Emergence and dynamical properties of stochastic branching in the electronic flows of disordered Dirac solids

2018-06-19T16:33:14+00:00 June 19th, 2018|Categories: Publications|Tags: |

Graphene as well as more generally Dirac solids constitute two-dimensional materials where the electronic flow is ultra-relativistic. When a Dirac solid is deposited on a different substrate surface with roughness, a local random potential develops through an inhomogeneous charge impurity distribution. This external potential affects profoundly the charge flow and induces a chaotic pattern of current branches that develops through focusing and defocusing effects produced by the randomness of the surface. An additional bias voltage may be used to tune the branching pattern of the charge carrier currents. We employ analytical and numerical techniques in order to investigate the onset and the statistical properties of carrier branches in Dirac solids. We find a specific scaling-type relationship that connects the physical scale for the occurrence of branches with the characteristic medium properties, such as disorder and bias field. We use numerics to test and verify the theoretica…

Published in: "EPL".

Efficient charge pump by pure mechanical resonators in graphene

2018-04-20T12:29:41+00:00 April 20th, 2018|Categories: Publications|Tags: |

Graphene is an ideal two-dimensional nanoelectromechanical material due to its outstanding elastic properties and superior electro-mechanical coupling. We study a graphene-based charge pump by two mechanical resonators out of phase. It is found that in the adiabatic limit, the pumped charge per mode is quantized in a pumping cycle and the electro-mechanical conversion efficiency is maximally saturated, as long as the mechanical lattice deformations produce a transport gap for massless Dirac electrons. The efficient charge pump originates from the definite chirality of Dirac electrons as well as the possible topological interface state forming in the evanescent modes. Our findings might shed light on enhancing the electro-mechanical conversion efficiency of graphene-based devices.

Published in: "EPL".

Strain-induced recovery of electronic anisotropy in 90°-twisted bilayer phosphorene

2018-03-15T12:31:49+00:00 March 15th, 2018|Categories: Publications|Tags: |

It is well known that anisotropy determines the preferred transport direction of carriers. To manipulate the anisotropy is an exciting topic in two-dimensional materials, where the carriers are confined within individual layers. In this work, it is found that uniaxial strain can tune the electronic anisotropy of the 90°-twisted bilayer phosphorene. In this unique bilayer structure, the zigzag direction of one layer corresponds to the armchair one of the other layer and vice versa. Owing to this complementary structure, the directional (zigzag or armchair) deformation response to strain of one layer is opposite to that of the other layer, where the in-plane positive Poisson’s ratio plays a key role. As a result, the doubly degenerate highest valence bands split, followed by a recovery of anisotropy. More interestingly, such an anisotropy, namely, the ratio of the effective mass along the ##IMG## [http://ej.iop.org/images/0295-5075/121/2/…] {$Gamma text{-} X$}

Published in: "EPL".

Theory of Goos-Hänchen shift in graphene: Energy-flux method

2018-03-01T12:29:39+00:00 March 1st, 2018|Categories: Publications|Tags: , |

We present a theoretical study of Goos-Hänchen shift and associated time delay of a p -polarized electromagnetic wave packet for total internal reflection at a plane interface between two media of different permittivity, with graphene at their interface. The study is based on the energy-flux method which takes into account the energy flux in graphene in addition to the energy flux in the incident, totally reflected, coupled incident-totally reflected and evanescent wave packets.

Published in: "EPL".

Switchable hyperbolic metamaterials based on the graphene-dielectric stacking structure and optical switches design

2018-01-22T14:29:23+00:00 January 22nd, 2018|Categories: Publications|Tags: |

We numerically demonstrate broadband optical switches which can control the propagation of mid-infrared waves that is incident to the interface between a switchable hyperbolic metamaterials (SHM) and air. The SHM consists of a one-dimensional periodic stacking of graphene layers and dielectric layers. The isofrequency curve of the structure can be switched between hyperbolic shape and elliptical shape by controlling the external gate voltage or the electrostatic field biasing. It is revealed that when the interface between air and the SHM is parallel to the graphene sheets, it can switch between positive refraction and total reflection; when the interface is perpendicular to the graphene sheets, it can switch between positive and negative refraction.

Published in: "EPL".

