Double-layer Graphene Structure Research Articles

Excitonic properties in a double-layer graphene

This paper investigates theoretically the excitonic condensation state at zero temperature in a double-layer graphene structure. In the framework of the unrestricted Hartree–Fock approximation, the electron-hole system in the structure described in the two-band electronic model is analyzed and one finds a set of self-consistent equations determining the excitonic order parameter. The optical properties of the excitonic condensation state then are examined in the Kubo linear optical response theory. Our results indicate that in the case of sufficiently large Coulomb interaction, the BEC excitonic condensation state might occur at low electronic excitation density. By turning the external electric field, the superfluid state stabilizes in the BCS-type excitonic condensate. The optical conductivity spectrum also provides us more insight into the excitonic condensation states.

Terahertz plasmon amplification in a double-layer graphene structure with direct electric current in hydrodynamic regime

The possibility of excitation and amplification of terahertz plasmons in a structure containing a layer of graphene with charge-carrier drift and a layer of graphene without carrier drift is theoretically investigated. The plasmon excitation in the graphene structure by an incident terahertz electromagnetic wave is calculated in the attenuated total reflection geometry. It is shown that, in graphene with charge-carrier drift, it is possible to achieve negative values of the real part of graphene conductivity for the phase velocity of plasmons exceeding the charge carrier's drift velocities, which indicates a non-Cherenkov amplification of terahertz plasmons in the double-layer graphene structure for practically achievable direct electric current values. Terahertz amplification originates due to the variation of the carrier mass density in graphene with terahertz electric field. In the case of weak deceleration of terahertz wave incident onto graphene structure, the value of the negative conductivity of graphene does not depend on the direction of the direct current in graphene (only the codirectional and counterdirectional currents with respect to the direction of plasmon propagation are considered). This is due to the insignificant spatial dispersion of the hydrodynamic conductivity of graphene in the case of weak deceleration of terahertz waves. The results of this work can be used to create compact terahertz radiation amplifiers operating at room temperature.

Hydrodynamic simulations of terahertz oscillation in double-layer graphene * * Project supported by the National Natural Science Foundation of China (No. 11604126).

We have theoretically studied current self-oscillations in double-layer graphene n+nn+ diodes driven by dc bias with the help of a time-dependent hydrodynamic model. The current self-oscillation results from resonant tunneling in the double-layer graphene structure. A detailed investigation of the dependence of the current self-oscillations on the applied bias has been carried out. The frequencies of current self-oscillations are in the terahertz (THz) region. The double-layer graphene n+nn+ device studied here may be presented as a THz source at room temperature.

Frequency-Reconfigurable Wide-Angle Terahertz Absorbers Using Single- and Double-Layer Decussate Graphene Ribbon Arrays.

We propose and numerically demonstrate two novel terahertz absorbers made up of periodic single- and double-layer decussate graphene ribbon arrays. The simulated results show that the proposed absorbers have narrowband near-unity terahertz absorption with ultra-wide frequency reconfiguration and angular stability. By tuning the Fermi level of graphene ribbons, the over 90% absorbance peak frequency of the absorber with single-layer graphene structure can be flexibly adjusted from 6.85 to 9.85 THz for both the transverse magnetic (TM) and transverse electric (TE) polarizations. This absorber with single-layer graphene demonstrates excellent angular stability with the absorbance peaks of the reconfigurable absorption bands remaining over 99.8% in a wide angle of incidence ranging from 0 to 70°. The tuning frequency can be significantly enhanced by using the absorber with double-layer graphene structure from 5.50 to 11.28 THz and 5.62 to 10.65 THz, approaching two octaves under TM and TE polarizations, respectively. The absorbance peaks of the reconfigurable absorption band of this absorber for both polarizations maintain over 70%, even at a large angle of incidence up to 70°. Furthermore, an analytical fitting model is also proposed to accurately predict the absorbance peak frequencies for this variety of absorbers. Benefitting from these attractive properties, the proposed absorber may have great potential applications in tunable terahertz trapping, detecting, sensing, and various terahertz optoelectronic devices.

Giant amplification of terahertz plasmons in a double-layer graphene

The amplification of terahertz plasmons in a pair of parallel active graphene monolayers is studied theoretically. The plasmon wave in a symmetric double-layer graphene structure splits into two branches with a symmetric and an antisymmetric distribution of tangential to graphene component of the electric field across the plane of symmetry of the structure. It is shown that, normalized to the wavelength, the terahertz plasmon amplification factor of the symmetric mode in the double-layer graphene structure could be greater than that in a single graphene layer by four orders of magnitude.

Metastable electron-electron states in double-layer graphene structures

The prototypical exciton model of two interacting Dirac particles in graphene was analyzed in [1] and it was found that in one of the electron-hole scattering channels the total kinetic energy vanishes, resulting in a singular behaviour. We show that this singularity can be removed by extending the quasiparticle dispersion, thus breaking the symmetry between upper and lower Dirac cones. The dynamics of an electron-electron pair are then mapped onto that of a single particle with negative mass and anisotropic dispersion. We show that the interplay between dispersion and repulsive interaction can result in the formation of bound, Cooper-pair-like, metastable states in double-layered hybrid structures.

