Instability Of Graphene Research Articles

Resolving the Mechanism of Acoustic Plasmon Instability in Graphene Doped by Alkali Metals.

Graphene doped by alkali atoms (AC) supports two heavily populated bands ( and ) crossing the Fermi level, which enables the formation of two intense two-dimensional plasmons: the Dirac plasmon (DP) and the acoustic plasmon (AP). Although the mechanism of the formation of these plasmons in electrostatically biased graphene or at noble metal surfaces is well known, the mechanism of their formation in alkali-doped graphenes is still not completely understood. We shall demonstrate that two isoelectronic systems, KC and CsC, support substantially different plasmonic spectra: the KC supports a sharp DP and a well-defined AP, while the CsC supports a broad DP and does not support an AP at all. We shall demonstrate that the AP in an AC is not, as previously believed, just a consequence of the interplay of the and intraband transitions, but a very subtle interplay between these transitions and the background screening, caused by the out-of-plane interband transitions.

Ballistic Injection Terahertz Plasma Instability in Graphene n+–i–n–n+ Field‐Effect Transistors and Lateral Diodes

Ballistic Electrons The Coulomb drag effect in lateral n+–i–n–n+ graphene diode/transistor structures with ballistic injection of graphene Dirac fermions makes it possible to strongly modify the current-voltage characteristics to produce “gain” in the terahertz frequency range, leading to voltage- and current-driven switches, frequency multipliers, and terahertz emitters. More details can be found in article number 2100694 by Victor Ryzhii and co-workers.

Magnetoelectronic Instability of Graphene on a Ferromagnetic Substrate

Magnetoelectronic Instability of Graphene on a Ferromagnetic Substrate

Fate of an electron beam in graphene: Coulomb relaxation or plasma instability?

Electron beams in two-dimensional systems can provide a useful tool to study energy-momentum relaxation of electrons and to generate microwave radiation stemming from plasma-beam instabilities. Naturally, these two applications cannot coexist: if beam electrons do relax, the beam is stabilized; if instability exists, it strongly distorts the distribution function of beam electrons. In this paper, we study the competition of beam relaxation due to electron-electron (e-e) collisions and development of plasma beam instability in graphene. We find that unstable plasma mode associated with a beam is stabilized already by weak e-e collisions. At intermediate e-e collision frequency, the instability re-appears at the ordinary graphene plasmon mode. Such instability is interpreted as viscous transfer of momentum from beam to 2d plasmons. Its growth rate reaches its maximum at hydrodynamic-to-ballistic crossover, when plasmon wavelength and electron mean free path are of the same order of magnitude.

Studying the Stability and Conductivity of the Composite Films via Electrospinning/Spray Process

The electrospinning/spray technique is a promising approach to fabricate composite films for many applications in flexible electronic devices. In this study, an elastic film with remarkable conductivity is synthesized by electrospinning TPU (thermoplastic polyurethanes) fibers and electrospraying graphene nanosheets into the single collector. This technique allows the graphene nanosheets to adhere uniformly to a carbon textile matrix for promoting its conductivity. But the graphene is possible to be oxidized into graphene oxide during electrospray, which is attributed to the high-voltage field, the rapid evaporation of the solvent and the full exposure to air. And the spinning fibers have the potential to induce the behavior as well. The instability of graphene during the process can make an impact on the overall conductivity and uniformity of composite films. So, we set up a series of experiments on the study of oxidization behavior of graphene and its influence on the performance of composite films. With different parameters, some samples are synthesized to conduct Raman spectroscopy analysis as well as take the measurements of overall conductivity through a 4-point probe electrical conductivity device. The oxidization behavior of graphene during the electrospinning/spray process can be well characterized by the results from the Raman spectrums. And the results of electrical conductivity measurements can provide powerful evidence to ensure the great conductivity of composite films via electrospinning/spray process.

