Session Y5: Graphene: Transport and Optical Phenomena: Heterostructures

Plasmons and Coulomb drag in Dirac/Schroedinger hybrid electron systems

We show that the plasmon spectrum of an ordinary two-dimensional electron gas (2DEG) hosted in a GaAs heterostructure is significantly modified when a graphene sheet is placed on the surface of the semiconductor in close proximity to the 2DEG. Long-range Coulomb interactions between massive electrons and massless Dirac fermions lead to a new set of optical and acoustic intra-subband plasmons. Here we compute the dispersion of these coupled modes within the Random Phase Approximation, providing analytical expressions in the long-wavelength limit that shed light on their dependence on the Dirac velocity and Dirac-fermion density. We also evaluate the resistivity in a Coulomb-drag transport setup. These Dirac/Schroedinger hybrid electron systems are experimentally feasible and open new research opportunities for fundamental studies of electron-electron interaction effects in two spatial dimensions.   

Enhancement of Coulomb drag in double-layer graphene structures by plasmons and dielectric background inhomogeneity

The drag of massless fermions in graphene double-layer structures has been investigated over a wide range of temperatures and interlayer separations. We have shown [1] that the inhomogeneity of the dielectric background in such graphene structures, for experimentally relevant parameters, results in a significant enhancement of the drag resistivity. At intermediate temperatures the dynamical screening via plasmon-mediated drag enhances the drag resistivity and results in an upturn in its behavior at large interlayer separations. In a range of interlayer separations, corresponding to the crossover from strong to weak coupling of graphene layers, we find that the decrease of the drag resistivity with interlayer spacing is approximately quadratic. This dependence weakens below this range of interlayer spacing while for larger separations we find a cubic (quartic) dependence at intermediate (low) temperatures. \\[4pt] [1] S. M. Badalyan and F. Peeters, Phys. Rev. B {\bf 86}, 121405(R) (2012).   

Energy-driven drag in Graphene

When solid surfaces slide against each other they experience friction which can be enhanced by inserting molasses between them or reduced by using a lubricant. In the same way, two spatially isolated conducting layers that are placed in close proximity with each other feel friction because the long-ranged Coulomb interaction allows electrons in adjacent layers to ``rub shoulders at a distance.'' Recent measurements of Coulomb drag in Graphene by Gorbachev and co-workers from Manchester (doi:10.1038/nphys2441) have found that it is dramatically enhanced near the Dirac point, in stark contradiction with earlier theories predicting vanishing drag. We argue that a new kind of drag develops when heat transport in the two layers becomes strongly coupled due to efficient energy transfer between the layers. As a result, spatial charge inhomogeneity couples the motion of the electron liquid with heat transport through it, damping motion of electron flow in one layer by heat dissipation in the other. Interestingly, and somewhat paradoxically, this leads to strong drag without momentum transfer between layers. We predict distinct experimental signatures and discuss its magnetic field dependence.   

Hydrodynamical Modes and New Transport Phenomena in Graphene: Nonlocality and Anomalous Drag

The semimetal band structure of graphene givs rise to an unusually strong coupling between electrical currents and charge-neutral currents. This coupling leads to new transport phenomena mediated by neutral modes. This talk will highlight two examples connected with ongoing experiments. One is giant nonlocality observed in electric measurements.[1] This effect was explained by spin transport made possible by novel spin-Hall response near the Dirac point.[2] Another example is anomalous drag observed at charge neutrality which was attributed to the effects mediated by energy transfer in graphene heterostructures.[3,4] Drag measurements thus afford a unique probe of energy transfer at the nanoscale, a fundamental process which is not easily amenable to more conventional techniques such as calorimetry, and is key for the physics of strong interactions that occur near neutrality. \\[4pt] [1] D. A. Abanin et al, Science 332, 328-330 (2011); \\[0pt] [2] D. A. Abanin et al, Phys. Rev. Lett. 107, 096601 (2011) \\[0pt] [3] R. V. Gorbachev et al, arXiv: 1206.6626, doi:10.1038/nphys2441 \\[0pt] [4] J. W. C. Song and L. S. Levitov, arXiv:1205.5257, Phys.Rev.Lett., to be published (2012)   

Insulating behavior at the neutrality point in dual-gated single-layer graphene

The conductivity at the neutrality point in single-layer graphene is known to saturate on the order of e$^{2}$/h due to disorder-induced density fluctuations. In this study, we report contrasting results using dual-gated graphene devices with a boron nitride back-gate dielectric and a suspended top-gate, allowing for carrier mobilities over 100 000 cm$^{2}$/Vs. As the temperature is lowered, the peak resistivity at the charge-neutrality point unexpectedly diverges with a power-law behavior and becomes as high as several megohms per square. As a transverse magnetic field is applied, our device remains insulating and directly transitions to the ?=0 quantum Hall state. We discuss possible origins for this insulating behavior.   

