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Controlled engineering of spin-polarized transport properties in a zigzag graphene nanojunction

2018-11-13T12:33:23+00:00November 13th, 2018|Categories: Publications|Tags: |

We investigate a novel way to manipulate the spin-polarized transmission in a two-terminal zigzag graphene nanoribbon in the presence of the Rashba spin-orbit (SO) interaction with a circular-shaped cavity engraved into it. A usual technique to control the spin-polarized transport behaviour of a nanoribbon can be achieved by tuning the strength of the SO coupling, while we show that an efficient engineering of the spin-polarized transport properties can also be done via cavities of different radii engraved in the nanoribbon. Simplicity of the technique in creating such cavities in the experiments renders an additional handle to explore transport properties as a function of the location of the cavity in the nanoribbon. Further, a systematic assessment of the interplay of the Rashba interaction and the dimensions of the nanoribbon is presented. These results should provide useful input to the spintronic behaviour of such devices. In addition to the spin polarization, we have also …

Published in: "EPL".

On the influence of dilute charged impurity and perpendicular electric field on the electronic phase of phosphorene: Band gap engineering

2018-11-13T12:33:20+00:00November 13th, 2018|Categories: Publications|Tags: |

Tuning the band gap plays an important role for applicability of 2D materials in the semiconductor industry. The present paper is a theoretical study on the band gap engineering using the electronic density of states (DOS) of phosphorene in the presence of dilute charged impurity and of a perpendicular electric field. The electronic DOS is numerically calculated using a combination of the continuum model Hamiltonian and the Green’s function approach. Our findings show that the band gap of phosphorene in the absence and presence of the perpendicular electric field decreases with increasing impurity concentration and/or impurity scattering potential. Further, we found that in the presence of opposite perpendicular electric fields, the electronic DOS of disordered phosphorene shows different changing behaviors stemming from the Stark effect: in the positive case the band gap increases with increasing electric-field strength; whereas in the negative case the band gap disappears. The…

Published in: "EPL".

Andreev reflection across a Kane-Mele normal-superconductor nano-junction

2018-11-08T16:34:09+00:00November 8th, 2018|Categories: Publications|Tags: |

We have investigated the transport properties of a Kane-Mele normal-superconductor (NS) nano-junction using the familiar Blonder-Tinkham-Klapwijk (BTK) theory. The effects of the Rashba and the intrinsic spin-orbit coupling are mimicked by the inclusion of different transition metal adatoms adsorbed in a graphene nanoribbon. Specifically, we have focussed on the Andreev reflection phenomena for a range of Rashba and intrinsic coupling strengths. We have computed the spin resolved tunneling conductance where we found that the conductance characteristics are very sensitive to the strengths of the spin-orbit couplings. Further an interesting interplay between the Rashba and the intrinsic spin-orbit couplings is observed and its effects on the tunneling conductance are explored in detail. The possibility of tuning the spin-orbit couplings via different metal adatoms provides an experimental handle for achieving tunable conductance properties of this Kane-Mele nano-junction.

Published in: "EPL".

Enhancing thermal rectification in graphene-carbon nanotube junctions by tuning the chirality of pillar

2018-09-21T14:33:18+00:00September 21st, 2018|Categories: Publications|Tags: |

This letter investigates thermal rectification (TR) in graphene-carbon nanotube (GN-CNT) junctions formed by SWCNT(12, 12) connected with a single-layer graphene nanosheet (GN-SWCNT(12, 12)). It is found that the TR ratio of GN-SWCNT junction can be enhanced dramatically by tuning the chirality of the pillar. TR ratio of the GN-SWCNT(12, 12) junction can respectively reach up to 1487% and 2586.4% at temperatures of 300 K and 200 K ##IMG## [http://ej.iop.org/images/0295-5075/123/4/44004/epl19293ieqn1.gif] {$(vert Delta vert =0.5)$} , much higher than those previously reported for the pillared graphene and GN-CNT junctions. The influences of the geometric parameters on the thermal rectification are discussed. The results could offer useful guidelines to the design and performance improvement of the GN-CNT–based thermal rectifier.

Published in: "EPL".

