arXiv subjects

S. V. Morozov

Publications and source records attributed to S. V. Morozov.

At least 37 records · Page 2Linked to original sources

arXiv2015

Resonant tunnelling between the chiral Landau states of twisted graphene lattices

A new class of multilayered functional materials has recently emerged in which the component atomic layers are held together by weak van der Waals forces that preserve the structural integrity and physical properties of each layer. An exemplar of such a structure is a transistor device in which relativistic Dirac Fermions can resonantly tunnel through a boron nitride barrier, a few atomic layers thick, sandwiched between two graphene electrodes. An applied magnetic field quantises graphene's gapless conduction and valence band states into discrete Landau levels, allowing us to resolve individual inter-Landau level transitions and thereby demonstrate that the energy, momentum and chiral properties of the electrons are conserved in the tunnelling process. We also demonstrate that the change in the semiclassical cyclotron trajectories, following a tunnelling event, is a form of Klein tunnelling for inter-layer transitions.

cond-mat.mes-hall↗

arXiv2014

Twist-controlled resonant tunnelling in graphene-boron nitride-graphene heterostructures

Recent developments in the technology of van der Waals heterostructures made from two-dimensional atomic crystals have already led to the observation of new physical phenomena, such as the metal-insulator transition and Coulomb drag, and to the realisation of functional devices, such as tunnel diodes, tunnel transistors and photovoltaic sensors. An unprecedented degree of control of the electronic properties is available not only by means of the selection of materials in the stack but also through the additional fine-tuning achievable by adjusting the built-in strain and relative orientation of the component layers. Here we demonstrate how careful alignment of the crystallographic orientation of two graphene electrodes, separated by a layer of hexagonal boron nitride (hBN) in a transistor device, can achieve resonant tunnelling with conservation of electron energy, momentum and, potentially, chirality. We show how the resonance peak and negative differential conductance in the device characteristics induces a tuneable radio-frequency oscillatory current which has potential for future high frequency technology.

cond-mat.mes-hall↗

arXiv2011

Field-effect tunneling transistor based on vertical graphene heterostructures

We report a bipolar field effect tunneling transistor that exploits to advantage the low density of states in graphene and its one atomic layer thickness. Our proof-of-concept devices are graphene heterostructures with atomically thin boron nitride acting as a tunnel barrier. They exhibit room temperature switching ratios ~50, a value that can be enhanced further by optimizing the device structure. These devices have potential for high frequency operation and large scale integration.

cond-mat.mes-hall↗

arXiv2011

Tunable metal-insulator transition in double-layer graphene heterostructures

We report a double-layer electronic system made of two closely-spaced but electrically isolated graphene monolayers sandwiched in boron nitride. For large carrier densities in one of the layers, the adjacent layer no longer exhibits a minimum metallic conductivity at the neutrality point, and its resistivity diverges at low temperatures. This divergence can be suppressed by magnetic field or by reducing the carrier density in the adjacent layer. We believe that the observed localization is intrinsic for neutral graphene with generic disorder if metallic electron-hole puddles are screened out.

cond-mat.mes-hall↗

arXiv2011

Terahertz detection in a slit-grating-gate field-effect-transistor structure

We have fabricated a grating-gate InGaAs/GaAs field-effect transistor structure with narrow slits between the grating gate fingers. The resonant photoconductive response of this structure has been measured in the sub-terahertz frequency range. The frequencies of the photoresponse peaks correspond to the excitation of the plasmon resonances in the structure channel. The obtained responsivity exceeds the responsivity reported previously for similar plasmonic terahertz detectors by two orders of magnitude due to enhanced coupling between incoming terahertz radiation and plasmon oscillations in the slit-grating-gate field-effect transistor structure.

cond-mat.mtrl-sci↗

arXiv2011

Dirac cones reshaped by interaction effects in suspended graphene

We report measurements of the cyclotron mass in graphene for carrier concentrations n varying over three orders of magnitude. In contrast to the single-particle picture, the real spectrum of graphene is profoundly nonlinear so that the Fermi velocity describing the spectral slope reaches ~3x10^6 m/s at n <10^10 cm^-2, three times the value commonly used for graphene. The observed changes are attributed to electron-electron interaction that renormalizes the Dirac spectrum because of weak screening. Our experiments also put an upper limit of ~0.1 meV on the possible gap in graphene.