Plasmon-induced transparency in graphene-based terahertz metamaterials

2018-01-15T16:29:56+00:00 January 15th, 2018|Categories: Publications|Tags: , |

Plasmon-induced transparency (PIT) effect in a terahertz graphene metamaterial is numerically and theoretically analyzed. The proposed metamaterial comprises of a pair of graphene split ring resonators placed alternately on both sides of a graphene strip of nanometer scale. The PIT effect in the graphene metamaterial is studied for different vertical and horizontal configurations. We have shown that the PIT effect can be tuned by varying the Fermi energy of the graphene layer. A theoretical model using the three-level plasmonic system is established in order to validate the numerical results. Our studies could be significant in designing graphene-based frequency agile ultra-thin devices for terahertz applications.

Published in: "EPL".

Polarization modulation based on the hybrid waveguide of graphene sandwiched structure

2017-11-28T14:30:41+00:00 November 28th, 2017|Categories: Publications|Tags: |

Polarization beam splitter (PBS) plays an important role to realize beam control and modulation. A novel hybrid structure of graphene sandwiched waveguide is proposed to fulfill polarization manipulation and selection based on the refractive index engineering techniques. The fundamental mode of TM cannot be supported in this case. However, both TE and TM mode are excited and transmitting in the hybrid waveguide if the design parameters, including the waveguide width and the waveguide height, are changed. The incident wavelength largely affects the effective index, which results in supporting/not supporting the TM mode. The proposed design exhibits high extinction ratio, compact in size, flexible to control, compatible with CMOS process, and easy to be integrated with other optoelectronic devices, allowing it to be used in optical communication and optical information processing.

Published in: "EPL".

Casimir energy for acoustic phonons in graphene

2017-11-06T14:30:17+00:00 November 6th, 2017|Categories: Publications|Tags: , |

We find the Casimir energy, at finite temperature, for acoustic phonons in a graphene sheet suspended over a rectangular trench, and the corresponding Casimir forces are interpreted as correction terms to the built-in tensions of the graphene. We show that these corrections generally break the tensional isotropy of the membrane, and can increase or decrease the membrane tension. We demonstrate that for a narrow rectangular trench with side-lengths in the order of few nanometers and few micrometers, these temperature corrections are expected to be noticeable ##IMG## [http://ej.iop.org/images/0295-5075/119/4/48002/epl18766ieqn1.gif] {$({sim}10^{-4} text{N/m})$} at room temperature. These corrections would be even more considerable by increasing the temperature, and can be applied for adjusting the built-in tension of the graphene. Consequently we introduce a corrected version for the fundamental resonance frequency of the graphene resonator.

Published in: "EPL".

Dynamically tunable electromagnetically-induced-transparency–like resonances in graphene nanoring and nanodisk hybrid metamaterials

2017-11-02T14:29:09+00:00 November 2nd, 2017|Categories: Publications|Tags: |

A tunable electromagnetically-induced-transparency–like (EIT-like) device is proposed numerically and theoretically in the mid-infrared region, which is composed of periodically patterned ring and disk graphene. Distinguished from the commonly used three-level system, the hybridization of the plasmon mode is applied to describing and explaining the EIT-like phenomenon in the proposed systems. What is more, further researches have revealed that the spectral position of the transparency window can be tuned not only by geometrically changing the couple distance in graphene nanostructures, but also by dynamically altering the radius of the graphene nanodisk and the chemical potential of the graphene. At the transparency window, there exist large optical delays, which can slow down the speed of light in vacuum. This work may pave the way to the development of applications including tunable sensors, slow-light devices, and optical switches.

Published in: "EPL".

Quantum Hall effect in monolayer and bilayer black phosphorus

2017-10-25T22:29:21+00:00 October 25th, 2017|Categories: Publications|Tags: |

We numerically study the quantum Hall effect in both monolayer and bilayer black phosphorus (BP) under an external electric field in the presence of both disorder and a strong perpendicular magnetic field based on the tight-binding model. In the monolayer BP case, the quantized Hall conductivity is similar to that of a two-dimensional electron gas (2DEG), but the positions of all the Hall plateaus shift to the left due to the spectral asymmetry, in agreement with the experimental observations. The width of ##IMG## [http://ej.iop.org/images/0295-5075/119/3/37005/epl18755ieqn1.gif] {$nu=0$} Hall plateau can be increased by applying a bias voltage to the top and bottom layers of the monolayer BP. The longitudinal conductivity exhibits a strong anisotropy that is large along the armchair direction. In the bilayer BP case, the Hall conductivity remains the same quantization rule as in the case of monolayer BP. Under a certain bias voltage, the ##IMG##

Published in: "EPL".

