Michihisa Yamamoto

Trilayer graphene is a semimetal with a gate-tunable band overlap

Graphene-based materials are promising candidates for nanoelectronic devices because very high carrier mobilities can be achieved without the use of sophisticated material preparation techniques. However, the carrier mobilities reported for single-layer and bilayer graphene are still less than those reported for graphite crystals at low temperatures, and the optimum number of graphene layers for any given application is currently unclear, because the charge transport properties of samples containing three or more graphene layers have not yet been investigated systematically. Here, we study charge transport through trilayer graphene as a function of carrier density, temperature, and perpendicular electric field. We find that trilayer graphene is a semimetal with a resistivity that decreases with increasing electric field, a behaviour that is markedly different from that of single-layer and bilayer graphene. We show that the phenomenon originates from an overlap between the conduction and valence bands that can be controlled by an electric field, a property that had never previously been observed in any other semimetal. We also determine the effective mass of the charge carriers, and show that it accounts for a large part of the variation in the carrier mobility as the number of layers in the sample is varied.

Contact resistance in graphene-based devices

We report a systematic study of the total contact resistance present at the interface between a metal (Ti) and graphene layers of different, known thickness. By comparing devices fabricated on many different graphene flakes we demonstrate that the contact resistance consists of a gate independent and a gate dependent part. We show that quantitatively the gate independent part of the contact resistance is the same for single-, bi-, and tri-layer graphene. We argue that this is the result of charge transfer from the metal, causing the Fermi level in the graphene region under the contacts to shift far away from the charge neutrality point.

Tuneable electronic properties in graphene

Novel materials are in great demand for future applications. The discovery of graphene, a one atom thick carbon layer, holds the promise for unique device architectures and functionalities exploiting unprecedented physical phenomena. The ability to embed graphene materials in a double gated structure allowed on-chip realization of relativistic tunneling experiments in single layer graphene, the discovery of a gate tunable band gap in bilayer graphene and of a gate tunable band overlap in trilayer graphene. Here we discuss recent advances in the physics and nanotechnology fabrication of double gated single- and few-layer graphene devices.

Electrons surfing on a sound wave as a platform for quantum optics with flying electrons

Electrons in a metal are indistinguishable particles that interact strongly with other electrons and their environment. Isolating and detecting a single flying electron after propagation, in a similar manner to quantum optics experiments with single photons, is therefore a challenging task. So far only a few experiments have been performed in a high-mobility two-dimensional electron gas in which the electron propagates almost ballistically. In these previous works, flying electrons were detected by means of the current generated by an ensemble of electrons, and electron correlations were encrypted in the current noise. Here we demonstrate the experimental realization of high-efficiency single-electron source and detector for a single electron propagating isolated from the other electrons through a one-dimensional channel. The moving potential is excited by a surface acoustic wave, which carries the single electron along the one-dimensional channel at a speed of 3 μm ns−1. When this quantum channel is placed between two quantum dots several micrometres apart, a single electron can be transported from one quantum dot to the other with quantum efficiencies of emission and detection of 96% and 92%, respectively. Furthermore, the transfer of the electron can be triggered on a timescale shorter than the coherence time T2* of GaAs spin qubits. Our work opens new avenues with which to study the teleportation of a single electron spin and the distant interaction between spatially separated qubits in a condensed-matter system.

Generation and detection of pure valley current by electrically induced Berry curvature in bilayer graphene

Bilayer graphene can host topological currents that are robust against defects and are associated with the electron valleys. It is now shown that electric fields can tune this topological valley transport over long distances at room temperature. The field of ‘Valleytronics’ has recently been attracting growing interest as a promising concept for the next generation electronics, because non-dissipative pure valley currents with no accompanying net charge flow can be manipulated for computational use, akin to pure spin currents1. Valley is a quantum number defined in an electronic system whose energy bands contain energetically degenerate but non-equivalent local minima (conduction band) or maxima (valence band) due to a certain crystal structure. Specifically, spatial inversion symmetry broken two-dimensional honeycomb lattice systems exhibiting Berry curvature is a subset of possible systems that enable optical2,3,4,5, magnetic6,7,8,9 and electrical control of the valley degree of freedom10,11,12. Here we use dual-gated bilayer graphene to electrically induce and control broken inversion symmetry (or Berry curvature) as well as the carrier density for generating and detecting the pure valley current. In the insulating regime, at zero-magnetic field, we observe a large nonlocal resistance that scales cubically with the local resistivity, which is evidence of pure valley current.