Broken Symmetry Quantum Hall states in Dual Gated ABA Trilayer Graphene

We perform low temperature transport measurements on dual-gated suspended trilayer graphene in the quantum Hall (QH) regime. We observe QH plateaus at filling factors $\nu =$-8, -2, 2, 6, and~10, in agreement with the full-parameter tight binding calculations. In high magnetic fields, oddinteger plateaus are also resolved,~indicating almost complete lifting of the 12-fold degeneracy of~the lowest Landau levels (LL). Under an out-of-plane electric field E$\bot $. We observe degeneracy~breaking and transitions between QH plateaus. Interestingly, depending on its direction, E$\bot $selectively breaks the LL degeneracies in the electron-doped or hole-doped regimes.~   

Comparison of mobility at the top and bottom surfaces of multilayer graphene placed on SiO$_2$ substrate

It is known that charged impurities attached to the surface of graphene films are the main source of deteriorating mobility in graphene flakes obtained by the mechanical exfoliation. There are several origins for charged impurities: charges in the substrate, to which the bottom surface of the graphene films faces, the adsorbed molecules and contaminations due to chemicals (resist residues and so on) mainly attached to the top surface of graphene. This paper aims to evaluate the influence of the charged impurities on the top and bottom surfaces separately. For this purpose, we used dual-gated multilayer graphene with a contactless top gate. We developed a method of estimating the mobility of the top and bottom surfaces of multilayer graphene (MLG), from the top- and bottom-gate voltage dependence of the conductivity. We find that in thick MLG, mobility of the top surface is more than three times larger than that of the bottom surface. This indicates that the influence of the SiO$_2$ substrate on the mobility is stronger than that of adsorbates and contaminations on the top surface of the MLG.   

Electric charge and potential distribution in twisted multilayer graphene

The specifics of charge screening and electrostatic potential spatial distribution in rotationally faulted multilayered graphene films with decoupled layers placed in between charged substrates is theoretically analyzed. The analysis is carried out using a nonlinear Thomas -Fermi approach. It is shown that by varying the areal charge densities on the substrates and/or the thickness of the graphene pack one may tune the screening length in the graphene pack. When the charge densities on the substrates are weak, the screening length is of the same order as the pack thickness, which agrees with semimetallic properties of graphene. When the amount of the donated charge is sufficiently large the screening length reduces indicating the transition to a metallic-like behavior of the graphene layers. The transition is shown to turn on rather quickly, and in occurs when the charge on the substrates/external electric field reaches a certain crossover magnitude. The possibilities for experimental observation of the predicted transition are discussed.   

Tunable van Hove Singularities and Optical Absorption of Twisted Bilayer Graphene

We perform the first-principles GW-Bethe-Salpeter Equation (BSE) simulation to study the optical absorption spectra of isolated twisted bilayer graphene (TBLG). The twisting generates new van Hove singularities (VHS), and these VHSs and corresponding optical absorption peaks can be tuned in a wide range by the twist angle. Enhanced electron-electron and electron-hole interactions are shown to be important to understand both optical absorption peak positions and their lineshapes. With these many-electron effects included, our calculation satisfactorily explains recent experimental measurements.   

Gate tunable quantum transport in double layer graphene heterostructures

Motivated by the recently observed highly resistive state in double layer graphene heterostructures [1] we consider a system of two layers of graphene, ``studied'' and ``control,'' separated by an insulating layer. We theoretically analyze the effect of additional screening provided by Dirac electrons in the ``control'' graphene layer on the transport characteristics of the ``studied'' graphene layer. We find that in a typical device geometry fabricated on top of SiO2 substrate [1] the suppression of charge inhomogeneity is less efficient than initially expected and is limited by about a factor of 2. We also analyze the effect of additional screening on the quantum correction to the conductivity of the ``studied'' layer in this system in the metallic regime. We find that ``control'' layer screening is very efficient at suppressing electron-electron interactions in the ``studied'' layer which results in improved coherence and a novel gate tunable quantum correction to conductivity. The results of this work are summarized in [2].\\[4pt] [1] L. A. Ponomarenko et. al. Nat. Phys. 7, 958 (2011).\\[0pt] [2] K. Kechedzhi, E. H. Hwang, and S. Das Sarma Phys. Rev. B 86, 165442 (2012).   

