EPC is found to be strongly related to the quasiparticle excitations, electronic states and optical properties. Thus, the EPC may be manipulated via external parameters, such as electric field or magnetic field, and provide alternative access to adjust the characteristics of carbon-based devices. Here, we explore the EPC in graphite via magnetophonon resonance using cyclotron resonance (CR) spectroscopy. A marked avoided-level-crossing splitting of the CR and a Fano resonance-like behavior are observed, when the CR energies intersect the specific phonon energies via tuning the magnitude of the magnetic field. We attribute these results to the resonance between the CR excitations and the large momentum zone-edge, and the long-wavelength zone-center phonons, respectively. This work suggests that rich interacting physics exists in graphitic materials, which may have profound implications in future optoelectronics.   

High frequency high magnetic field response of graphene monolayers

We study the electronic magnetotransport in graphene at rf frequencies (5-50GHz). Our aim is to investigate the dynamics of charge carriers in the quantum Hall regime. The graphene sample is placed in a break made in a coplanar waveguide and the transmitted power is measured. In order to isolate the response of the sample from the direct transmission between the input and output waveguides, the graphene electron density distribution is modulated with a side gate and the resulting modulation in the transmitted power detected via a standard lock-in technique. The fixed frequency graphene response as a function of magnetic field reveals two different components. One is symmetric in B and dominates under large side gate voltage, and the other shows reproducible fluctuations revealed only at low gate voltage modulation amplitude. The first part is thought to be related to the bulk conductivity and the fluctuations to the carrier dynamics close to the edge. The amplitude of the fluctuations depends on the trajectory of the carriers, since the parity with respect to magnetic field reversal is not conserved. We thus demonstrate the chiral nature of the transport. We assume that the fluctuations of impedance originate in the scattering from localized states close to the sample edge.   

Impurity-limited carrier transport in graphene nanoribbons

We have measured the transport property of graphene nanoribbons as a function of impurity density in ultra high vacuum. Specifically, the impact of Coulomb and van der Waals impurities on the transport and source-drain gap of nanoribbons is investigated. Our results have direct consequences on fundamental science using graphene constrictions and graphene-based devices.   

Electrical Transport Study of Suspended Graphene Nanoribbons

Suspended graphene nanoribbon field effect transistors from unzipped multiwall carbon nanotubes have been fabricated. Electrical transport measurements show that current-annealing effectively removes the adsorbed impurities on the suspended graphene nanoribbons. Further increasing the annealing current creates a narrow constriction in the ribbons with non-negligible disorder, leading to the formation of a large band-gap and subsequent high on/off ratio. On the other hand, uniform suspended graphene nanoribbons with ultra-low-disorder reveal a high mobility exceeding 3000 cm2 V-1 s-1 and an intrinsic band gap. The width and length dependence of the electrical transport properties of ultra-low-disorder graphene nanoribbons with nearly atomically smooth edges will also be discussed.   

Quantum Transport in Graphene Nanoribbon Networks

Focusing on systems that can be realized experimentally, both in-plane conductance of inter-connected graphene nanoribbons and tunneling conductance in out-of-plane nanoribbon intersections are investigated. The quantum transport properties of such networks are computed using first-principles calculations based on the density functional theory formalism. The electronic transport through in-plane nanoribbon cross-points is found to be significantly affected by scattering at the intersections with the exception of all zigzag nanoribbon terminals arranged at a 60 degree angle. This result demonstrates the possibility of designing graphene nanoribbon networks capable of guiding electron along desired and predetermined paths. In addition, the electron transport properties of out-of-plane nanoribbons cross-points with realistic size are described within a simple tight-binding approach. The stacking angle is predicted to play a key role on the electronic transmission through nanoribbon networks.   

Current Quantization in Corrugated Graphene Nanoribbons

Corrugated graphene provides such a phenomenon as curvature induced $p-n$ junction band gap opening and decoherence. We report yet another effect of current quantization in graphene nanoribbons via energy minigaps induced by the corrugation. Effects of edge roughness and long range charged scatterers on the quantization are investigated. Comparison is drawn with acoustically induced minigaps in carbon nanotubes [Talyanskii et al.,PRL 87,276082(2001)].