Tensile properties of three-dimensionally interconnected graphene-networked Cu composite and changes in its microstructure in relation to heat treatment temperature

An investigation of microstructure changes in a new, in situ-fabricated, three-dimensionally interconnected graphene-networked Cu composite was performed in relation to the heat treatment temperature in H2 and in air. Heat-treated in H2 revealed that recrystallization commenced at ~ 300 ℃ and the crystal grew afterward. However, the crystal growth was constrained within the initial Cu grains and inhibited due to Cu grains being wrapped by the stable graphene, even at 1000 ℃. This resulted in similar or narrower grain-size distribution. Meanwhile heat treatment in air resulted in the oxidation of carbon, instability of graphene, and crystal growth over the grain boundaries, which resulted in a wider grain-size distribution. Increases in tensile and plastic deformation (30% and 144%, respectively) were observed due to the grain size refinement and the strong interfacial bonds between graphene and the Cu matrix. The result was greater load bearing and toleration of a substantial load during plastic deformation.

Electron-phonon instability in graphene revealed by global and local noise probes.

Understanding and controlling nonequilibrium electronic phenomena is an outstanding challenge in science and engineering. By electrically driving ultraclean graphene devices out of equilibrium, we observe an instability that is manifested as substantially enhanced current fluctuations and suppressed conductivity at microwave frequencies. Spatial mapping of the nonequilibrium current fluctuations using nanoscale magnetic field sensors reveals that the fluctuations grow exponentially along the direction of carrier flow. Our observations, including the dependence on density and temperature, are consistently explained by the emergence of an electron-phonon Cerenkov instability at supersonic drift velocities. These results offer the opportunity for tunable terahertz generation and active phononic devices based on two-dimensional materials.

Comprehensive analysis of the effects of bending strain on GFET on ultra‐flat flexible PI substrate using varnish PI

The study investigated how bending strain affected graphene field-effect transistors (GFETs) that had been fabricated on ultra-flat flexible polyimide (PI) film, and the instability of graphene from repeated bending strain. An ultra-flat surface of PI film was achieved by using varnish PI with gradually increasing temperature. The average surface roughness was reduced from 23.23 to 0.32 nm. Bending strain distorted the symmetric atomic structure of graphene, and thereby caused a shift of two-dimensional peak by -42.67 cm -1 /% in the Raman spectrum and acted such as uniaxial strain. Bending strain on the GFET increased its energy bandgap, and the increased energy bandgap decreased the `off' current and increased the current on/off ratio. Bending strain increased carrier concentration and decreased carrier mobility. Repeated bending strain damaged graphene by increasing defects, and thereby increased its resistivity by ~25%. The damage could be recovered by thermal annealing at 200°C.

Plasma wave instabilities in nonequilibrium graphene

We study two-stream instabilities in a non-equilibrium system in which a stream of electrons is injected into doped graphene. As with equivalent non-equilibrium parabolic band systems, we find that the graphene systems can support unstable charge-density waves whose amplitudes grow with time. We determine the range of wavevector $\boldsymbol{q}$ that are unstable, and their growth rates. We find no instability for waves with wavevectors parallel or perpendicular to the direction of the injected carriers. We find that, within the small wavevector approximation, the angle between $\boldsymbol{q}$ and the direction of the injected electrons that maximizes the growth rate increases with increasing $\boldsymbol{|q|}$. We compare the range and strength of the instability in graphene to that of two and three dimensional parabolic band systems.

Rayleigh-Bénard instability in graphene

Motivated by the observation that electrons in graphene, in the hydrodynamic regime of transport, can be treated as a two-dimensional ultra-relativistic gas with very low shear viscosity, we examine the existence of the Rayleigh-B\'enard instability in a massless electron-hole plasma. Firstly, we perform a linear stability analysis, derive the leading contributions to the relativistic Rayleigh number, and calculate the critical value above which the instability develops. By replacing typical values for graphene, such as thermal conductivity, shear viscosity, temperature, and sample sizes, we find that the instability might be experimentally observed in the near future. Additionally, we have performed simulations for vanishing reduced chemical potential and compare the measured critical Rayleigh number with the theoretical prediction, finding good agreement.