Wannier-Koopmans method calculations of organic molecule crystal band gaps

2018-09-04T14:33:11+00:00September 4th, 2018|Categories: Publications|

It is important to accurately predict the band gaps of crystals, including organic crystals, with low computational cost. Despite the significant underestimation of the crystal band gap by the density functional theory (DFT), a recently proposed Wannier-Koopmans method (WKM) based on DFT calculations seems to yield accurate band gaps for a wide class of materials including common semiconductors, alkali halides and 2D materials. It is nevertheless important to test the limit of WKM, in particular in systems with unique characteristics. In this work, we apply the WKM to 10 organic small molecule crystals and find that the WKM calculated band gaps agree well with GW results. We also introduce a new way to calculate the Wannier functions in the WKM calculations.

Published in: "EPL".

Edge-mode–based graphene nanomechanical resonators for high-sensitivity mass sensor

2018-09-04T14:33:09+00:00September 4th, 2018|Categories: Publications|Tags: |

We perform both molecular dynamics simulations and theoretical analysis to study the sensitivity of the mass sensor based on graphene nanomechanical resonators, which are actuated following the global extended mode or the localized edge mode. We find that the mass detection sensitivity corresponding to the edge mode is about three times higher than that corresponding to the extended mode. Our analytic derivations reveal that the enhancement of the sensitivity originates from the reduction of the effective mass for the edge mode due to its localizing feature. Our findings shed light on improving the sensitivity of the graphene-based mass spectrometry by utilizing the localized edge mode.

Published in: "EPL".

Electron scattering in gapped graphene quantum dots

2018-08-20T14:33:36+00:00August 20th, 2018|Categories: Publications|Tags: , |

Due to Klein tunneling in graphene only quasi-bound states are realized in graphene quantum dots by electrostatic gating. Particles in the quasi-bound states are trapped inside the dot for a finite time and they keep bouncing back and forth till they find their way out. Here we study the effect of an induced gap on the scattering problem of Dirac electrons on a circular electrostatically confined quantum dot. Introducing an energy gap inside the quantum dot enables us to distinguish three scattering regimes instead of two in the case of gapless graphene quantum dot. We will focus on these regimes and analyze the scattering efficiency as a function of the electron energy, the dot radius and the energy gap. Moreover, we will discuss how the system parameters can affect the scattering resonances inside the dot.

Published in: "EPL".

A continuum model of lithium ion transport inside graphene

2018-08-13T18:33:27+00:00August 13th, 2018|Categories: Publications|Tags: , |

In this letter, we demonstrate the usage of a continuum equation in conjunction with Poisson equation and mean-field theory to investigate the ion transport and storage pattern of lithium ions between double-layer graphene. The majority of recent research on the ion transport for such batteries merely focuses on the effect of an external electric field acting on ions. Here, we emphasize the nanoscale mechanics of the graphene anode so that the forces between ions and the host material, and steric effects between lithium ions are incorporated. Under certain electric fields, multi-layers are formed between graphene sheets, and some related storage phenomenon is also investigated for potential applications in lithium ion battery and providing further understanding of ion transport inside biological channels.

Published in: "EPL".

Tunable Anderson localization of propagating graphene surface plasmon modes in a random modulated graphene monolayer

2018-08-03T14:33:15+00:00August 3rd, 2018|Categories: Publications|Tags: , |

Graphene is a two-dimensional material that has attracted a lot of attention due to the excitation of surface plasmons in the mid-infrared region. In this letter, we present the emergence of Anderson localization of propagating surface plasmon in a monolayer of graphene imbedded on a silicon random grating nanostructure. Several new localized graphene surface plasmon (GSP) modes with higher intensity and quality factor occur with respect to corresponding modes in the periodic substrate structure. Although tuning the resonance wavelengths belonging to Anderson localized GSPs is a difficult task due to its random nature, here we tune these wavelengths by adjusting the Fermi energy of the monolayer graphene.

Published in: "EPL".

Phase diagrams for superfluidity of indirect excitons in double Hall systems GaAs/GaAlAs/GaAs and bilayer-graphene/hBN/bilayer-graphene

2018-07-31T14:33:29+00:00July 31st, 2018|Categories: Publications|Tags: , , |

The energy competition of the reentrant integer quantum Hall state and the superfluid Bose Einstein condensate of indirect excitons in twin 2D Hall systems separated by an insulating barrier and with complementary layer fillings, ##IMG## [http://ej.iop.org/images/0295-5075/123/1/16001/epl19212ieqn1.gif] {$nu_{textit{bot}}+nu_{textit{top}}=1$} , has been analysed and compared with experimental observations in GaAs/GaAlAs/GaAs and bilayer-graphene/hBN/bilayer-graphene in the so-called counterflow and drag configurations. The related phase diagrams for varying fillings of layers and barrier thickness have been determined. A modification of the phase transition diagrams due to a change of the drag to the counterflow configuration is illustrated in agreement with the observations. The presented model appears to be consistent with the experimental observations.