Transport properties of monolayer and bilayer graphene supported by hexagonal boron nitride

We present transport studies on hexagonal boron nitride (h-BN) supported monolayer and bilayer graphene. Following the method introduced by Dean et al, we first exfoliate thin sheets of h-BN (15-20 nm) to SiO$_{\mathrm{2}}$/Si substrate then align and transfer exfoliated graphene flakes onto the h-BN sheets. E-beam lithography is used to process the samples into Hall bar devices. We find that current annealing at low temperature can increase the mobility of as-fabricated devices but often introduces large density inhomogeneity at the same time. AFM images of annealed devices reveal the limitations of this technique. In comparison, thermal annealing is much more reliable in improving the sample quality. Bilayer devices annealed in a flow of Ar/H$_{\mathrm{2}}$ at 450C for 5 hours show high mobility of 30,000 cm$^{\mathrm{2}}$/Vs at low temperature. We observe high-quality Shubnikov-de Hass (SdH) oscillations and degeneracy-lifted Landau levels in these samples. We extend existing measurements of the electron and hole effective mass in bilayer graphene[1] to lower carrier density regimes and discuss the implications of the results.[1] K. Zou, X. Hong, and J. Zhu, Phys. Rev. B 84, 085408 (2011).   

Ground state of double layer graphene heterostructures in the presence of charged impurities

A graphene double layer heterostructure is formed by two sheets of graphene separated by a thin dielectric film. Using the Thomas-Fermi-Dirac theory we have studied the carrier density profile in the presence of charged impurities. In this talk I will present our results for the case of heterostructures formed by two sheets of single-layer-graphene (SLG) and two sheets of bilayer-graphene (BLG). As for isolated layers, we find that the presence of charged impurities induces strong carrier density inhomogeneities, especially at low dopings where the density landscape breaks up in electron-hole puddles. We find that the amplitude of the carrier density inhomogeneities in double layers can be much lower than in isolated layers due to the better screening properties of double layer systems. I will then present results for the case of ``hybrid'' structures formed by one sheet of SLG and one sheet of BLG.   

Electronic and thermoelectric transport in graphene double layer structures with boron nitride spacers

Recently, much attention has been devoted to electrically isolated graphene-graphene double layers in which interaction-driven novel physics such as exciton condensation are predicted. We have used polyvinyl alcohol (PVA) based carrier films and a micro-manipulator to transfer mechanically exfoliated flakes onto desired locations with accuracy of $\sim$1 $\mu$m. We have fabricated graphene/boron nitride (BN)/graphene stacking structures on BN substrates to study their electronic and thermoelectric transport properties. We observed the low temperature mobility of graphene as high as 75000 cm$^2$/V-s. We have performed Coulomb drag measurements and observed the sign and magnitude dependence of the drag resistivity on the carrier types and densities of both graphene layers, consistent with the previous reports. We also performed thermoelectric transport measurements in such graphene double layer structures, especially in the complementary doped regime (so called excitonic regime) with one layer of electrons and the other layer of holes. Our approach may be useful to probe exciton condensation and other novel physics driven by electron-electron interactions in graphene double layers.   

Photo doping effect in graphene/BN heterostructure

Boron nitride has been demonstrated as an ideal substrate to achieve high mobility in graphene. At the same time We observed strong change of graphene transport properties by shining light on graphene/BN heterostructure. This is attributed to photo doping effect induced by impurity excitation in BN. Optical spectroscopy based on this photo-doping effects enables us to probe impurities in crystalline BN. Such information will be important for potential applications based on graphene/BN heterostructures. The potential of applying similar technique to probe defects in other insulators and semiconductors will also be discussed.   

Photon Induced Transport in Graphene-Boron Nitride-Graphene Heterostructures

Monolayer graphene, an atomically thin sheet of hexagonally oriented carbon, is a zero band gap conductor that exhibits strong electron-electron interactions and broadband optical absorption. By combining MLG and hexagonal boron nitride into ultrathin vertical stacks, experiments have demonstrated improved mobility, Coulomb drag, and field-effect tunneling across few-layer boron nitride barriers. Here, we report on the photon-induced transport of charge carriers through a graphene-boron nitride-graphene heterostructure. The dependence of the generated photocurrent on photon energy and interlayer bias voltage is studied. The photocurrent is found to depend strongly on both these parameters, showing several interesting features. We consider several processes that may serve to explain the rich dependence of photoconductance on applied bias voltage and photon energy.