Role of metallic substrate on the plasmon modes in double-layer graphene structures

Abstract Novel heterostructures combining different layered materials offer new opportunities for applications and fundamental studies of collective excitations driven by interlayer Coulomb interactions. In this work, we have investigated the influence of the metallic-like substrate on the plasmon spectrum of a double layer graphene system and a structure consisting of conventional two-dimensional electron gas (2DEG) immersed in a semiconductor quantum well and a graphene sheet with an interlayer separation of d . Long-range Coulomb interactions between substrate and graphene layered systems lead a new set of spectrum plasmons. At long wavelengths ( q → 0 ) the acoustic modes ( ω ~ q ) depend, besides on the carrier density in each layer, on the distance between the first carrier layer and the substrate in both structures. Furthermore, in the relativistic/nonrelativistic layered structure an undamped acoustic mode emerges for a certain interlayer critical distance d c . On the other hand, the optical plasmon modes emerging from the coupling of the double-layer systems and the substrate, both start at finite frequency at q= 0 in contrast to the collective excitation spectrum ω ~ q 1/2 reported in the literature for double-layer graphene structures.

Magneto-plasmonics in graphene-dielectric sandwich.

In this paper, dispersion properties and field distributions of surface magneto plasmons (SMPs) in double-layer graphene structures at room temperature are studied. It is found that, the dispersion curves of both symmetric and antisymmetric SMPs modes split into several branches/bands when a magnetic field is applied perpendicularly to the graphene surface. Surprisingly, the lowest energy SMP band has anomalous dependence on the applied magnetic field, different to the other higher bands. In addition, the symmetric and antisymmetric modes can be decoupled if the two graphene layers possess different properties, such as different Fermi energies. Furthermore, electric components of the surface modes which are parallel to the graphene surfaces but perpendicular to the propagation direction (i.e. the transverse-electric mode) are no longer zero caused by the Lorentz force on the free electrons.

Charge-density-wave states in double-layer graphene structures at a high magnetic field

We study the phases of correlated charge-density waves that form in a high magnetic field in two parallel graphene flakes separated by a thin insulator. The predicted phases include the square and hexagonal charge-density-wave bubbles, and a quasi-one-dimensional stripe phase. We find that the transition temperature for such phases is within the experimentally accessible range and that formation of interlayer-correlated states produces a negative compressibility contribution to the differential capacitance of this system.

Coulomb drag between in-plane graphene double ribbons and the impact of the dielectric constant

With recent developments in the search for novel device ideas, understanding electron-electron interaction in low dimensional systems is of particular interest. Coulomb drag measurements can provide critical insights in this context. In this article, we present a novel planar graphene double ribbon structure that shows for the first time that Coulomb drag is observable in two adjacent monolayer ribbons in the same plane at room temperature. Moreover, our planar devices enable experimentally study of the impact of the dielectric constant on Coulomb drag which is difficult to explore in the typically used double layer graphene structures. Our experimental findings indicate in particular that the drag resistance is proportional to the dielectric constant (e) and does not, as recently reported, show an increasing trend of interaction strength for small e-values. In fact, we find that the drag resistance follows approximately an e 1.2-dependence. The exponent of “1.2” is consistent with the theory considering the carrier concentration in our samples, and positions our results in between the weak and strong coupling limits.

Plasmonic excitations in Coulomb-coupledN-layer graphene structures

We study Dirac plasmons and their damping in spatially separated $N$-layer graphene structures at finite doping and temperatures. The plasmon spectrum consists of one optical excitation with a square-root dispersion and $N-1$ acoustical excitations with linear dispersions, which are undamped at zero temperature within a triangular energy region outside the electron-hole continuum. For any finite number of graphene layers we have found that the energy and weight of the optical plasmon increase in the long wavelength limit, respectively, as square-root and linear functions of $N$. This is in agreement with recent experimental findings. With an increase of the number of multilayer acoustical plasmon modes, the energy and weight of the upper lying branches also exhibit an enhancement with $N$. This increase is strongest for the uppermost acoustical mode so that its energy can exceed at some value of momentum the plasmon energy in an individual graphene sheet. Meanwhile, the energy of the low lying acoustical branches decreases weakly with $N$ as compared with the single acoustical mode in double-layer graphene structures. Our numerical calculations provide a detailed understanding of the overall behavior of the wave vector dependence of the optical and acoustical multilayer plasmon modes and show how their dispersion and damping are modified as a function of temperature, interlayer spacing, and inlayer carrier density in (un)balanced graphene multilayer structures.

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 is investigated over a wide range of temperatures and interlayer separations. We show 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.

Coulomb interaction effects in graphene bilayers: electron–hole pairing and plasmaron formation

We report a theoretical study of the many-body effects of electron–electron interaction on the ground-state and spectral properties of double-layer graphene. Using a projector-based renormalization method we show that if a finite-voltage difference is applied between the graphene layers, electron–hole pairs can be formed and—at very low temperatures—an excitonic instability might emerge in a double-layer graphene structure. The single-particle spectral function near the Fermi surface exhibits a prominent quasiparticle peak, different from neutral (undoped) graphene bilayers. Away from the Fermi surface, we find that the charge carriers strongly interact with plasmons, thereby giving rise to a broad plasmaron peak in the angle-resolved photoemission spectrum.