Published in: "EPL".

Phase diagrams for superfluidity of indirect excitons in double Hall systems GaAs/GaAlAs/GaAs and bilayer-graphene/hBN/bilayer-graphene

2018-07-31T14:33:28+00:00July 31st, 2018|Categories: Publications|Tags: , , |

The energy competition of the reentrant integer quantum Hall state and the superfluid Bose Einstein condensate of indirect excitons in twin 2D Hall systems separated by an insulating barrier and with complementary layer fillings, ##IMG## [http://ej.iop.org/images/0295-5075/123/1/16001/epl19212ieqn1.gif] {$nu_{textit{bot}}+nu_{textit{top}}=1$} , has been analysed and compared with experimental observations in GaAs/GaAlAs/GaAs and bilayer-graphene/hBN/bilayer-graphene in the so-called counterflow and drag configurations. The related phase diagrams for varying fillings of layers and barrier thickness have been determined. A modification of the phase transition diagrams due to a change of the drag to the counterflow configuration is illustrated in agreement with the observations. The presented model appears to be consistent with the experimental observations.

Published in: "EPL".

Cross-Kerr nonlinearity in the surface plasmon polariton waves generated at the interface of graphene and gain medium

2018-07-18T12:33:52+00:00July 18th, 2018|Categories: Publications|Tags: |

Surface plasmon polariton (SPP) waves generated at the interface of graphene and a gain-assisted medium due to the effective Kerr nonlinear variation of the dielectric function are controlled and modified. The gain doublet in the SPP waves is measured at the interface with collective cross-Kerr nonlinear probe signals which generates Kerr nonlinearity in the SPPs. The alternate normal and anomalous dispersion of SPPs is controlled under Kerr nonlinearity and strength of the control fields, leading to slow and fast SPPs propagation. Further, the propagation length of SPPs is controlled under Kerr nonlinearity and strength of the control fields. The controlled SPPs show significant importance for optical tweezers, radiations guiding, nano-photonics, plasmonster technology, photovoltaic devices, data storage, solar cells and biosensor technology.

Published in: "EPL".

High intrinsic dissipation of graphyne nanotubes

2018-06-27T16:33:15+00:00June 27th, 2018|Categories: Publications|Tags: |

We utilize molecular-dynamics simulations to report the first investigation of energy dissipation of two different doubly clamped graphyne nanotubes (GNTs), where the point of emphasis is to compare their dissipation characteristics with those of carbon nanotube (CNT). The obtained results demonstrate that: (a) GNTs exhibit significantly higher energy dissipation, and thus lower quality ( Q ) factor which is generally five times lower than that of CNT; (b) the Q factor of GNT further reduces with the increasing percentage of acetylenic linkages, which originates from the larger vibrational mismatch between the acetylenic linkage ( sp C-C bonds) and the hexagonal ring ( sp 2 C-C bonds); (c) the application of tensile strain is found to be highly beneficial to improving the Q factor of GNTs, especially for those with higher percentage of acetylenic linkages. These findings enable a first insight into the damping behavior of GNTs and also o…

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Atomically thin mononitrides SiN and GeN: New two-dimensional wide band gap semiconductors

2018-06-27T16:33:13+00:00June 27th, 2018|Categories: Publications|

To match well with the Si-based electronics and satisfy the demand of miniaturization in modern industry, low-dimensional Si-based semiconductors maybe are the best candidates. Owing to the lack of such materials, in this work, we predicted two-dimensional atomically thin mononitrides SiN and GeN via a swarm structure search method and density functional theory. By the employment of HSE functional, both SiN and GeN are calculated as indirect wide band gap semiconductors with the gaps of ##IMG## [http://ej.iop.org/images/0295-5075/122/4/47002/epl19156ieqn1.gif] {${sim}2.75$} and ##IMG## [http://ej.iop.org/images/0295-5075/122/4/47002/epl19156ieqn2.gif] {${sim}2.25 text{eV}$} , respectively. Ab initio molecular-dynamics calculation displays that both mononitrides can exist stably even at an extremely high temperature of ##IMG## [http://ej.iop.org/images/0295-5075/122/4/47002/epl19…] {${sim}2000 text{K}$}

Published in: "EPL".

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

2018-06-19T16:33:14+00:00June 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+00:00April 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+00:00March 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+00:00March 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+00:00January 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+00:00January 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".

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