Stacking-order dependent transport properties of trilayer graphene

We report markedly different transport properties of ABA- and ABC-stacked trilayer graphenes. Our experiments in double-gated trilayer devices provide evidence that a perpendicular electric field opens an energy gap in the ABC trilayer, while it causes the increase of a band overlap in the ABA trilayer. In a perpendicular magnetic field, the ABA trilayer develops quantum Hall plateaus at filling factors of \nu = 2, 4, 6... with a step of \Delta \nu = 2, whereas the inversion symmetric ABC trilayer exhibits plateaus at \nu = 6 and 10 with 4-fold spin and valley degeneracy.

Straining Graphene Using Thin Film Shrinkage Methods

Theoretical works suggest the possibility and usefulness of strain engineering of graphene by predicting remarkable properties, such as Dirac cone merging, bandgap opening and pseudo magnetic field generation. However, most of these predictions have not yet been confirmed because it is experimentally difficult to control the magnitude and type (e.g., uniaxial, biaxial, and so forth) of strain in graphene devices. Here we report two novel methods to apply strain without bending the substrate. We employ thin films of evaporated metal and organic insulator deposited on graphene, which shrink after electron beam irradiation or heat application. These methods make it possible to apply both biaxial strain and in-plane isotropic compressive strain in a well-controlled manner. Raman spectroscopy measurements show a clear splitting of the degenerate states of the G-band in the case of biaxial strain, and G-band blue shift without splitting in the case of in-plane isotropic compressive strain. In the case of biaxial strain application, we find out the ratio of the strain component perpendicular to the stretching direction is at least three times larger than what was previously observed, indicating that shrinkage of the metal or organic insulator deposited on graphene induces both tensile and compressive strain in this atomically thin material. Our studies present for the first time a viable way to apply strain to graphene without the need to bend the substrate.

Supercurrent in the quantum Hall regime.

Making a graphene super-edge In superconductors, the electrical current is carried by “Cooper pairs,” formed out of an electron and a hole. This supercurrent will happily cross a thin barrier between two superconductors. But what if a strong magnetic field were applied at the barrier, forcing charge carriers to travel only along the edge of the barrier? Amet et al. explored this regime in a sample consisting of two superconducting electrodes and a graphene barrier under magnetic fields of up to 2 tesla (see the Perspective by Mason). Their transport measurements were consistent with a model in which the supercurrent was carried by the edge states in graphene. Science, this issue p. 966; see also p. 891 Transport measurements show that quantum Hall edge states carry the supercurrent in a graphene Josephson junction. A promising route for creating topological states and excitations is to combine superconductivity and the quantum Hall (QH) effect. Despite this potential, signatures of superconductivity in the QH regime remain scarce, and a superconducting current through a QH weak link has been challenging to observe. We demonstrate the existence of a distinct supercurrent mechanism in encapsulated graphene samples contacted by superconducting electrodes, in magnetic fields as high as 2 tesla. The observation of a supercurrent in the QH regime marks an important step in the quest for exotic topological excitations, such as Majorana fermions and parafermions, which may find applications in fault-tolerant quantum computing.

Double-gated graphene-based devices

We discuss transport through double-gated single- and few-layer graphene devices. This kind of device configuration has been used to investigate the modulation of the energy band structure through the application of an external perpendicular electric field, a unique property of few-layer graphene systems. Here we discuss technological details that are important for the fabrication of top-gated structures, based on electron-gun evaporation of SiO2. We perform a statistical study that demonstrates how—contrary to expectations—the breakdown field of electron-gun evaporated thin SiO2 films is comparable to that of thermally grown oxide layers. We find that a high breakdown field can be achieved in evaporated SiO2 only if the oxide deposition is directly followed by metallization of the top electrodes, without exposure of the SiO2 layer to air.

Noise correlations in a Coulomb-blockaded quantum dot.

We report measurements of current noise auto- and cross correlation in a tunable quantum dot with two or three leads. As the Coulomb blockade is lifted at finite source-drain bias, the autocorrelation evolves from super- to sub-Poissonian in the two-lead case, and the cross correlation evolves from positive to negative in the three-lead case, consistent with transport through multiple levels. Cross correlations in the three-lead dot are found to be proportional to the noise in excess of the Poissonian value in the limit of weak output tunneling.