Investigation on the extended range of absorbing film for a microcavity enhanced graphene photodetector

2017-10-06T14:30:52+00:00 October 6th, 2017|Categories: Publications|Tags: |

Microcavity is the preferred graphene-based photodetector structure for its perfect feature of narrow spectral width and absorption enhancement, thus its application sheds light on ultra-fast detection in optic telecommunication and sensing fields. Due to an extremely thin film of graphene, the present study naturally deems it essential to locate the graphene in the exact position of resonant peak intensity. Here an extended graphene position margin in asymmetric planar microcavity with absorption higher than 97.35% was demonstrated. The shift of the centre wavelength caused by graphene was revealed to be non-negligible for telecommunication applications and graphene applied devices. The maximum shift beyond the designed wavelength of 1550 nm has reached 1.07 nm in the ##IMG## [http://ej.iop.org/images/0295-5075/119/2/24003/epl18709ieqn1.gif] {$lambda/2$} cavity, which may have severe impact on the DWDM system. Our theoretical investigation amplifies the …

Published in: "EPL".

Lattice symmetries, spectral topology and opto-electronic properties of graphene-like materials

2017-09-19T14:30:49+00:00 September 19th, 2017|Categories: Publications|Tags: |

The topology of the band structure, which is determined by the lattice symmetries, has a strong influence on the transport properties. Here we consider an anisotropic honeycomb lattice and study the effect of a continuously deformed band structure on the optical conductivity and on diffusion due to quantum fluctuations. In contrast to the behavior at an isotropic node we find super- and subdiffusion for the anisotropic node. The spectral saddle points create van Hove singularities in the optical conductivity, which could be used to characterize the spectral properties experimentally.

Published in: "EPL".

Enhancing light absorption in graphene with plasmonic lattices

2017-09-18T16:29:45+00:00 September 18th, 2017|Categories: Publications|Tags: |

We present a novel strategy based on metallic arrays of nanoparticles for enhancing the optical absorption in a monolayer of graphene. The collective resonances sustained by the metallic array and arising from the radiative coupling of localized surface plasmon resonances favour the interaction between the graphene layer and the light distributed in the structure. We measure the dispersion diagram of the allowed hybrid plasmonic-photonic modes and we calculate that one of these modes leads to an enhancement of the optical absorption of graphene by a factor 7. We propose to exploit this result for the rational design of graphene-based photodetectors.

Published in: "EPL".

Strong enhancement of third-harmonic generation in a double layer graphene system caused by electron-hole pairing

2017-09-04T14:29:35+00:00 September 4th, 2017|Categories: Publications|Tags: |

A manifestation of electron-hole pairing in nonlinear electromagnetic response of a double layer graphene system is studied. It is shown that the pairing causes the appearance of a number of peaks in the frequency dependence of the intensity of the third-harmonic generation (THG). The highest peak corresponds to ##IMG## [http://ej.iop.org/images/0295-5075/118/6/67008/epl18672ieqn1.gif] {$hbaromega = (2/3)Delta$} , where ω is the incident wave frequency, and Δ is the order parameter of the electron-hole pairing. The absolute value of the THG intensity in the systems with electron-hole pairing is of several orders of magnitude greater than the THG intensity in the unpaired state. It is shown that huge enhancement of the THG intensity occurs both in the double monolayer and double bilayer graphene systems.

Published in: "EPL".

Site-dependent lattice dynamics in periodically rippled graphene on Ru(0001)

2017-06-23T13:16:11+00:00 June 23rd, 2017|Categories: Publications|Tags: , |

Phonon dispersion in periodically rippled graphene/Ru(0001) has been studied by high-resolution electron energy loss spectroscopy and density functional theory. We show that, in addition to the usual phonon modes of the free-standing graphene lattice, the phonon spectrum of graphene/Ru(0001) exhibits additional modes arising from the peculiar nanostructured morphology of this system. In particular, the out-of-plane optical phonon has two components at 83 and 105 meV, related to the spectral contribution arising from carbon atoms in the valleys and in the hills of the ripples, respectively.

Published in: "EPL".