cond-mat.mes-hall↗

arXiv2011

Interaction-Driven Spectrum Reconstruction in Bilayer Graphene

The nematic phase transition in electronic liquids, driven by Coulomb interactions, represents a new class of strongly correlated electronic ground states. We studied suspended samples of bilayer graphene, annealed so that it achieves very high quasiparticle mobilities. Bilayer graphene is a truly two-dimensional material with complex chiral electronic spectra and the high quality of our samples allowed us to observe strong spectrum reconstructions and electron topological transitions that can be attributed to a nematic phase transition and a decrease in rotational symmetry. These results are especially surprising because no interaction effects have been observed so far in bilayer graphene in the absence of an applied magnetic field.

cond-mat.mes-hall↗

arXiv2011

Micrometer-scale ballistic transport in encapsulated graphene at room temperature

Devices made from graphene encapsulated in hexagonal boron-nitride exhibit pronounced negative bend resistance and an anomalous Hall effect, which are a direct consequence of room-temperature ballistic transport on a micrometer scale for a wide range of carrier concentrations. The encapsulation makes graphene practically insusceptible to the ambient atmosphere and, simultaneously, allows the use of boron nitride as an ultrathin top gate dielectric.

cond-mat.mtrl-sci↗

arXiv2011

Giant Nonlocality near the Dirac Point in Graphene

Transport measurements have been a powerful tool for uncovering new electronic phenomena in graphene. We report nonlocal measurements performed in the Hall bar geometry with voltage probes far away from the classical path of charge flow. We observe a large nonlocal response near the Dirac point in fields as low as 0.1T, which persists up to room temperature. The nonlocality is consistent with the long-range flavor currents induced by lifting of spin/valley degeneracy. The effect is expected to contribute strongly to all magnetotransport phenomena near the neutrality point.

cond-mat.mes-hall↗

arXiv2010

Graphene as a transparent conductive support for studying biological molecules by transmission electron microscopy

We demonstrate the application of graphene as a support for imaging individual biological molecules in transmission electron microscope (TEM). A simple procedure to produce free-standing graphene membranes has been designed. Such membranes are extremely robust and can support practically any sub-micrometer object. Tobacco mosaic virus has been deposited on graphene samples and observed in a TEM. High contrast has been achieved even though no staining has been applied.

cond-mat.mtrl-sci↗

arXiv2010

From One Electron to One Hole: Quasiparticle Counting in Graphene Quantum Dots Determined by Electrochemical and Plasma Etching

Graphene is considered to be a promising material for future electronics. The envisaged transistor applications often rely on precision cutting of graphene sheets with nanometer accuracy. In this letter we demonstrate graphene-based quantum dots created by using atomic force microscopy (AFM) with tip-assisted electrochemical etching. This lithography technique provides resolution of about 20 nm, which can probably be further improved by employing sharper tips and better humidity control. The behavior of our smallest dots in magnetic field has allowed us to identify the charge neutrality point and distinguish the states with one electron, no charge and one hole left inside the quantum dot.

cond-mat.mes-hall↗

arXiv2010

Electronic properties of a biased graphene bilayer

We study, within the tight-binding approximation, the electronic properties of a graphene bilayer in the presence of an external electric field applied perpendicular to the system -- \emph{biased bilayer}. The effect of the perpendicular electric field is included through a parallel plate capacitor model, with screening correction at the Hartree level. The full tight-binding description is compared with its 4-band and 2-band continuum approximations, and the 4-band model is shown to be always a suitable approximation for the conditions realized in experiments. The model is applied to real biased bilayer devices, either made out of SiC or exfoliated graphene, and good agreement with experimental results is found, indicating that the model is capturing the key ingredients, and that a finite gap is effectively being controlled externally. Analysis of experimental results regarding the electrical noise and cyclotron resonance further suggests that the model can be seen as a good starting point to understand the electronic properties of graphene bilayer. Also, we study the effect of electron-hole asymmetry terms, as the second-nearest-neighbor hopping energies $t'$ (in-plane) and $γ_{4}$ (inter-layer), and the on-site energy $Δ$.

cond-mat.mes-hall↗

arXiv2009

Influence of metal contacts and charge inhomogeneity on transport properties of graphene near the neutrality point

There is an increasing amount of literature concerning electronic properties of graphene close to the neutrality point. Many experiments continue using the two-probe geometry or invasive contacts or do not control samples' macroscopic homogeneity. We believe that it is helpful to point out some problems related to such measurements. By using experimental examples, we illustrate that the charge inhomogeneity induced by spurious chemical doping or metal contacts can lead to large systematic errors in assessing graphene's transport properties and, in particular, its minimal conductivity. The problems are most severe in the case of two-probe measurements where the contact resistance is found to strongly vary as a function of gate voltage.