Supercritical instability in graphene with two charged impurities

We study the supercritical instability in gapped graphene with two charged impurities separated by distance R using the two-dimensional Dirac equation for electron quasiparticles. Attention is paid to a situation when charges of impurities are subcritical, whereas their total charge exceeds a critical one. The critical distance R_{cr} in the system of two charged centers is defined as that at which the electron bound state with the lowest energy reaches the boundary of the lower continuum. A variational calculation of the critical distance R_{cr} separating the supercritical (R<R_{cr}) and subcritical (R>R_{cr}) regimes is carried out. It is shown that the critical distance R_{cr} increases as the quasiparticle gap decreases. The energy and width of a quasistationary state as functions of the distance between two impurities are derived in the quasiclassical approximation.

Observation of a snap-through instability in graphene

We examine the competition between adhesive and bending energies for few-layer graphene (FLG) samples placed on microscale-corrugated metallic substrates. Using atomic force microscopy, we show that the graphene undergoes a sharp “snap-through” transition as a function of layer thickness, where the material transitions between conforming to the substrate and lying flat on top of the substrate. By utilizing the critical snap-through thickness in an elasticity model for the FLG’s bending, we extract a value for graphene-surface adhesion energy of 1.1 eV/nm2.

Spin-density-wave instability in graphene doped near the van Hove singularity

We study the instability of the metallic state towards the formation of a new ground state in graphene doped near the van Hove singularity. The system is described by the Hubbard model and a field theoretical approach is used to calculate the charge and spin susceptibility. We find that for repulsive interactions, within the random phase approximation, there is a competition between ferromagnetism and spin-density wave (SDW). It turns out that a SDW with a triangular geometry is more favorable when the Hubbard parameter is above the critical value U_c(T), which depends on the temperature T, even if there are small variations in the doping. Our results can be verified by ARPES or neutron scattering experiments in highly doped graphene.

Band Gap Opening by Two-Dimensional Manifestation of Peierls Instability in Graphene

Using first-principles calculations of graphene having high-symmetry distortion or defects, we investigate band gap opening by chiral symmetry breaking, or intervalley mixing, in graphene and show an intuitive picture of understanding the gap opening in terms of local bonding and antibonding hybridizations. We identify that the gap opening by chiral symmetry breaking in honeycomb lattices is an ideal two-dimensional (2D) extension of the Peierls metal-insulator transition in 1D linear lattices. We show that the spontaneous Kekule distortion, a 2D version of the Peierls distortion, takes place in biaxially strained graphene, leading to structural failure. We also show that the gap opening in graphene antidots and armchair nanoribbons, which has been usually attributed to quantum confinement effects, can be understood with the chiral symmetry breaking.

Supercritical Coulomb center and excitonic instability in graphene

It is well known that there are resonant states with complex energy for the supercritical Coulomb impurity in graphene. We show that opening of a quasiparticle gap decreases the imaginary part of energy, $|\text{Im}\text{ }E|$, of these states and stabilizes the system. For gapless quasiparticles with strong Coulomb interaction in graphene, we solve the Bethe-Salpeter equation for the electron-hole bound state and show that it has a tachyonic solution for strong enough coupling $\ensuremath{\alpha}={e}^{2}/\ensuremath{\kappa}\ensuremath{\hbar}{v}_{F}$ leading to instability of the system. In the random-phase approximation, the critical coupling is estimated to be ${\ensuremath{\alpha}}_{c}=1.62$ and is an analog of the critical charge in the Coulomb center problem. We argue that the excitonic instability should be resolved through the formation of an excitonic condensate and gap generation in the quasiparticle spectrum.

Kekule-distortion-induced exciton instability in graphene

Effects of a Kekule distortion on exciton instability in single-layer graphene are discussed. In the framework of quantum electrodynamics the mass of the electron generated dynamically is worked out using a Schwinger-Dyson equation. For homogeneous lattice distortion it is shown that the generated mass is independent of the amplitude of the lattice distortion at the one-loop approximation. Formation of excitons induced by the homogeneous Kekule distortion could appear only through direct dependence of the lattice distortion.