A new material property of graphene: The bending Poisson coefficient

2017-06-23T13:16:08+00:00 June 23rd, 2017|Categories: Publications|Tags: |

The in-plane infinitesimal deformations of graphene are well understood: they can be computed by solving the equilibrium problem for a sheet of isotropic elastic material with suitable stretching stiffness and Poisson coefficient ##IMG## [http://ej.iop.org/images/0295-5075/118/2/26001/epl18559ieqn1.gif] {$nu^{(m)}$} . Here, we pose the following question: does the Poisson coefficient ##IMG## [http://ej.iop.org/images/0295-5075/118/2/26001/epl18559ieqn2.gif] {$nu^{(m)}$} affect the response to bending of graphene? Despite what happens if one adopts classical structural models, it does not. In this letter we show that a new material property, conceptually and quantitatively different from ##IMG## [http://ej.iop.org/images/0295-5075/118/2/26001/epl18559ieqn3.gif] {$nu^{(m)}$} , has to be introduced. We term this new parameter bending Poisson coefficient ; we propose for it a physical interpretation in terms…

Published in: "EPL".

Double valley Dirac fermions for 3D and 2D Hg 1− x Cd x Te with strong asymmetry

2017-06-22T13:15:54+00:00 June 22nd, 2017|Categories: Publications|

In this paper the possibility to bring about the double-valley Dirac fermions in some quantum structures is predicted. These quantum structures are: strained 3D Hg 1− x Cd x Te topological insulator (TI) with strong interface inversion asymmetry and the asymmetric Hg 1− x Cd x Te double quantum wells (DQW). The numerical analysis of the dispersion relation for 3D TI Hg 1− x Cd x Te for the proper Cd ( x )-content of the Hg 1− x Cd x Te compound clearly shows that the inversion symmetry breaking together with the unaxial tensile strain causes the splitting of each of the Dirac nodes (two belonging to two interfaces) into two in the proximity of the Γ-point. Similar effects can be obtained for asymmetric Hg 1− x Cd x Te DQW with the proper content of Cd and proper width of the quantum wells. The aim of this w…

Published in: "EPL".

Effect of edge vacancies on performance of planar graphene tunnel field-effect transistor

2017-06-20T13:15:36+00:00 June 20th, 2017|Categories: Publications|Tags: |

The influence of edge vacancies on the working ability of the planar graphene tunnel field-effect transistor (TFET) is studied at various concentrations and distributions (normal, uniform, periodic) of defects. All calculations are performed by using the Green’s function method and the tight-binding approximation. It is shown that the transistor performance depends critically on two important factors associated with the defects: the destruction of the edge-localized electronic states and the emergence of subpeaks near the Fermi level. The supportable operation conditions of the TFET are found to be ensured at 30 percent or less of edge vacancies regardless of the type of their distribution.

Published in: "EPL".

Spin torques due to various linear spin-orbit coupling in semiconductor and graphene systems in the adiabatic limit

2017-06-19T13:15:40+00:00 June 19th, 2017|Categories: Publications|Tags: |

We use the gauge formalism to investigate the current-induced spin dynamics in ferromagnetic media with spatially varying local magnetization, which is coupled to various material systems exhibiting linear spin-orbit coupling (SOC) effects, such as semiconductor and graphene materials. We perform a gauge transformation to the system, and obtain a gauge field (vector potential) in the adiabatic limit, i.e. , strong coupling between the spin of the conduction electrons to the magnetization. The gauge field interacts with the applied current, resulting in a current-driven effective magnetic field and the corresponding spin torque acting on the magnetization of the FM media. We find that the current-driven spin orbit torque in various linear SOC systems and graphene systems can be described by a unified way. We propose a generalized Landau-Lifshitz-Gilbert (LLG) equation which includes this effective field term.

Published in: "EPL".

Analytic solution for gauged Dirac-Weyl equation in (2 + 1)-dimensions

2017-06-12T15:15:46+00:00 June 12th, 2017|Categories: Publications|Tags: |

A gauged Dirac-Weyl equation in ##IMG## [http://ej.iop.org/images/0295-5075/118/2/21001/epl18543ieqn3.gif] {$(2+1)$} -dimensions is considered. This equation has the particularity to describe the states of a graphene Dirac matter. In particular we are interested in matter interacting with a Chern-Simons gauge field. We show that exact self-dual solutions are admitted. These solutions are the same as those supported by nonrelativistic matter interacting with a Chern-Simons gauge field.

Published in: "EPL".

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