cond-mat.mes-hall↗

arXiv2008

Control of graphene's properties by reversible hydrogenation

Graphene - a monolayer of carbon atoms densely packed into a hexagonal lattice - has one of the strongest possible atomic bonds and can be viewed as a robust atomic-scale scaffold, to which other chemical species can be attached without destroying it. This notion of graphene as a giant flat molecule that can be altered chemically is supported by the observation of so-called graphene oxide, that is graphene densely covered with hydroxyl and other groups. Unfortunately, graphene oxide is strongly disordered, poorly conductive and difficult to reduce to the original state. Nevertheless, one can imagine atoms or molecules being attached to the atomic scaffold in a strictly periodic manner, which should result in a different electronic structure and, essentially, a different crystalline material. A hypothetical example for this is graphane, a wide-gap semiconductor, in which hydrogen is bonded to each carbon site of graphene. Here we show that by exposing graphene to atomic hydrogen, it is possible to transform this highly-conductive semimetal into an insulator. Transmission electron microscopy reveals that the material retains the hexagonal lattice but its period becomes markedly shorter than that of graphene, providing direct evidence for a new graphene-based derivative. The reaction with hydrogen is found to be reversible so that the original metallic state and lattice spacing are restored by annealing and even the quantum Hall effect recovers. Our work proves the concept of chemical modification of graphene, which promises a whole range of new two-dimensional crystals with designed electronic and other properties.

cond-mat.mes-hall↗

arXiv2008

Graphene-Based Liquid Crystal Device

Graphene is only one atom thick, optically transparent, chemically inert and an excellent conductor. These properties seem to make this material an excellent candidate for applications in various photonic devices that require conducting but transparent thin films. In this letter we demonstrate liquid crystal devices with electrodes made of graphene which show excellent performance with a high contrast ratio. We also discuss the advantages of graphene compared to conventionally-used metal oxides in terms of low resistivity, high transparency and chemical stability.

cond-mat.mes-hall↗

arXiv2008

Giant Intrinsic Carrier Mobilities in Graphene and Its Bilayer

We have studied temperature dependences of electron transport in graphene and its bilayer and found extremely low electron-phonon scattering rates that set the fundamental limit on possible charge carrier mobilities at room temperature. Our measurements have shown that mobilities significantly higher than 200,000 cm2/Vs are achievable, if extrinsic disorder is eliminated. A sharp (threshold-like) increase in resistivity observed above approximately 200K is unexpected but can qualitatively be understood within a model of a rippled graphene sheet in which scattering occurs on intra-ripple flexural phonons.

cond-mat.mes-hall↗

arXiv2007

Biased bilayer graphene: semiconductor with a gap tunable by electric field effect

We demonstrate that the electronic gap of a graphene bilayer can be controlled externally by applying a gate bias. From the magneto-transport data (Shubnikov-de Haas measurements of the cyclotron mass), and using a tight binding model, we extract the value of the gap as a function of the electronic density. We show that the gap can be changed from zero to mid-infrared energies to a value as large as 0.3 eV by using fields of < 1 V/nm, below the electric breakdown of SiO2. The opening of a gap is clearly seen in the quantum Hall regime.

cond-mat.mtrl-sci↗

arXiv2007

Detection of Individual Gas Molecules Absorbed on Graphene

The ultimate aspiration of any detection method is to achieve such a level of sensitivity that individual quanta of a measured value can be resolved. In the case of chemical sensors, the quantum is one atom or molecule. Such resolution has so far been beyond the reach of any detection technique, including solid-state gas sensors hailed for their exceptional sensitivity. The fundamental reason limiting the resolution of such sensors is fluctuations due to thermal motion of charges and defects which lead to intrinsic noise exceeding the sought-after signal from individual molecules, usually by many orders of magnitude. Here we show that micrometre-size sensors made from graphene are capable of detecting individual events when a gas molecule attaches to or detaches from graphenes surface. The adsorbed molecules change the local carrier concentration in graphene one by one electron, which leads to step-like changes in resistance. The achieved sensitivity is due to the fact that graphene is an exceptionally low-noise material electronically, which makes it a promising candidate not only for chemical detectors but also for other applications where local probes sensitive to external charge, magnetic field or mechanical strain are required.

cond-mat.mtrl-sci↗